title_ integrated analyses of submarine volcanic structure…

UNIVERSITÀ di ROMA

SAPIENZA

Dipartimento di Scienze della Terra

Tesi di Dottorato di Ricerca

XXII° Ciclo

Integrated analyses of the submarine volcanic

structures offshore Pantelleria

Dottoranda: Dott.ssa Marilena Calarco

Docente guida: Prof. F.L.Chiocci

Referees: Dott.ssa C.Romagnoli e Dott. D.A. Clague Coordinatore dottorato: Prof.ssa L.Corda

“Do you get aroused from squeezing balloons until

they explode? Have you ever been patient enough to stand in front of a ripe flower bud for hours to witness its slow explosion into full opening? If you answered yes lots of things are going to change into something else through the process of eruption or sprouting or bursting forth. I bet you'll dream of undersea volcanoes spurting”

R. Brezsney - Aries - August 7, 2008

Acknowledgements

I would like to express my gratitude to the official referees of my thesis, Claudia

Romagnoli and Dave Clague, for their helpfulness and comments. Their critical and constructive reviews of my work helped to improve this thesis considerably and stimulate me to do better and better.

I would like to also thank Jenny Paduan for her revisions and for the enthusiasm

and encouragement during the last years. Thanks are due to my supervisor Francesco L. Chiocci for the possibility to work

on these data and for valuable discussions during the last months. I greatly appreciated the possibility to work at the University of Rome, as well as

the period I spent at the Monterey Bay Aquarium Research Institute, which not only gave me the possibility to learn more and grow my passion for volcanism, but also gave me new scientific contacts and personal friendships with the people I meet in California.

My thanks go to my colleagues at the department that I have collaborated with

and shared lunch-time or coffee together. I always enjoyed the helpful collaboration and nice discussions (about science and other exciting things of life) especially with Eleonora, Aida, and Andrea who showed me that the “fog” could be also out of my thoughts!

Thanks to captain and the crew of the R/V Urania, and all researchers and

students that participated in the cruises at Pantelleria. Further thanks go to all my friends from all over the world for their huge

support. I greatly appreciate your everlasting encouragement to stay strong and positive, and for distracting me from work at the right times.

Moreover, the financial support of the University of Rome, Sapienza is gratefully acknowledged as well as other “sponsorship” during the last months.

Last but not least, I deeply thank my parents, my sister, and Keith. Without their

encouragement, assistance, and understanding, it would have been impossible for me to finish this work.

Pursuing this Ph.D. project was challenging for me. I hope I have learned as much

as possible both from the “not always positive” and the enjoyable times.

Table of contents

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Table of contents

INTRODUCTION………………………………………………………………………….1

1 GEOLOGICAL SETTING OF PANTELLERIA VOLCANO

1.1 GEOLOGICAL AND STRUCTURAL SETTING OF THE SICILY CHANNEL ……..3

1.1.1 SICILY CHANNEL VOLCANISM ………………………………………………5

1.2 PANTELLERIA VOLCANIC COMPLEX ……………………….………………………6

1.2.1 STRUCTURAL SETTING OF PANTELLERIA ……………………………….8

1.2.2 VOLCANIC HISTORY OF PANTELLERIA ISLAND …………………………9

1.2.2.1 PALEO-PANTELLERIA (324±10.5 KA–~50 KA) …………………..10

1.2.2.2 NEO-PANTELLERIA (<50 KA) .……………………………………11

• RECENT VOLCANIC ACTIVITY - SUBMARINE ERUPTION OF 1891

1.2.3 VOLCANIC MORPHOLOGIES OF PANTELLERIA ISLAND ..…………..17

1.2.4 OFFSHORE GEOPHYSICAL STUDIES …………………………………….18

2 GROWTH AND EVOLUTION OF VOLCANIC ISLANDS

2.1 EDIFICE GROWTH……………………………………………………………………..20

2.2 EROSIONAL AND INSTABILITY PROCESSES ……………………………………23

2.3 VOLCANICLASTIC PARTICLE TYPES ……………………………………………...24

2.3.1 MORPHOLOGY OF VOLCANIC GLASS FRAGMENTS ………………….25

2.4 SUBMARINE SEDIMENTATION AROUND VOLCANIC ISLANDS ……………….27

3 METHODOLOGIES AND DATASET

3.1 MULTIBEAM ECHO-SOUNDER AND DTMs ………………………………………..32

3.1.1 MBE DATASET: PROCESSING AND ANALYSIS …………………………33

3.2 SINGLE-CHANNEL REFLECTION SEISMIC SURVEYS ………………………….35

3.2.1 SEISMIC DATASET: PROCESSING AND ANALYSIS ……………………37

3.3 SEAFLOOR SAMPLING METHODS …………………………………………………38

3.3.1 SAMPLING DATASET ………………………………………………………...40

3.3.2 ANALYSIS METHODS ………………………………………………………..41

3.3.2.1 ELECTRON MICROPROBE ANALYSIS (EMPA) AND SCANNING

ELECTRON MICROSCOPE (SEM) ………………………………………….41

3.3.2.2 GRAIN-SIZE AND CALCIMETRY ANALYSES ..………………….42

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4 ANALYSIS OF SUBMARINE FLANKS OF PANTELLERIA VOLCANO

• General morphologies of Pantelleria offshore

• Pantelleria volcano

4.1 NORTHWESTERN SECTOR ………………………………………………………….51

4.1.1 THE NW INSULAR SHELF ……………………………………………………51

4.1.1.1 UNDERWATER EXTENSION OF SUBAERIAL LAVAS …………..53

4.1.1.2 DEEP FEATURES ON THE NW INSULAR SHELF ……...……….57

4.1.2 THE NW DEEP FLANK …..………………………………………………….58

4.1.2.1 THE NW VOLCANIC FIELD ..……………………………………….59

• 1891 VENT

4.1.2.2 INSTABILITY ALONG THE SLOPE ..………………………………61

4.2 SOUTHWESTERN SECTOR ………………………………………………………….66

4.2.1 THE SW INSULAR SHELF …………………………….……………………...66

4.2.2 SW DEEP FLANK …………………..……………………….………………..68

4.3 NORTHEASTERN SECTOR …………………………………………………………..73

4.3.1 THE NE INSULAR SHELF………………………………………….………….73

4.3.2 NE DEEP FLANK .…………………………………………….……………….78

4.4 SOUTHEASTERN SECTOR ………………………………………………………….80

4.4.1 THE SE INSULAR SHELF ……….……………………………………………80

4.4.2 SE DEEP FLANK ………………………………………………………………82

4.5 SUB-SURFACE FACIES MAP ..…………………………………………………..…88

5 SEAFLOOR SAMPLING

5.1 SAMPLES FROM THE 1891 ERUPTION AND SURROUNDING AREA …..…89

5.1.1 GRAIN-SIZE AND CALCIMETRY ANALYSES ON THE PC1 CORE .….96

5.1.2 THE MORPHOLOGY OF GLASS FRAGMENTS …………………………..99

5.1.3 GLASS AND SCORIA COMPOSITION ……………………………...……106

5.2 OTHER SAMPLES COLLECTED OFFSHORE PANTELLERIA ………………….110 6 DISCUSSION AND CONCLUSION

6.1 THE SUBMARINE FLANKS OF PANTELLERIA VOLCANO …………………….119

6.1.1 OVERALL PHYSIOGRAPHY OF PANTELLERIA AND ITS

RELATIONSHIP WITH THE CONTINENTAL RIFT ……………………………..121

6.1.2 GENERAL MORPHOLOGY OF THE INSULAR SHELF …………………122

• Shelf break

6.1.3 MASS WASTING AND EROSIONAL PROCESSES ON THE

PANTELLERIA FLANKS …………………………………………………………...127

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6.1.4 SUBMARINE VOLCANIC MORPHOLOGIES ……………………………130

• Shallow lava lobes

• Deep lava lobes

• Shallow volcanic centers

• Deep volcanic centers

• Volcanic outcrops and dikes

6.2 CONSIDERATION OF THE SHELF GEOLOGY AND ENVIRONMENT ………140

6.2.1 COASTAL LAVA LOBES AND CORRELATION WITH ON LAND ACTIVITY

6.2.2 DEEP PORTION OF THE NW SHELF ……………………………….……145

6.2.3 CONCLUDING REMARKS OF THE SHELF GEOLOGY ………………..148

6.3 QUANTITATIVE GEO-MORPHOMETRIC ANALYSIS OF THE VOLCANIC

CENTERS ………………………………………………………………………………..…149

6.3.1 METHOD ………………………………………………………………………149

6.3.2 GEOMETRY OF THE VOLCANIC CENTERS …………………………….155

6.3.3 MORPHOLOGICAL PARAMETERS OF THE VOLCANIC CENTERS …159

6.3.4 DISTRIBUTION OF VOLCANIC CONES IN THE NW FIELD …………...165

6.3.5 SUMMIT COLLAPSES ..……………………………………………………167

6.3.6 SUMMARY OF THE GEO-MORPHOMETRIC ANALYSIS ………………173

6.4 SAMPLES FROM THE NW VOLCANIC FIELD AND RELATION WITH THE 1891

ERUPTION …………………………………………………………………………………174

6.4.1 CASE HISTORY OF FLOATING BASALTIC LAVA BLOCKS …………..174

6.4.2 PRODUCTS AND ERUPTIVE STYLE OF THE NW VOLCANIC

FIELD………………………………………………………………………………….176

• Re-location of the 1891 vent

• Eruption style of the 1891 eruption

• Evidence of earlier eruptions

6.4.3 CONCLUDING REMARKS ..……………………………………………….182

6.5 MARINE GEOHAZARDS ALONG THE ITALIAN COASTS ………………………183

6.6 MAIN CONCLUSION ...………………………………………………………………185

6.7 FUTURE WORK ………………………………………………………………………187

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References …………………………………..…………………………………………………188

APPENDIX A

APPENDIX B1, B2, B3

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Introduction Detailed investigations of the submarine portions of Pantelleria were carried out

in 2006 and 2008 (aboard R\V Urania) using a 50 kHz multibeam bathymetric

system and through collecting seafloor samples by dredging, grabbing and coring.

The high resolution bathymetric data, acquired from –20 m down to -1300 m,

furnished a digital terrain model allowing a detailed morphological reconstruction

of the seafloor geological features. The main focus of this work is the

characterization of the submarine flanks of Pantelleria. This was accomplished

through observations and geo-morphometric analysis particularly of the volcanic

morphologies.

For the first time, numerous volcanic cones were identified on the NW offshore

of Pantelleria and have been investigated based on morphological and

morphometric parameters, with the aim of summarizing the most significant

volcano-morphological parameters of these edifices.

Moreover, marine surveys used high resolution bathymetry and sampling to

locate and characterize the 1891 submarine eruption that occurred offshore

Pantelleria Island.

During the 1891 eruption, the observed activity lasted for 8-days with the

presence of steam plumes and floating lava basaltic bombs (“lava balloons”),

rising buoyantly to the sea surface 5 km W-NW from Pantelleria village (Riccò,

1892). The study of the products recovered in the area nearby the eruption

allowed for a rigorous characterization of this eruption in term of vent location and

eruptive style.

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Finally, the morphological mapping of the area offshore Pantelleria volcano was

made with the aim to assess potential marine geohazards.

This work provides a detailed overview of the area and a valuable road map for

designing further studies. The characterization and study of the collected samples

is still ongoing and offer a great opportunity not only for geology but also for other

natural science disciplines (e.g., biology, palaeontology).

Ch. 1 – Geological setting of Pantelleria volcano

3

Chapter 1

Geological setting of Pantelleria volcano

This chapter presents the geological and structural setting of the Pantelleria

volcanic complex. The geological description begins with an illustration of its

general location in the NW-SE Sicily Channel Rift Zone (SCRZ), located between

Sicily and Tunisia. The Pantelleria volcanic complex and its subaerial eruptive

history spanning more than 300 ka is presented next. This section ends with a

short summary of the current state-of-the-art of geophysical research in the area

offshore Pantelleria Island.

1.1 Geological and structural setting of the Sicily Channel

The Sicily Channel (Colantoni, 1975; Calanchi et al., 1989) is located between

Sicily and Tunisia (Fig. 1.1). Along its eastern margin it drops off into the Ionian

Sea along the Malta escarpment, a sharp fault system that reaches depths of

more than 3000 m and separates the Sicily Channel from the Ionian Abyssal Plain.

The northwestern limit of the SCRZ is less clearly delimited by a NE-SW

escarpment in a region between Tunisia and Sardinia (Morelli et al., 1975).

Fig. 1.1 Morphological diagram of the Sicily Channel (after Colantoni, 1975).


4

The Sicily Channel is a shallow epicontinental sea (mean depth of some 350

m); a large part of it was exposed when sea-level was lower during Pleistocene

glacial lowstands (Morelli et al., 1975). Basins up to 1700 m deep are located

along graben structures (e.g., the Pantelleria Rift), mainly in the axial zone of the

channel (Colantoni, 1975). From a physiographic point of view, about 35% of the

area belongs to the continental shelf, 55% to the continental slope, 3.5% to the

banks, and 5% to basins. Volcanic islands and seamounts occupy <1.5% of the

area.

The rift zone is a geologically recent feature, with a well-preserved topographic

expression. The central part of the Sicily Channel (Fig. 1.1) is a foundered area

characterized by three main deep and narrow tectonic depressions, namely the

Pantelleria, Linosa, and Malta basins.

These basins reach maximum depths of 1350 m , 1529 m , and 1721 m,

respectively, and are filled by more than 2000 m of Plio-Quaternary sediments.

Recent paleontological studies of cores show a sediment accumulation rate

around 20-25 cm/1000 yr (Colantoni, 1975).

The basins are bounded by steep NW-SE faults (Morelli et al., 1975). Deviation

from the main trend occurs due to the presence of transfer zones, most probably

controlled by E-W or N-S transverse structures (Corti et al., 2004). The NW-SE

trend of the Pantelleria basin (about 87x28 km wide and 1350 m deep) represents

the best example of a rift valley in the area (Calanchi et al., 1989). The basin

narrows westward into a depression trending roughly N-S (Fig. 1.1), which is

between the shallow Adventure Bank and Tunisian slope (Argnani, 1990).

The Sicily Channel is the most rifted and thinned crustal sector of the African

foreland south of the Maghrebian chain; it is made up of Mesozoic-Cenozoic

carbonate units which are underlain by older continental crust (Reuther and

Eisbacher, 1985). Rifting began in the Late Miocene and achieved its maximum in

the Pliocene to Late Quaternary. The crust thins from 25 km at the periphery of the

SCRZ to 18 km in the axial sector (Scarascia et al., 2000). At present, evidence of

rifting is concentrated along the axial area, where the asthenosphere shoals to ~60

km depth (Della Vedova et al., 1989).

The structural interpretation of the SCRZ is still debated and a clear relationship

between tectonic structures and volcanism has yet to be defined (Civile et al.,


5

2008). The Pantelleria, Linosa, and Malta basins are considered as pull-apart

basins with dextral transcurrent motion along the basin master faults (Cello et al.,

1985), or as extensional en-echelon grabens due to either 1) a dextral E-W

megashear with NW-SE maximum horizontal compression (Illies, 1981; Reuther

and Eisbacher, 1985) or 2) mantle convections developed during the roll-back of

the African lithosphere slab beneath the Tyrrhenian basin (Argnani, 1990).

Fig. 1.2 Schematic block diagram illustrating overlapping processes in the SCRZ where the growing accretionary prism crosscuts while at the same time being crosscut by the Sicily Channel rift (from Corti et al., 2004).

Moreover, the structural pattern of the northwestern side of the Sicily Channel is

characterized by a NE-SW striking accretionary prism. This prism is composed of

thrusts and folds and is cut by a system of NW-SE trending normal faults and

grabens. Observations from the structural and geophysical data suggest that the

Sicily Channel may have been shaped through two independent tectonic

processes: the Maghrebides-Apennines accretionary prism and the Sicily Channel

Rift. It is possible that these processes act simultaneously and overlap each other

(Fig.1.2) (Corti et al., 2004).

1.1.1 Sicily Channel volcanism

The Sicily Channel, as revealed by geophysical studies (Finetti, 1972), is a

Miocene carbonate platform with a system of grabens that foundered mostly

during the Pliocene, with basalt injections along the faults. Apart from the

injections, the area is magnetically uniform; this is confirmed by the presence of

continental crust and a thick sedimentary series in the area (Morelli et al., 1975).


6

The two SCRZ emergent volcanoes of Pantelleria and Linosa represent intense

Plio-Pleistocenic volcanism (Civetta et al., 1998). Many other submerged

volcanoes are located either in the rift zone or on the continental slope, and some

were active in the last few centuries (Imbo`, 1965; Zarudzki, 1972). The volcanism

has mainly basaltic affinity with the exception of the conspicuous presence of

pantellerites, pantelleritic trachytes, and trachytes outcropping only on Pantelleria

Island (Corti et al., 2006).

The volcanic edifice of Linosa is located in an off-axis position on the rift flank

(Fig. 1.1), possibly indicating lateral magma migration toward the footwall of major

boundary faults (Corti et al., 2004). Conversely, the Pantelleria edifice lies within

the graben (Fig. 1.1. & 1.2), suggesting that control may be exerted by a transfer

zone in favoring a rift-parallel magma migration and melt accumulation at the

transfer zone itself (Corti et al., 2004). Chemical and isotopic characteristics of

basalts from the Sicily Channel suggest that they were generated from a

subcrustal magma without direct sialic contamination (Colantoni, 1975). The

general petrologic features of Sicily Channel volcanism point to anorogenic

magmatism akin to that found in continental rift zones (Corti et al., 2006).

The volume of volcanic rocks emplaced in the Sicily Channel is not presently

known. A rough estimate suggests that it is less than 2000-3000 km3, which

supports its classification as a low-volcanicity rift (Barberi et al., 1982). This

volume is similar to that of the Western African Rift and the Rhine graben. In

contrast, the petrogenetic affinity of the volcanism (Calanchi et al., 1989) indicates

a good analogy with the high-volcanicity of the Eastern African Rift (Barberi et al.,

1982).

1.2 Pantelleria volcanic complex

The Pantelleria volcanic complex has a eruptive history older than 300 ka

(Mahood and Hildreth, 1986). The subaerial peralkaline volcanism of Pantelleria is

well known in the field of volcanology and the island is the type locality for

pantellerite. Pantellerites are Fe-rich, peralkaline, oversaturated volcanic rocks,

occasionally representing the felsic end-member of some bimodal suites in

oceanic islands and continental grabens. Pantelleria Island has such a bimodal


7

suite. On one extreme, there are mafic lavas, which include transitional basalt and

hawaiite that range in composition from ~46 to 49 wt.% SiO2. On the other, there

are felsic lavas and tuffs, which include metaluminous trachyte, peralkaline

trachyte, and pantellerite that range in composition from ~62 to 72 wt.% SiO2.

Rocks of intermediate composition (falling in the so-called ‘Daly Gap’) are rare

and occur in the form of enclaves in trachytes and pantellerites (Ferla and Meli,

2006). The origin of the ‘Daly gap’ and the petrogenetic relationships between

transitional basalt, trachyte, peralkaline trachyte, and pantellerite are still

controversial at Pantelleria and other similar volcanic complexes (White et al.,

2009).

Over the last 100 years, Pantelleria geological evolution and eruptive dynamics

have been studied by numerous researchers, reaching a peak period of

publications in the 1980’s. The conclusions of these different studies commonly

differ substantially.

The 13x8 km pear-shaped Pantelleria Island is the emergent part of a

submarine volcano structure, reaching 836 m above and about 1200 m below sea

level, with a surface area of about 84 km2. The subaerial portion of the island

consists of pantellerite and trachyte lava-flows and domes, ignimbrites from

caldera-forming pyroclastic eruptions, and alkali basalt, in decreasing order of

abundance (Fulignati et al., 1997).

The island can largely be described as a mosaic of coalescing pantelleritic

centers. Most of the explosive eruption cycles have built up monogenetic pumice

cones with heights varying between 25 and 100 m, with basal sections of 100 to

350 m in diameter, and with outer cone slopes ranging from between 15 and 35

degrees (Orsi et al., 1991). The explosive eruptions were often followed by

effusion of lavas.

The island is topographically dominated by a 6 km wide caldera that encloses

the Montagna Grande, a broad postcaldera cone of trachytic lavas that rises to

more than 800 m above sea level (Fig. 1.3; Mahood and Hildreth, 1986).

It appears that volcanic activity has propagated to the northwest over time. On

the southern coast of the island, erosion rates have exceeded constructional

volcanism resulting in 200 m sea cliffs cut into lavas and tuffs; these expose the

oldest subaerial rocks outcropping on the island. By contrast, the northwest lobe of

the island slopes gently to the sea and is mantled by young basalts and the


8

distinctive and widespread Green Tuff (Mahood and Hildreth, 1986). The most

recent activity was a submarine eruption of basalts, which occurred 4 km beyond

the northwest coast in 1891 (Riccò, 1892; Washington, 1909). Throughout the

island, evidence of an active hydrothermal field exists with steaming ground,

mofettes, and hot springs (Tmax 98°C), particularly in the intra and peri-caldera

zones (Fulignati et al., 1997).

1.2.1 Structural setting of Pantelleria

Both tectonic and volcano-tectonic features are present at Pantelleria.

The island’s structural setting reflects that of the SCRZ (Fig. 1.1). Tectonic

faults and fractures related to the regional stress regime have the same orientation

as the rift-bounding faults. In this regard, NW–SE trending fractures and right-

lateral strike-slip faults are the dominant lineaments, whereas NE–SW and N–S

trending lineaments are also common (Colantoni, 1975; Cello et al., 1985;

Catalano et al., 2009).

Fig. 1.3 Digital elevation model of Pantelleria Island (generated with a 40 m resolution

grid), including a structural sketch of the island (modified from Civile et al., 2008).


9

Moreover, a NNE–SSW fault, called Zinedi fault (ZF in Fig. 1.3), divides the NW

sector of the island (characterized by the occurrence of alkali-basalt effusion) from

the SE portion, where only peralkaline products (ranging from pantellerites to

trachytes) have erupted (Civile et al., 2008). The NW sector is possibly located on

thinner crust, which allows for the direct rise of magma from the mantle (Berrino,

1997). Taking into account that several volcanic belts trend NNE–SSW, a WNW–

ESE directed extension is the dominant deformation mode of Pantelleria Island

(Catalano et al., 2009).

The NW-trending fault system is also represented by the Scauri fault (SF in Fig.

1.3).

This fault is around 5 km long and extends primarily along the shore, defining

the island’s southwestern cliff (Fig. 1.3). It is here that the oldest rocks of the island

(from ~320 to 80 ka B.P.) crop out and are locally draped by the Green Tuff.

The volcano-tectonic features of the island are related to caldera collapses and

resurgence of the Montagna Grande inside the youngest caldera (Fig. 1.3). At

least two caldera collapses affected the island in recent times. The oldest caldera,

named Serra Ghirlanda (Cello et al., 1985) or La Vecchia (Mahood and Hildreth,

1986) is dated to 114 ka BP (LVCF in Fig. 1.3). The youngest is related to the

eruption of the Green Tuff (Orsi and Sheridan, 1984) and has been identified and

named differently by various authors (CDCF in Fig. 1.3) as Zighidi caldera (Cello

et al., 1985), Monastero caldera (Cornette et al., 1983), and Cinque Denti caldera

(Mahood and Hildreth, 1983); the latter term will be used in this thesis.

1.2.2 Volcanic history of Pantelleria Island

The volcanic history of Pantelleria, as reconstructed from terrestrial records,

started around 300 ka ago. It is characterized by cyclic activity, with periods of

quiescence followed by periods of low-energy eruption and large explosive

eruptions, some of which produced caldera collapses (La Vecchia and Cinque

Denti calderas). Before 50 ka BP, the history of the island cannot be reconstructed

in detail due to paucity of outcrops because of repeated caldera collapses, the

erosion of coastal cliffs, and the blanketing of the whole island by the Green Tuff

eruption at ~50 ka BP (Cornette et al., 1983).


10

The volcanological history of the Pantelleria complex has been divided by

several authors into two distinct phases separated by the Green Tuff eruption

(Villari, 1974; Cornette et al., 1983; Mahood and Hildreth, 1983; Orsi and

Sheridan, 1984; Mahood and Hildreth, 1986; Civetta et al., 1988; Orsi et al., 1991;

Civetta et al., 1998). These two phases are the Paleo-Pantelleria (324±10.5 ka -

~50 ka) and the neo-Pantelleria (~50 ka–recent) (Mahood and Hildreth 1983;

Civetta, D’Antonio et al. 1998).

1.2.2.1 Paleo-Pantelleria (324 ±10.5 ka–~50 ka)

Paleo-Pantelleria comprises different magmatic episodes with silicic lava flows

and pyroclastic rocks predominating over alkali basaltic lava flows. Ordering the

episodes is challenging because of the very few and scattered outcrops exposed.

The Paleo-Pantelleria activity can be subdivided in pre- and post-La Vecchia

intervals, divided by the formation of the La Vecchia caldera around 114 ka.

The pre-La Vecchia eruptive interval was dominated by lavas. The oldest rocks

exposed on Pantelleria occur at the southern shoreline and are pantellerite lavas

(Mahood and Hildreth, 1986). Early outcrops also were identified at Cuddia di

Scauri, Punta del Duce, La Ficara, and Cala dell'Altura (Fig. 1.4) and consist of

thin flows interbedded with variably welded fall deposits. Other lavas, such as

those exposed above Punta di Sciaccazza, beneath the village of Scauri and just

north of Cala della Capre (Fig. 1.4), are much thicker and aerially extensive.

Another eruptive center related to this period is Cuddia di Khamma (124 ± 6 Ka), a

pantelleritic lava shield well-exposed on sea cliffs near Punta Zinedi (Fig. 1.4).

La Vecchia caldera, which formed about 114 ka ago, is exposed

discontinuously in the central part of the island (Mahood and Hildreth, 1986). This

depression measures 7.8 x 6.8 km and occupies an area of approximately 42 km2,

using the Zinedi Fault as its northwest wall.

Post-La Vecchia volcanism took place along ring-fracture zones and built

edifices at Costa Monastero, Cuddia Attalora, and the string of about 14

pantellerite vents called the Cuddioli di Dietro Isola (Fig. 1.4). It produced

voluminous lavas, pumice falls, and welded tuffs, which partially filled La Vecchia

caldera (Mahood and Hildreth, 1986).


11

Fig. 1.4 Map of Pantelleria Island, with locations of pre-La Vecchia, post-La Vecchia, and mafic outcrops. The felsic eruptive centers were classified by Mahood & Hildreth (1986). CDCF and LVCF are Cinque Denti and La Vecchia caldera fault scarps. M.=Monte; C.=cuddia (hill) or cuddle (hills); cuddioli=little hills; cala=cove; contrada=district; costa=ridgecrest; p.ta= cape; +=elevations in meters (modified from Mahood and Hildreth, 1986).

1.2.2.2 Neo-Pantelleria (<50 ka)

The second period of the geological history of Pantelleria began with the Green

Tuff eruption (~50 ka), which covered over 50% of the island’s surface, outside the

related 6 km wide Cinque Denti caldera collapse structure (Mahood and Hildreth,

1983). The depression reaches a maximum of about 80 m, and the average


12

values are around 50-60 m (measured as the difference in the altitude of the rim

and the basal floor, Villari, 1970).

This Neo-Pantelleria period has been subdivided by Civetta et al. (1984; 1988)

into six silicic episodes sometimes intercalated with basaltic eruptions (Fig. 1.5).

As detailed below, the definition of the younger cycles was mainly based on

stratigraphy, occurrence of paleosoils, and location of vents.

The Green Tuff is considered representative of the first silicic cycle. The name

of this largely welded, compositionally-zoned sheet reflects its typical pistachio-

green color. The Green Tuff is the product of a complex eruption that included

phases of ignimbrites, fall, and surge. The deposits are primarily rheomorphic and

locally display extensive flow folding (Orsi and Sheridan, 1984). Offshore, this

eruption is correlated with the Y-6 ash layer, widespread in the central

Mediterranean Sea (Keller et al., 1978; Orsi and Sheridan, 1984; Anastasakis and

Pe-Piper, 2006).

Fig. 1.5 Geological sketch map of Pantelleria Island (modified from Civetta et al.,

1988).


13

Most of the activity of the second cycle is recorded in three volcanic edifices

aligned on a N-S trend: Mt. Gibele, Cuddia Gadir, and a third which is now only

partially cropping out in the Sidor area (Fig. 1.5). Part of Mt. Gibele, the only center

located within the younger caldera, subsequently underwent uplift and tilt to form

the Montagna Grande block (Washington, 1913-14; Mahood and Hildreth, 1983).

Mahood and Hildreth (1986) proposed that the Mt. Gibele-Montagna Grande

volcanic activity followed the Green Tuff eruption without a significant delay.

Subsequent geo-chronological studies by Civetta et al. (1988) indicate a period of

repose longer than 10 ka preceding this cycle.

The third silicic cycle includes pyroclastic-fall deposits, lava domes, and lava

flows (Fig. 1.5). Most of the vents are located outside the caldera's walls and align

on features following a NW-SE regional trend (Civetta et al., 1988). Dating of

sampled rocks of this cycle show similar ages grouped around 22 ka.

The fourth silicic cycle is mainly represented by pyroclastic-fall deposits, lava

domes, and lava flows. The products of this cycle (Fig. 1.5) have been grouped

together mostly on the basis of the location of their vents. They occur in a circular

array which was interpreted as the rim of the Cinque Denti caldera (Cornette et al.,

1983). All of the dated rocks range from 15 to 20 ka. Most of the eruptions show

two different phases: 1) an explosive phase with the formation of pantelleritic

pumice-fall deposits and 2) an effusive phase of less-evolved lava flows. Mt.

Gelfiser volcano is the best example of such an evolution. The rocks of this cycle

range in composition from pantellerite to pantelleritic-trachyte.

The fifth silicic cycle is recorded in pumice-fall deposits, lava domes, and lava

flows (Fig. 1.5). The vents show N-S and NW-SE trending alignments. K-Ar ages

range from 14 to 12 ka. During this cycle, only a very small amount of magma was

discharged. The rocks of this cycle have similar composition as the fourth cycle

ones.

The products of the sixth and last recognized silicic eruptive cycle on the island

range from pantellerite to trachyte (Civetta et al., 1988) and include pyroclastic-fall

deposits, lava domes, and lava flows (Fig. 1.5). The outcrops attributed to this

cycle have been grouped together according to the location of their vents and

stratigraphic position. Many vents of this cycle are located on three sides of the

Montagna Grande uplifted block (Fig. 1.5). K-Ar ages range between 10 and 8 ka.


14

The analysis of volcanic rocks younger than 45 ka (erupted since the Green

Tuff) shows that the silicic products erupted in the SE portion of the island derive

from the differentiation of primary basic magmas stored in a shallow-depth (3–4

km) chamber (Civetta et al., 1988).

Exposures of basalts, which erupted intermittently for at least 114 ka, are limited

almost exclusively to the extracaldera northwestern lobe of the island (Figs. 1.4-

1.5) Mafic rocks (transitional basalt to hawaiite) outcrop over ~6% of the area of

Pantelleria (Mahood and Hildreth, 1983; Mahood and Hildreth, 1986) as basaltic

scoria cones and lava flows (Fig. 1.4). These include flows that erupted at ~114 ka

at Monte Sant'Elmo-Cuddia del Cat; at ~30 ka from the Cuddia Bruciata (i.e., P.ta

San Leonardo Basalts in Fig. 1.5), Ferle (which is now leveled for a lengthened

airstrip), and Cuddia del Monte scoria cones; at ~10 ka from the Cuddie Rosse

scoria cones located in Mursia (Cornette et al., 1983; Mahood and Hildreth, 1986;

Civetta et al., 1988); and offshore in 1891 (see below). The Green Tuff overlies

basaltic flows near Punta Ficara and drapes basaltic scoria cones at Stufe di

Kazen, Acropoli, and Cala dell'Alca (Fig. 1.4) (Mahood and Hildreth, 1986).

The recurrent basaltic volcanism north of Monte Gelkhamar and Costa di Zinedi

scarp indicates that a major high-level felsic reservoir is not and has not been

present there. The extended basaltic activity in the northwestern lobe contrasts

with the southeastern two-thirds of the island, which has no basalt flows and only a

few dikes. The absence of mafic rocks in the region of the nested calderas

suggests that a magma chamber of low-density felsic magma prevents basaltic

liquids from reaching the surface (Mahood and Hildreth, 1986).

On Pantelleria there is no clear evidence for hydrovolcanic features such as wet

surges, littoral cones, peperites, and hyaloclastite deposits (with the exception of

one that is discussed below). Submarine or littoral interactions might have taken

place seaward of the present coastline during eruptive episodes when lava flows

entered the sea (Mahood and Hildreth, 1986).

The only hydrovolcanic evidence at Pantelleria is a proximal, non-welded, ash-

rich, pumice-fall deposit (Mahood and Hildreth, 1986). This deposit is exposed in

low sea cliffs at Cala delle Giache and is associated with the nearby Cuddia di

Khamma shield. The age of this deposit (~124±6 ka) falls near the peak of the last

major interglacial period. This was the only time in the subaerial history of the

island that sea level was as high as (or slightly higher) than today and may explain


15

why the Cala delle Giache deposit is the only preserved evidence of shallow

magma-water interaction (Mahood and Hildreth, 1986).

• Recent volcanic activity - Submarine eruption of 18 91

The most recent activity at Pantelleria was a basaltic submarine eruption that

occurred in 1891, four km northwest of the island (Judd and Butler, 1891; Butler,

1892; Butler and Perry, 1892; Riccò, 1892; Washington, 1909; Imbo`, 1965). In

1890, several small earthquakes occurred in Pantelleria causing an uplift of ~0.80

m on part of the northern coast (visible from P.ta Karuscia to P.ta Tracino) (Riccò,

1892), the cracking of cisterns, and an increase in the number and activity of

fumaroles.

After more than one year’s time, swarms again commenced October 14, 1891,

three days before the eruption. These were accompanied by drying up of some

springs, and further uplift of the north coast (Butler, 1892; Riccò, 1892) that ended

at the beginning of the eruption.

The 8-day submarine eruption (17-24 October 1891) produced floating black

scoriaceous bombs up to one meter in diameter, that rose to the surface along a

line some 200 m in length and 50 m in width, extending NE-SW. The center of this

line was estimated to be at Lat 36° 50’ ¾ - Long 0° 33’ ½ (Riccò, 1892). The

persistence of this linear band of scoriaceous bombs during the eruption may

indicate a submarine fissure (Butler, 1892; Riccò, 1892). Some of the bombs,

discharging steam, ran hissing over the water with recoil. Some pieces were

thrown 20 m in the air. After floating, many bombs exploded and the resulting

fragments became water-logged and sank. Many were still very hot inside, fusing

zinc (415°C) (Butler and Perry, 1892; Riccò, 1892; Washington, 1909). The

subspherical bombs were not only porous in texture, but contained large cavities.

The bombs showed a distinct brownish layer about 2.5 cm thick, rich in crystals

(i.e., plagioclase, olivine, clinopyroxene, oxides) with an outermost glassy skin.

Beneath this brownish layer, a darker layer (about 1 cm thick) occurred. It was

mostly glass with the same crystals. The majority of the bomb was black, coarsely

spongy, with a pitchstone-like luster, and more highly crystalline. In thin section, it

was composed of one third triclinic feldspar, olivine, and augite and the rest was a

black opaque groundmass (Washingthon, 1909).


16

Foerstner (1891) Perry (1892)

Washington (1909)

SiO2 44.64 46.40 44.83

Al2O3 12.74 21.84 11.73

Fe2O3 4.21 9.53 1.35

FeO 11.17 2.04 11.79

MgO 5.82 5.37 5.50

CaO 10.12 10.33 9.63

Na2O 4.31 3.27 3.34

K2O 1.41 1.69 1.40

TiO2 5.86 n.d. 6.88

P2O5 n.d. n.d. 2.14

MnO 0.20 n.d. 0.20

H2O+ 0.81

H2O- 0.51 n.d. 0.10

Total 100.99 100.47 99.7 Table 1.1: Chemical analysis of basalt scoria from the 1891 submarine eruption in

Pantelleria analyzed by Foerstner (1891), Perry (Judd and Butler, 1891), and Washington (1909). The analyses show the percent composition of washed powder.

The petrographical character of the bombs was briefly described by Butler and

in greater detail by Foerstner (1891). Analyses of the bombs were made

separately by Foerstner (1891), Perry (Judd and Butler, 1891), and Washington

(1909) (Table 1.1), and show an alkali basalt affinity. Most of the differences

between the analyses seems to be analytical. These analyses were done by

sequentially precipitating, one element after another, and always ended up with

totals near 100% because the final precipitation removed everything that

remained. The apparent high Al2O3 in the Perry analysis, for example, probably

included the TiO2 which he did not separately analyze. His higher SiO2 may

include P2O5. That all three analyses have similar MgO and CaO suggesting that

most of the differences were simply analytical.

Similar submarine eruptions have been reported in only a few other cases [i.e.,

Mauna Loa, Hawaii, in 1877 (Moore et al., 1989); Socorro Island, Mexico, in 1993

(Siebe et al., 1995); Terceira Island, Azores, in 1998-01 (Gaspar et al., 2003)]

(See Sec. 6.4).


17

1.2.3 Volcanic morphologies of Pantelleria Island

The following volcanic structures of Pantelleria Island are described below to

give an idea of the different morphologies present on land. These terrestrial

morphologies will be considered later for correlation and comparison with

submarine ones.

Many lava edifices at Pantelleria differ morphologically from typical silicic calc-

alkalic centers, for which much of the nomenclature was developed. However,

features similar to those at Pantelleria have been described for other peralkaline

centers. In particular, Bryan's (1966) account and illustrations of Socorro Island

(Mexico) edifices strikingly evoke those at Pantelleria, where principally three

edifice types are recognized: lava cone, lava dome, and lava shield (Fig. 1.6).

Rittman (1967), Villari (1968; 1974), and Cornette et al. (1983) describes M.te

Gelkhamar and other lava cones that resemble it as "endogenous domes". The

term refers to a lava dome that grows by internal expansion (Williams and

McBirney, 1979). As such, true endogenous domes are not present on Pantelleria,

as all five dome-like edifices [i.e., Cuddia del Gallo, Monti Gibbile (2), Fossa del

Russo, and Fossa Carbonara, see Fig.1.4] have been breached by lava flows and

were classified by Mahood and Hildreth (1986) as lava cones (blue points in Fig.

1.4).

The pantellerite lava cones (Fig. 1.6) represent an intermediate formation mode

between an exogenous dome (Fig. 1.6) and a silicic andesite stratocone.

Eruptions begin with a dome, which is soon breached by lava flows. Then, the lava

cone develops differently from an exogenous dome and, instead of stubby flows,

lava flows as long as 3 km are erupted (Mahood and Hildreth, 1986). M.te

Gelkhamar, M.te Gelfiser, and Cuddia Randazzo (orange points in Fig. 1.4)

consist predominantly of thick lava flows and were classified as lava domes

(Mahood and Hildreth, 1986).

The pantellerite lava shields are characterized by more subdued morphologies

than the lava cones (Fig. 1.6), owing both to their spatter-fed eruptions and to their

generally wider pumice blanketing. Similar edifices make up a significant

proportion of the Pre-La Vecchia stratigraphic records, and their deposits

volumetrically dominate the cliffs exposed in the southeastern sector. The main


18

shields are Cuddia di Scauri, C. Gadir, C. del Moro, C. Sciuvechi, C. Valletta, C.

Maccotta, C. Mueggen, and C. Patite (green points in Fig. 1.4).

The morphologies of lava cones may be related to magmas having viscosities

intermediate between those that form the lava domes and the less steep

pantellerite shields (Mahood and Hildreth, 1986).

Fig. 1.6 Pantelleritic dome, cone, and shield sketches approximately to scale (from

Mahood and Hildreth, 1986).

1.2.4 Offshore geophysical studies

Geophysical studies in the area offshore Pantelleria mainly consist of seismic,

gravity, and oceanographic surveys that were conducted in the last few decades.

Recently, field geological and structural data of Pantelleria Island were integrated

with offshore geophysical measurements of the surrounding region by Civile et al.

(2008; 2010), with the aim of mapping the main structural trends of the region and

analyzing the relationships between tectonic processes and magmatic

manifestations.

Several multichannel seismic reflection profiles (i.e., Italian Commercial Zone

“G” lines, Mediterranean Sea “MS” lines, and “CROP” lines) acquired in the area

surrounding Pantelleria Island were used (Civile et al., 2008) to broadly map the

structural features and the volcanic bodies of the offshore area (Fig. 1.7).


19

The study of seismic facies by Civile et al. (2008) shows the presence of a large

magmatic field bordered by faults, entirely surrounding Pantelleria, and almost

coinciding with the submerged part of the Pantelleria volcanic edifice (Fig. 1.7).

The mapped faults (Fig. 1.7) can be grouped into two main systems, with trends

NW–SE (N120°–140°) and NNE–SSW (N20°–35°). Some NNE –SSW faults affect

the Quaternary magmatic rocks of the Pantelleria plateau and limit the wide

volcanic field located about 25 km south-westward from Pantelleria (Fig. 1.7).

Some of the identified faults recognized from the seismic profiles were

correlated to the structures observed onshore. In particular, it was found that the

Zinedi Fault continues offshore the northeastern sector of the island (Civile et al.,

2008) (Fig. 1.7).

Fig. 1.7 Simplified tectonic and volcanic map of the Pantelleria region (modified from

Civile et al., 2008).

Ch 2 - Growth and evolution of volcanic islands

20

Chapter 2

Growth and evolution of volcanic islands

Volcanic islands evolve in different stages that involve a wide range of

processes ranging from construction to destruction processes, which often act at

the same time, leading to the formation of the structures we see. Actually, the

growth of volcanic islands typically is controlled by a complex interplay among the

tectonic evolution of volcanoes, the rate and type of volcanic activity, the intensity

of dismantling processes, and the nature of the transporting and depositional

processes (Schmincke, 2004).

2.1 Edifice growth

Volcanic islands start to grow at great water depths, most of the time over 2000

m b.s.l. (meters below sea level), and represent the tip of large seamounts that are

temporarily exposed above sea level (Staudigel and Clague, 2010).

Fig. 2.1 Schematic cross section of a seamount and the clastic fan surrounding the

island based on drilling into the flanks of Gran Canaria (from Schneider, 2000). Some 90% by volume of these volcanic island complexes accumulate

underwater and hence are extremely poorly known (Schmincke, 2004). The

evolutionary pattern of volcanic islands has been deducted principally from the


21

subaerial edifices (i.e., Clague and Dalrymple, 1987; Kano et al., 1993; Sohn,

1995), deep sea drilling cores (Schmincke and Sumita, 1998; Schneider, 2000),

and dredging on oceanic islands (Moore and Fiske, 1969; Davis et al., 2003).

In the last decades, studies of swath bathymetry mapping and deep

submersible observations have increasingly helped to document the submarine

history of volcanic islands .

When magma rises to the seafloor, covered by soft sediments, it is likely to

intrude laterally into low density muddy layers, if they are present (Schneider,

2000). This complex of intrusions into sediments represents the base above which

the submarine edifice grows. Initially, a deep-sea seamount will consist mainly of

pillow lava, with intrusions in the interior of the edifice (Fig. 2.1). Subsequently,

seamounts begin to grow in height mainly due to lava effusions and intrusions into

their substructure (Schneider, 2000). Explosive activity at seamounts may begin

at abyssal depth, however, at depths less than ~700 m, the relatively low

hydrostatic pressure leads to an increase of explosive volcanism (Staudigel and

Clague, 2010).

Eruption styles (whether underwater eruptions are effusive or explosive) hence

the products that comprises island volcanoes, might vary with 1) the depth of

eruption beneath water, 2) the magma composition (especially the amount of

volatiles), and 3) the magma-water interactions (Fisher, 1984).

Head & Wilson (2003) summarized the conditions under which submarine

basaltic eruptions could be pyroclastic. Taking into account only magmatic

explosivity (i.e. where fragmentation is driven by magmatic volatiles), there are six

theoretical mechanisms of basaltic magma ascent and eruption (Fig. 2.2):

1) no gas exsolution (uncommon) with emission of lava flow;

2) gas exsolution but no magmatic disruption where a vesicular magma

is erupted effusively from the vent;

3) gas exsolution, magma disruption, and hawaiian-style fountaining.

The appearance and products will be different from those of a subaerial

lava fountain eruption, because of the immediate contact with seawater;

4) low magma rise speed, which leads to bubble coalescence process

in the dike/conduit that feeds the vent, and results in strombolian activity;

5) volatile content building up in the top of a solidified dike leading to

vulcanian eruptions;


22

6) volatile content building up in the magma reservoir leading to foam

formation with the build-up of a seamount and the eruption of lava

fountains.

Fig. 2.2 Configuration of the ascent and eruption of basaltic magma in several possible

submarine examples (from Head and Wilson, 2003).

Moreover, based on observations in a variety of seafloor environments and

theoretical considerations of the ascent and eruption of magma under similar

submarine conditions with varying amounts of H2O and CO2, Head & Wilson

(2003) summarized a set of predictors to aid in the recognition and interpretation

of underwater landforms, and described the expected deposits that magmatic

explosivity eruptions might form.

On the other hand, the range of magma-water interactions that might occur in

subaqueous and emergent volcanism has been described by Kokelaar (1986) and

Wohletz (2003).


23

When a seamount breaches the sea surface, islands are formed and shallow

submarine volcanism allows the structure to emerge above sea level, passing into

subaerial volcanic activity (Staudigel and Clague, 2010). The newly born islands

might easily disappear after their emergence. This is due to destruction processes

which can occur simultaneously with construction processes and mainly related to

sea-surface wave erosion of volcaniclastic rocks, which are the dominant eruptive

rock types at shallow-water seamounts. However, persistent emergence is

possible during periods when eruptive rates exceed erosional rates (Schmincke,

2004).

2.2 Erosional and instability processes

Marine geophysical surveys have revealed that most of the volcanic islands not

only grow through the addition of eruptive products to their surface and flanks, but

change their shape and volume by slow erosion or sudden collapse processes

(e.g., Moore, 1964; Lénat et al., 1989; Moore et al., 1989; Deplus et al., 2001;

Krastel et al., 2001; Masson et al., 2002). Mass wasting processes play a

significant role in the growth and evolution of volcanic islands and the surrounding

volcaniclastic apron (Labauzy, 1996; Wright, 1996; Ollier et al., 1998; Schmincke

and Sumita, 1998; Chadwick Jr. et al., 2005).

The causes of destabilization of volcanic flanks are complex and vary between

volcanoes (McGuire, 2003). These mainly include: 1) magma intrusion as a

cryptodome or dike (Elsworth and Voight, 1995; Elsworth and Voight, 1996;

Tibaldi, 1996), 2) overloading of the edifice flanks by magma extrusion, 3) a

phreatic eruption (Yamamoto et al., 1999), 4) ground-shaking during an

earthquake, 5) tectonic movements, 6) climate change and sea-level fluctuation

(Capra, 2006), 7) change in water pore pressures, and 8) erosion and

oversteepening of volcano flanks.

Amongst the wide range of mass movement processes that affect volcanic

islands, two main types have been recognized in the last decades: slumps and

debris avalanches (Moore et al., 1989). Slumps are relatively short and wide; they

show a steep front, terraces, and move on slopes greater than 3 degrees

episodically with ca. 10m/100 a. Their thickness can be in excess of 1000 m and

are often associated with large earthquakes (Smith et al., 1999). A classic


24

example of a slowly deforming slump is Hilina slump on the south flank of Kilauea

(Moore et al., 1989; Smith et al., 1999). The second type, debris avalanches,

generally represents a single episode of rapid failure, occur on gradients of less

than 1.5-3 degrees, and are long (up to 230 km) and thin (0.05-2 km) compared to

width. They commonly have a well-defined amphitheater at their head and

hummocky terrain in their lower part (Moore et al., 1989). The Nuuano Alika debris

avalanches on the west flank of Mauna Loa volcano is a good example of slide

(Lipman et al., 1988).

Geomorphologic analyses of submarine and subaerial surface features of

several volcanic islands using a combined topographic/bathymetric digital

elevation model coupled with onshore geological and geophysical data have

added to the understanding about the distribution and cyclicity of mass wasting

processes around volcanic islands [e.g., Hawaii’s (Moore and Fiske, 1969; Moore

et al., 1994; Clague and Moore, 2002; Coombs et al., 2004), Canary (Watts and

Masson, 1995; Urgeles et al., 1997; Masson et al., 2002), La Reunion (Lénat et

al., 1989; Ollier et al., 1998; Oehler et al., 2004; Oehler et al., 2008), Lesser

Antilles Arc (Deplus et al., 2001; Boudon et al., 2007), Aleutian arc (Coombs et al.,

2007), and Aeolian Islands (Kokelaar and Romagnoli, 1995; Chiocci et al., 2008)].

2.3 Volcaniclastic particle types

Volcaniclastic sediments are broadly defined as clastic deposits derived from

the transport and deposition/redeposition of volcanic activity products. The

classification of these sediments has long been discussed by geoscientists

(Manville et al., 2009) due to their inherent complexity in fragmentation, underlying

transport, and depositional processes that operate in volcanically-impacted

environments.

Volcaniclastic studies require a truly interdisciplinary approach bringing in many

areas of physical volcanology, classical clastic sedimentology, fluid dynamics,

hydrology, and geomorphology. Therefore, it was comparatively recent that this

discipline has developed into a specify area of research.

The nomenclature used to classify these volcaniclastic deposits follows the

terminology used by Schneider (2000):


25

Primary volcaniclastic deposits may be defined as assemblages of fragmental

products deposited directly by explosive or effusive eruption. The fragments,

based on their origin, can be defined as:

Pyroclasts, which are the most common volcaniclasts and are formed by

explosive fragmentation of the magma emitted from vents into particles. Explosive

activity results from the expansion of magmatic gas or from vaporization of

external water at the contact of rising magma (i.e., hydromagmatic or

phreatomagmatic eruption). Pyroclast fragments, smaller than 2 millimeters in

average diameter, compose the volcanic ash. Ashes’ characteristics and shapes,

along with chemical characterization, are fundamental for tephrochronology, the

study and correlation of ash layers in the geologic record;

Autoclastics are formed by mechanical self-fragmentation of lava flows or

domes during their progression. They also result from minor explosions, either

magmatic or hydromagmatic during the flow of lavas. Hypabyssal intrusions which

are in contact with water-saturated sediments might form a kind of hydroclastic

breccias, called peperites (Goto and McPhie, 1996).

Primary volcaniclastic deposits, if not cemented, are easily resedimented.

Resedimented or reworked pyroclastics and autoclastics are still classified as

primary volcaniclastics.

Secondary volcaniclastic rocks are formed from fragments (i.e., epiclastics) of

volcanic origin resulting from weathering or physical erosion of pre-existing

coherent rocks, and excludes reworking of particles from non-welded or

unconsolidated materials (Manville et al., 2009). The main variation of this general

behavior is related to a catastrophic flank failure during eruptive activity. Such an

event is likely the most important source of secondary volcaniclastics in the life of

many volcanoes.

2.3.1 Morphology of volcanic glass fragments

Submarine eruptions usually produce volcaniclastic particles that result from the

fragmentation of volcanic material during eruptive activity (Fisher, 1961; Fisher

and Schmincke, 1984; Fisher and Smith, 1991).

The study of volcanic glass fragment (either subaerial and submarine) and their

morphologies is becoming important in the interpretation of explosive eruption


26

phenomena and physical characterization of the magmas. In fact, the shape of

glassy lava fragments may be used to interpret physical properties of an erupting

magma and its volatile content, and can also indicate the degree of interaction

between the magma and water (Heiken and Wohletz, 1985).

Currently, a systematic literature on submarine glassy lava fragments is lacking

and reports are limited to site-specific accounts mainly concentrated around the

Hawaiian Islands (Clague et al., 2000a; Clague et al., 2003; Davis and Clague,

2006; Schipper et al., 2010), from seamounts near the East Pacific Rise (Batiza et

al., 1984), and ocean ridges, (Maicher and White, 2001; Clague et al., 2003b;

Sohn et al., 2008; Clague et al., 2009a). This is due to the recent development of

this topic; therefore, a lot of work is still required in this field.

Several laboratory simulations have helped our understanding of how the

explosive processes are related to morphologies and surface texture (Smith and

Batiza, 1989; Mastin et al., 2008). These accounts focus on local settings and try

first to explain under what conditions glasses can form, then to relate them to

different known cases.

Fig. 2.3 Different morphologic types of limu o Pele fragments from the northern East

Pacific Rise. In A) and B) flat to gently curved sheets; in C) thick folded fragments; in D) and E) stretched glass ribbons and Pele's hair; in F) and G) thin folded fragments. Scale bars are 1 mm. (from Clague et al., 2009a).

Pyroclasts formed during submarine eruptions show a wide diversity of

morphologies, textures, and vesicularity (e.g., rounded, elongated vesicles). The

morphologies (Fig. 2.3) might include angular vesicular to dense glassy shards,

fluidal shapes ranging from Pele’s hair (long glassy strands) to limu o Pele,


27

defined as lava bubble walls shards (Hon et al., 1988). The limu o Pele presents

different shapes such as rods types (simple, twisted, bent, folded, or keeled),

ribbons (flat, twisted, bent, folded, or keeled), and small fragments of pumice. The

maximum dimensions are up to several mm across (Clague et al., 2009a).

2.4 Submarine sedimentation around volcanic islands

Submarine sedimentation around volcanic islands is related to a spectrum of

eruptive and sedimentary mechanisms.

Volcaniclastic particles deposited in marine settings can originate both from

subaerial and submarine environments (Carey and Sigurdsson, 1984). Pyroclastic

sediments introduced into shallow subaqueous settings are sorted by wind (when

the activity is subaerial or the pyroclasts reach the sea-surface), tidal and coastal

currents (e.g., Sohn and Yoon, 2010), and gravitational mass-flow processes.

The volcaniclastic products are widely dispersed in the submarine setting

adjacent to extrusion sources by three principal kinds of marine transport

processes: suspension fallout, sediment gravity flows, and instability phenomena

(Fisher, 1984). Therefore, the dominant transport mechanism mostly depends

upon currents and slope.

The diversity of the submarine volcaniclastic processes is related to the

succession of syn-eruptive and non-eruptive periods (Fig. 2.4). During these

periods, the volcaniclastic sediments can be deposited (i) directly from eruptions,

(ii) from remobilization of juvenile volcaniclastics, or (iii) from fragments derived

from erosion of pre-existing rocks and weathering.

During syn-eruptive periods (Fig. 2.4), volcaniclastic material can be emplaced

by primary volcanic mechanisms. Syn-eruptive volcaniclastic deposits can be

reworked by sedimentary processes occurring instantaneously with volcanic

eruption or during non-volcanic periods (Fig. 2.4).

Volcaniclastic gravity flows can be subdivided into two categories: (1) primary

volcaniclastic gravity flows that are directly related to the eruption-like pyroclastic

flows or hyaloclastite mass flows, and (2) secondary volcaniclastic gravity flows

that are derived from flow transformations and are genetically related to primary

eruptive gravity flows (Fisher, 1984; Schneider, 2000). In marine settings, the


28

largest syn-eruptive transport processes in volume are primary volcaniclastic

gravity flows, mainly related to subaqueous pyroclastic flows, whereas ash

turbidites are secondary flows. These secondary flows can form by transformation

of a primary pyroclastic flow by mixing with water. There is a wide literature about

the formation of ash turbidities from pyroclastic flows entering the sea both from

subaerial (Carey et al., 1996; Mandeville et al., 1996; Hart et al., 2004) and

submarine settings (Fiske and Matsuda, 1964; Cas and Wright, 1991), mostly

derived from direct observations, subaqueous ignimbrites deposits (Yamada,

1984; Kokelaar and Koniger, 2000), and laboratory experiments (Legros and

Druitt, 2000; Freundt, 2003).

Moreover, especially during non-eruptive periods, reworking can result from

erosion of pre-existing rocks and weathering from emerged volcanic terranes,

leading to epiclastic processes (Carey and Sigurdsson, 1984).

Redistribution of non-welded deposits along the island’s flanks and into a

marginal basin is mainly driven by bottom currents or sedimentary gravity flows,

which might be triggered by seismic activity.

Fig. 2.4 Schematic diagram of the diversity and mutual genetic relationship of

volcaniclastic transport processes. Primary processes occur during active volcanic phases. Sedimentary processes can be genetically related to transformation of primary mechanisms into gravity flows. During non-eruptive stages, primary deposits are remobilized and older formations are eroded (from Schneider, 2000).

Sedimentary reworking of volcaniclastic material is important in marine settings

because it provides insight into the eruptive mechanisms and post-eruptive

processes. Recognition of the different kinds of volcanic particles is important to

establish the stratigraphy of the volcaniclastic successions, because the presence


29

of primary or “eruption fed” deposits (White, 2000) and reworked pyroclastic

particles may give indication about the relative timing of the volcanic activity (Fig.

2.5). Moreover, marine fallout ash layers, if not reworked, form important

chronostratigraphic markers that can be correlated over large areas (Schneider,

2000). Tephra layers are useful for correlation and dating of volcanic events,

determination of eruption frequency, and estimation of erupted volumes.

As a whole, accumulation of volcaniclastic sediments around a volcano

generates a volcaniclastic apron that records its magmatic, volcanic, and

sedimentary evolution (Menard, 1956). The main factors controlling the

sedimentary evolution of the submarine volcaniclastic aprons of active volcanic

island are primarily the magmatic productivity and sea level fluctuations. In

general, high rates of submarine volcaniclastic sedimentation occurs mainly during

eruptive events (Fig. 2.5). During periods of volcanic quiescence, sedimentation

rates are very low and volcaniclastic influxes mostly result from the transfer of

materials from the emerged part of the island to the submarine flanks. Sea-level

oscillations might influence sedimentation along the flanks by inducing formation of

insular shelves.

Fig. 2.5 Sketch of the dynamics of a submarine volcaniclastic apron during eruptive

and non-eruptive stages of a volcanic island. Magmatic productivity, load subsidence, and volcano-seismic activity are the main factors that control the dynamics during eruptive periods, with clastic influx dominated by syn-eruptive sedimentation. During non-eruptive periods, due to the sediment stored in the insular shelf, epiclastic influx is very minor. The sedimentation rate is very low and the growth of the apron is mostly controlled by hemipelagic sedimentation and eustatic changes (from Schneider, 2000).


30

As a whole, the submarine flanks of volcanic islands are shaped by different

processes acting at various time scales and at different periods. These mainly

include volcanic constructional processes (Chadwick Jr. et al., 2005; Wanless et

al., 2006; Mitchell et al., 2008), instability phenomena (Carracedo, 1999; Mitchell

et al., 2002), weathering and erosion (Llanes et al., 2009), tectonic movements

(Moore et al., 1996; Webster et al., 2006), and sediment deposition.

Today, morphological studies of the seafloor surrounding densely populated

volcanic islands have importance for hazard evaluation of volcanic activity and

landslides.

Ch. 3 - Methodology and dataset

31

Chapter 3

Methodology and dataset

Nowadays, seafloor exploration is mainly achieved through the use of

geophysical instruments that exploit acoustic wave propagation in water and by

direct seafloor sampling. Improvements in seafloor survey technology are

continuously occurring, particularly with the use of remote sampling tools and

marine robots.

Accurate marine surveys require the use of a geospatial positioning system to

locate the submarine structures. The system currently used is the NAVSTAR/GPS

(NAVigation Satellite Timing And Ranging Global Positioning System). This is a

satellite-based radio navigation system, referenced to the WGS 84 ellipsoid,

providing precise three-dimensional position and time information. It was

developed for the U.S. Air Force and is being used in a growing number of

commercial products (Logsdon, 1995). The precision of the measurements is

linked to the GPS receivers and is on the order of about 10 m in the horizontal

plane but might be improved using a differential correction technique; therefore,

ship positioning was measured utilizing with a Differential Global Positioning

System (DGPS). This enhancement of GPS provides the possibility for post-

processing or real-time corrections. In land and hydrographic surveys, the Real

Time Kinematic (RTK) technique is frequently used, despite its decrease in

accuracy with distance from the reference station.

This work is based on multibeam data collected during two research cruises

(Zibibbo ’06 and Passito ’08) carried out by the R/V Urania of CNR (National

Research Council). The surveys cover a comprehensive area of about 2000 km2

(310 km2 of which consists of the Pantelleria submarine flanks). The integrated

data provide the highest resolution bathymetry currently available for this portion of

the SCRZ (Bosman et al., 2007). I helped process the dataset collected during the

first cruise, then actively collaborated to plan and acquire/process the geophysical

data and samples during the second cruise.

In this chapter, the dataset utilized for the thesis will be introduced and a brief

review will be given of the different technologies (acoustic methods and seafloor

sampling) employed during marine surveys performed offshore of Pantelleria


32

volcano. An account of processing techniques employed and analysis methods is

also presented.

3.1 Multibeam echo-sounder and DTMs

Multibeam echo sounders (MBES) are now the most common technology used

to survey the seafloor. As with other sonar systems, they transmit and receive

high-frequency sound energy. Vertical depths are obtained through processing of

the returning echo signals reflected by the seafloor. MBES produces a “swath” of

soundings (i.e. depth), with each beam width ranging between 1.5° and 6° along

track and 1.5° (at nadir) across track. This techn ique can provide high-resolution

footprints when a narrowly focused beam is formed. Generating many ping per

second (between 15 to 0.141 depending on range), a multibeam system can

obtain full coverage of an area by measuring and recording the time for the

acoustic signal to travel from the transmitter (transducer) to the seafloor and back

to the receiver (two way travel time). The computation of signal parameters (i.e.,

amplitude, frequency, phase) is the product of the characteristics of the seafloor,

namely 1) slope angle of incidence of the beam and 2) bottom reflectivity. The

quality of the return signal is dependent upon the primary projector/receiver

characteristics. The signal intensity time series of the bottom return is produced by

the reflective and geometrical properties of the seafloor. It must be noted that the

footprint size increases with grazing angle; therefore, this technique does not give

a point measurement but the average value for an area. This value must be

integrated with the other sensors to determine the total sounding (i.e. x-y-z)

relative to the coordinate system chosen. The accuracy of the horizontal

positioning relies on the ability to compensate for errors caused by vessel roll,

pitch, and yaw. Vertical resolution decreases with increasing depth, but is linked to

sound speed velocity, calibration parameters, and tide, for example. The coverage

area on the seafloor is given by adding each bathymetry profile acquired across

track (Fig. 3.1).


33

Fig. 3.1 A multibeam sonar swath below a ship. The area highlighted in yellow shows the wide area covered across track, while the beam is narrow along track (black line). The area highlighted in green is an example of a received beam. The average depth is obtained in the area where the transmission and received beams intersect (shown in blue) (from http://www.amloceanographic.com).

The full multibeam measurement suite consists of 1) a dynamic positioning

system, 2) a gyro to orient the system (yaw), 3) a Motion Reference Unit (MRU)

specifically designed to measure roll, pitch, yaw, and heave motions in marine

applications, 4) an arrangement of emitter and receiver transducers, and 5) a data

management system. During the survey, it is crucial to frequently measure sound

velocities (variable according to pressure, salinity, and temperature) through the

vertical water column with a sound velocity profiler (SVP-CTD) to correct travel

time, hence sounding depths.

3.1.1 MBE Dataset: processing and analysis

Bathymetric data were acquired by a SEABAT Multibeam Echo-Sounder, using

a frequency of 50 kHz (Reson 8160 system -126 beams), that gives the highest

performance at medium water depths (-300 to -1500 m). Pitch, roll, and heading

values are provided by the Inertial Navigation System Mahrs TSS. Sound velocity

distribution profiles with depth, needed for acoustic refraction corrections of raw

water depth data, were recorded with a Navitronic SVP-25 velocity probe.

MBE raw data were processed using dedicated commercial software (i.e., Caris

Hips & SIPs, Microstation, Surfer), daily sound speed profiles performed during the

acquisition, and calibration of transducers in areas close to the survey zone were

performed.

The protocol employed for data processing included: 1) preliminary data re-

calibration with correct transducer parameters and tide corrections, 2) depth

recalculation using correct sound speed profiles, 3) manual cleaning of spikes and


34

statistical treatment of soundings on each swath with removal of organized/non-

organized noise, and 4) merging of the swaths and successive application of

statistic and geometric filters in order to eliminate residual noise. Processed x,y,z

data were interpolated to create a regular grid using known algorithms (e.g.,

kriking, simple binning). Algorithms were chosen depending on the situation. For

example, a simple binning algorithm, which attributes each sounding to adjacent

grid nodes giving it a decreasing weight with distance from the sounding, is

efficient with high data densities but creates artificial terracing on steep gradients

where data are sparse. Grid node spacing was optimized for the depths of areas

mapped and the resulting Digital Terrain Models (DTMs) have been computed with

cell sizes ranging from 2 m in shallow water (< 100 m b.s.l.) to 20 m (down to 1300

m b.s.l.) (Fig.3.2).

Fig.3.2 Shaded relief map showing topographic and bathymetric data (cell grid size 20 m) at Pantelleria volcano. The coastline is shown in black; gaps in the submarine coverage are in white.

DTMs were the base for quantitative analysis. Quantitative analysis methods

have not been extensively employed on submarine landscapes as compared with

subaerial ones, The application of quantitative geomorphometric techniques has


35

been increasing (Mitchell et al., 2002; Llanes et al., 2009) and numerous authors

have demonstrated its utility in improving the geological interpretation of

submarine features in different settings (Romero Ruiz et al., 2000; Micallef et al.,

2008).

Terrain morphometric attributes (Wood, 1996) were extracted from these DTMs

using dedicated commercial software (i.e., Global Mapper, Microstation, Surfer,

and ArcGis), and several data layers were computed as morphometric maps:

� shaded relief map

� slope gradient and aspect maps

� elevation map 3D map

� profile curvature

� plan curvature

� submarine drainage system

Afterwards most of the attributes and morphometric parameters of surface

features were extracted (e.g., slope) or semi-automatically calculated (e.g.,

directions, volumes). The multibeam sonar also recorded acoustic backscatter

data, which were mapped using the CARIS software.

All the resulting maps (Ch. 4) are shown in Universal Transverse Mercator

projection (Zone 33, World Geodetic System 1984, WGS84) with ship positioning

achieved by GPS/DGPS.

A subaerial DTM of Pantelleria islands was created from shuttle radar

topography mission (SRTM) data available from http://www.jpl.nasa.gov/. A more

detailed map was made by digitizing the Regional Technical Map (Carta Tecnica

Regionale - CTR 1:10.000) of Pantelleria.

3.2 Single-channel reflection seismic surveys

Marine seismic surveying uses the propagation of acoustic waves in water to

explore the structure of the water-sediment interface and underlying

sediment/rocks layers. Seismic techniques generally involve measuring the travel

time from superficial shots through the subsurface to arrays of hydrophones to

obtain the subbottom stratigraphy (in time). A generated seismic wave penetrates

the seafloor at different angles, depending on the varying impedance of the sub-


36

surface. Impedance (I) is a measure of the wave velocity (v) and the density (ρ) of

the medium (i.e., I= vm x ρm). For example, reflections would differ for a wave

passing from a sandy layer to a more clay-rich layer. Reflection waves follow

Snell’s law and are recorded by subsurface receivers.

There are two basically different kinds of energy sources that are used for

marine reflection surveys: 1) impulsive sources (e.g., Sparker, SPK) and 2)

resonant sources (e.g., Sub-bottom Profiler, SBP). It is conventional to refer to the

signals produced by impulsive sources as seismic wavelets and to those produced

by resonant sources as sonar pulses.

A SPK device is a pulse plasma jet acoustic source that produces a very low-

frequency sonar pulse underwater. For each firing, it stores electric charge in a

large high-voltage bank of capacitors, and then releases the stored energy in an

arc across electrodes in the sea. The underwater spark discharge produces a

plasma that increases its volume and creates a vapor bubble, which expands and

collapses, making a loud sound. The transmitted energy range is between 0.2-30

kJ and the frequency varies between 100-1000 HZ. The short-pulse length of the

plasma gun permits good penetration of the signal but results in a low capability to

resolve reflectors (about a few meters). Its reflector resolving capability can also

be reduced by secondary reflections that occur especially in shallow water,

referred to as “ringing”.

The SBP exploits piezoelectric crystal properties to generate an electric

potential in response to applied mechanical stress, producing a pressure pulse

shaped like a portion of a sinusoid. Typically, SBP systems send narrow-angle

acoustic waves frequency modulated with two main bands. The high-frequency

signal yields high-resolution returns from the water–sediment interface and a

strong bathymetric response, while the low-frequency signal can penetrate into the

sub-bottom formations for mapping of sediment layers. The effectiveness of

subbottom profiling depends on the nature of sediments. Coarse-grained

sediments and sand provide stronger bathymetric reflections and poor penetration

into subbottom layers; however, fine-grained sediments yield weaker bathymetric

reflections and deeper penetration into subbottom layers.

The resolution of the technique depends on the signal frequency (f) and the P-

wave velocity (VP) in water and sediment layers (Lin et al., 2009). The resolution


37

(in meters) is approximately equal to a quarter of the wavelength: ResSBP≈Vp/ (4 f)

(e.g. in soft sediment, about 20 cm).

3.2.1 Seismic Dataset: processing and analysis

Analyzed mono-channel seismic surveys included several profiles (total of ~630

n.m. length) acquired during two cruises (2006 and 2008) on board of the R/V

Urania (Fig. 3.3).

High-resolution single-channel seismic Sparker profiles (at 4.5 kJ and 1.5 kJ)

were collected (analog in 2006 and digital in 2008) on specific areas around

Pantelleria, in order to characterize shallow stratigraphy of a landslide deposits.

The total length of the survey was about 54 n.m. (Fig. 3.3).

Fig. 3.3 Location map of the seismic profiles (sparker, sub-bottom profiles and G82

Ministerial lines) examined for this study.


38

Seismic acoustic data were acquired on board the R/V Urania with the hull-

mounted Datasonic CAP-6600 CHIRP-II sub-bottom profiler using a frequency of

3.5 kHz. The acquisition system on board was SwanPro SW and data (about 580

n.m) were recorded in SEG-Y format on magneto-optical disks.

Multichannel seismic profiles from the Mediterranean Sea “MS” lines, also

called Ministerial Seismic lines1 (Italian Commercial Zone “G”, lines G82: 103/110;

149/153), were also examined (see Fig. 3.3 for location).

Selected profiles are described in Ch. 4. Moreover, several profiles were

examined, and even though most of them will not be directly presented in this work

a sub-surface interpretative map is given in Ch.4.6.

3.3 Seafloor sampling methods

Seafloor samples were collected offshore Pantelleria in 2006 and 2008 with a

box-corer, gravity core, dredge, and grab sampler. Specific sites were targeted in

order to ground-truth bathymetry data and validate the interpretation of sea-floor

processes, such as sedimentary processes and the last volcanic activity, or to

investigate in situ rocks.

A short description of these sampling methods is briefly given.

A grab is a simple method of bringing up surface sediments from the seafloor. It

cannot be used to characterize different sedimentary layers since it mixes the

sediments upon collection; therefore, it is only used for characterizing superficial

sediment. The sampler consists of a pair of weighted cylindrical (i.e., Shipek type)

or semicylindrical (i.e., Van Veen type) jaws with a chain suspension (Fig. 3.4).

Once it is launched, the jaws of the grab sampler open and it descends to the

seafloor. The impact on the bottom surface triggers the spring loaded release

mechanism that closes the jaws, trapping sediments. The grab sampler is then

recovered. The capacity depends on device characteristics and seafloor

properties.

1 Rilievi di sismica riconoscitiva o ministeriale sono stati eseguiti dall'Agip, quale Operatore per conto dello Stato, nelle differenti Zone economiche del sottofondo marino, in accordo con la legge n° 613 de l 21/7/1967 e successive modifiche, che disciplina l'esplorazione di idrocarburi in mare.


39

Fig. 3.4 Seafloor sampling methods used at Pantelleria (grab, box-corer, rock dredge,

and gravity core).

A box-corer is designed to take a sample of the surface and shallow sediment

and bring it back to the ship intact. The device consists of a stainless steel box that

penetrates into soft sediments up to 100 cm depending on its dimensions (Fig.

3.4). The closing mechanism consists of a blade that traps the sediment in the

box. The lack of disturbance of recovered sediments can be ascertained by their

appearance. Sediments are usually sub-sampled by inserting core tubes.

A rock dredge consists of an open-bottomed steel box that it is attached to a

thick cable, with “teeth” along its rim and a large-mesh chain-link bag hanging from

it for retaining rocks (Fig. 3.4). The sampling procedure is to bring the ship onto

the chosen site, lower the dredge slowly to the seafloor, drive the ship a short

distance, then reel in the dredge, dragging it along the bottom (upslope) while

monitoring tension (how strongly it grips, and hopefully detaching rocks from the

sea floor). Once enough cable has been reeled in to bring the dredge off the

bottom, the dredge is recovered.

A gravity core penetrates the bottom due to the force of gravity (Fig. 3.4) and

retains sediment in a long PVC tube. Since the speed of penetration is high, it

barely disrupts the sedimentary layers. A cable attached to the ship retrieves the

core.


40

3.3.1 Sampling dataset

Seafloor sampling was performed during both cruises.

During the first cruise, sampling was focused on finding deposits related to the

submarine eruption of 1891 offshore Pantelleria. Several other samples, mainly

collected from dredge and grab methods, were planned day by day as bathymetry

data showed the surveyed morphologies.

During the last cruise (in which the writer actively participated), sampling was

mainly focused on specific targets such as volcanic outcrops and cones to validate

the interpretation of geological processes mapped around Pantelleria volcano on

the base of bathymetry data, and to characterize the cyclicity and dispersal of the

products of the last eruptions.

Fig. 3.5 Location map of the of the sampling (gravity corer, box-corer grabbing, and

dredging,) examined for this study.


41

The samples recovered in the studied area and examined for this study (Fig.

3.5) comprise different rock types and lithologies, as well as different biogenic

components and corals that are briefly described in Ch.5.

3.3.2 Analysis methods

A selection of samples was analyzed using microscope, electron microprobe,

scanning electron microscope, and sieving in order to better define volcanological

and sedimentological characters and to better understand depositional processes

in the studied area.

3.3.2.1 Electron microprobe analysis (EMPA) and scanning electron

microscope (SEM)

Electron microprobe analysis (EMPA) is a non-destructive technique for

chemically analyzing the composition of microscopic areas of solid samples in

which X-rays are excited by a focused electron beam. The X-ray spectrum

contains characteristic lines of the elements analyzed; hence a qualitative analysis

can be achieved by identifying the lines from their wavelengths. By comparing

their intensities with those emitted from standard samples (pure elements or

compounds of known composition) it is possible to quantitatively determine the

concentrations of elements from fluorine (Z=9) to uranium (Z=92) at levels as low

as 100 ppm. The microprobe uses an electron beam current between 10 and 200

nanoamperes, roughly 1000 times greater than that in a scanning-electron

microscope (SEM). The user can view the sample surface at high magnification

(up to 40,000X) through a transmitted-light optical microscope, identify the

features of interest, and then analyze the composition of those features. The

resulting data yield quantitative chemical information in a textural context.

Chemical composition variations from within a sample can be readily determined.

About 30 samples from of rock dredge, core, and box-corer collected from the

submerged NW flank of Pantelleria were analyzed with EMPA (at University of

Rome Sapienza by Calarco M., Conte, A., Martorelli, E., Sposato, A.; at University


42

of California, Davis, California, by D.A. Clague; and at U.S. Geological Survey in

Menlo Park, California, by A. S. Davis). The results are described in Ch. 5.

The design and function of the SEM are very similar to the EPMA but this

instrument is primarily for imaging rather than analysis (Reed, 2005). Scanning

electron microscopes commonly have an X-ray spectrometer attached, enabling

the characteristic X-rays generated from a sample to be used to produce an image

and reveal information about external morphology (texture), chemical composition,

crystalline structure, and orientation of materials making up the sample. Also, with

a stationary beam, point analyses can be obtained, as in EMPA. However the

spatial resolution of SEM point analysis is limited to about 1 µm due to beam

spreading; higher resolution is obtained in SEM scanned images.

A selection of about 20 sub-samples from box-core and gravity core was

analyzed for shape and texture of glass shards (see Sec. 2.3.1) with SEM (at

INGV-Catania by Calarco M., Conte A., Martorelli E., Sposato, A.).

3.3.2.2 Grain-size and calcimetry analyses

Grain-size and calcimetry analyses were carried out on sediment samples at

University of Rome Sapienza (by Calarco, M.).

Grain-size analysis, also known as particle-size analysis or granulometric

analysis, is perhaps the most basic sedimentological technique to characterize

and interpret sediments and sedimentary rocks.

Twenty-four core samples were sieved. Low percent silty and mud samples

were dry-sieved using a sieve shaker, and the others were wet-sieved. Many

sieves were used (63µm, 88µm, 125µm, 177µm, 250µm, 354µm, 500µm, 707µm,

1 mm, 1.41 mm, and 2 mm) allowing fine resolution of the grain sizes. The <63 µm

size fraction was analyzed by a laser granulometer. The principle of this sensor is

based on the analysis of the diffraction pattern of a laser beam created by particles

in water. This tool directly measures the amount of particles in water and indicates

their size distribution. Granulometer statistical parameters include mean grain size,

variance, symmetry, and skewness. Results were calculated, merged, and plotted

as triangular diagrams using both Folk (1954) and Tortora (1999) classification

methods for sediments.


43

Calcimetry was performed using a Dietrich-Fruhling Calcimeter to determine the

amount of calcium carbonate in a sample. The method exploits the reaction of the

HCl attack of CaCO3 that produces CO2. The instrument consists of a sample-

holder, a cooling coil, and a graduated glass cylinder to capture CO2 produced

during the reaction between calcium carbonate and diluted hydrochloric acid.

Since the volume of CO2 released is related to CaCO3 content in the sample, it is

possible to calculate the amount of CaCO3.

The results of all analyses are described in Sec. 5.1.1.

Ch. 4 - Analysis of submarine flanks of Pantelleria volcano

44

Chapter 4

Analysis of submarine flanks of Pantelleria

volcano

Until recent surveys (2006 - 2008), little was known about the submarine

portions of the Pantelleria volcano. Preliminary results of the surveys were

presented by Bosman et al. (2007). The new data from this work will provide

information on the depth and morphology of the seafloor, as well as the shape and

size of the never before mapped submarine features.

At Pantelleria, underwater flanks show different morphologies. The data

presented in this chapter are subdivided in four distinctive sectors (northwestern,

northeastern, southwestern, and southeastern, respectively Sec. 4.1, 4.2, 4.3,

4.4). These sectors are shown in Fig. 4.1. After a brief introduction of the main

morphologies of the area surrounding Pantelleria, detailed morphological accounts

will be given for the various features for each sector of the volcano. This will

allowed for a more systematic approach in analyzing and comparing the flanks in

terms of features, depths, and average slopes.


45

• General morphologies of Pantelleria offshore

Fig. 4.1 Shaded relief map showing topographic and bathymetric data at Pantelleria volcano and the surrounding area. (C=channel; L= landslide scars; and R1, R2= ridges). Different sectors are marked by the colored areas. The coastline is shown in black. Brown contour every 150 m.

Pantelleria volcano is located at the center of a small sub-basin, at the apex of

the Pantelleria Rift (Fig.4.1 and Sec. 1.1).

The Pantelleria Rift is elongated NW-SE and is characterized by a regional

slope (0.45°-1.8°) dipping south-eastwards, as a re sult of increasing depth from

the shallower sill systems, which delimit the Sicily Channel from the West

Mediterranean basin in the north, to the deeper portion of the Pantelleria Rift (Fig.

4.1). In correspondence of Pantelleria Island, the Pantelleria Rift broadens to

about 30 km. The island divides the northern edge of the rift into two fault-bounded

valleys. Its morphology is almost flat on the NW sector, where it reaches a

maximum depth of 840 m, whereas it becomes highly irregular on the SE sector,

where the two large valleys dip toward the deeper portion of the Pantelleria Rift.

Both the northern and southern valleys show an increase in regional slope towards


46

the SE, in an area corresponding to a lineament roughly elongated NNE-SSW

(Fig. 4.1). This lineament corresponds well with the orientation of the Zinedi fault

(Fig. 4.1), and seems to be its extension offshore.

The valleys are roughly elongated NW-SE and are bounded on the north by the

Adventure Bank slope, and on the W and SW by the Tunisian continental slope

(Fig. 4.1). These slopes are characterized by tectonic deformation and locally by

contourite deposits (Martorelli et al., 2010).

The Adventure Bank gently inclines toward the rift, and is bounded by a steep

slope up to 700 m high. The SW flank of the rift, which is part of the Tunisian

continental slope, gently inclines toward the rift and is morphologically more

complex, with the presence of some structural highs.

The valley on the northern side of Pantelleria is about 8 km wide and 41 km

long and shows increasing regional slope below -600 m up to ~6°. It is

characterized by several minor escarpments between 20-50 m high likely related

to widespread erosive processes and/or to the action of currents. Erosive

processes and currents were inferred based on the peculiar fanning pattern, which

is hard to associate with a structural control.

At the base of the Adventure Bank slope, erosional processes seem to be better

organized in a channel oriented WNW-ESE (C in Fig.4.1). This channel is about 1

km wide and 50 m deep and continuous for about 6 km. It has a “U” shape profile

and ~75 m high steep side walls.

The southern valley of the rift is about 5 km wide and 41 km long and shows

increasing regional slope below -750 m up to ~4°. M inor escarpments are present,

but are less diffuse than on the northern one. Landslide scars (L in Fig. 4.1) are

seen in the lower SE portion of the Tunisian continental slope, between -430 m

and the basin, and affected an area of some 41 km2 with a ~5° gradient.

Moreover, the bathymetry data revealed two constructional ridges. The

westernmost one (R1 in Fig.6.1) is located about 23 km south-westward from

Pantelleria Island, along the Tunisian continental slope. The 300-400 m-high relief

shows a linear trend with a maximum length of 11 km and a maximum width of 3

km, and a shallow and almost flat crest, with a peak at -60 m. Its major axis is

aligned in the NW-SE direction, akin to the axis of the Pantelleria Rift and the

Pantelleria volcano. The flanks of the ridge are steep (with slope gradients up to

27°) and almost symmetric. In detail, the SW flank is affected by instability


47

phenomena, whereas the NE side seems covered by sediment. Along the whole

base of the SW flank an erosive channel (moat) is present (Martorelli et al., 2010).

The summit occupies an area of some 9 km2, and shows a terraced morphology

punctuated by several narrow ridges, likely related to the exposure of dike

intrusions. The break in slope is almost continuous at about -140 m and, along

with the terraced morphology, testifies that the ridge underwent eustatic sea level

variations which cause erosion. Based on seismic facies, Civile et al. (2008)

inferred it is a wide volcanic field bounded by normal faults.

The second ridge is located along the Tunisian slope (R2 in Fig.6.1), about 10

km south-westward from Pantelleria Island and at -760 m depth, with a summit at -

560 m. The 200 m-high ridge is about 3 km long and 1.6 km wide, and it is

elongated in the NNW-SSE direction. It exhibits a stubby shape and a strong

asymmetry, with linear 200-m-high steep (up to 20°) flank to the east and a less

steep flank to the west. In the southern sector, the western flank exhibits a 50-m-

high scarp.

• Pantelleria volcano

Pantelleria Island represents the emergent tip of an underwater volcanic

structure. The underwater portion descends to depths of 1200-1300 m comprising

a surface of 308 km2; therefore, around 72% of the volcanic complex lies below

sea level (Fig. 4.1 & 4.2).

The island is symmetrically elongated along the NW-SE trend of the SCRZ

(Sec.1.1). Likewise, below sea level, both the insular shelf and the majority of the

deeper outcrops have a greater extent on the NW and SE flanks (Fig. 4.1 &

section A-B in Fig. 4.2). On the other hand, the SW and NE flanks are steep and

narrow (sections E-F & G-H in Fig 4.2). Moreover, sections C-D and I-L in Fig.4.2

show depths for distances at 6 km offshore the island, indicating a plateau

punctuated by relief in the NW and deeper outcrops in the SE.


48

Fig. 4.2 Profiles along and perpendicular to the NW-SE axis of the Pantelleria volcanic

complex showing the symmetrical emerged and submerged flanks of the volcano. The coastline is shown in red and the base of the volcano is in black.

Depth distributions in each sector are shown in Fig. 4.3, and give a general

indication about the nature of the Pantelleria flanks. Of the sectors defined (Fig.

4.1), the NW sector of Pantelleria is the shallowest one. Depths progressively

increase from NW toward SE, where the base on the volcano reaches about -1300

m.

Even though insular shelf depths were observed at large-scale and a more

accurate definition of the insular shelf will be presented in the following sections,

the interval between 0-120 m b.s.l (yellow in Fig.4.3) shows its rough distribution.

This interval is the greatest in the NW sector and constitutes its highest depth

range percentage (22% in Fig. 4.3). Though less than the NW sector, the 0-120 m

interval is also the largest depth interval in the NE sector (17%). This interval

makes up a relatively smaller proportion of the SW sector (12%) and is minimal for

the SE sector (5%).

The depth distributions of the SW and NE sectors look quite similar with some

differences between -500 and -800 m, as the intervals between -600/-800 m are

better represented in the SW sector.

From the pie charts (Fig. 4.3), it is evident that the depth distributions are not

uniform both within and between each sector. The extreme case is the SE sector

where depth distributions are irregular (from 2% to 16%), with shallower depths

less represented than the deeper depths.


49

Fig. 4.3 Top: Pie charts showing depth distributions (in meters) for each sector of the Pantelleria volcano, as defined in Fig. 4.1. The interval approximately corresponding to the insular shelf break is shown in yellow. Bottom: Dispersion diagrams of each sector showing depth (ranging from <-50 m to -1300 m) versus slope. Gradients extracted from the bathymetry were binned into 50 m depth intervals and the median (50%) and the interquartile values (25%, 75%) of the data were found within each depth interval.

-1000

-800

-600

-400

-200

0

0 10 20 30 40

50%75%25%

-1200

-1000

-800

-600

-400

-200

0

0 10 20 30 40

-1200

-1000

-800

-600

-400

-200

0

0 10 20 30 40

-1400

-1200

-1000

-800

-600

-400

-200

00 10 20 30 40

NW NE

SW SE

depth (m) versus slope (o)


50

As a way of better understand the processes acting along the flanks of

Pantelleria volcano, a morphometric analysis of the flanks of Pantelleria was

developed.

By employing statistical analysis based on the approach of Mitchell et al.

(2002), dispersion diagrams (Fig. 4.3, bottom) were created as a way of

summarizing how the average slope of each sector changes with depth. This

analysis allows to compare different flanks in term of slope and break in slope.

Moreover the dispersion is indicative of the processes along the flanks as for low

value it points to a regularization of the slope in contrast to constructional

processes. Gradients were computed for each sector defined in Fig. 4.1 from the

bathymetry over a 20-m length scale, and sorted into 50-m depth intervals. The

median (50%) and interquartile ranges (25%, 75%) of the data were found within

each depth interval. In these diagrams, the average value of a break in slope

should correspond approximately to when the change in gradient is minimized; the

foot of the slope occurs when the change in gradient is at a maximum.

Average slope values in the four sectors (Fig. 4.3, bottom) show that trends in

slope are quite irregular and vary between a few degrees to a maximum of 30°

(i.e., NE sector). When comparing the slope distributions of the four sectors, the

SW and NE sectors showed similar distributions.

The shallow portion of the NW sector is characterized as having a shallow slope

corresponding to the insular shelf, with an abrupt increase in slope around -130 m

down to -250 m. The other sectors are characterized by steadily increasing slopes

from -50 m down to the foot of the slope (-250 m in the NE and SE sectors and -

200 m in the SW sector). The greatest slope in each sector is between 200-250 m.

The maximum observed slope is greatest in the NE sector (~30°). The NW had the

smallest maximum slope value for all the sectors at ~22°. The highest slopes for

each sector (>15°) are distributed in depths rangin g between ~150 m and ~500 m.

By -600 m, there is a decrease in slope to around 10°-12° in each sector. The

NE sector has a relatively constant slope of ~10° f rom -600 to -900 m, before

reducing to around 5°. On the other hand, the SE an d SW sectors show a general

decreasing trend of slope with depth. The SW reaches its minimum slope by -800

m, while the slope of the SE sector continues to gradually reduce down to -1200

m.


51

4.1 Northwestern sector

The northwestern sector covers an area of about 110 km2 and reaches a

maximum depth of about 700 m. It is characterized by a well-developed insular

shelf and by a cluster of pointed and elongated volcanic cones that mainly crop out

at greater depths.

4.1.1 The NW insular shelf

Fig. 4.4 Shaded relief map of the insular shelf on the northwestern sector of the Pantelleria volcano. Black contours are at 5 m depth intervals, the blue contour represents the coastline, and brown contours are at 20 m intervals on land. Submarine data and contours represent only surveyed areas; gaps in the submarine coverage are in white.

From Punta Fram to Punta Karuscia, the insular shelf (Fig. 4.4) includes a wide

area (about 20 km2) between the north coastline and the shelf break, located at

about -120 m. The shelf break is mainly uninterrupted except in the NW prominent

portion where it is hidden by a lava flow front, which connects the shelf with a

deeper outcrop.

The insular shelf extends offshore for a few hundreds of meters in the NE and

W and to about 4 km in the NW direction, consistent with the general elongation of


52

the whole volcanic complex (Figs. 4.1& 4.4). The shelf gently dips with an average

seaward gradient of 2°.

The insular slope descends abruptly to depths ranging between ~280 m to ~500

m with a gradient of 18-20°.

By employing statistical analysis (See Sec. 4), a dispersion diagram (Fig. 4.5A)

was created as a way of summarizing how the average slope of the shelf changes

with depth. Gradients were computed for the shelf area from the bathymetry over a

20-m length scale and sorted into 2-m depth intervals. The median (50%) range on

the data was found within each depth interval.

Fig. 4.5A highlights the presence of discontinuities in the slope distribution at

~66 m, ~78 m, ~114 m, and ~120 m. These discontinuities indicate a series of

minor shelf breaks, that might correspond to the edges of flows or paleo-

shorelines. The major shelf break occurs at the largest discontinuity at ~120 m

depth.

Fig. 4.5 A) Dispersion diagram of NW sector insular shelf depth (ranging from -15 to -

140 m) versus median slope (calculated every 2 m). Gradients extracted from the bathymetry were binned into 2-m depth intervals and the median value (50%) of the data was found within each depth interval. B) Pie chart showing the distribution of depths in the sector.

In the pie chart (Fig.4.5 B), the shelf depths were divided in 20 m-intervals to

show their distribution. The diagram highlights that the majority of the shelf lies

between -40 m and -80 m. It must to noted that data shallower than -15 m were

not available in the dataset used and this explains why this depth range is

underrepresented (1%).


53

The insular shelf is not flat (Fig. 4.4). The data show a variety of well preserved,

rough lava lobes and a shallow outcrop (described below in 4.1.1.1 and 4.1.1.2)

emplaced from different constructional processes alternating with erosional

activity.

4.1.1.1 Underwater extension of subaerial lavas

Fig. 4.6 Correlations between on land vents and lava flows from Mursia, Cuddia del

Monte, Cuddia Bruciata, and Cuddia Ferle (contour lines are spaced every 10 m offshore and every 20 m on land, the black contour is the coastline). Submarine data represent only surveyed areas; gaps are in white.

Shallow lava-fed deltas and flows, mapped with high resolution bathymetry data

were correlated with subaerial outcrops (Fig. 4.6) based on their proximity, on the

small difference in altitude as well as the paucity of other recent flows on the NW

shelf. On land lava flows from Mursia and Punta San Leonardo (including the lava

flow fields of Cuddia del Monte, Cuddia Bruciata, and Cuddia Ferle) advanced

offshore to a depth of ~55 m. The subaerial portions of these flows, were

geochemically analyzed and dated (Civetta et al., 1984; Mahood and Hildreth,

1986; Civetta et al., 1988; Avanzinelli et al., 2004). Civetta et al. (1998) reported


54

an age of <10 ka for Mursia basalts and ~27-29 ka for P.ta San Leonardo basalts

on land. Submarine portions of these lava flows are unsampled and undated.

Mursia basalts (Figs. 4.6 & 4.7) extend WNW offshore for about 1.6 km from the

vent, which is located close to the shoreline. The flow covers an area of

approximately 1 km2 offshore, reaching a maximum depth of 52 m (Fig. 4.6). The

flow shows a massive, elongate shape and divides into two lobes at about -40 m.

The individual lobes have rough surfaces, and minor scarps. The vertical relief of

the flow is about 5 m at a depth of 37 m (Fig. 4.4), and becomes more subtle with

increasing depth.

Fig. 4.7 Left: Offshore extension of the Mursia lava flow (location indicated in Fig. 4.6). Contours on the island are every 3 m. It partly overlays another, more subdued outcrop to the south. Coastline is in black. Right: Cross-section at about 37 m depth, location indicated on the map.

The offshore Mursia lava flow may partly overlie a more subdued outcrop

offshore Cala dell’ Alca (Fig. 4.7). This outcrop is characterized by a scarp that has

about 12 m vertical relief and a sharp concave edge at -40 m.

The Punta San Leonardo Basalts [(27-29 ka; lava flow fields of Cuddia del

Monte, Cuddia Bruciata, and Cuddia Ferle (Civetta et al., 1998)], are well

represented in the submarine data, where two different lava outcrops are

recognized. The first of these is a wide fan-shaped lava delta in front of P.ta Sidere

to P.ta San Leonardo, which extends seaward about 1.4 km with an overall area of

3.4 km2. Unfortunately, the lack of very shallow bathymetry (0-30 m) immediately

offshore Cuddia del Monte (located at the present southern shoreline of Pantelleria

village) and Cuddia Bruciata (located east of Pantelleria village) prevents


55

distinguishing the offshore extensions of lava flows emitted from each vent (Figs.

4.6 & 4.8).

Fig. 4.8 Top: Offshore extension of the Cuddia del Monte and Cuddia Bruciata lava flows (location in Fig. 4.6). Contours on the island are 3 m. Coastline is in black. Bottom: Cross-section at about 48 m depth, location is indicated on the map.

The flow front of the fan-shaped lava delta reaches a maximum depth of 54 m

and is characterized by an irregular edge. The submarine extension of the lava

flow has an overall width of 2.7 km and seems to be constituted by several narrow

lobes that have a rugged small-scale morphology resulting from lava tongues,

furrows, clefts, and tumuli, for example. In the shallow portions above 35 m depth,

the flow is characterized by relief up to 10 m (see profile in Fig. 4.8). The lava flow

has transverse clefts of a few meters, and narrow tongues, which decrease in

number with increasing depth.

The second delta, located in the offshore area between Cala Bue Marino and

Punta Karuscia, represents the submarine extension of the lava flows likely

emitted from Cuddia Ferle, which is located further inland (Figs. 4.6 & 4.9) and has

now been leveled for a lengthened airstrip. Cuddia Ferle lavas created a fan-

shaped lava delta on the shore (Figs. 4.6 & 4.9), which at present-day shows


56

fingers of lava into the water and covers an overall area of 5 km2. The massive

lava outcrop is up to 10 m high (a-b profile in Fig. 4.9) and shows a strongly

crenulated lava front, reaching a depth that varies from 38 m to 47 m in the

northern area. Single lobes are similar to those of Cuddia del Monte; although they

protrude less far into the sea. A few lava fingers, up to 4 m high, have been

preserved in front of P.ta Karuscia. Offshore P.ta La Guardia, at -37 m, the flow

partly converges on a 5 m cliff (c in Fig. 4.9), oriented obliquely to the flow axis.

Fig. 4.9 Top: Offshore extension of the San Leonardo basalt flows [c=cliff] Contours

every 3 m. Coastline is in black. (Location in Fig. 4.6). Bottom: Cross-section of flow at about 40 m depth, location is indicated on the map.

A volcanic outcrop (Fig. 4.10), which covers an area of 0.3 km2, is located 1.2

km from the N shore, at 52 m depth. The outcrop has cone-shaped morphology as

high as 15 m and some lobes on the NE flank.


57

Fig. 4.10 Left: Shaded relief map of the cone-shaped outcrop on the offshore extension

of the San Leonardo basalt flows. Lower right: Location of the cone on the northern shelf. Upper right: Cross-section (a-b) of the cone (location on the shaded relief map).

4.1.1.2 Deep features on the NW insular shelf

The deeper NW insular shelf (i.e., below -60 m) is overlaid by lobate

morphologies, characterized by an undulate shape and escarpments that

produced a backstepping morphology (Figs. 4.4 & 4.11). The main break in slope

is almost continuous at about -120 m and corresponds to the shelf break.

A 15 m-high scarp at about -80 m is continuous for about 3.2 km on the NNW

shelf (a1 and a2 in Fig. 4.11). It could be a paleo-shoreline, and its possible age

will be discussed in Sec. 6.2.

The scarp appears smoothed and partly draped by successive flows that

reached its edge (i.e., b & d in Fig. 4.11). In fact, narrow lobes are present at the

foot of the scarp on the northern side (b in Fig. 4.11). These 400 m wide narrow

lobes have 6 m vertical relief. They lie on a flat area inshore of a 1 km-long

elongated feature that has 3 m vertical relief and a width of 60 m. This feature runs

parallel to the edge of the major break-in-slope at -120 m and partially isolates an

inner bay at -112 m (c in Fig. 4.11).

On the western side of the NW shelf (a2 in Fig. 4.11), the break-in-slope at -80

m is covered by a different flow lobe and its original geometry is shown only along

a few places. A fan-shaped lobe to the north (d in Fig. 4.11) almost completely

covers a terrace at -120 m and is characterized by a smooth surface and an

irregular edge at depths between -112 m to -128 m.


58

Fig. 4.11 Left: Shaded relief map of the insular shelf on the northwestern sector of the Pantelleria volcano. Contour every 20 m. Right: 3D view of the lower NW shelf from the north [a1, a2) scarps; b) narrow lobes; c) inner bay; d) fan-shaped-flow; e) lava flow front; f) narrow chutes]. The near-shore lava flows and the island are in yellow.

Narrow and 3-4 m deep chutes are visible in the middle of the upper shelf,

between -60 m down to -100 m (f in Fig. 4.11).

4.1.2 The NW deep flank

The deeper portion of the NW sector below the shelf edge (Fig. 4.1) reaches a

maximum depth of about 700 m, has an average slope of 5°-12°, and covers an

area of about 85 km2. The area is located on a saddle between the two valleys of

the rift, where a 3 km wide passage separates the volcanic complex from the

WNW-ESE escarpment of the Adventure Bank (Fig. 4.1). The deep flank is

connected to the insular shelf through a steep slope (18-20°), which descends

abruptly from ~280 to ~550 m depth.

From the high-resolution multibeam bathymetric data, a cluster of pointed

volcanic cones and fissure vents have been identified. Most of the volcanic cones

show well-preserved morphology, despite the area being locally affected by small

to medium scales instability phenomena. This volcanic field was previously

unknown, although the last eruption (1891) of the volcano occurred in this area. A

discussion about evidence of this eruption will be presented in Sec. 6.4

f


59

4.1.2.1 The NW volcanic field

Detailed quantitative description and morphometric analysis of the NW volcanic

field will be presented in Sec. 6.3 with the discussion of the results. As a

consequence, only a brief illustration is given here.

Fig. 4.12 Top: shaded relief map of the NW volcanic field and IDs of the cones, outlined in red. Interpreted eruptive centers are red dots. Bottom: the columns in the plot show the depth of each cone considering its basal depth (Zb) and summit depth (Zs). The value indicated in the column correspond to the maximum difference in elevation (i.e., |Zb-Zs|).

The NW volcanic field mainly consists of well-preserved volcanic cones. The

volcanic cone closest to the island is only 2 km W of the shoreline at a depth of

~500 m, while the farthest one is located 9 km to the NW at ~700 m depth.


60

In the volcanic field, 26 centers were identified (Fig. 4.12 top) that range

between simple conical to composite, elongated morphologies. Some edifices

were formed by coalescent centers (i.e., Cone 2), creating elongated features with

a length up to 2.7 km. The volcanic centers are characterized by high relief, no

summit craters, and smooth flanks, with the exception of a few (i.e., Cones 1, 3,

10, and 14), which were affected by summit collapses (Sec. 6.3.5).

The cones occur at depths ranging between 328 m to 730 m at their base (Zb),

while the summits (Zs) range between -614 m and ~165 m (i.e., Cones 9, 12, and

14). The maximum difference in elevation between the deeper basal edge and the

summit of individual cones (i.e., Zb-Zs) ranges from 63 m to 520 m (Fig. 4.12

bottom). The maximum heights, considering the basal slope, range between 41 m

to 346 m. Basal diameters vary between 0.2 km to 3.5 km and slope angles at the

summit range from 21° to 42°. The basal areas are b etween 0.04 km2 and 6 km2,

and the volumes vary from 6x10-4-0.37 km3, for a total amount of 1.75 km3 with an

average volume of 0.067 km3.

The majority of cones display a preferred morphological orientation mostly

resulting from the alignment of simple conical shapes. These alignments will be

discussed in Sec. 6.3.4.

• 1891 vent

Fig. 4.13 Location of the strip of floating scoria observed during the 1891 eruption,

defined by the coordinates given by Riccò (1892). Another possible vent location is marked as “a” (See location in Fig. 4.12).

The last activity of the volcano occurred in 1891 (Riccò, 1892) in this area,

about 5 km northwest of the coast of Pantelleria. This vent was named Foerstner


61

volcano following that eruption (Washington, 1909). However, it had not

previously been mapped.

The location of the coordinates (N36° 50’ 45’’; E0° 33’ 30’’) given by Riccò

(1892) for the center of the strip of NE-SW oriented floating scoria (200 m long, 50

m wide) originating during the eruption is shown in Fig. 4.13. At the time of the

eruption the depth of the seabed was measured and the position was calculated

on board of the vessel Bausan. The depth was measured with a 25 kg lead line

and found to be greater than -320 m. The position was measured using

triangulation via two fixed angles from Semaforo P.ta Caruscia and Semaforo P.ta

Fram. The coordinates of the center (N36° 50’ 45’’; E0° 33’ 30’’), which were

referred to in the current-time system as Bessel ellipsoid with prime meridian

Rome-Monte Mario, were transformed here into the UTM-WGS84 system. The

transformed coordinates (N36° 50’ 51”, E11° 53’, 34 .7”) correspond to a depth of

215 m. Combining the historical reports and the detailed bathymetric map, the

most likely eruptive site is a 200-m wide cone that rises from -350 m and has a

summit peak at -260 m (16 in Fig. 4.13). This 90 m-high cone is located along a

steep slope, has two peaks aligned NE-SW, and has a volume of some 600,000

m3. Another possible vent (a in Fig 4.13) that might be associated with the

emission of floating scoria was identified immediately north of the cone, and closer

to the location given. This 400 m long outcrop, oriented NNE-SSW, is similar to a

rampart and extends on the slope (between -120 m and -330 m). However, as the

estimated depth given by historical report and the bathymetric map do not match,

I prefer the cone 16 location.

4.1.2.2 Instability along the slope

The shelf edge in the NW sector is locally affected by erosional processes that

mostly developed along the slope. Some of these processes have been

investigated by coupling swath bathymetry and seismic (SBP, SPK) data.

Narrow scarps along the slope (between -200 and -300 m) are present but not

widespread. Their widths range between 400-700 m with vertical relief up to 20-30

m. These features seem to be due to the erosion of the slope.


62

Fig. 4.14 Mass wasting features on the NW slope of the Pantelleria flank (contour every

25 m). Upper right: The a-b section shows the “U”-shaped cross-section of the valley. Lower right: the c-d section shows the slope of the valley.

The valley between two outcrops is affected by narrow scars (Fig. 4.14), and a

debris is present at its foot. To the west of the outcrop, mass wasting processes

created a steep scarp (~16°) and developed downslop e in a scar. The scar is

about 4 km in length and 1 km wide. It is characterized by a “U”-shaped cross-

section (profile a-b in Fig. 4.14), which narrows downslope where it is confined by

an elongate smooth feature. Narrow steps are present along the slope (profile c-d

in Fig. 4.14). On the east flank of the outcrop, the slope gradient is less (~7°) than

in the west, and only few narrow scars between -175 m to -300 m are present.

A 1 km long E-W closed depression occurs at the foot of the slope, while a

possible current deposit (contourite in Fig. 4.14), about 1.5 km in length and 10 m

thick, was identified at the base of the shelf at -350 m, suggesting that this area is

affected by intense oceanographic dynamics.

The area mostly affected by instability processes is located on the western

portion of the shelf (Fig. 4.15). Here, three different deposits were identified along

the slope and at the toe of the volcanic cone on the basis of their different

morphological facies (A, B, & C in Fig. 4.15).

The facies A is located at the base of the slope, between -670 m and -720 m

and covers an area of about 5 km2, with a gradient of 2°. The upper slope is

characterized by an 8° gradient slope and is affect ed by several narrow arcuate


63

slump scars, which continue on the facies A and produce an irregular morphology.

These features are spaced between each other some 150-180 m. They range in

scale from 0.3 km to 0.8 km in length, 5-20 m in height, and often have a plunge

pool 2 m deep (profile a-b in Fig. 4.15). A possible mechanism for their origin is

scouring during submarine debris flows. The scars decrease in frequency and

extension downslope, but also continue laterally over the C facies. The C facies,

which covers an area of 1.2 km2, is similar to A but seems to be covering the base

of a volcanic edifice (Fig. 4.15).

The B facies (Fig. 4.15) covers an area of 8 km2 down to -730 m. It is

characterized by a hummocky morphology due to both several blocks appearing

elongated to semi-circular in shape (20-100 m wide, ranging from 3 to 10 m in

height) and semi-circular volcanic outcrops (Fig. 4.15 and profile e-f). The volcanic

outcrops are 200-300 m wide and range from 20-25 m in height. They are partly

covered by sediment (profile e-f in Fig. 4.15); this suggests that some of the other

blocks located further downslope might also be narrow primary structures

completely covered by the sediment.

High-resolution seismic surveys were carried out in this area to investigate the

deposits (Sec. 3.2.1). Transparent facies indicative of mass movements were

mapped on the Sparker (SPK, total length of profiles about 75 km) and sub-bottom

seismic profiles (SBP, total length of profiles about 100 km) (e.g., Fig. 4.15 middle)

from the base of the edifice to about -735 m. This allowed for a detailed isopach

map to be created (Fig. 4.15 top left) illustrating the variation in thickness of the

deposits, with the aim of estimating their overall volume. The seismic profiles show

that the deposits extend further downslope than it appears from the surface

morphology, and covered an extensive area of 25 km2. The acoustic thickness

varies from 65 ms to 10 ms or less. The two-way travel times were converted to

meters using a reasonable seismic velocity of 1800 m/s for the landslide deposits

interval, and the overall volume was estimated to be ~0.4 km3.


64

Fig. 4.15 Top left: Shaded relief map and interpretation of the A, B, and C facies and

isopach lines of slide deposits mapped with SPK and SBP data. Top right: The a-b section shows the profile along the slope. Middle: In the c-d section, the SPK profile_L11 shows a transparent unit indicative of slide deposits. Bottom: In the e-f section, the SBP 071208_016 profile shows some volcanic outcrops. The outcrops shown on the left side are partly covered by sediment. Profile locations marked on the map.


65

A multichannel seismic reflection profile (Italian Commercial Zone “G” lines

G82-103B in Fig. 4.16) carried out in the same area indicates the presence of a

chaotic facies beneath the cone.

Fig. 4.16 Interpretation of a multichannel seismic reflection profile (Italian Commercial

Zone “G” lines G82-103B) showing magmatic bodies and a chaotic facies below and at the base of the volcanic cone exposed on the left.


66

4.2 Southwestern sector

The southwestern sector (Fig. 4.1) covers an area of about 50 km2 offshore

Cala dell’Altura-Balata dei Turchi and reaches a maximum depth of about 1000 m.

It is characterized by a narrow insular shelf and by different volcanic features (lava

flows, cones, and other volcanic outcrops) at depths greater than 120 m.

4.2.1 The SW insular shelf

From Cala dell’Altura to Balata dei Turchi, the insular shelf includes a narrow

section (<0.7 km wide) between the coastline and the shelf break. The break lies

at varying depths, ranging between –82 m and -112 m. The shelf is relatively steep

and dips with an average seaward gradient of 10°.

By employing statistical analysis, a dispersion diagram (Fig. 4.17) was created

to summarize how the slope varies with depth according to the approach

described in Sec. 4. Discontinuities occur at depths of ~40 m, ~50 m, ~82 m, ~112

m, ~132 m, and ~142 m (Fig. 4.17). There is almost a continuous increase in the

average slope value with depth and variations corresponding to breaks in the

slope are not markedly defined.

Fig. 4.17 A) Dispersion diagram of the SW sector insular shelf depth versus median slope (calculated every 2 m). Gradients extracted from the bathymetry were sorted into 2-m depth intervals and the median value (50%) of the data was found within each depth interval. B) Shaded relief map combined with an aerial photo of the SW flank of the island is shown.

A

B


67

A variety of features related to widespread mass wasting processes were

identified on the shelf. The most important one is a 400 m long slide scar located

offshore Cala di Licata-P.ta Polacco (a in Fig. 4.18). The scar seems to affect

smooth deposits (b in Fig. 4.18). Based on their relatively flat and smooth

morphology, the deposits were identified as depositional terraces [(i.e.,

sedimentary bodies outcropping on the sea floors at a shallow depth (generally

within -150 meters), having a wedge-shaped geometry and a terraced

morphology, Agate et al., 2004)].The depositional terraces can be traced

discontinuously along the SW shelf.

The headwall scarp has a 400 m rectilinear edge and cuts the edge of the

depositional terraces (b in Fig. 4.18) at about -30 m. The slide surface has a slope

of 10° and is incised by several narrow scars ( c in Fig. 4.18), which appear to be

characterized by retrogressive mechanisms. The failure is deflected by an outcrop

(f in Fig. 4.18) and does not reach the shelf break (d in Fig. 4.18), which has a

regular edge throughout this area at about -100 m. Overall, the slide scar is

between 30 and 100 m deep, with a total area of 0.15 km2 and an estimated

volume of slumped sediment of 9x10-3 km3.

Fig. 4.18 Left: Shaded relief map with the morphological interpretation of the slide scar

affecting the shelf offshore Cala di Licata. Contour every 10 m. Right: profile A-B showing the lower portion of the scar and 3D view. [a) slide scar; b) edge of depositional terrace; c) narrow scars; d) shelf break; e) scarps, f) outcrop]. See location in Fig. 4.17.

A few volcanic outcrops were identified on the SW shelf. The largest is a 25 m-

high cone-shaped volcanic outcrop (Fig. 4.19), about 400 m offshore P.ta Ferreri.


68

The outcrop has a very shallow summit peak (-12 m; ∗ in Fig. 4.19). Its NE flank is

characterized by a sub-circular edge, while the rest of it is shaped by several lobes

(<4 m in height), mainly oriented NW-SE and NE-SW. It covers an area of ~0.1

km2 and has a volume of 7x10-3 km3 above the surrounding shelf. Moreover, it is

aligned NE-SW with the subaerial cones of Cuddie di Bellizzi (Fig. 4.19), which

erupted pantellerite rocks (18±5 ka, Civetta et al., 1988).

Fig.4.19 Shaded relief map of the cone-shaped outcrop offshore P.ta Ferreri (∗) peak.

Contour every 10 m.

4.2.2 SW deep flank

The SW flank consists of a steep slope with a gradient of 18-20° that descends

abruptly to depths ranging between ~660 m to ~1000 m. The flank extends

offshore to a maximum distance of 4 km and is highly affected by dismantling

processes. The main features identified in this area consist of deep lava flows,

volcanic outcrop, volcanic centers, and a channel (Fig. 4.20).

Two main lava flows were observed along this flank. Both have distinctive

structures and morphology. The sketch in Fig. 4.20 shows their main flow

dimensional parameters . Their upper limits are both confined in a scar or chute

near-shore (a and b in Fig. 4.20), but no sign of them crossing the shoreline was

identified. They are oriented approximately ENE-WSW (via the downslope

direction) and are characterized in the upper portion (between -280 m to -560 m)

by roughly linear channel-levee structures, with levee deposits along the sides up


69

to 20 m in vertical relief and between ~60 m to ~120 m wide (Fig. 4.20). Also,

there are lava delta lobes at the flow toe, which is located at the base of the edifice

(at ~700 m). The lava channel-levee affects the upper steep slope (average 16°),

while the lobes deposits lay on a less steep slope (2–7°) forming a lower-fan

(distal) segment.

Fig. 4.20 Left: Shaded relief map of the flank offshore Cala dell’Altura-Balata dei Turchi. [a, b) channel-levee structures with arrows indicating the levees; a1, a2, a3) lava lobes; c) lava flow; d, e, f) volcanic outcrops; g) channel]. Contour every 100 m. Right: Cross-sections of the channel-levee structures. Bottom right: channel-levee and flow toe section sketch showing the main flow dimensional parameters described in this study (after Harris et al., 2004).

The lava flow a identified offshore Cala di Sataria and P.ta di Tre Pietre covers

an area of about 3.2 km2, is about 0.7 km wide, and 1.8 km in length; while the

flow channel is ~0.3 km wide. The levees developed just deeper than -275 m, with

about 20 m in vertical relief and ~60 m in width. The 3 km long channel fed two

lower delta lobes (a1 and a2 in Figs. 4.20 & 4.21). The lower portion of the channel

is characterized by a flat top surface and an almost continuous 10 m-high rim at

the edge, which seems to indicate that the lava drained and flowed laterally (a3 in

Figs. 4.20 & 4.21). The front of the top lobe is rugged and about 75 m thick. It has

a perimeter of 1.3 km. The delta lobes are oriented in different directions,

indicating that the lava flow deflected during its emplacement. Lobes a1 and a2


70

have a ropy to blocky texture, a regular front that has a perimeter of ~1.5 km, and

are about 50 m thick at about -680 m. Along lobe a2, stair-stepped terraces and

levee remnants are present. Lava flow front perimeters are about 1.5 km each.

The lava channel b identified offshore Scauri covers an area of about 1 km2, is

about 0.4 km wide and 2 km long (b in Figs. 4.20 & 4.21). The levees are deeper

than -280 m, with about 20 m in vertical relief, and up to ~120 m wide. The flow

created a flat surface at -490 m, with a rugged front that is about 50 m thick. Some

erosional features, at the base of the flow, were identified between -550 m an the

base of the edifice.

Another flow is present immediately south of b, down to -820 m (c in Figs. 4.20

& 4.21). The upper portion of this flow (down to -640 m) possibly formed a

separate lobe about 50 m thick. The toe portion is a flat surface at about -700 m

and a thickness of 75 m. The flow covers an area of about 1.4 km2 and the front

has a perimeter of ~1.6 km.

Fig. 4.21 Left: Shaded relief map of the volcanic field offshore P.ta Ferreri [a, b) channel-levee structures; a1, a2, a3) lava lobes; c) lava lobe; d, e, f, n) volcanic outcrops; d1, d2) cone-shaped structures; i) cliff; l, m) lava lobes] (see Fig. 4.20 for location). The cones in the cluster are marked. Right: the columns in the plot show the depth of each cone considering its basal depth (Zb) and summit depth (Zs). The value indicated in the column correspond to the maximum difference in elevation (i.e., |Zb-Zs|).

Volcanic outcrops were identified offshore Cala dell’Altura and P.ta del

Molinazzo (Fig. 4.20). They cover an area of ~3.5 km2 and are oriented ENE-


71

WSW and N-S (d and e in Fig. 4.20). The first outcrop (d in Fig. 4.20) consists of a

volcanic massif carved by erosional processes. The outcrop, about 200 m-high in

cross-section, is characterized by a long smooth ridge with a flat summit at -115

m, and by an erosional channel oriented E-W (d1 in Fig. 4.21). The channel is 1.2

km long and 200 m wide. Two narrow cone-shaped structures, both with a

diameter of less than 100 m, were identified on its northern flank and at its base

(d2 in Fig. 4.21).

The second outcrop (e in Figs. 4.22) is 250 m high in cross-section. The summit

is a ~1 long ridge, which has a peak at about -410 m. The flanks are characterized

by steep slopes (up to 24°) and by a 1 km long stru cture oriented E-W that

intersects the peak.

Along the flank offshore P.ta Ferreri, were identified four steep cones (cones 27,

28, 29, and 30 in Figs. 4.20 & 4.21). They are conical-shaped structures with two

or more peaks at the summit. Their bases (Zb) occur at depths ranging from 500 m

to 850 m, while the shallowest summit is ~238 m deep (i.e., Cone 27) (Fig. 4.21).

The maximum differences between the deeper basal edges and the summits (i.e.,

Zb-Zs) of individual cones range from 240 m to 338 m (Fig. 4.21). Whereas, the

heights, which considers the basal slopes, range from 126 m to 213 m. Basal

diameters vary between 0.5 km to 0.8 km and measured slope angles range from

36° to 40°. The basal areas are between 0.2 km 2 and 0.6 km2, with volumes

between 0.1–0.5 km3.

West of Cone 27, there is a 130 m-high volcanic outcrop (f in Figs. 4.20 & 4.21).

The outcrop shows a star-shaped structure. The cone lies on a terraced area (i in

Fig. 4.21) that is overlaid by lava lobes encroaching from two different directions (c

and l in Fig. 4.21), suggesting that these lobes were emplaced over the previous

flow unit i.

To the SW of Cone 28, there are two deep lava lobes (l and m in Fig. 4. 21),

each about 50 m thick. The upper one is oriented NE-SW, covers an area of 0.5

km2, and overlies the lower lobe that is oriented N-S and occupies an area of 0.2

km2. The lava flow front is at -820 m and at the same depth as the lava flow to the

north. On a broader scale, those two lava flow fronts and a less discernible one in

the south (n in Fig. 4.21), show a distinctive NW-SE alignment.

Mass wasting processes mainly affected the upper slope of the SW sector

(between -180 m and -250 m) (Fig. 4.20). Moreover, offshore Salto la Vecchia,


72

sediment transport is channelized between two outcrops (g in Figs. 4.20 & 4.22).

The erosive channel developed on a steep slope (<20°) and reached the base of

Pantelleria edifice, at about -1000 m. It is 3 km long and on the average of 0.8 km

wide, has a “U”-shaped section profile, and is oriented NNE-SSW. The head of

this channel system (at about -200 m) is ~2 km wide and shows several scars with

a ~80 m high scarps. Narrow incisive valleys (< 0.4 km wide) developed from the

scars.

Fig. 4.22 Shaded relief map offshore Salto la Vecchia [e) volcanic outcrops; g) channel;

h) distal conical-shaped structures, o) scours]. (See Fig. 4.20 for location). The sediment is funneled along the SW flank down to the base of the edifice.

Afterwards, the transport of sediment towards the basin follows less incised

scours, on a slope of 3-4° ( o in 4.22).


73

4.3 Northeastern sector

The northeastern sector (Fig. 4.1) covers an area of about 42 km2 and reaches

a maximum depth of about 1000 m. It is characterized by a narrow insular shelf

and by several erosional features. Protrusions of ancient lava flows emitted on

land have been identified where the shelf widens, in the northern and southern

extents of this area. Erosive channels are widespread across the slope and

represent the main active dismantling process affecting the area.

4.3.1 The NE insular shelf

Fig. 4.23 A) Dispersion diagram of the NE sector insular shelf depth (ranging from -15

to -160 m) versus median slope (calculated every 2 m). Gradients extracted from the bathymetry were sorted into 2-m depth intervals and the median value (50%) of the data was found within each depth interval. B) Shaded relief map combined with an aerial photo of the flank in the NE sector of Pantelleria Island. Contour every 100 m.

The NE shelf can be divided in three different portions (Fig. 4.23): 1) from P.ta

Pozzolana to P.ta Spadillo where the shelf includes a 1 km wide area between the

coastline and the shelf break; 2) between P.ta Spadillo to P.ta dell’Arco, where the

shelf is less than 0.4 km wide; and 3) from P.ta dell’Arco to P.ta del Duce, the

shelf widens again. Deeper than -40 m, the shelf is quite steep and dips with an

average seaward gradient of 12°. The shelf break va ries between –78 m and -108

m.

B A


74

Statistical analysis was applied also to this sector with the aim of summarizing

how the average slope changes with depth (Fig. 4.23). The dispersion diagram

highlights the presence of discontinuities (e.g., at ~38 m, and ~78 m) that

represent the average value of the shelf break and secondary breaks in slope.

These results compare well with the changes in slope and scarps observed in the

bathymetry. The discontinuity at ~98 m is well-represented in the northern portion

of the NE sector, while the ~108 m is observed in the southern portion.

A terrace (a in Fig. 4.24) between P.ta Pozzolana to P.ta Spadillo has an

almost rectilinear break in slope at about -50 m. The terrace forms a gentle-

dipping slope, with an average seaward gradient of 1°.

Fig. 4.24 Shaded relief map of bathymetry to -120 m offshore P.ta Pozzolana to P.ta

Spadillo in the NE sector [a) depositional terrace; b) lava flow related structure; c) cone-shaped structure; d) star-shaped structure; e) Khaggiar lava flow]. The bathymetry data represents only the areas surveyed at high resolution; gaps in data are represented in white (coastline is in black, contour every 20 m).

Offshore Spiaggia della Balata, the terrace is overlain by a lava flow (b in Fig.

4.24), which covers an area of 0.3 km2. The flow can be correlated to the

Khartibucale hawaiite flow (~29 ka following the geologic map by Mahood and

Hildreth, 1986), which erupted on land over the Green Tuff unit (~45 ka; Mahood


75

and Hildreth, 1986). The submarine extension of the lava flow front reaches a

maximum depth of 40 m, has vertical relief up to 10 m (Fig. 4.24), and a rugged

small-scale morphology.

A 20 m-high outcrop (c in Fig. 4.24) was identified 1 km from the shore at ~40 m

depth, covering an area of 0.06 km2. It has a cone-shaped morphology, although

bathymetry data at the summit is lacking. The dendritic smooth structure (d in Fig.

4.24) north of this outcrop, is the summit of another volcanic outcrop, with the base

at -510 m.

From P.ta Spadillo to P.ta dell’Arco (Fig. 4.23 B), the shelf varies from narrow (<

0.4 km) to nearly nonexistent, and steeply dips with an average seaward gradient

of >25°. The shelf break is between -60 m to -100 m , and is relatively straight on

the large scale with an orientation NW-SE. The shelf is cut by narrow scars and

furrows, most of which are organized in channels further downslope. The extent of

the volcanic products erupted on land and occupying the coastal zone is also

limited due to the constricted shelf.

The submarine extension of the 2.5 km long Khaggiar lava flow, erupted about

5.5 ka from Cuddia Randazzo (Fig. 4.23 B) (Mahood and Hildreth, 1986), has a 40

m near-vertical cliff on the shelf (e in Fig. 4.24). There is no evidence of a lava flow

on the steep slope (>30°) below. Instead, a smooth, fan-shaped deposit, which

might be volcaniclastic debris, lies at greater depth (Sec. 4.3.2) offshore of the

steepest Khaggiar lava lobes down to -500 m. The shallow portion of this deposit

is characterized by a 150-wide scar, which is probably the main transit path for

sediment (a in Fig. 4.25).

Thick stubby outcrops (~5 m vertical relief) were identified within 120 m from the

shoreline, down to -50 m. These constitute an offshore extension of the Cuddia

Gadir flows (b in Fig. 4.25) that are dated at 14 ± 7 ka (Civetta et al., 1988).

A ~10 m thick, 420 m wide lava lobe was identified offshore P.ta Carace (c in

Fig. 4.25). The flow has a rough surface and its northern lobe was extruded on a

steep slope (20°) down to -90 m. The lava flow is c onnected with a subaerial

outcrop (Fig. 4.25), whose identification is still controversial since it has been

ascribed to both Cuddia Maccotta (dated at 12.0 ± 2.2 ka) (Cornette et al., 1983;

Orsi et al., 1991), and Cuddia Mueggen (dated at 13 ± 6 ka) (Mahood and

Hildreth, 1986).


76

Fig. 4.25 Shaded relief map combined with an aerial photo of the NE sector, between

P.ta Spadillo-P.ta Zinedi. [a) the scar affecting the fan-shaped feature of the offshore Khaggiar lava flows; b, c) lava flow-related structures]. This area was surveyed at high-resolution down to -150 m; gaps in data are represented in white (coastline is in black, contour every 25 m).

Shallow volcanic outcrops were also identified between Cala delle Giache and

Cala di Tramontana and correspond with a widening of the shelf (a and b in Fig.

4.26). One of the outcrops extends nearly 250 m offshore, on a steep slope (10°),

and reaches a depth of 75 m. The outcrop may be related to Cuddia Mueggen

activity (Mahood and Hildreth, 1986).

The overall slope of the shelf between Cala delle Giache to P.ta dell’Arco (Fig.

4.26) is highly incised by narrow channels and gullies that reach the shelf break,

which here lies at depths ranging between 50 m to 80 m.


77

Fig. 4.26 Shaded relief map combined with an aerial photo between P.ta Zinedi-P.ta

del Duce [a) shallow volcanic outcrops from Cuddia Mueggen; b) volcanic outcrop; c) depositional terrace; d) channels; e) entrenched narrow gullies]. Bathymetry data shown only to -150 m (deeper, lower resolution data is presented in Fig. 4.27). Gaps in data are in white (coastline is in black, contour every 25 m).

From P.ta dell’Arco to P.ta del Duce, the shelf widens to 1.3 km, with an

average slope of 3° (Fig. 4.26). The shelf break is highly irregular both in shape

and depth. A depositional terrace is locally identified down to -60 m (c in Fig.

4.26). Features identified offshore P.ta dell’Arco include two shallow channels,

each about 300 m wide, oriented ENE-WSW (d in Fig. 4.26). The heads of the

channels are being carved by entrenched narrow gullies (50 m and 30 m wide,

respectively, and a few meters deep), indicating active erosional processes (e in

Fig. 4.26). Due to the lack of both shallow data and high resolution topography, it

is not possible to establish if these channels are connected to features on land.


78

4.3.2 NE deep flank

Fig. 4.27 Shaded relief map of the NE flank offshore P.ta Pozzolana-P.ta del Duce [a)

channel; b, c, d) volcanic outcrops]. (Coastline is in black, contour every 100 m).

The NE deep flank consists of a narrow, steep slope (16°). Its base increases in

depth from 600 m to 900 m toward the SE and extends offshore up to 2 km (Fig.

4.27).

The flank is highly affected by dismantling processes acting along the overall

slope.

Erosive processes are mainly organized in narrow, incised gullies and chutes,

with only a few of them carving the shelf. The chutes range from 20 m to 250 m in

width, and are often less than 20 m deep. With only a few exceptions, the chutes

seem to have not deeply incised into the substrate. Below -300 m, chutes are

mainly oriented N45°E, along the maximum downhill s lope angle of the flank. At

the toe of the edifice, chutes completely disappear, and no fan deposits were

clearly identified.


79

Fig. 4.28 Left: Gradient map draped over bathymetry of the channel offshore P.ta

dell’Arco; arrows point to the slope break in the thalweg of the channel. Contour every 200 m. (See Fig. 4.27 for location) [a) channel; d) volcanic outcrops]. Right: Cross-section along the thalweg of the channel.

A larger channel has developed offshore P.ta dell’Arco (a in Figs. 4.27 & 4.28),

in correspondence with erosive features affecting the shelf (d & e in Fig. 4.26).

The channel is 2.6 km long and about 250 m wide. It has a “U”-shaped cross-

section and steep flanks (<30°). The left flank is characterized by a rectilinear

steep side wall with about 100 m of vertical relief; while the right flank is affected

by secondary valleys. The longitudinal profile is characterized by a high slope

(22°) that decreases down slope (average slope: 15° ) (Fig. 4.28). The thalweg

shows three secondary breaks in slope at ~225 m, ~450 m, and ~770 m (Fig.

4.28). The channel is oriented N45°E down to -780 m , where it changes direction

abruptly towards N125°E. This change is related to the outcrop (d in Fig. 4.27)

located at the foot of slope.

Many volcanic outcrops are present within the channels and at the foot of the

slope. The main ones include a 150 m-high outcrop offshore Cuddia Gadir (b in

Fig. 4.27), a 200 m-high elongated outcrop identified at ~525 m at about 3 km N of

P.ta Pozzolana (c in Fig. 4.27), and a 200 m-high sub-circular outcrop identified at

the foot of P.ta dell’Arco channel (d in Figs. 4.27 & 4.28). The last two outcrops

have similar sizes (basal area: ~0.8 km2), and steep flanks (up to 40°). Moreover,

they have a radial direction from the island, and appear smoother sloped

compared with the well-preserved cones of the NW sector.


80

4.4 Southeastern sector

The southeastern sector (Fig. 4.1) covers an area of about 110 km2 and

reaches a maximum depth of about 1300 m. The flank extends about 12 km

offshore, predominantly toward the SE, and has an average slope of 12°. It is

characterized by an uneven insular shelf and by erosional remnants of volcanic

cones and dikes that crop out along the slope. Erosional-depositional features

such as channels, scars, and narrow fans are widespread.

4.4.1 The SE insular shelf

From P.ta del Duce to Balata dei Turchi, the SE insular shelf has an irregular

shape and extends up to 1.5 km offshore (Fig. 4.29). The shelf covers an area of

about 7 km2 and dips with an average seaward gradient of 6°. I n this sector, the

shelf break is highly irregular and variable in depth, ranging from -90 m to -125 m.

The dispersion diagram (Fig. 4.29A) shows that the slope tends to increase in

the shallow section (>-52 m), and is highly variable down to -120 m. The slope

then sharply increases from -120 to -128, followed by a more gradually decrease

down to -160 m. The variability of the shelf break apparent in the bathymetry data

is well represented in the statistical analysis, which highlights the presence of

several local minimums in gradient change at various depths (e.g., at ~24 m, ~30

m, ~72 m, ~112 m, and ~122 m). However, no marked correspondence was found

between the values highlighted from the statistic analysis and the depths of the

shelf breaks identified by bathymetry.


81

Fig. 4.29 A) Dispersion diagram of the SE sector insular shelf depth (ranging from -15

to -160 m) versus median slope (calculated every 2 m). Gradients extracted from the bathymetry were binned into 2-m depth intervals and the median value (50%) of the data was found within each depth interval. B) Shaded relief map combined with aerial photo of the shallow SE flank of the island. [a) depositional terraces; b) shelf; c) scars, d) lava lobes; e) scarp; f) erosional processes; g) terraced surface]. This area was surveyed at high-resolution down to -150 m; gaps in data are represented in white (contour every 50 m). The location is outlined on the inset map of the island.

The main features identified on the SE shelf are a submerged terrace, volcanic

outcrops, and scars.

The shallow submerged terrace (a in Fig. 4.29B) lies between -40 m to -50 m

and runs continuously parallel to the coastline. On the dispersion diagram (Fig.

4.29A), the base of the depositional terrace corresponds to the maximum change

in gradient found at -52 m. The terrace forms a gently-dipping slope, with an

average seaward gradient of 4°, and extends offshor e for some 0.6 km.

Between Martingana and P.ta Limarsi (i.e., Contrada Dietro lsola), narrow

outcrops occur down to -35 m (d in Fig. 4.29B). These outcrops are continuous

with subaerial ones. As reported by Mahood and Hildreth (1986), the lava flows of

Cuddia Attalora (69±9 ka) are overlaid along the cliff of Contrada Dietro Isola by a

superficial 1-3 m thick deposit of Green Tuff. Hence, the outcrops found on the

shelf might be related to the Green Tuff or to the higher portion of Cuddia Attalora

lava flows.

Moreover, the shelf (b in Fig. 4.29B) is affected by erosive-instability processes

(e.g., c in Fig. 4.29B) and several narrow scars were identified. About 0.7 km NE

B A


82

of P.ta Limarsi, 100 m from the shoreline, the shelf is cut by a 250 m wide scarp

with 40 m of vertical relief (e in Fig. 4.29B). This is the headwall of a 250 m wide

channel that develops further downslope (see Fig. 4.30). It was carved by

entrenched narrow gullies (20-30 m wide and 10 m deep), which indicates active

erosional downcutting processes (f in Fig. 4.29B).

In the submarine area offshore P.ta Limarsi-Balata dei Turchi, the shelf is

characterized by a sub-circular, gently sloping (< 1°) terraced surface ( g in Fig.

4.29B) which is bounded by a break in slope at -110 m.

4.4.2 SE deep flank

Fig. 4.30 Shaded relief map of the SE flank offshore P.ta del Duce-Balata dei Turchi.

[a) volcanic outcrop with terraced surface at the summit; b) valley; c) erosive channel; d) volcanic cones; e, g, h, i, l) volcanic outcrops; f, m) channels; n) transverse escarpments; arrows= inferred dikes]. (Coastline is in black, contour interval 250 m, data gaps are in white).


83

The SE flank has a slope with an average gradient of 13° that gradually

descends to -1300 m towards the SE. The overall morphology is complex and

appears smoothed by erosion. The flank is characterized by volcanic cones,

volcanic outcrops, dikes, escarpments, and erosive channels (Fig. 4.30). Volcanic

outcrops oriented NW-SE and steep escarpments, up to 200 m in vertical relief,

delimit the SE sector towards the NE, S, and SW. Each of these outcrops consists

of numerous narrow conical structures up to 60 m in vertical relief and resistant

ridges, while the escarpments are mainly transverse features.

The main features of the SE flank are presented here based on location starting

in the SW, then moving down the flank from NE to SW.

The sub-circular, terraced surface described in Sec. 4.4.1 (a in Fig. 4.30)

represents the summit of an outcrop that descends to -1000 m. The outcrop is

characterized by NW-SE aligned ridges (b in Fig. 4.30) and erosive processes.

The ridges are cut by a rectilinear erosive channel (c in Fig. 4.30). The channel is

NE-SW oriented, 200 m long, and about 150 m wide. Moreover, the SW-facing

flank of the outcrop is carved by narrow erosive channels, showing retrogressive

scars and fresh morphologies.

In the area offshore P.ta Limarsi-Balata dei Turchi down to the base of the

edifice, elongated structures are present (arrows in Fig. 4.30). These are aligned

for several kilometers along the NW-SE, NE-SW, and NNW-SSE trends and are

characterized by different vertical reliefs, ranging from 10 m to 60 m. They are

possibly feeder dikes exposed at the surface by erosional processes. Along these

structures, several peaks were identified. Because the peaks are not always

perfectly aligned along the same direction, the lineaments appear often

disarticulated.

A volcanic outcrop was identified offshore P.ta Salina at ~500 m depth (e in Fig.

4.30). It has about 300 m vertical relief and covers an area of 4.5 km2. The summit

portion and the east-facing flank contain crests and radial ridges. One of these

ridges, about ~1.3 km long, is oriented about E-W. This orientation seems to

indicate the presence of a radial structural pattern of the volcano, as it is also

aligned with a shallower channel (f in Figs. 4.30 & 4.33).

Volcanic outcrops were also identified in the submerged area between P.ta del

Duce and P.ta dell’Arco (g, h, i in Fig. 4.30). Some of these outcrops are deeply

scarred by erosional processes that affected their summit and flanks. The northern


84

outcrops (g in Fig. 4.30) cover an area of about 2.9 km2, while h & i cover 1.6 km2

and 0.7 km2, respectively. The bases of these outcrops are aligned NW-SE. The

deepest of these outcrops is found at ~900 m depth, and is characterized by a 1.4

km elongated ridge which has a radial orientation with respect to the shelf (i in Fig.

4.30). At 3.5 km NNE from the previous outcrop, a 200 m-high outcrop was

identified (l in Fig. 4.30). It is isolated from the others and rises from about -1000

m, with a WNW-ESE orientation.

The area offshore these outcrops, between –950 m and 1100– m, shows a

rough and coarse morphology. A 3.5 kHz profile acquired in the area shows small

hyperbolic facies (Fig. 4.31). Along this profile, the facies is about 25 ms thick.

This facies might be related to a landslide deposit.

Fig. 4.31 Sub-bottom 3.5 kHz profiler data showing small hyperbolas. See location on

Fig. 4.30. Overall, most of the SE flank between -740 m and -1000 m is characterized by

irregular morphologies (Fig. 4.32), which on the seismic data corresponds to the

hyperbolic acoustic facies described above.

Several escarpments were also identified from -740 m down to the base of the

edifice (e.g., n1, n2, n3 in Fig. 4.30 & Fig.4.32). These escarpments are between

0.7 km and 2.5 km long, show different orientations, and have vertical relief from

50 to 150 m. These features often isolate small depressions about 100-200 m

wide.


85

Fig. 4.32 3D view of the deeper SE flank characterized by irregular morphologies and

transverse escarpments (n1, n2, n3). See location on Fig. 4.30.

Erosional processes are widespread across the slope and are expressed as

scars, gullies, and erosive channels. The scars affecting the shelf, are often the

headwalls of erosive channel structures, such as one offshore P.ta del Duce (m in

Fig. 4.30) and P.ta Limarsi (m in Figs. 4.30 & 4.33). They range from 100 m to 300

m in width, several km in length, and are up to 30 m deep. Their general

longitudinal profile is characterized by a medium slope (15°) that decreases down

slope (average slope: 5°). Some of the channels (su ch as f in Fig. 4.33) show a

radial pattern with respect to the volcano edifice and might have re-utilized ancient

structural lineaments. Most of them, with the exception of the one offshore P.ta

Limarsi, do not affected the submerged depositional terraces (Fig. 4.33).

As a whole, erosion seems to be both downward, with deepening of the valleys,

and headward, as the shelf break is deeply carved (Fig. 4.33).


86

Fig. 4.33 Shaded relief map offshore P.ta Limarsi-P.ta Salina [f, m) channels; n) transverse escarpment; O1, O2=cone-shaped outcrops] (Coastline is in black, contour interval: 200 m).

Fig. 4.34 Left: Detailed map of the SE flank [a) depression; b1, b2) ridges; n1)

escarpment]. Dashed lines identify trends. The cones in the cluster are marked with numbers. Right: the columns in the plot show the depth of each cone considering its basal depth (Zb) and summit depth (Zs). The value indicated in the column correspond to the maximum difference in elevation (i.e., |Zb-Zs|).


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Four volcanic cones are 5 km offshore P.ta Limarsi (d in Fig. 4.30 & cones 31,

32, 33, and 34 in Fig.4.34). The bases (Zb in Fig.4.34) of the cones occur at

depths ranging from between 720 m to 770 m, while the shallowest summit (Cone

31) reaches ~330 m depth. The maximum difference in elevation between the

deeper basal edges and the summits of individual cones (i.e., Zb-Zs) ranges from

285 m to 390 m (Fig. 4.34). Basal diameters vary from 0.9 km to 1.3 km, basal

areas are between 0.6 km2 and 1.5 km2, and flank slopes range between 32° and

38°.

The peaks of these cones show different trends (Fig. 4.34) and giving a sub-

circular crown feature, with a 50 m deep depression in the middle (a in Fig. 4.34).

Cone 33 shows two peaks inside the rim on the W facing flank and a NNW-SSE

elongated ridge on the southern flank. The peaks are aligned NNW-SSE, as other

ridges in the nearby area (b1 & b2 in Fig. 4.34), for an overall elongation of ~4 km.

Both cones 31 and 32 have steep slopes (up to 32°) along the SE-facing flanks

(Fig. 4.34). The slope is continuous with a sharp, 1.5 km long, ~150 m high, NE-

SW trending escarpment to the NE (n1 in Fig. 4.34; cliff face from -725 m to -875

m). The escarpment has about 75 m vertical relief. Because of their linear

alignment, these features seem to be the expression of a fault-related lineament

(i.e., fault scarps).


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4.5 Sub-surface seismic facies map

Several sub-bottom profiles from the area surrounding Pantelleria (Sec. 3.2.1.)

were examined. The interpretation was often difficult due to low to no penetration

in correspondence of volcanic outcrops and to the irregular navigation during the

acquisition of the data (see map Sec. 3.2.1). However, the interpretation was

tentatively used to create an acoustic map of the sub-surface seismic facies. The

map gives a broad idea of the sedimentary pattern of the area (Fig. 4.35).

Fig. 4.35 Schematic map of the sub-surface seismic facies offshore Pantelleria volcano.

From the SBP, the area surrounding Pantelleria is characterized by four main

sub-surface facies (Fig. 4.35): stratified, low or no acoustic penetration,

transparent, and hyperbolic. Each of these facies might be associated with


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different features and processes. The stratified facies consists of low amplitude

parallel reflections related to sediment covering. The low or no acoustic

penetration facies is likely associated to volcanic outcrops. The transparent and

the hyperbolic facies might indicate the occurrence of landslide deposits and

volcanic outcrops.

Ch. 5 - Seafloor sampling

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Chapter 5

Seafloor sampling

Seafloor samples off Pantelleria were recovered on cruises in 2006 and 2008.

The first cruise (2006) was mainly focused on finding products of the 1891

eruption from the area surrounding where the event was reported (see Sec.

4.1.2.1). A selection of these samples was analyzed to better characterize the

event in terms of dispersal and eruptive style.

In addition to the 1891 eruption products, the samples contained samples useful

in characterizing the flanks of Pantelleria. They included a wide variety of rock

types and lithologies, and biogenic fragments such as corals.

5.1 Samples from the 1891 eruption and surrounding area

Most of the sampling of the 2006 cruise was focused on finding products from

the last eruption (1891), which was excellently described in a historical report by

Riccò (1892). The eruption produced floating black scoriaceous bombs that

showed a basalt affinity, based on independent analyses by Foerstner (1891),

Perry (Judd and Butler, 1891), and Washington (1909) (See Table 1.1). None of

the historic 1891 samples have been preserved in a museum collection.

The investigation concentrated on an area 5 km NW of Pantelleria village

covering approximately 3 km2 (Fig. 5.1). The area was selected based on its close

proximity to the location of the 1891 eruption given by Riccò (1892) (see Sec.

1.2.2.3 & 4.1.2.1).

Sampling (Fig. 5.1 & Table 5.1) encompassed dredging (PD 36, PD 37, PPD 5),

box coring (PX1, PX2, PX3, PX4, PX5, PX6, PX7, PX8), gravity coring (PC1), and

grab sampling (PB3, PB5, PB6, PB7, PB8, PB9, PB10, PB11, PB12, PB13, PB14,

PB15, PB16, PB17, PB18, PB20bis, PB21, PB22, PB23, PB24, PB26, PB29,

PB29bis, PB30, PB31tris, PPB 28). Sampled materials from this area includes

scoriaceous block fragments, volcaniclastic sands, silt, and maerl sediments (Fig.

5.1). The term maerl refers to sediments characterized by accumulations of live

and dead unattached coralline algae (e.g., rhodoliths).


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Sample Id Sample type Northing Easting Depth (m) Recovery(cm)

PD 36 Dredge 4082111 → 4082162 222756 → 222789 270-300 ---

PD 37 “ 4082355 → 4082417 222778 → 222942 225-310 ---

PPD 5 “ 4082321 → 4082565 221040 → 222420 190-240 ---

PX 1 Box corer 4083630 222549 330 8

PX 2 “ 4083043 221275 360 ~23

PX 3 “ 4083320 222264 344 ~12

PX 4 “ 4082533 222480 345 ---

PX 5 “ 4082871 222277 315 ~12

PX 6 “ 4082250 222590 351 1

PX 7 “ 4082550 222196 280 ~14

PX 8 “ 4082074 222664 350 ---

PC 1 Gravity core 4083652 222543 330 183

PB 3 Grab 4082440 221550 180 ---

PB 5 “ 4083385 223320 148 ---

PB 6 “ 4082747 222456 290 ---

PB 7 “ 4083362 221761 398 ---

PB 8 “ 4083254 223907 83 ---

PB 9 “ 4083305 223723 93 ---

PB 10 “ 4083204 223735 86 ---

PB 11 “ 4083265 223634 93 ---

PB 12 “ 4083594 223167 187 ---

PB 13 “ 4083287 222776 298 ---

PB 14 “ 4083669 222525 330 ---

PB 15 “ 4083481 223057 180 ---

PB 16 “ 4083514 222868 258 ---

PB 17 “ 4083507 222657 318 ---

PB 18 “ 4083904 222516 325 ---

PB20bis “ 4083634 222262 270

PB 21 “ 4083492 222461 338 ---

PB 22 “ 4083787 222500 329 ---

PB 23 “ 4083075 223061 198 ---

PB 24 “ 4083037 221280 350 ---

PB 26 “ 4083169 221992 363 ---

PB 29 “ 4082212 222817 270 ---

PB29 bis “ 4082179 222801 255 ---

PB 30 “ 4082238 222572 350 ---

PB31tris “ 4082386 222881 251 ---

PPB 28 “ 4082545 221600 189 --- Table 5.1 Sample description (method, depth, location, and recovery) for the inferred 1891 debris samples and surrounding area.


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Most of the scoriaceous block fragments (i.e., PD 36, PD37, PB29, & PB29bis)

and lapilli (i.e., PB30 & PX6) were collected near a narrow cone that rises ~90 m

from 350 m depth (white arrow in Fig. 5.1). However, some scoriaceous fragments

and lapilli were also collected further to the NW (PPD 5, PX7, and PX4).

These scoriaceous block fragments (Fig. 5.2) range in size from ~0.30 cm to

~20 cm and are very fragile. They are inferred to be debris produced during the

volcanic activity described by Riccò (1892), both in terms of time and eruptive

style. Only a few samples (e.g., PX4) showed a thick brownish ferromanganese

oxide coat upon recovery . The majority of samples had a fresh appearance with a

distinctly black-brownish outermost layer, about 2.5 cm in thickness, with rare

biological deposits on their surfaces.

Fig. 5.1 Map of the samples collected on the NW flank of Pantelleria. A general classification of the sediment is given by the colored circles, showing also the different layers and components identified. The white arrow indicates the narrow cone (ID:16). The tan outline marks the inferred area of dispersion of the thin top layer of volcaniclastic sediment based on collected samples. (Red labels are grab and dredge volcaniclastic samples; blue labels are grab samples rich in sediment; black labels are box-corer samples and the gravity core). Contour interval: 25 m.

The scoriaceous block fragments are highly porous, have a sponge-like texture,

and contain large vesicles. The vesicles vary in shape from rounded to elongated,

and they are sometimes coalesced (Fig. 5.2). Their sizes range from less than one


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millimeter and up to a few centimeters. The vesicles are enlarged particularly

toward the interior of the fragments. A concavity could be detected in some of the

larger block fragments; whereas, it was not possible to detect this in most of the

smaller fragments. This concavity could be related to a large internal cavity that is

a characteristic of floating lava bombs (Siebe et al., 1995; Gaspar et al., 2003).

The density of the block fragments was 1.40 ± 0.05 g/cm3, using a rudimentary

pycnometer.

On the microscopic scale, the fragments have a porphyritic texture in the inner

portion with phenocrysts of plagioclase and clinopyroxene (see C in Fig. 5.15).

Fig. 5.2 Selection of scoriaceous block fragments from PD36 on the left and PPD5 on the right. Arrows indicate large vesicles. Scale bars: 10 cm.

Samples from box coring (i.e., PX) contained small scoria, lapilli, and fine

sediment in the area immediately NW of where the larger scoriaceous blocks were

found (Fig. 5.1). The recovered sediment, which ranges from 8 cm to 22 cm in

thickness, is characterized by two layers of blackish volcaniclastic fine sand, which

are interbedded by a clastic layer of grayish-beige silt (Fig. 5.3). To preserve the

layering of the sediment, the intact box core samples (with the exception of PX7)

was sub-sampled with smaller tube cores, which were split open for analysis.

The sample PX2 reached a deeper layer of beige pelitic sand. The layer

thicknesses for this and the other layered box core samples are shown in Fig. 5.3.

A lateral variability in the layer thickness of few cm or less was generally observed

before the sediment was sub-sampled (e.g., PX5 in Fig. 5.3).


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Fig. 5.3 Box corer samples recovered in the study area. Scoria and lapilli dominated PX4 and PX7; the others were layered and finer grained. Box core samples PX1, PX2, PX3, and PX5 were sub-sampled with small cores; the lengthwise splits are shown. Sampling locations are shown in Fig. 5.1.

The gravity core PC1 (total recovery ~2 m) sampled a flat area (<2° slope) at

330 m depth, between few volcanic edifices (Fig. 5.1). From initial observations, it

is possible to distinguish three different units (Fig. 5.4). Taking into account that

the underlying deposit was not reached and that the uppermost part of the core

was not recovered, the core consisted of two dark volcaniclastic units interbedded

by a pelitic sand unit of ~50 cm thickness (Fig. 5.4). The core sheet (Fig. 5.4)


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reports the macro-scale description of units, based on their aspect morphology

and X-ray results. The upper massive volcaniclastic unit (~17 cm in thickness,

from 15 cm to 32 cm) has a sharp basal contact; while the upper contact of the

lower volcaniclastic unit (~1 m in thickness, from 83 cm to 198 cm) is irregular and

bioturbated down to 10 cm. The lower volcaniclastic unit (~1 m in thickness) shows

parallel laminations and cross bedding, which were identified by color variations of

the beds and from the orientation of coarser fragments shown by the X-ray sheet

(Fig.5.4).

Fig. 5.4 PC1 Core sheet with a macro-scale description of units based on their appearance and X-ray analysis. Selections of the 24 sub-samples (indicated in cm from the core top) that were analyzed for grain size and carbonate content.


95

The core was sub-sampled in 2-cm layers and 24 sub-samples were chosen for

sieving and calcimetry analysis (Fig. 5.4). The selection criteria were based on

their location (i.e., top or bottom layers) and macro-scale appearance (i.e.,

massive or bedded layers). The result of the analysis will be presented in Sec.

5.1.1.

Fig. 5.5 Sample PB8. In a) bioclastic beige sand; in b) nodules of coralline red algae.

The grab samples (i.e., PB & PPB in Fig. 5.1) recovered primarily surface

sediment, which is locally constituted by a thin cover (1-3 cm) of grayish-blackish

fine volcaniclastic sand, beige sand-silt rich deposits, and bioclastic sand (i.e.,

PB8, PB9, PB10, & PB11 in Figs. 5.1 & 5.5). The bioclastic sand, collected on the

lower portion of the insular shelf, was rich in coralline red algae (b in Fig. 5.5),

which probably formed a maerl bed.

Based on the collected samples, the area of dispersion of the thin top cover of

blackish fine volcaniclastic sediment is about 2 km2 (Fig. 5.1). This area is

surrounded by several edifices and acted as a small basin for the deposits.

Scoriaceous block fragments and glassy fragments of different morphologies

constituted the bulk of the volcaniclastic sediments recovered in this area.

Therefore, a representative sub-set of collected samples was analyzed for glass

morphology and composition with the aim of characterizing the 1891 event

products, the activity of the area, and identification of different events. Particular

focus was given to the scoriaceous block fragments collected nearby the narrow

cone (i.e., PD36), distal scoria and lapilli (i.e., PX4, PX7, & PX8), 14 intervals from

the two volcaniclastic units of the core (upper unit: cm 17-19, cm 19-21, cm 25-27,

cm 31-33, reticulite from cm 17-19; lower unit: cm 83-85, cm 109-111, cm 129-


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131, cm 135-137, cm 157-159, cm 163-165, cm 169-171, cm 193-195), and 9

levels from the box-corer samples (PX: 1 (2), PX2 (1), PX3 (2), PX5 (2), PX7 (2)).

Glassy fragment morphologies are illustrated in Sec. 5.1.2.

A detailed description of the chemical and petrographic analyses has been

performed (Conte et al., in preparation). The results from these analyses which

relate to this work will be presented in Sec. 5.1.3 and incorporated into the

discussion (Sec. 6.2).

5.1.1 Grain-size and calcimetry analyses on the PC1 core

Core samples were analyzed for grain size and carbonate content using the

method described in Sec. 3.3.2.2. Table 5.2 summarizes the results of analyzed

samples and shows the sedimentary classes, following the classification method of

Folk (1954) and Tortora (1999) for sand–silt–clay components (Fig. 5.6), and the

total amount of calcium carbonate in weight percent.

Fig. 5.6 Ternary diagrams showing the sediment classification schemes of Folk (1954) and Tortora (1999).

The volcaniclastic samples analyzed ranged in grain-size from sand to silt. The

upper unit is characterized by coarser sediment (i.e., sand and silty sand)

compared to the lower unit, which is characterized by finer sediment (i.e., sandy

silt and sand-rich silt). Tortora’s classification (Table 5.2), which has more classes

(Fig. 5.6) and is specific for marine sediment, shows the difference between the

two volcaniclastic units better than the Folk classification. Small amounts of

carbonate sediment (2.3% to 6.6%) occurred throughout these units. There was

not a strong correlation between different sediment colors and carbonate content.


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The pelitic sand unit (yellow layers in Table 5.2) is characterized by muddy sand

and sand-rich mud and has high carbonate contents (46.6% to 55.6%).

PC1 layer Folk's Tortora's % CaCo3

19-21 Sand Sand 3.87

25-27 Silty sand Silty sand 5.30

29-31 Silty sand Silty sand 6.57

33-35 Muddy sand Muddy sand 51.52


49-51 Muddy sand Sand-rich mud 46.63




83-85 Silty sand Sand-rich silt 4.50



109-111 Sandy silt Sand-rich silt 3.89



135-137 c Silty sand Silty sand 4.30

139-141 d Silty sand Silty sand 5.13



157-159 l Sandy silt Sand-rich silt 2.47

163-165 d Silty sand Sand-rich silt 2.27



189-191 Sandy silt Sand-rich silt 3.07 Table 5.2 Classification according to the methods from Folk (1954) and Tortora (1999) and total amount of calcium carbonate (in weight percent) of analyzed samples for 2 cm-interval sub-samples of core PC1. Layers are indicated in cm from the top of the core (c=crystal-rich layer, d=dark layer, l=light layer).

The results of the complete analyses for the 24 samples are given in Appendix

A, while a summary of the statistical parameters is given in Table 5.3. These

includes the cumulative curves that were used as a graphic device for determining

granulometer statistical parameters (i.e., average particle size, sorting, symmetry,


98

skewness, and kurtosis) using the formulas for graphical determination given by

Folk and Ward (1957) and triangular plot diagrams, according to both the Folk

(1954) and Tortora (1999) classification methods.

PC19-21 is the only well-sorted (sorting value <0.5 in Table 5.3) volcaniclastic

layer, while the others are from medium to poorly sorted (sorting value 0.5-1.6).

The highest value for volcaniclastic sediment, indicating poorly sorted sediment, is

at the top of the lower volcaniclastic unit (PC83-85), which probably contained

both medium sorted volcaniclastic sediment and poorly sorted pelitic sediment

(average sorting value: 2.6) (Table 5.3).

Parameter (Folk e Ward, 1957) Population (Wentworth, 1922)

Sample Mode Sorting Skewness Kurtosis Population >2 (%)

Sand (%) Silt (%) Mud (%)

19-21 2.03 0.41 0.79 9.27 0.00% 97.47 2.39 0.14

25-27 3.48 1.19 0.19 1.47 0.00 77.93 20.77 1.30

29-31 3.42 1.27 0.24 1.90 0.00 81.15 16.18 2.67

33-35 4.06 2.40 0.59 2.01 0.00 75,20 12,88 11,92

39-41 3.6 2.80 0.4 1.35 0.36 75.1 12.73 11.81

49-51 4.64 2.61 0.57 1.27 0.05 69.5 14.68 15.77

59-61 4.89 2.71 0.45 0.79 0.82 57.5 23.45 18.23

69-71 4.64 2.61 0.57 1.27 0.05 69.5 14.68 15.77

79-81 4.02 2.61 0.57 1.27 0.97 73.95 12.45 12.63

83-85 4.38 1.59 0.39 1.48 0.19 56.47 38.81 4.53

88-91 4.25 1.19 0.38 1.30 0.07 59.12 38.45 2.36

99-101 4.42 1.23 0.50 1.36 0.17 51.74 45.09 3.00

109-111 4.49 1.17 0.50 1.16 0.00 47.80 49.36 2.84

119-121 4.48 1.34 0.53 1.25 0.00 51.28 45.14 3.58

129-131 4.40 1.23 0.50 1.29 0.00 53.34 43.75 2.91

135-137 3.85 0.69 0.17 1.87 0.00 77.90 21.13 0.97

139-141 3.83 0.76 0.20 1.97 0.00 78.21 20.62 1.17

141-143 4.34 1.21 0.53 1.18 0.00 53.06 44.01 2.93

149-151 4.44 1.21 0.59 1.25 0.00 51.95 44.90 3.15

157-159 4.68 1.23 0.50 1.21 0.00 42.27 53.90 3.83

163-165 4.31 1.17 0.50 1.26 0.00 57.83 39.56 2.61

169-171 4.42 1.15 0.56 1.22 0.00 50.69 46.52 2.79

179-181 4.33 1.03 0.53 1.29 0.00 53.19 44.52 2.29

189-191 4.51 1.19 0.47 1.25 0.00 45.69 51.10 3.21 Table 5.3 Summary of the statistical parameters of the 24 samples analyzed for grain-

size. Layers are indicated in cm from the top of the core.


99

5.1.2 The morphology of glass fragments

In order to understand fragmentation mechanisms of the explosions, some

glass fractions from the core and box corer samples were sieved and the particles

imaged with a binocular microscope and a scanning electron microscope (SEM).

3D imagery allowed for better characterization of the grain shapes than did 2D

microscopy of the thin sections.

Glassy fragments from the volcaniclastic units of core and box-corers are

translucent, between light and dark brown in color, and appear unaltered. The

fragments vary from medium to highly vesiculated and show a wide range of

morphologies. Those morphologies include fluidal-shaped, rare limu o Pele, Pele’s

hair fragments, highly vesicular to scoriaceous fragments, and reticulite. Moreover,

crystals (i.e., feldspar, clinopyroxene, & Fe-Ti oxides) were often embedded in the

fragments and occasionally formed small lumps. Microscope and SEM images

showing the morphologies of glass fragments are shown in Figs 5.7-5.12.

Fluidal glasses were characterized by a huge variety of shapes (Figs. 5.7 &

5.8). The most common are thick fragments (a in Fig. 5.7) and rods, which are

simple (c in Fig. 5.7), bent (e in Fig.5.7), folded (g in Fig. 5.7), branched (b and d

in Fig.5.7), or stretched (f and h in Fig. 5.7).

The fluidal-shaped particles, which have striated surfaces (Fig. 5.8), generally

have highly stretched, elongate vesicles parallel to their length, with characteristic

cigar-like shapes (e.g., c, f, h in Fig. 5.7 and bottom Fig. 5.7). These elongate

vesicles are between sub-circular to tabular in cross-section. Where they extend

beyond the edges, the vesicles show different sized diameters (Fig. 5.8).


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Fig. 5.7 Top: Photomicrographs of fluidal-shaped glass fragments (a-h) recovered in sediment samples from the NW of Pantelleria. See text for detail. Scale bars are 1 mm. Bottom: thin-section photograph of highly stretched vesicles from PC17-19.

PC 17-19 0.1 mm


101

Fig.5.8 A selection of SEM photomicrographs of glass fragments from NW of

Pantelleria showing a variety of fluidal striated fragments and their cross-sections. The glassy fragment section shows different sized diameters for the elongated sub-circular and tubular vesicles

Pele’s hair fragments were common (Fig. 5.9), but difficult to preserve intact.

The length of observed strands is <2 mm, while the diameter is less than 0.01 mm.

Most of them taper from one end to the other, and often contain a small lump

created by a crystal, which has a greater thickness than the glass (e.g., PX7 in

Fig. 5.9).

The highly vesicular (50–75 vol.% vesicles) to scoriaceous (>75vol.% vesicles)

rounded pyroclasts are composed of brownish translucent sideromelane (i.e., a

vitreous, basaltic volcanic glass of yellow-brown color) with a characteristic

iridescent sheen (e.g., PC1 17-19 in Fig. 5.10). Most of the clasts are equant in

shape, with very irregular outlines and a disordered foamy texture (Fig. 5.10, a-d

in Fig. 5.11). The vesicles are sub-spherical, occasionally coalesced, and

frequently present two different and distinct size domains (a-d in Fig. 5.11). Some

of these pyroclasts enclosed crystals, suggesting continued post-eruptive

degassing (a in Fig. 5.11).


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Fig. 5.9 Photomicrographs of Pele’s hair fragments recovered in volcaniclastic clasts from the NW of Pantelleria. Scale bars are 1 mm

Fig. 5.10 Photomicrographs of highly vesicular to scoriaceous rounded volcaniclastic clasts recovered in sediment samples from the NW of Pantelleria. Scale bars are 1 mm. See text for details.


103

Fig. 5.11 A selection of SEM photomicrographs of highly vesicular to scoriaceous clasts (a-d); reticulite fragments (in a, e-h). Scale bars 100 µm. See text for details.


104

A large fraction of the glassy fragments is represented by reticulite (a, e-h in

Fig. 5.11 & b-f in Fig. 5.12), a “lattice work made up of triangular glass rods”

(Heiken and Wohletz, 1985). Reticulite has vesicularity of 75-98%, and represents

magmatic foam (Mangan and Cashman, 1996). The reticulite is an extremely

delicate 3D network of glass rods, which creates the hexagonally closest-packing

of spheres similar to generic foam (a in Fig. 5.12). The rods connect at plateau

borders, triple junctions of coalescing vesicles (a & e in Fig. 5.11). Plateau borders

represent the intersection of three vesicle walls or films at angles of 120° and they

formed as the melt is drawn away from the inter-cell films by capillary, expulsive

and gravitational forces (Mangan and Cashman, 1996). The area of contact

between vesicles forms a very thin film.

Viewed with binocular microscope and Scanning Electron Microscope (SEM),

reticulite fragments are composed of fragile yellowish to brownish translucent

sideromelane glass rods, devoid of any crystals, with characteristic triangular

cross-sections (e-h in Figs. 5.11 & b-f in Fig. 5.12). The fragments are less that 5

mm in size, while the vesicles are between ~0.1 mm to ~1.2 mm in diameter.

Vesicles show spherical (e.g., b in Fig. 5.12) or polyhedral (e.g., c in Fig. 5.12)

shapes. Polyhedral shapes can be achieved by sphere flattening and deformation

(g in Fig. 5.11) that produce a more densely packed structure. The open network

is made up of glass rods, which formed by an extreme expansion of vesicles

bursting at their thinnest points (c in Fig. 5.12). Therefore, a thin intact glass wall

between the vesicles has been preserved only in few fragments (e.g., d & f in Fig.

5.12), and intact vesicles containing original volatiles were not identified. The films

are less than 0.3 µm in thickness (a in Fig. 5.11).

A progressive order of the structure in a regular network along with the

expansion of bubbles was identified from highly vesicular to reticulite pyroclasts (c

& e in Fig. 5.11), showing that basaltic foams evolve through an initially

disordered, to a well-ordered structure. Moreover, some of the clasts show that the

reticulite might have been formed at the core of less vesiculated scoria clasts (e.g.,

d in Fig. 5.11).


105

Fig. 5.12 a) Sketch of the ideal 2-D foam construction (from Mangan and Cashman, 1996). b-f) Photomicrographs of recovered reticulite fragment samples from the NW of Pantelleria. Scale bars 1 mm.


106

5.1.3 Glass and scoria composition

Electron microprobe (EMPA) and whole rock (ICP-MS) analyses have been

performed for a selection of glass fragments (including reticulite) and scoriaceous

lava fragments. Representative analyses of the glass and scoria fragments are

given in Table 5.4, while the results for total alkali-silica (TAS) analysis are shown

in Fig. 5.13. Moreover, the TAS plot (Fig. 5.13) shows: 1) the whole rock analyses

for a few of the samples [i.e., PX4, PX7, PX8, & PD36 (A. Conte, personal

communication)], 2) the analysis given by Butler and Perry (1892) for the sample

collected at sea surface during the 1891 eruption of Pantelleria, and 3) the

analyses of subaerial lava flows from the NW sector of Pantelleria given by Civetta

et al. (1998), White et al. (2009), and Giocanda and Landi (2010) (See also Table

5.4).

All the glass fragments from the upper and lower core unit, and box-corer layers

straddle the boundary between the hawaiite/basalt fields, and range from 45.9 to

47.2 wt.% SiO2, with 4.5-5.2 wt% MgO (Fig. 5.13). In the TAS diagram (Fig. 5.13),

the layers analyzed from the core were grouped in upper core unit (PC-1 Upper,

which includes PC17-19, PC19-21, PC25-27, & PC31-33) and lower core unit (PC-

1 Lower, which includes PC109-111, PC135-137, PC163-165, & PC167-169). The

reticulite pyroclasts from the top of the core (retic. from PC17-19 in Fig. 5.13) have

the same composition as the other glass fragments from the upper core unit,

indicating that they are submarine erupted material from the same eruption.

The analyses of the layers from the box-corer show significant variability

amongst the analysis. The variability in composition prevented identification of

major differences between the two layers sampled from the box-corers [i.e., PX1

(0-1), PX1 (4.5-9), PX2 (3.5-4.5), PX3 (0-1), PX3 (10-12), PX5 (0.5), PX5 (5.5-

9.5), PX5 (9.5-12), PX 7 top, & PX-7bottom], and the upper layers of the gravity

core (PC-1 Upper); whereas, a variability between the upper and the lower units of

the core is more pronounced. The lower unit of the core, with respect to the upper

one, is characterized by higher SiO2, Al2O3, and MgO contents, and lower K2O,

TiO2, and FeO contents (see Table 5.4). Moreover, trachytic or pantelleritic glass

and crystals were found in this unit during the analysis. Therefore, it seems that

the lower unit had incorporated some pre-existing debris from the slope of the

island, which could be related to the subaerial eruptions of the island.


107

Whole rock analyses of the scoria (i.e., PX-4, PX7, PX-8, & PD36) are

comparable in composition with the 1891 bomb (Butler and Perry, 1892), and with

a subaerial basalt from P.ta Guardia (sample Opl 1001 in Civetta et al., 1998)

(Fig. 5.13).

Fig. 5.13 Top: Total alkali-silica (TAS, Le Bas et al., 1986) plot for sand grains and scoria collected on the NW volcanic field of Pantelleria. WR = whole rock analysis. Analyses of a bomb from the 1891 eruption of Pantelleria (Butler and Perry, 1892) and of subaerial lava flows from the NW sector of Pantelleria (Civetta et al., 1998; White et al., 2009; Gioncada and Landi, 2010) are also reported. Bottom: MgO vs. SiO2 (wt %) contents for the same samples.

0

1

2

3

4

5

6

7

8

40 42 44 46 48 50 52 54

SiO2 wt%

MgO

wt%

0

1

2

3

4

5

6

7

8

9

10

40 42 44 46 48 50 52

SiO2 wt%

Na 2

O+

K2O

wt%

PC 1 upper

PC1 lower

PC 1 ret

PX1 0-1

PX1 4.5-9

PX2 3.5-4.5

PX3 0-1

PX3 10-12

PX5 0.5

PX5 5.5-9.5

PX5 9.5-12

PX7 top

PX7 bottom

PX7

PX4

PX 8

PD 36

PX4

PX7

PX8

PD36

Butler and Perry (1892)

Civetta et al. (1998)

White et al. (2009)

Gioncada and Landi (2010)

Basalt

Hawaiite

Basanite

Glas

s in

shar

dsGl

ass i

n sc

oria

Scor

ia W

hole

Roc

k


108

The compositions found in the NW offshore of Pantelleria are compatible with

other lava erupted in the NW sector of the island (Civetta et al., 1998; White et al.,

2009; Gioncada and Landi, 2010), even if they are shifted towards slightly more

evolved basaltic composition.

As a whole, chemical data indicate that the scoria samples have, as expected, a

similar composition classified as basalt according to the total alkali-silica diagram

(TAS, Le Bas et al., 1986; Fig. 5.13). Glassy groundmass compositions fall within

the field for hawaiite/basalt due to the significantly different phenocrysts and

microlite crystallization.

sample PC-1 PC-1 PC1ret Px 1 PX-1 PX 2 PX3 PX 3 PX5 PX5 PX5 PX7 PX7 layer upper lower 17-19 (0-1) (4.5-9) (3.5-4.5) (0-1) (10-12) (0.5) (5.5-9.5 )(9.5-12) top bottom

SiO2 46.63 47.17 46.56 46.81 46.61 45.94 45.99 46.20 46.12 46.01 46.02 47.20 46.08TiO2 4.42 3.83 4.44 4.26 4.22 4.31 4.35 4.32 4.33 4.30 4.28 4.16 4.24Al2O3 13.21 14.18 13.05 12.37 12.51 12.64 12.81 12.87 13.41 13.65 13.51 12.51 13.05FeO 13.83 12.89 13.93 15.09 14.94 15.32 14.92 14.91 14.98 14.75 14.93 14.95 15.30MnO 0.26 0.23 0.23 0.26 0.29 0.29 0.26 0.28 0.24 0.25 0.29 0.26 0.26MgO 4.74 5.17 4.69 4.58 4.67 4.87 4.86 4.81 4.69 4.59 4.77 4.49 4.68CaO 9.61 9.79 9.74 9.48 9.56 9.62 9.56 9.37 9.41 9.36 9.49 9.33 9.47Na2O 3.77 3.74 3.75 3.61 3.68 3.75 3.76 3.74 3.61 3.83 3.60 3.66 3.66K2O 1.40 1.26 1.42 1.40 1.38 1.40 1.36 1.43 1.41 1.43 1.39 1.36 1.40P2O5 2.14 1.73 2.20 2.14 2.13 1.87 2.13 2.07 1.79 1.83 1.72 2.06 1.86Total 100 100 100 100 100 100 100 100 100 100 100 100 100

Na2O+K2O 5.17 5.00 5.16 5.01 5.06 5.15 5.11 5.18 5.03 5.25 4.99 5.03 5.06CaO/Al2O3 0.73 0.69 0.75 0.77 0.76 0.76 0.75 0.73 0.70 0.69 0.70 0.75 0.73Na2O/K2O 2.69 2.97 2.64 2.58 2.66 2.67 2.76 2.61 2.56 2.69 2.58 2.69 2.61Mg#(mol.FeOt) 40.42 44.31 40.00 37.58 38.23 38.62 39.25 39.01 38.30 38.12 38.74 37.31 37.76

glass in shards

sample PX7 PX4 PX 8 PD 36 PX4 PX7 PX8 PD36

SiO2 45.80 46.03 47.81 47.27 45.36 45.34 45.14 45.32TiO2 4.32 4.05 4.06 4.18 4.33 4.27 4.31 4.16Al2O3 13.08 12.69 12.60 12.52 13.39 13.35 13.41 13.78Fe2O3(T) 15.31 15.61 14.51 15.40 15.31 15.71 15.59 15.19MnO 0.31 0.29 0.28 0.30 0.21 0.22 0.22 0.23MgO 4.71 4.76 4.47 4.19 5.48 5.57 5.46 5.23CaO 9.44 9.55 9.13 9.06 9.52 9.59 9.43 9.74Na2O 3.59 3.48 3.59 3.38 3.37 3.28 3.42 3.51K2O 1.41 1.45 1.53 1.61 1.16 1.10 1.18 1.15P2O5 2.03 2.07 2.02 2.08 1.87 1.58 1.84 1.70Total 100 100 100 100 100 100 100 100FeOt 13.78 14.13 14.02 13.67

Na2O+K2O 5.00 4.93 5.12 4.99 4.52 4.38 4.60 4.66CaO/Al2O3 0.72 0.75 0.72 0.72 0.71 0.72 0.70 0.71Na2O/K2O 2.54 2.40 2.36 2.10 2.91 2.99 2.90 3.05Mg#(mol.FeOt) 37.90 37.66 37.89 35.01 41.50 41.26 40.97 40.53

glass in scoria whole rock


109

Cuddia Cuddia Cuddia Cuddie Punta Contrada Kharti

Bruciata Bruciata Monte Rosse Guardia Karuscia bucale Mursia

SiO2 47.94 47.83 47.63 47.01 46.86 47.75 49.21 46.87TiO2 2.66 2.61 2.63 3.16 3.19 2.93 3.01 3.47Al2O3 15.51 15.69 15.39 14.37 14.89 14.96 16.01 14.14Fe2O3(T) 12.17 11.84 11.70 13.78 13.04 12.61 11.64 14.53MnO 0.16 0.16 0.17 0.18 0.18 0.18 0.19 0.20MgO 6.28 6.18 6.43 6.18 5.86 5.80 4.12 5.73CaO 11.11 11.24 11.15 10.59 10.70 10.52 8.34 10.27Na2O 2.93 3.06 3.20 3.19 3.37 3.51 4.14 3.10K2O 0.76 0.88 1.03 0.91 0.99 1.04 2.02 0.95P2O5 0.48 0.52 0.67 0.62 0.92 0.72 1.31 0.74Total 100 100 100 100 100 100 100 100FeOt 10.95 10.66 10.53 12.40 11.74 11.34 10.48 13.07

Na2O+K2O 3.69 3.95 4.23 4.10 4.36 4.54 6.16 4.05CaO/Al2O3 0.72 0.72 0.72 0.74 0.72 0.70 0.52 0.73Na2O/K2O 3.84 3.47 3.12 3.51 3.40 3.38 2.06 3.27Mg#(mol.FeOt) 50.57 50.82 52.12 47.07 47.09 47.68 41.22 43.88

Giocanda & Landi (2010)Sample

White et al. (2009)

Butler and Perry (1892)Sample 1891

eruptionCuddie Punta Punta Cuddia Cuddia Punta Punta Punta Punta

eruption Rosse S.Leonardo

Sidere del Cat del Cat S.Leonardo

S.Leonardo

Karuscia Guardia

SiO2 46.08 45.58 47.00 50.56 47.02 46.31 47.54 47.31 47.48 45.57TiO2 0.00 3.36 2.59 2.09 3.39 3.41 2.88 2.89 2.91 3.93Al2O3 21.69 14.87 15.78 16.06 14.24 14.09 15.22 15.25 15.19 13.90Fe2O3(T) 11.72 12.40 11.73 10.55 13.28 13.21 12.81 12.78 12.82 15.28MnO 0.00 0.17 0.17 0.15 0.18 0.18 0.18 0.18 0.18 0.20MgO 5.33 6.86 6.33 5.55 6.57 6.43 5.83 5.77 5.77 5.25CaO 10.26 10.82 11.63 9.28 10.52 10.38 10.44 10.70 10.49 10.13Na2O 3.25 3.54 3.17 3.76 2.67 3.54 3.44 3.45 3.48 3.46K2O 1.68 1.45 1.00 1.52 0.88 1.17 0.98 0.99 1.00 1.04P2O5 0.00 0.96 0.60 0.48 1.25 1.28 0.68 0.68 0.68 1.26Total 100 100 100 100 100 100 100 100 100 100FeOt 10.54 11.15 10.56 9.49 11.95 11.89 11.53 11.50 11.54 13.74

Na2O+K2O 4.93 4.99 4.17 5.28 3.55 4.71 4.42 4.44 4.48 4.49CaO/Al2O3 0.47 0.73 0.74 0.58 0.74 0.74 0.69 0.70 0.69 0.73Na2O/K2O 1.93 2.45 3.19 2.47 3.02 3.03 3.52 3.48 3.47 3.34Mg#(mol.FeOt) 47.42 52.31 51.66 51.06 49.48 49.10 47.40 47.20 47.14 40.52

Civetta et al. (1998)

Table 5.4 The table shows the chemical compositions for : 1) glass and scoria fragments sampled offshore Pantelleria, including whole rock analyses for PX4, PX7, PX8, & PD36; 2) a bomb from the 1891 eruption of Pantelleria (Butler and Perry, 1892); subaerial lava flows from the NW sector of Pantelleria (Civetta et al., 1998; White et al., 2009; Gioncada and Landi, 2010).


110

5.2 Other samples collected offshore Pantelleria

Fig.5.14 Location map of volcanic samples. Contour interval: 150 m. For the dredge track coordinates, see Table. 5.4. (Contour interval: 150 m).

More than 50 dredge samples (Table 5.5) were collected offshore

Pantelleria during the two cruises (Figs. 5.14 & 5.17). To date, detailed analysis of

these samples has not been performed. However, a sub-set of these samples,

useful in characterizing the area through their macro-scale characteristics, will be

described. The samples include scoriaceous lava, massive lava, limestone, and

corals (Figs. 5.14-5.22; Table 5.5).

Scoriaceous basaltic lava samples show sub-aphyric to porphyric texture with

plagioclase and clinopyroxene phenocrysts (Fig. 5.15). Basaltic scoria with

vesicles that are totally or partially filled with abundant carbonate cement dominate

(i.e., PD5, PD11, PD20, PD22, PD24, PD25; PPD1, PPD6, PPD14, PPD25 &

PPD27, in Fig. 5.14 and A & B in Fig. 5.15). The white clay-like cement that fills

the vugs was analyzed at IR spectroscopy and found to be motukoreaite, an

aluminum-magnesium hydrosulfate mineral. This mineral is a quite common low-

temperature alteration product of seawater-basaltic glass interaction (Zamarreno


111

et al., 1989). Scoria with empty vugs were locally found (i.e., PD31, PD36, PD37,

& PPD5 in Fig. 5.14 and C in Fig. 5.15).

Table 5.5 Locations of the dredge samples from the 2006 (i.e., PD) and 2008 (i.e., PPD) cruises given by the start and end coordinates of each of the dredge tracks.

PD1 4083332 221205 4083311 221161 X X XPD 4 4081096 220288 4081109 220326 XPD 5 4079304 222607 4079247 222622 X XPD 7 4083462 222701 4083463 222733 XPD 8 4084237 223249 4084209 223262 X X XPD 9 4089195 222177 4088608 222429 X X

PD 10 4088428 225583 4088359 225632 XPD 11 4088350 226539 4088176 226378 X XPD 12 4087352 225020 4087327 224995 XPD 13 4085765 225758 4085765 225758 X XPD 14 4086378 227246 4086414 226983 X XPD 15 4082663 222082 4082956 222208 X X XPD 16 4081887 220694 4081635 220954 X X X XPD 17 4081868 218943 4081876 218989 XPD 19 4084087 222590 4083952 222553 X XPD 20 4083618 222696 4083653 223019 X XPD 21 4083714 223045 4083700 223046 XPD 22 4083668 222738 4083719 222950 X XPD 23 4083897 221998 4083694 222118 XPD 24 4083645 221719 4083687 222043 X X X XPD 25 4083331 220669 4083189 220756 X XPD 27 4073724 226753 4073681 226802 X XPD 28 4067986 228603 4067905 228666 X XPD 29 4077514 240268 4077584 240159 X X XPD 30 4073802 241714 4073957 241569 X XPD 31 4073867 241675 4073946 241647 X X XPD 32 4070968 240939 4070968 240939 X XPD 33 4066832 239443 4067020 239276 X X XPD 35 4063555 238831 4063555 238831 XPD 36 4082106 222804 4082595 222951 XPD 37 4082339 222743 4082620 223367 XPPD 1 4086912 223263 4086201 222803 XPPD 2 4085010 220562 4085551 221122 XPPD 4 4083264 221007 4083012 219873 X X XPPD 5 4082322 221893 4082565 221041 XPPD 6 4083718 222421 4083639 221841 X X XPPD 7 4086145 226333 4085304 225684 X XPPD 9 4085426 223167 4085437 223741 X XPPD13 4088465 223953 4087677 224093 XPPD14 4075788 223618 4075987 223353 X XPPD15 4068026 235637 4068291 235085 X XPPD16 4067688 234349 4067828 234818 X X XPPD17 4065970 236397 4066557 235877 XPPD18 4066225 239088 4066766 238607 X XPPD19 4067296 239999 4067154 238957 XPPD23 4075192 240940 4075318 240321 X XPPD24 4077843 240236 4077827 239773 XPPD25 4082828 231704 4082805 230905 X X XPPD26 4083777 223612 4083510 223870 XPPD27 4081047 222242 4081322 223180 X X XPPD28 4067580 232055 4068264 232261 XPPD29 4068070 228572 4068303 229051 X X XPPD30 4070075 228363 4070052 229056 XPPD31 4071404 227443 4071783 227892 X X

Northing Volcanic LimestoneBioclastic limestoneEastingID Northing Easting

Coral limestone

Convolute limestone Corals

Living corals & bryozoa

Start End Contents of dredged sample


112

Fig. 5.15 Crossed nicols (left) and parallel nicols (right) thin-section photomicrographs of some volcanic samples. A) scoriaceous lava with partially cement-filled vugs from PD25; B) scoriaceous lava with cement-filled vugs from PD20, C) scoriaceous lava with empty vugs from PD36; D) non-vesicular lava from PD7. Ol=olivine; Pl=plagioclase; Cpx= clinopyroxene.


113

The non-vesicular lava includes basalts (i.e., PD7, PD8, PPD4, PPD9, &

PPD26) and felsic rocks (i.e., PD27, PD28, PD33, & PPD29).

Sampled pantellerite lavas had a fresh surface cut from dredging and some of

them had glassy outer rinds (Fig. 5.16). Pantellerite lavas have a porphyritic

texture with alkali feldspar crystals (Fig. 5.16). They were sampled at depths

ranging between 370 m and 830 m, close to the levee of the channel-flow

structure along the SW flank (PD27 in Figs. 5.14 & 5.16), and along the flank of

the deepest cone identified on the SW flank (PD28 & PPD29 in Fig. 5.14). A highly

altered felsic rock was collected along the flank of a cone in the SE sector (PD33

in Fig. 5.14).

Fig. 5.16 Pantellerite samples from the channel-levee lava flow. On the right, the blocks with a glassy outermost layer. Scale bar is 10 cm.

Limestone samples were collected around the whole island (Fig. 5.17) and most

of them showed a black outermost layer. They can be tentatively classified into

four different groups: bioclastic limestone, coral limestone, unclassified limestone,

and convolute micrite limestone (Fig. 5.17).

The bioclastic limestone, collected between -360 m to -415 m (i.e., PD 11,

PD12, PD33; PPD15, PPD16, & PPD27), is made up of abundant pieces of

reticulate bryozoans and bivalve shells, and rare gastropods (Fig. 5.18).


114

Fig. 5.17 Location map of the limestone and coral samples. For the dredge track coordinates, see Table 5.5.

Fig. 5.18 Bioclastic limestone sample showing abundant bryozoa and bivalve shells.


115

The coral limestone (PD1, PD8, PD9, PD13, PD14, PD15, PD16, PD17, PD19,

PD20, PD24, PD25, PD27, PD28, PD29, PD30, PD31, PD32, PD33, PD35; PPD6,

PPD7, PPD13, PPD15, PPD16, PPD18, PPD23, PPD24, PPD25, PPD27, PPD28,

PPD30, & PPD31 in Fig. 5.17) is mainly made up by Lophelia pertusa and

Madrepora oculata, (Fig. 5.19). Lophelia pertusa (Linnaeus, 1758) is a reef-

building cold-water coral, while Madrepora oculata (Linnaeus, 1758), also called

zigzag coral, is a deep stony coral that grows in small (30 - 50 cm high) fan-

shaped colonies. Madreopora oculata is often found among Lophelia colonies

(Schroeder et al., 2005), but unlike Lophelia, the skeleton of Madrepora is very

fragile, limiting its framework building capability. At Pantelleria these samples were

found between ~250 m and ~800 m b.s.l.

Fig. 5.19 Coral limestone collected NW of Pantelleria. Scale bars: 10 cm.

The unclassified type of limestone is mainly made up by light brown micrite (i.e.,

PD4, PD5, PD10, PD21, & PD22; PPD14 & PPD17). These samples were found

at depth ranging between ~190 m and ~610 m (Fig. 5.17).

Fig. 5.20 Convolute micrite limestone. Scale bars: 10 cm.


116

The convolute micrite limestone (i.e., PD1, PD13, PD15, PD16, PD23, & PD24;

PPD4, PPD6, & PPD9) displays uncommon structures created by folded thin

layers (<1 cm), of unknown origin (Fig. 5.20). These samples were found between

~210 m and ~820 m b.s.l.

Abundant coral fragments of different species and size were sampled in the

area. These included large accumulations of deep-sea corals Lophelia pertusa

(i.e., PD1, PD8, PD9, PD14, PD16, PD19, PD32; PPD2, PPD4, PPD18, PPD19, &

PPD31) and Madrepora oculata (i.e., PD15, PD16, PD32; PPD19, PPD25, &

PPD31), and solitary deep-sea corals Desmophyllum dianthus up to few cm (i.e.,

PD1, PD29, PD30, & PD31) (Fig. 5.21). Desmophyllum is a solitary, non-

constructional cup coral used in long-term climate studies (Etnoyer, 2010), which

can be used to help revealing sea’s past conditions.

Fig. 5.21 Coral fragments of Lophelia pertusa and Madrepora oculata from PD15,

PD16, and PPD18; Desmophyllum dianthus from PD 30. Scale bars 10 cm.


117

Living specimens of hexacorallia Gerardia savalia (i.e., PD 16 sampled at ~350

m b.s.l, and PPD7 at ~250 m b.s.l), solitary coral Desmophyllum dianthus (i.e.,

PPD29 at ~650 m b.s.l), and bryozoa (i.e., PD16, PD24, & PD29; PPD16 &

PPD23) were also collected in the area (Fig. 5.22).

Fig. 5.22 Living association of corals and bryozoa: a) Gerardia savalia from PD16; b) Desmophyllum dianthus from PPD29; and c) bryozoa from PD29.

Ch. 6 - Discussion and conclusions

118

Chapter 6

Discussion and conclusions In order to discuss the new data presented in the previous chapters, an

overview of the processes that characterize the flanks of the Pantelleria volcano is

provided here.

In the first section, morphologic comparisons will be made between the

features observed in the various sectors and combined with the other available

data, with the aim to improve the geological interpretation of the history of the

volcano.

In the second section, some of the volcanic features (i.e., lava flows) identified

offshore the island are correlated with onshore vents and compared with the sea

level curve.

In the third section, a quantitative geo-morphometric analysis has been

performed to investigate the bathymetry data set. The volcanic centers identified

offshore the island were analyzed in detail, by comparison with available literature

and by morphometric analyses. Morphometric parameters of the volcanic centers

were statistically treated to identify common trends that might be the result of

analogous constructional and erosional processes.

Finally, a discussion of the volcanic deposits that were sampled in the area NW

of Pantelleria and likely related to the last eruptive activity of the island will be

presented in the fourth section.

As a whole, the morphological mapping and interpretation of the submarine

flanks of Pantelleria and the surrounding area will be a product of this thesis

included in the national project MaGIC (Marine Geohazards along the Italian

Coasts) for the definition of geohazards along the Italian coast (Sec. 6.5).


119

6.1 The submarine flanks of Pantelleria volcano

The area surrounding the Pantelleria volcano comprises different geomorphic

features that were mapped and classified based on the combined interpretation of

the new bathymetry data, high resolution seismic, and seafloor sampling

presented in the previous chapters (Ch. 4 & 5).

A first subdivision of the studied area in morphological domains is between the

northern sector of the Pantelleria Rift and the volcanic terrain of the Pantelleria

volcano (Figs. 4.1 & 6.1). The northern sector of Pantelleria Rift was described in

Ch. 4 and includes: ridges, abyssal sediment/abyssal channels, and minor and

major escarpments (Fig. 4.1). The volcanic terrain that makes up the Pantelleria

complex can be subdivided into the following constructive geomorphic features

(roughly in order of increasing “apparent” age): lava lobes, volcanic centers, dikes,

and volcanic outcrops. Non volcanic geomorphic features partly related to

destructive processes, such as erosive channels, major and minor escarpments,

scars, and landslide deposits are present (Sec. 6.1.3). Constructive-destructive

processes shaped the insular shelf.

In the following, the main geomorphic features present along the flanks of

Pantelleria are briefly described (Fig. 6.1).

The lava lobes identified in shallow-water (i.e., between the shoreline and the

shelf break) (Sec. 6.1.4 and 6.2.1) are characterized by distinctive lobate outlines,

low relief, a rugged surface texture, and a steep front. They are inferred to include

extensions of lava flows erupted on land (i.e., coastal lava lobes), and activity not

directly connected with onshore vents. They are diffuse on the NW and NE insular

shelf, although the ones occurring on the NW shelf present the greatest extension.

The deep lava flows (Sec. 6.1.4) are present only on the SW flank and are often

characterized by channel-levee structures and several lobes with relatively smooth

surfaces.

Volcanic centers (Sec. 6.1.4 and 6.3) have a conical morphology with medium-

high relief. They show different shapes, mainly consisting of sub-circular,

elongated, composite, and occasionally breached. They are often found in clusters

or aligned along an axial direction. A few of them show signs of a summit sector

collapse, with a narrow cone within them. In few cases, deep lava flows extend

downslope around them. Both the lava flows and most of the cones appear to be


120

relatively young because of the well-preserved and undegraded morphology,

despite some cones being affected by erosion. Most of the volcanic centers were

identified in the NW sector, in the same area where the last eruption of the volcano

(1891) occurred.

Fig. 6.1 Geo-morphological map of the submarine flanks of Pantelleria volcano. Legend shows mapped geomorphic features. A description of the geomorphic features is provided in the text.

The volcanic outcrops (Sec. 6.1.4) are coherent, massive ridges that are

generally linear and oriented in a radial direction from the coast. The SBP data

revealed that some of them are mantled by muddy sediment (see map in Sec.

4.5). As they appear eroded and degraded, and often covered by sediment, they

are thought to represent older features than the volcanic centers.

The insular shelf (Sec. 6.1.2 and 6.2) is the area that extends from the shoreline

to the shelf break, which occurs at variable depths (between -80 and -120 m). The

shelf at Pantelleria is mainly characterized by the presence of coastal lava lobes

on the NW and by depositional terraces on the SE.


121

6.1.1 Overall physiography of Pantelleria and its relationship with the

continental rift

Pantelleria Island represents only the emergent tip of a large Quaternary

volcano, while about the 72% of the area of the volcano lies below sea level (Figs.

4.1 & 4.2).

The NW-SE elongation trend of the island is lengthened by the submerged

structures since the insular shelf and the majority of the volcanic outcrops, as well

as a large volcanic field, have a greater extent on the NW and SE flanks (Fig. 6.1

& cross-section A-B Fig. 4.2). On the contrary, the SW and NE flanks have a

higher slope gradient (Fig 4.3), less evidence of volcanic activity, and are affected

by intense erosive processes. Moreover, the base of the volcanic edifice is rather

shallow in the NW sector, while it deepens towards the SE where it reaches a

depth of 1300 m.

The profiles of the flanks (Fig. 4.3) show the highest slope values below the

shelf break, around -200 m to -250 m (median slope gradient 24.7° at -250 m).

Toward the base of the edifice, the slope gradient decreases; local increases in

slope are due to the occurrence of volcanic outcrops along the flanks. Although

the flanks are punctuated by several outcrops, the morphology of the flanks of

Pantelleria is upward concave. Moreover, even though Pantelleria volcano is

located in a relatively shallow setting, the morphology of the flanks is comparable

to those of other volcanic islands with extent at major depth (Mitchell et al., 2002).

As a whole, the profiles (Fig. 4.3) of the Pantelleria flanks seem more equivalent to

constructional flanks than to flanks shaped by large collapses, which generally

have lower gradients (Mitchell et al., 2002).

Therefore, on a regional scale, the elongation of the edifice along a NW-SE

direction, as well as the slope of the NE and SW flanks, seems due to a structural

control. Whereas, on a local scale, different constructive and destructive

processes shaped the flanks of the volcano.

Overall, the peculiarity of Pantelleria is that it occurs and reflects the structural

pattern of the Sicily Channel Rift Zone. In fact, the structural high of Pantelleria,

according to Civile et al. (2008) (See Sec. 1.2.4), consists in a large fault-bounded

magmatic field entirely surrounding the island, and nearly coinciding with the

submerged part of the Pantelleria volcanic edifice. Moreover, as the volcanic


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activity decreased, dismantling processes were likely allowed to advance and

shape the flanks of the edifice.

6.1.2 General morphology of the insular shelf

Fig. 6.2 Slope aspect (0°-40°) draped over a bathym etry map of Pantelleria Island. The

red line is the shelf break. Coastline in black. An insular shelf was identified all around the island (Fig. 6.2), however the shelf

is characterized by an uneven morphology and show relevant differences between

the four sectors (sectors shown in Fig. 4.1) in terms of the areal extent, slope,

depth of the shelf break, and small scale features. The NW sector is the only one

with a significant wide shelf (up to 4 km). In the NE and SW sectors, the insular

shelf is rather absent or very narrow; while it is uneven and shows variable extent

(between 0.5 km and 1.6 km) in the SE sector (Fig. 6.2), as a consequence of the

differential gradual retrogradation of the shelf break.

The dispersion diagram of the median slope gradient (Fig. 6.3) is presented to

compare the shallow portions of the Pantelleria sectors, in terms of slope gradient

and depth of the slope breaks. The diagram verifies similar trends between the

gradients in the SE and NW sectors below -65m, and between the gradients in the

NE and SW sectors, where the highest slope values (>10°) are also found.


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Fig. 6.3 Dispersion diagram of each sector (NW, NE, SW, and SE) showing depth

(ranging from >-14 m to -160 m) versus slope. Gradients extracted from the bathymetry were binned into 2 m depth intervals and the median (50%) of the data were found within each depth interval.

Fig. 6.4 Distribution of submerged depositional terraces around Pantelleria Island

based on bathymetry data.

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Slope median value (°)

Dep

th (

m)

NW

NE

SW

SE


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Moreover, the characteristics of the shelf depend on processes that occurred on

the island. As an example, a variety of remarkably preserved lava lobes extent on

the NW shelf; while marine depositional terraces are mainly distributed in the SE

and SW flanks, between Scauri and P.ta dell’Arco, as a result of the intense

erosion of the sea cliff (Fig. 6.4).

The submerged depositional terraces were identified on the shelf on the basis

of their relatively flat and smooth morphology (Fig. 6.4). They are not continuous

and occur with variable thicknesses at depths between -25 m and -75 m.

The general coastal uplifting of the volcano along the NE flank (De Guidi and

Monaco, 2009) might have produced the conditions for strong erosion and

production of sediments. However, the narrow and steep shelf of this sector might

have made it difficult for these depositional structures to be maintained. On the

southern shelf, most of the depositional terraces occur on relatively flat areas

offshore sea cliffs, which expose silicic rocks older than 50 ka (Fig. 1.5, modified

from Civetta et al., 1988). The coastal cliff erosion (e.g., P.ta Limarsi in Fig. 6.4)

seems to be one of the main processes for the production of sediment that feeds

the marine terraces. Detailed seismostratigraphic analysis and sampling are

required to establish the nature of these deposits.

Overall, volcanic activity and re-sedimentation processes have modified the

shelf at Pantelleria, which is far from being a simple product of erosional truncation

as one may have supposed.

• Shelf break

The depths of the shelf break around the island were analyzed, as they could

give an indication of the vertical movement occurred at Pantelleria. The dispersion

diagram (Fig. 6.3) shows the possible depths for the shelf break at ~120 m in the

NW sector, between ~ 82-112 m in the SW, between ~ 78-108 m in the NE sector,

and between ~90-125 m in the SE sector. At this scale, the comparison of shelf

break depths does not show evidence that differential vertical movements

occurred around the island, rather it points out the complexity of the NE and SW

sectors with respect to the NW and SE, where a clear break in slope, consistent

with the Last Glacial Maximum sea level, occurs.


125

The depth of the shelf break is analyzed in detail by the bathymetric profiles in

Fig. 6.5. The shelf breaks depths were projected on rectilinear transects in order to

allow for comparisons between different sectors. The shelf break is quite irregular

and occurs at various depths that vary between ~80 m and ~120 m b.s.l. The shelf

break in the NW sector is located at around -120 m. This constant value for the

NW sector is peculiar when compared with the behavior in the other sectors,

where it is highly irregular and often shallower (varying between ~80-110 m b.s.l.).

While the origin of the shelf break of continental shelves has been widely

discussed, there are only a few studies describing volcanic island shelves. Menard

(1983; 1986) proposed that they evolved through competition between processes

that enlarge them, through wave erosion during sea-level rise, and processes that

fill in the erosional spaces, including volcanism and other constructional

mechanisms. The effects of tectonic subsidence, uplift, and sea level variations

complicate this model (Quartau et al., 2010).

In the case of Pantelleria, it was not possible to discriminate between primary

structural factors (i.e., structural and isostatic factors), which controlled the shelf

formation, and superimposed effects acting at different time scale (such as the

large-scale eustatic oscillations in sea level and present-day physical shelf

processes), which modified and continue to modify the superficial sedimentary

cover of the shelf by erosion and transport. In fact, the projected profiles (Fig. 6.5)

show that an accurate evaluation of the shelf break trend is difficult to perform

based only on bathymetry data. This is mainly due to the presence of scars and

furrows carving the shelf break. Thus mass wasting processes and intense erosion

of the shelf cause the abrupt changes and irregularities along the shelf break

profiles and prevent one from identifying if the trend of the shelf break is related to

vertical movements and/or sea level oscillations.

Only in the case of the NW sector can the depth of the shelf break (~ 120 m

b.s.l.) be associated with a major low stand of sea-level during the last glacial

period (Würm), which had its maximum (Last Glacial Maximum) about 20,000

years ago.

The others sectors show a different setting. According to the ground

deformation and gravity changes, the southern portion of the island, separated

from the NW portion by the NNE–SSW Zinedi fault, has been and continues to be

affected by an intense vertical tectonic or volcano-tectonic deformation (Behncke


126

et al., 2006). A recent study (De Guidi and Monaco, 2009) suggested that in the

last 900 years the NE coastal sector has undergone strong uplift (~5 mm/yr) and

the island has been tilted towards the SE. The uplift is in agreement with the

historical accounts given by Riccò (1892), who described that the NE cliff was

uplifted almost 1 m in the year preceding the 1891 eruptive event.

NW SE

NE


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Fig. 6.5 Cross-sections showing the projections of the shelf break edge (in red) around

Pantelleria edifice on transects (in black). The shore line is in black. Sectors: NW, SE, NE, and SW.

6.1.3 Mass wasting and erosional processes on the Pantelleria flanks

The uneven, degraded morphology of most of the Pantelleria volcano flanks

indicates that the flanks are affected by mass wasting and erosional dismantling

processes at various scales. In particular, widespread dismantling processes

mainly affect the NE flank, which is the steepest sector. Erosional channels are

diffuse in the southern portion of the island, and a few landslides occurred both in

shallow and deep depths. Any features related to the collapses (e.g., debris

avalanches or large slide scars) have been found from the submarine

morphologies.

Mass wasting and erosional processes mainly shaped the NE flank of

Pantelleria, which is characterized by several narrow erosive chutes. Although the

slope of the SW flank is close to that of the NE, the occurrence of constructive

features is greater than features related to destructive processes. However, the

lack of a wide shelf and the high slope gradient seems to be the factors that led to

S


128

a more efficient downslope sediment transport, with the development of

widespread erosional features on the submerged flanks.

Fig. 6.6 Example of a channel showing both headward erosion and downcutting

processes. The black arrows point to the entrenched gullies, which indicate active erosive processes along the thalweg.

Although all the insular slopes and shelf edges are carved by several scars and

gullies, large erosive channels mainly occur in the southern portion of the island.

The channels are bounded and diverted by volcanic outcrops and ridges, and a

few of them show an evident structural control. In fact, some of them utilized and

carved fissures related to dike intrusions, as it is testified by the presence of

ridges, interpreted as well preserved dikes further downslope. The channels show

headward erosion that may reach the insular shelf (e.g., Fig. 6.6). In a few cases,

the head of the channel is carved by entrenched narrow gullies, indicating active

erosional downcutting processes. Erosive channels seems the main process

acting on high slope gradients, while diffuse erosion occurs where the slope

gradient decreases, and results in a general degraded morphology that is

distinctive of the southern flank (Fig. 4.30).

Minor slides have affected the insular shelf. Specifically, a minor slide scar was

identified about 4 km S of the Scauri village, ~170 m from the coastline (Sec. 4.2.1

& Fig. 4.18). Based on the bathymetry data, the volume of eroded sediment was

estimated to be 9x10-3 km3. The scar affected the depositional terrace and shows


129

a retrogressive mechanism. As the retrogression seems active, this coastal

landslide poses a hazard to nearby infrastructures.

At Pantelleria, three slide deposits, characterized by different superficial facies,

were identified on the NW flank (Fig. 6.1) at the base of some volcanic centers

(Sec. 4.1.2.2). The deposits were investigated by high-resolution seismic surveys

(i.e., SPK and SBP) (Fig. 4.15). This allowed the overall volume to be estimated at

~0.4 km3. These deposits are the largest mass wasting products found at

Pantelleria, however they have very small dimensions if compared with deposits

found in other volcanic islands (e.g., up to 10,000–12,000 km3 for Hilina slump,

Hawaii, Smith et al., 1999). Moreover, their source area of the slides is still not

clear. Two possible alternative hypothesizes can be given to explain these

deposits. The deposits might be related to instability processes along the slope,

previous to the emplacement of volcanic centers, or they might be related to the

growth and collapse of one of the nearby cones as the volume is comparable with

some of the largest volcanic edifices of the NW volcanic field. The lack of any

erosional remnants of volcanic centers and the lack of intermediate-size cone

mass wasting (summit collapse are about one order of magnitude lower) lead us to

prefer the first hypothesis.

On the southern flank, the morphology is characterized by long escarpments

often transverse oriented with respect to the flank of the volcano (Figs. 4.32 & 6.1),

which might indicate the occurrence of instability phenomena. Based on a few

seismic profiles, potential landslide deposits were recognized downslope the SE

flank (between –950 m and -1100 m) (Fig. 6.1). However, due to the paucity of

seismic data in this area, further investigations are required to understand their

nature.

Although Pantelleria Rift is not characterized strong seismicity (Reuther and

Eisbacher, 1985), the occurrence of earthquake swarms, reported before the 1891

eruption (Riccò, 1892), leads one to consider that instability processes could be

trigger by seismic events related to the volcano-tectonic activity and/or to the

volcanic activity by itself, as the occurrence of the summit collapse of the cones

might suggest.

It is worth noting that compared with other Italian volcanic islands [(i.e.,

Stromboli (Romagnoli et al., 1993; Romagnoli et al., 2009) and Ischia (Chiocci and

De Alteriis, 2006)], Pantelleria does not show evidence of major flank collapse


130

and/or debris avalanche deposits. At Ischia Island, the caldera resurgence

triggered the flank collapse (Carlino et al., 2006), which produced a large debris

avalanche along the submerged south flank. At Pantelleria, the caldera

resurgence does not seem to have generated similar event. This is probably due

to a different structural pattern, its relatively shallow setting in the rift, and the lack

of unbutressed submarine flanks.

6.1.4 Submarine volcanic morphologies

The onshore volcanic activity of Pantelleria mainly produced cones (locally

called “cuddie”) often aligned on fissures; however, the major volcano and

volcano-tectonic features exposed are represented also by lava flows, pyroclastic

deposits, caldera rims, and dike swarms (Villari, 1970; Villari, 1974; Mahood and

Hildreth, 1986; Orsi et al., 1991; Catalano et al., 2009).

A wide variety of underwater constructional volcanic structures, mainly

consisting of lava flows and lobes, volcanic centers, erosional remnants of

volcanic outcrops, and dikes were identified offshore Pantelleria (Fig. 6.1). Their

distribution and characteristics were investigated through bathymetry data,

coupled with observations and analyses performed on available samples.

• Shallow lava lobes

Subaerial coastal lava flows can easily be followed underwater along the NW,

NE, and SE portions of the shelf (Fig. 6.1). Therefore, based on morphological

continuity, some submarine lobes on the shelf (Sec. 4.1.1.1, 4.3.1, & 4.4.1) are

inferred to represent the extension at sea of some basaltic and felsic flows.

The shallow lava lobes cover an area of about 6 km2, most of which is related to

the lobes covering the NW shelf. In fact, in this area, the flows have a greater

extent as they flowed for long distances on a gently dipping slope, while they are

restricted elsewhere where the shelf is steeper. Even though no direct sampling is

available for the NW lobes, based on their continuity with on land basalts, the

offshore data broadens up to double the extent of the basaltic lava flows known of

the island than estimated to-date.


131

Detailed observations of these structures and the correlation with onshore flows

will be presented in Sec. 6.2.

• Deep lava lobes

The deep lava flows extending downward of the SW flank of Pantelleria (Sec.

4.2.2 and Fig. 6.1) at ~700 m depth, occupy an overall area of 7.2 km2.

The deep lava flows offshore Scauri are dissimilar from those on the shelf.

The main differences that characterize these flows (Fig. 4.20 & 4.21, Fig. 6.7)

are:

1) the absence of a developed insular shelf along the flank;

2) the occurrence along the flank down to the base of the volcano, here

located at ~700 m b.s.l.;

3) the steep slope (average 16°) of the shallow por tion of the flank where the

lava flowed;

4) the well-developed lateral ridges, interpreted as channel-levee structures,

which characterize two of these flows in their upper portion;

5) the presence of a terraced surface both at intermediate depths and at the

toe;

6) the composition of the lava that has been characterized as pantellerite

(Sec. 5.2);

7) the long distance (~4 km) reached from the shoreline by the flows identified

offshore P.ta Tre Pietre and Scauri (Fig. 6.7), which are also confined in a chute;

8) the occurrence of some of these lobes nearby some cones below -600 m.

Based on these differences observed between the SW flank lavas and those on

the NW and NE flanks, a different emplacement process is likely.

Interestingly, channel-levee structures are observed with these flows. The

channel-levee structure is a typical lava flow morphology that is mainly associated

with basaltic composition at sea (Gregg and Smith, 2003), but on land can also be

created by lava of different compositions (e.g., Harris et al., 2004). Also at

Pantelleria island, the Khaggiar lava flow (pantelleritic trachyte) from Cuddia

Randazzo (Fig. 6.12) represents a long-distance and high volume subaerial

viscous flow partly channelized along the flank. While common on land, the

explosive interactions, which can occur when lava contacts water make the


132

formation of channel-levee structures along the submerged slope rare, and usually

associated with high eruptive rate and high volume (Gregg and Smith, 2003).

Fig. 6.7 Shaded relief map combined with the aerial photo of the SW flank between

P.ta Tre Pietre and P.ta Ferreri, showing deep lava flows (in red) and the location of PD 27. Contour every 100 m.

In the case of these flows, the formation of levees might be the result of lava

accretion on the slower-moving parts of a high emission rate flow, while the

formation of overlapping lobes seems to indicate that the activity along this flank

was intense and flows were diverted, creating different lobes with prominent flow

fronts.

As it was not possible to identify the vents, the main issue was trying to

understand if the two lava flows identified below ~200 m depth north of P.ta di Tre

Pietre and offshore Scauri (Fig. 6.7) were erupted subaerially or underwater. The

hypothesis that these lavas were completely emplaced in a subaerial setting was

rejected, as this would imply a very high rate of subsidence of the entire edifice.

Therefore, for these two flows, the lava was most likely erupted 1) from a

submarine vent, or 2) from an onshore vent and then entered the sea. The

submerged vents for the lava channels might be located in the upper chutes, while


133

for the lava lobes found at greater depth might be one of the nearby cones (Fig.

6.7). Whereas if we considered that those flows were erupted subaerially, the

inferred vents might have been covered by a succeeding volcanic activity.

If the lavas were erupted from a subaerial vent and then entered into water, or

from a shallow vent, no model that encompasses silicic flows has been proposed

to date. In fact, it has been observed (Skilling, 2002), and generally accepted, that

those flows might be easily prone to be destroyed at the shoreline and produce

hyaloclastites feeding a variety of flows (i.e., grain flow, debris flow, and distal

turbidity current).

From a preliminary analysis of the sample (i.e., PD 27 in Sec. 5.2 and Fig. 6.7)

collected from the levee of the northern flow, it was found to be a pantellerite lava

with a glassy outer rind, likely due to the rapid chilling. This indicates this flow to

be part of the silicic activity of Pantelleria volcano.

Furthermore, it has to be taken into account that along the SW cliff the oldest

outcrops of Pantelleria are exposed (Fig. 1.4). These outcrops consist primarily of

pantellerite lavas (Mahood and Hildreth, 1986) and related to the pre-La Vecchia

eruptive interval (325-114 ka). Therefore, even though the age of these submarine

lavas is unknown and no direct relationship with on land outcrops is present, a

comparison between these two should be considered in future.

• Shallow volcanic centers

Three sub-circular shaped outcrops were identified on the shelf of Pantelleria

(see also Sec. 4.1.1, 4.2.1, & 4.3.1), at depths ranging between -40 m and -50 m,

with the shallowest peak at -12 m. The first outcrop is 2 km NNE from Pantelleria

village, the second is on the SW shelf offshore of P.ta Ferreri, and the third is on

the NE shelf offshore P.ta Pozzolana (Figs. 6.8 & 6.12). The outcrops are

characterized by similar size and morphology, with the one offshore Pantelleria

showing some flows (A in Fig. 6.8) and the one offshore P.ta Ferreri showing

radial ridges (C in Fig. 6.8). From their sub-circular shape, the outcrops seem to

be volcanic centers with the radial ridges, likely due to the exposure of dikes.

However, when compared with other volcanic centers offshore Pantelleria, their

aspect ratio (height versus diameter) is lower and ranges from 0.04 to 0.06.

Typically cones have higher aspect ratios (0.22) and the ratio found for these

outcrops seems to be due to a lower height rather than a larger diameter. The


134

relatively smaller height versus diameter for these outcrops may indicate the lava

did not stack up to form a steep cone or that erosion played a key role in their

shaping.

Fig. 6.8 Shaded relief maps combined with aerial photos of the shallow volcanic centers offshore Pantelleria village, P.ta Pozzolana, and P.ta Ferreri (*=cuddie).

Erosive processes of weaker strata might be related to wave erosion as well as

weathering, if the outcrops were emerged or partly emerged during the sea level

lowstand. Moreover, the presence of a depositional terrace, in the case of the

cone offshore P.ta Pozzolana (B in Fig. 6.8), indicates that part of the outcrop is

covered by sediment.

The possibility that these outcrops are littoral cones (i.e., mounds of hyaloclastic

debris constructed by hydroclastic explosions at the point where lava enters the

sea, Moore and Ault, 1965; Fisher, 1968) was also evaluated. However, if we

consider that the outcrop offshore Pantelleria village (A in Fig. 6.8) is related to the


135

San Leonardo basalts outcropping on the facing coast, its present basal depth

(~52 m) is not deep enough to be consistent with littoral hydrovolcanic explosions

caused by the lava entering the sea, as the shoreline was below -80 m

(Waelbroeck et al., 2002) when those lavas were erupted (27-29 ka, Mahood and

Hildreth, 1986; Civetta et al., 1998).

Therefore, the outcrops might be the peak of larger cones partly mantled by

sediment and in part dismantled. However, since the outcrops A shows flows, it is

unlikely that it is only the summit of a buried larger cone, and its morphology is due

to the emplacement of lava.

Based on the evidence presented, the morphologies of these shallow-water

volcanic cones (a in Fig. 6.9) seems due both to constructional and erosional

processes (Fig. 6.9). Two of them seem to be the peaks of larger cones, partly

covered by sediment (b in Fig. 6.9), or the core structures of dismantled cones (c

in Fig.6.9) (or a combination of the two processes).

Fig. 6.9 Inferred evolution of the volcanic cones on the insular shelf. In a) the initial stage; in b) the effect of the depositional process; in c) the effect of the dismantling process.

Moreover, although there are no indications about the activity and age of these

cones, the one offshore P.ta Ferreri can be part of a 2 km-long NE-SW alignment

with the on land Cuddie di Bellizzi (C in Fig. 6.8), which erupted pantellerite lavas

around 18-22 ka ago (Civetta et al., 1988).


136

• Deep volcanic centers

The most surprising finding in the bathymetry is the large number of submarine

volcanic centers (i.e., cones and eruptive fissures) offshore Pantelleria, with the

farthest one located only 8.5 km offshore. The volcanic cones are the most

common volcanic structures at Pantelleria and were mostly identified on the NW

flank (Sec. 4.1.2.1); whereas a few occurred on the SW (Sec. 4.2.2) and SE (Sec.

4.4.2) flanks (Fig. 6.1).

The cones are essentially symmetric with slopes between 21° to 42°, heights up

to 350 m, and basal widths of 0.5-2.3 km. Their volumes vary between 7x10-4–0.37

km3. Their morphology and distribution seem not related to depth since they range

from 330 m to 850 m b.s.l., with the shallowest ones located on the NW flank, and

the deepest ones on the SE flank. The volcanic centers exhibit a well-preserved

morphology; a few centers present small-scale craters and a newly re-constructed

cone within it.

Cones with morphologies similar to the ones at Pantelleria were also identified

in different submarine volcanic settings, such as oceanic hot spots [e.g., Hawaii

(Clague et al., 2000b; Wanless et al., 2006), Galapagos (Glass et al., 2007)], and

the volcanic arc [e.g., Anatahan volcano (Chadwick Jr. et al., 2005)]. Moreover,

the morphology of these cones is similar to that of the emerged scoria cones,

although they do not show a crater at their top. As a comparison, scoria cone

heights vary between 50 and 600 m, basal diameters range from 0.25 to 2.5, and

slope gradients vary narrowly between 32° and 33° ( Schmincke, 2004).

The NW volcanic field has a total volume of about 1.75 km3 and accounts for

about 9% of the estimate volume of the subaerial volcano (~18.8 km3). The

volcanic rocks sampled on the cones are with cement filled vesicles or empty

vesicles (Sec. 5.2). Scoriaceous basalts with vesicles totally or partially filled with

abundant carbonate cement dominate (Figs. 5.14 & 5.15), and constitute the older

products that crop out in this area; however no dating of the samples is available

to date. Scoria with empty vugs were locally found, and some of them were

correlated with the 1891 eruption from their fresh appearance, lack of alteration,

and proximity with the vent location (Sec. 6.4).

The four cones identified offshore Scauri (i.e., SW flank in Fig. 6.1) are

characterized by undegraded conical-shaped structures, which have two or more


137

peaks at the summit. Different from the cones of the NW volcanic field, these

cones have less variability in shape and size (mean diameter: 750 m), a mean

slope gradient of 25°, and comprise a total volume of about 0.1 km3. A few of them

have a lava flow at their toe. Similarly to on land scoria cones, these lava flows

might be related to effusion activity, which usually follows the explosive phase.

However, different from the onshore “cuddie” classified as lava cones or lava

domes by Mahood and Hildreth (1986) (Sec. 1.2.3), they were not breached by the

lava flows. The two samples collected near one of the cones were pantellerite

rocks (Fig. 5.14), indicating a different composition with respect to the NW field,

and similar to the activity that occurred on land. Moreover, these cones are aligned

NE-SW, in the same direction of a shallow outcrop, interpreted as the peak of an

ancient cone partly dismantled by erosion (C in Fig. 6.8), and some onshore cones

(i.e., Cuddie di Bellizzi in Fig. 6.8), which developed along the flank of the volcano.

During their activity (15-20 ka), the Cuddie di Bellizzi erupted pantellerite rocks

(Civetta et al., 1988). Although the eruptive style and age of the submarine cones

are unknown, their occurrence outside the caldera rim, the composition of the

collected samples, and their alignment with the on land “cuddie” suggest that they

could represent eruptions from fractures along the underwater flank of the volcano,

and might be considered satellite cones.

The cones identified offshore P.ta Limarsi (i.e., SE flank in Fig. 6.1) comprise a

total volume of about 0.4 km3. The heights of these cones are up to 500 m and

their basal widths range between 0.8-1.1 km. The cones are characterized by

irregular shapes and their morphology appears degraded. Erosion has led to the

exposure of dikes. The distribution of the volcanic centers and dikes shows an

overall alignment along NNW-SSE and NE-SW directions (Fig. 4.34). The samples

collected nearby these cones (PDD 18 Fig. 5.17) mainly recovered corals, coral

limestone, and a felsic rock (PD33 in Fig. 5.14), covered by muddy sediments, and

indicate that in recent times the volcanic centers were not active.

As a whole, the volcanic centers represent a significant contribution to the

growth of the Pantelleria edifice. It is clear that they developed on an “undisturbed”

seafloor in the NW sector and possibly indicate a migration of the volcanic activity

toward the NW of the Pantelleria Rift. However, a few volcanic centers have been

detected also elsewhere, on older volcanic landforms.


138

From the genetic point of view, according to Clague et al. (2000b), pointed

cones formation at Hawaii likely required high initial contents of magmatic volatiles

to cause a vigorous eruption. Moreover, the Hawaiian cones turn out to be made

of highly vesicular pillows where they have sampled them (D. Clague personal

communication). At Pantelleria, where most of the cones might be considered

pointed or resulting from coalescence together, high volatile content might be

inferred from the presence of the highly vesicular basalts, the occurrence of

floating lava during the 1891 eruption, and the glassy fragments found in the NW

volcanic field. However, the quantification of magmatic volatiles and degassing

processes is fundamental to a better characterization of these eruptions, in term of

eruptive style and hence morphology.

Furthermore, along with the volcanic samples, several limestone rocks of

different types, as well as corals and bryozoa (Sec. 5.2) were collected on these

volcanic centers. These might indicate that during periods without activity the

environmental conditions are favorable to the growth of fauna on a hard substrate

and formation of limestone. Moreover, it was noted that the convolute limestone

(Sec. 5.2) is typical of the NW sector and might indicate the occurrence of

hydrothermal activity or other unknown processes.

• Volcanic outcrops and dikes

Fig. 6.10 3D-model close-ups of the two types of volcanic outcrops identified on the submerged flank of Pantelleria. A) Massive volcanic outcrop from the NW flank. B): volcanic outcrop showing possible radial dikes from the SE flank.


139

Erosional remnants of volcanic outcrops are located in the deeper portion of the

NE, SW, and SE flanks where they form radial ridges which develop outward

(downslope) from the volcano.

Two different types of volcanic outcrops morphologies were distinguished (Fig.

6.10). The outcrops from the NW and SE flanks usually present a massive shape

(A in Fig. 6.10), while the outcrops from the SE and SW flanks appear to be

dismantled by erosive processes, which often leads to the exposure of coherent

ridges, (B in Fig. 6.10). The different shapes that characterize these outcrops

might indicate a different origin. The first type are similar to those described by

Chadwick et al. (2005), and interpreted as thick pasty lava flows, which fragment

along steep slopes, while the second seems the result of dismantling processes of

older volcanic centers, which might be related to the exposure of their inner

structure, exposing radial dikes.

The dikes exposed by erosion (e.g., f in Fig. 4.21) are coherent linear features

identified at the summit of most of the SW and SE outcrops. Furthermore, a large

number of isolated ridges, which may be as well interpreted as dikes, protrude up

to 40 m above the SE flank. The dikes are mainly oriented in a radial direction

from the volcano and can be several km long (Fig. 6.10).

Volcanic outcrops are, from one side, degraded by erosion and, on the other,

seem partially buried by sediment, which mantles their morphologies (see map in

Sec. 4.5). The samples recovered on the outcrops mainly consisted of coral

limestone, bioclastic limestone, live corals and bryozoa (Fig. 5.17; Sec. 5.2),

and/or a thick cover of sediment. Only two volcanic rocks, similar to the ones

sampled on the NW field, were recovered on the volcanic outcrops (i.e., PPD25 on

the NE flank, and PD31 on the SW flank; Fig. 5.14). The volcanic sample collected

from the SW flank (i.e., PD31) is of interest, as it seems the only fresh one

collected outside of the NW field. The southern offshore sector shows a very

complex and uneven morphology primary volcanic morphologies Primary volcanic

morphologies, which often present a structural alignment, seem totally altered by

dismantling and gravity driven processes.

Coupling all available information, it can be inferred that most of the volcanic

outcrops are likely older features than some of the volcanic cones identified in the

NW field.


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6.2 Consideration of the shelf geology and environment

6.2.1 Coastal lava lobes and correlation with on land activity

The shallow lava lobes on the insular shelf formed either as lobes that crossed

the shoreline and were emplaced under water, or they were emplaced during

periods of lower sea level as subaerial flows. To differentiate their mode of

emplacement requires knowledge of their ages and the changes in sea level

through time. The reconstruction of the sea-level curve suggested that in the last

~300 ka ago (corresponding to the time span of the volcanic activity known on

land), sea level ranged on the average from 0, during interglacial intervals, to ~120

m, during glacial periods (Fig. 6.11) (Waelbroeck et al., 2002).

Fig. 6.11 Relative sea-level variation curve for the last 300 ka (after Waelbroeck et al., 2002).

A detailed analysis of the vertical movements of the area offshore Pantelleria is

lacking, local vertical movements are considered negligible in the analysis

reported here because the shelf break does not show great vertical variability in

the NW sector, where most of the lava flows are located. The shelf break,

however, is less-marked, locally shallower (between -80 m and -110 m), and

highly irregular in the other sectors.

Many underwater lobes (Table 6.1), most likely made up of massive lava with

minor volcaniclastic debris, have been identified on the shelf (Fig. 6.12). These

underwater lobes, on the basis of their morphological continuity, probably are

0

100 150 200 250 3000ka

-140

-120

-100

-80

-60

-40

-20

50

Relat

ive se

a lev

el (m

eters)


141

continuations of the subaerial basaltic flows from Mursia and Punta San Leonardo,

Khartibucale hawaiite lava flow, Khaggiar felsic flows from Cuddia Randazzo, and

felsic flows from Cuddia Gadir, Cuddia Maccotta, Cuddia Mueggen, and Cuddia

Attalora (Table 6.1) (Civetta et al., 1984; Mahood and Hildreth, 1986; Civetta et al.,

1988).

Lava type Lava flow / field Approximately Age (ka) See Also

basaltic Murisa <10 Sec. 4.1.1.1

basaltic Punta San Leonardo 27-29 Sec. 4.1.1.1

hawaiite Khartibucale ~29 Sec. 4.3.1

felsic Khaggiar, Cuddia Randazzo 8.2 ± 1.7 Sec. 4.4.1

felsic Cuddia Gadir 28 ± 3.5 Sec. 4.4.1

felsic Cuddia Maccotta 12 ± 2.2 Sec. 4.4.1

felsic Cuddia Mueggen 13 ± 6 Sec. 4.4.1

felsic Cuddia Attalora 69 ± 9 Sec. 4.4.1

Table 6.1 List of underwater lobes identified on the shelf.

The ages of the underwater lava lobes identified on the shallow shelf of the NW,

NE, and SE sectors were inferred from previous dating performed for the on land

portion of the lava flows. By combining the age of a given lava flow with sea level

at the time of its emplacement, it is possible to estimate if the mapped flows were

subaerially or submarine emplaced and submerged afterwards. This allows

morphological characteristics of these lobes to be attributed to the sea level

condition at the time of their emplacement.

Mursia basalt flows were erupted from a vent located close to the shoreline (Fig.

6.12). The distal edge of the flow correlated with this vent reaches a maximum

depth of ~52 m, at about 1.6 km from the shoreline (See 4.1.1.1). At the time that

Mursia basalts were erupted (<10 ka, Civetta et al., 1998) , the eustatic curve

suggests that sea level was no deeper than about -40 m (Waelbroeck et al., 2002;

Lambeck et al., 2004). If we consider the sea-level at -40 m, taking into account

that it was also rising, this implies that the lava flow reached the shoreline and

partly entered the sea, although no bathymetric evidence of the previous shoreline

is apparent. Thus, although the majority of the lava flow was likely emplaced on

land, it is possible that the flow front crossed the shoreline. Lava flows usually cool

rapidly when they encounter water. This could causes the lava to shatter into

fragments creating volcaniclastic debris (Umino et al., 2006). The gentle slope of

the NW shelf might have favored a more passive lava entry or the growth of the

volcaniclastic deposits due to the interaction with water. In this case, this deposit


142

might have stagnated underwater and eventually supported overlying lava flows

that built up above sea level. The underwater lobes of Mursia are characterized by

a quite rough surface, but small-scale features are less evident when compared

with the lobes offshore P.ta San Leonardo and Khartibucale.

Fig. 6.12 Shaded relief map combined with the aerial photo of Pantelleria Island

showing the correlation between the subaerial eruptive centres (cuddie) and the underwater lava lobes, and few volcanic outcrops identified on Pantelleria shelf.

Mursia basalts partly overlays another, more subdued outcrop offshore Cala

dell’ Alca (Fig. 4.7). This might be associated with some of the lava flows that

erupted from the Mt. Gelkhamar (23 ± 3 ka, Mahood and Hildreth, 1986), from

which relatively fluid flows spread westward and reached the sea (Villari, 1968).

The possibility that the flows came from the Cala dell’Alca cinder cone seems less


143

likely due to the older age of those flows, which on land were covered by the

Green Tuff eruption (Mahood and Hildreth, 1986).

Punta San Leonardo basalts (which include the lava flow fields of Cuddia del

Monte, Cuddia Bruciata, and Cuddia Ferle, see Sec. 4.1.1.1 and Fig. 6.12) and the

Khartibucale hawaiite flow were erupted around 27-29 ka (Mahood and Hildreth,

1986; Civetta et al., 1998). At that time, the shelf was partly emerged and the

shoreline was at ~80 m. Submarine lava lobes that can be correlated with these

basaltic and hawaiite lava flows, at present-day, are found at a maximum depth of

54 m and 40 m, respectively. Therefore, they were emplaced completely on land

and did not reach the sea. The evidence that these flows were erupted in a

subaerial environment and covered by the sea only afterwards indicates that the

present-day depths of the lava fronts were not controlled by sea level and the

extensions of the P.ta San Leonardo basalts and the Khartibucale hawaiites on the

shelf are mainly related to the effusion rate of the vents. Moreover, these lobes,

when compared to the submerged lobes of Mursia basalts, are characterized by

more-rugged small-scale features, probably due to their subaerial emplacement.

As these features at present-day are still preserved with a rather fresh

morphology, it can be inferred that surf erosion was not intense during the rise of

sea-level.

In summary, it can be determined that the steps that the P.ta San Leonardo and

Khartibucale lava flows went through during their emplacement created the lava

fields. Afterwards, due to sea level rise, the lava fields were progressively

submerged and likely underwent weak surf erosion.

The underwater lava lobes on Cuddia del Monte, Cuddia Bruciata, and Cuddia

Ferle overlay a wide portion of the NW shelf and are the largest ones identified

offshore Pantelleria.

On the NW shelf, the basaltic flows are characterized by two different

morphologies. The first is represented by the elongate flow of Mursia, and the

second by the fan-shaped lava deltas of P.ta San Leonardo. Comparing these

flows with subaerial ones, the Mursia morphologic type seems similar to fluidal

pahoehoe flows, which commonly produces a large amount of volcaniclastic debris

upon entering into water (Umino et al., 2006). The P.ta San Leonardo lava delta

seems comparable to a’a flows, which commonly form a fan-shaped lava delta on


144

the shore which might also extruded fingers of lava into the water (Umino et al.,

2006). A comparison with Pantelleria basaltic lavas would be of interest.

Lava lobes on the NE and SE shelf (see Sec. 4.3.1, Sec. 4.4.1, and Fig. 6.12)

were correlated with on land pantelleritic trachytic flows. Most of these submerged

lobes extend short distances from the shoreline, also in response of their different

physical characteristics, as they should be more viscous lavas with respect to the

basalts.

The young Khaggiar pantelleritic trachite lava flows are associated with the

activity of Cuddia Randazzo (dated at 5.42 ± 0.22 ka, Civetta et al., 1998). At

present-day, the flow front is found at sea and it forms a 40-m-high cliff on the low

gradient surface on the W side (Fig. 4.24), while it is absent on the steep slope

facing NE. However, a fan-shaped deposit is present downslope of the NE flank.

At the time of the emplacement of these flows, the sea level was about at its

present-day level; hence the lava entered the sea. The absence of a front on the

NE flank might be due to a more efficient cooling and breaking of the lava entering

the sea on a steep slope, with respect to the W side. Moreover, this interaction

with water might have created the volcaniclastic debris which formed the fan-

shaped deposit downslope. As a whole, the underwater features associated with

the Khaggiar flow were likely created by interactions between water and magma

from a flow that was erupted from a center located further inland and then reached

the shoreline (Fig. 6.12).

Cuddia Gadir (dated at 14 ± 7 ka, Civetta et al., 1988) and Cuddia Mueggen

(dated at 13 ± 6 ka, Mahood and Hildreth, 1986) lavas (Fig. 4.25, 4.26 and Fig.

6.12) were erupted when the sea level was lower than -40 m. Therefore, the lava

lobes were submerged after their emplacement and had no interaction with water.

The narrow lobe identified offshore P.ta Carace down to -90 m (c in Fig. 4.25) was

ascribed to either Cuddia Maccotta (dated at 12.0 ± 2.2 ka, Cornette et al., 1983;

Orsi et al., 1991) or Cuddia Mueggen and might have been entered the sea

passively and prevented from fully breaking and collapsing at the contact with

water since it flowed over a deeper outcrop (c in Fig. 4.25). Similar process was

observed during in the 2002 for the lava delta at Stromboli (Chiocci et al., 2007).

The narrow lobes offshore Contrada dietro Isola might be ascribed to Cuddia

Attalora activity (dated at 69± 9 ka, Mahood and Hildreth, 1986) (Figs. 4.29 &


145

6.12), and are the oldest lobes identified on the shallow shelf and they were likely

emplaced during a period of low sea level (Waelbroeck et al., 2002).

Based on the eustatic sea level fluctuations experienced by the shelf over time,

it can be inferred that not all the existing lava lobes identified on the shelf were

originally emplaced into water. The overall picture gained from these data

indicates that most of the coastal lavas represent subaerial lava flows erupted

when sea-level was lower, which afterwards were partly covered by the sea.

Whereas, the lava lobes from Mursia and some narrow lobes of Mueggen lavas

represent subaerial lava flows whose distal ends entered the sea during the

Holocene period.

These submarine outcrops are minimally affected by surf erosion. The role of

differing resistance of volcanic products to erosion by surf was considered by

Carracedo et al. (1999) who suggested that the volcaniclastic deposits produced

by magma-water interaction should be more easily prone to erosion than

subaerially-erupted lava. In contrast, the well-preserved morphology of some of

the observed lava flows, including the ancient lavas such as those from P.ta San

Leonardo, is consistent with dense subaerial lava that strongly resists marine

erosion.

• Deep portion of the NW shelf

During the history of the Pantelleria volcano, the insular shelf underwent sea-

level fluctuations, which partly or completely exposed it to subaerial processes.

The NW shelf might represent a record of these fluctuations as it is

characterized by two main breaks in slope, which are at ~80 m and ~120 m b.s.l.

(see Sec. 4.1.1). By comparing the depths of the breaks in slope with the eustatic

sea level curve (Fig. 6.11) (Waelbroeck et al., 2002) the break in slope at ~80 m

b.s.l. is consistent with a low stand of sea-level at ~67 ka, while the other (at ~120

m b.s.l) corresponds to sea level during the Last Glacial Maximum (~20 ka), or

during the previous one (~137 ka). Therefore, these breaks in slope (Fig. 6.13)

seem related to the sea-level fluctuations and might represent two paleo-

shorelines resulting from erosion during periods of lower sea level.


146

Moreover, the NW shelf also recorded part of the activity of the volcano, since it

is covered by lava lobes and likely other volcaniclastic deposits of diverse ages,

with the sources seemingly localized on land or at present-day submerged vents.

The products of the Green Tuff eruption at ~50 ka (Cornette et al., 1983) are

widespread and mantled the northwest lobe of the island (Mahood and Hildreth,

1986), successively affected by younger basaltic activity. When the Green Tuff

erupted, part of the shelf (down to the break in slope at some ~80 m b.s.l) was

emerged. From the bathymetry data, the break in slope appears partly mantled by

lobes, which reached its edge. These lobes might be related to the Green Tuff

eruption or to successive activity. However, these lobes can be relatively dated as

older than 29 ka, since they are covered by the P.ta San Leonardo basalts in the

shallower portion of the shelf. Furthermore, if the few narrow lobes (b in Fig. 4.11),

which crossed the break in slope on the eastern portion of the shelf, were also

erupted subaerially, their emplacement is consistent with a sea level at about -110

m, and can be relatively dated as successive to ~67 ka, probably during a

subsequent eustatic fall (~20 ka).

Fig.6.13 Left: 3D view of the NW shelf from the north. The near-shore lava flows and the island are in yellow in the distance. Right: Sketch of the evolution for lava flows on the NW insular shelf and hypothesized paleo-shorelines.

Moreover, the western side of the abovementioned break in slope at -80 m is

covered by a fan-shaped flow that reached the terrace that lies at -120 m (d in Fig.

4.11). The flow has a highly crenulated and jagged front. Taking into account the

observations made for the shallower P.ta San Leonardo basalts, the irregular edge

might be considered as an indication that the flow was erupted subaerially, even

though the vent was not identified. The flow crossed the scarp at -80 m and


147

reached the edge of the shelf; there, it deflected along the shore, as suggested by

its morphology (Fig. 4.11 & Fig. 6.13). In the literature, there are examples of

subaerial lava flows encountering the sea and not entering the water but flowing

sub-parallel to the original coastline, hence forming elongated flow lobes curved

along the coast (Umino et al., 2006). Such a mechanism of emplacement of flow

lobes at the coastline is not uncommon, but not readily explained. Mitchell et al.

(2008) proposed that sub-parallel emplacement of the primary lobes could be

caused by topographic control of the previous morphology and damming up by

lavas fed at low extrusion rates. Based on the morphology, this mode is inferred

for this lava flow. Moreover it gives a strong indication of the sea level depth at the

time of its emission (i.e., ~115 m) and consequently that its emplacement occurred

during the last fall of sea level (~20 ka).

An evolution model for the NW insular shelf (Fig. 6.13) can be inferred by

combining all observations and evidence, taking into account uncertainties in the

volcanic activity ages reported. This sketch model (Fig.6.13) summarizes the

possible relationship between the various structures identified on the NW shelf.

During the last glacial maximum (LGM) about 20,000 years ago, sea level was

at about -120 m (Waelbroeck et al., 2002) and the shelf was exposed to subaerial

erosive processes, which likely carved the narrow chutes observed between -60 m

and -100 m (f in Fig. 4.11).

Although Pantelleria volcano erupted compositionally diverse lava types (mainly

basalts and pantellerites), the correlation between on land products and the

shallowest lavas of the NW shelf allows for most of them to be characterized as

basalts. This evidence not only is in agreement with the observation that basaltic

lava-fed deltas are significant coastal constructors for many oceanic islands and

continental flood basalt provinces (Skilling, 2002), but it also increases the area

and volume of basaltic products known for Pantelleria.

As a whole, the lava flows that were identified on NW shelf of Pantelleria are

important both for the coastal evolution and palaeo-environmental studies.

Subaqueous structures and mechanisms at Pantelleria, with the exception of the

submarine extensions of coastal flows, remain poorly understood and several

questions arise. The main issue is whether these ancient lavas flowed subaerially

or underwater. In fact, whilst coastal processes, formation, and evolution of lava

deltas and shallow associated products have been previously described in


148

literature (e.g., Moore et al., 1973; Moore and Clague, 1987; Tribble, 1991; Mattox

and Mangan, 1997; Skilling, 2002; Kauahikaua et al., 2003; Umino et al., 2006;

Chiocci et al., 2008; Mitchell et al., 2008), little is known about deeper subaqueous

processes and products on the shelf.

6.2.3 Concluding remarks of the shelf geology

The main volcanic features identified on the shelf around Pantelleria include

coastal lava lobes and lava lobes affecting the lower shelf. Some possible

explanations for the observed features have been proposed. It appears clear that

the current shelf morphology is due to different lava emplacements and erosional

processes, which are also related to the last glacial period. At the same time,

when the shelf was exposed subaerially, lava advanced to the shore, with the

amount depending on volcanic centers distribution, eruptive rate, and shelf

geometry.

Furthermore, this research shows that a proportion of the submarine flanks of

volcanic islands comprises both lavas originated from land, that afterwards were

submerged, and lavas that partly passed through the surf zone. The current shape

of the northern insular shelf, resulting from both lava emplacement and erosional

processes, leads to the consideration that the shelves of volcanic islands are

active geologically. This has also been suggested for Pico Island (Mitchell et al.,

2008). In addition, although surf erosion has played a major role during time, the

case of the northern shelf of Pantelleria is different from other volcanic islands that

were more deeply affected by marine erosion (e.g., Canary Island, Teide Group

1997; Acosta et al., 2003; Mitchell et al., 2003)

At present-day, the sampling of the NW shelf has revealed the presence of

maerl facies at ~100 m b.s.l (Sec. 5.1). Similar facies characterized by coralgal

nodules were also observed for the deep drowned terraces at Hawaii (Clague and

Moore, 1991; Webster et al., 2006). In future research, it should also be taken into

account that this shelf constitutes a perfect setting for this facies. These coralgal

nodules and related biogenic sedimentation form a unique ecosystem with a high

benthic biodiversity that is fragile and can be easily disrupted. By understanding

the nature of the shelf as well as the distribution of this facies in the area,

recommendations for the preservation of the ecosystem can be also made.


149

6.3 Quantitative geo-morphometric analysis of the

volcanic centers

High-resolution digital terrain models allow for morphological and morphometric

approaches, including statistical analysis to understand the formation of submarine

volcanic edifices. The geo-morphometric analysis presented here for the volcanic

field identified in the NW sector (Sec. 4.1.2.1) followed the general procedures

used for oceanic seamounts (Smith, 1988; Scheirer and Macdonald, 1995; Smith,

1996; Ruiz et al., 2000), and for scoria cones (Tibaldi, 1995; Dóniz et al., 2008;

Favalli et al., 2009). As a comparison, the analysis will also include other cones

identified offshore Pantelleria and a few on land cuddie of the NW sector of the

island.

The main dimensions (e.g., diameter, height) and associated parameters (e.g.,

area, slope angle, height/diameter) of the cones were measured. Volume

computation was performed by a semi-automatic procedure. All information was

combined to create a database, which allowed for a morphometric and

morphological classification on the basis of shape. Moreover, azimuths between

the peaks of aligned cones have been measured to contribute to the

understanding of the structural setting of the area.

6.3.1 Method

The approach of this study was to conduct a statistical analysis of the

morphology and distribution of the volcanic structures, which were identified as

discrete features. Identification of each edifice, as well as defining its base through

the break in slope at its foot, is not trivial.

A qualitative classification was applied to subdivide the volcanic structures.

Constructs with a conical shape were classified as “cones” and a database was

created mainly following the criteria given by Smith (1996), Ruiz et al. (2000), and

Favalli et al. (2009). The position (in UTM coordinates) of cone centers and

morphological and morphometric parameters were tabulated (Table 6.2 & 6.3).

Morphological characteristics include height, basal diameter, slope, basal area,

and volume. Identification and measurement of the sizes, in particular the heights

and volumes, of many of these volcanic cones were somewhat arbitrary because


150

they are overlapping and their base lie on a dipping plane. Moreover, restrictive

criteria were applied to the investigated area in order to discern the cones.

Volcanic structures that rise less than 40 m from the seafloor have not been

included in the database as they might be outcrops. At the same time, some of the

overlapping cones constituting composite structures have been studied both in

their individual and overall morphologies to better understand their growth.

Therefore, this method produced a rigorous although not univocal classification,

because of inherent subjectivity in defining the cones.

Previous studies of volcanic cones approximated individual edifices as regular

truncated cones (e.g., Smith, 1988; Scheirer and Macdonald, 1995) or regular

elliptical cones (e.g., Rappaport et al., 1997), in order to approximate their shape

parameters and measure their volumes. In this study, a semi-automatic procedure

to measure the real shape of the edifices has been applied in order to provide the

closest estimation to their basal areas and volumes.

Moreover, the resolution of the data gave a unique opportunity to study in detail

the geomorphologic features associated with some edifices, such as the

occurrence of summit craters in some of them (Sec. 6.1.3.5).

The morphometric parameters used to describe shape are the depth of the

cone summit (Zs), the depth of the cone base (Zb), maximum height (hco) that is

measured as the maximum elevation above the fitting basal plane (Favalli et al.,

2009) (Fig. 6.14), and basal diameter (db, if elongated the minimum dmin and the

maximum diameter dmax). However, resulting associated parameters are also of

interest. Such associated parameters include perimeter, area, volume, aspect ratio

(or index of profile), basal ratio, slope angle, and azimuth (see Table 6.2).

Perimeter, basal area, and volume were estimated for each cone. The volumes of

the cones were measured (Vcom) by a 3D procedure, and compared with the

estimated values (Vcof) extracted by the cone volume formula, which is generally

applied in similar studies. The semi-automatic 3D procedure consists of first re-

creating manually a 3D base surface for a desired cone to be investigated in

detail. Afterwards, the volume for each cone identified is calculated by running a

volume computation tool with Microstation software. This tool is able to measure

the volume between two 3D surfaces, which will be selected as the original

surface and the 3D re-created base surface. This method takes into account that

most of the cones lie on a slope, have variable height, and a rough surface.


151

Therefore it allows for a more accurate estimation of volume, whereas the use of

the traditional method might overestimate the volume (a difference from 1% to

20% between the two methods was observed).

The aspect ratio (hco/db) was calculated by dividing a cone’s maximum height by

its average basal diameter value. Two different values of db were tested: the first

was extracted from measuring the perimeter of the cone’s shape and dividing by

π, and the other as the cone width ratio that uses the arithmetic mean of the

cone’s major and minor diameters ((dmax+dmin)/2). Comparing the results of the two

different computations, the estimated error was determined to be about 0.5%.

Moreover, as previously considered by Favalli et al. (2009) for the scoria cones

located on the Etna flanks, the maximum height above the fitting basal plane was

preferred to the maximum height above the average basal elevation of cone

(Settle method) (Fig. 6.14). This significantly lowered the standard aspect ratio,

which is calculated using the classical method of Settle (1979). For the volcanic

field at Pantelleria, a difference in the aspect ratio value of 0.3 (1%) was calculated

between the two methods.

Fig. 6.14 Methods to calculate height (hco) and diameter (db) of cones. The classical

method of Settle (1979) defines hm as the maximum elevation above the average basal elevation of cone. In the Favalli method, hco is the maximum elevation above the fitting basal plane. The two methods give significantly different results in the case of a dipping plane. Zs= depth of the cone summit m b.s.l., Zb: depth of the cone base b.s.l. (after Favalli et al., 2009)

The basal ratio (bsr) was calculated as the maximum-to-minimum diameter ratio

(dmax/ dmin). Slope angle (εm) for the highest portion of the flanks of individual cones

was calculated from the DTM using the interpolate-slope tool in ArcView. Due to

the variability of slope angle observed with height, average slope values for each

cone were extracted using the formula [ (εf)=tan-1(2(Zb-Zs)/d) ]. Azimuths between

the peaks of the cones and along the eruptive fissures of the volcanic field have

also been measured using the direction tool in ArcView.


152

The different physical parameters of the cones were plotted against each other

and a Pearson correlation analysis was applied to examine whether correlations

exist. The Pearson correlation coefficient (r) is a widely used method in defining

the linear relationship between two continuous quantitative variables (x, y). The

sample correlation coefficient rX,Y between two measured variables x and y with

the mean values µX and µY and standard deviations σX and σY is defined as:

∑=

−

−

−= n

i y

yi

x

xiyx

yx

nr

1, 1

1

σµ

σµ

where n is the number of samples. The Pearson correlation coefficient varies

between -1 to 1, indicating the degree of linear dependence between the

variables: 1 indicates a strong linear positive relationship, -1 a strong negative

linear relationship, and 0 indicates that there is no correlation .The statistical

analysis of data was performed using Microsoft Excel.

Parameter Unit Description Position (X and Y)

m X and Y location (WGS84, UTM zone 33N) of the baricenter of the polygon enclosing the base of the cone

Zs m Cone summit depth b.s.l.

Zb m Cone basal depth b.s.l.

|Zb-Zs| m Maximum difference in elevation

hco m Maximum cone height, defined as maximum elevation above fitting basal plane (Favalli method)

hm m Maximum cone height, defined as maximum elevation above average basal elevation (Seattle method)

hco,v m Average cone height extracted from Vco

dmin m Minimal cone basal diameter

dmax m Maximum cone basal diameter

db m Average cone basal diameter (calculated by Pc/π )

WRc m Cone width ratio, calculated by (dmax+dmin)/2

Pc m Cone perimeter

Ac km2 Cone basal area

Vco km3 Cone volume (determined by two different methods: Vcom, Vcof)

Vcom km3 Estimated cone volume using a semi-automatic 3D procedure

Vcof km3 Estimated cone volume calculated by the volume formula for a regular cone

ARc - Cone aspect ratio (or index of profile), calculated by h/db where h = hco

(Favalli method) or hm (Settle method) bsr - Cone basal ratio, defined as dmax/dmin

εm degree Slope angle, calculated on the DTM for the highest portion of the flanks of individual cones

εf degree Average slope angle, from a singular cone profile calculated by tan-1(2(Zb-Zs)/db)

azimuth degree Azimuth along fissures and between cone peaks

Table 6.2 Morphometric parameters used to describe the cones.


153

Moreover, morphometric analyses have allowed for the establishment of

quantitative comparisons between the NW cones, the other cones offshore Scauri

and P.ta Limarsi, and some cuddie from the NW of Pantelleria Island (i.e., Cuddia

del Monte, Cuddia del Cat, M.te Gelkhamar, & Cuddia Bruciata). The analyses of

the cuddie from the digital terrain model was tricky since the resolution of available

data is low, therefore it should be taken into account that those values are affected

by a higher error.

The final aim of this study is a morphological definition and determination of

cones size and the reconstruction of the magma-feeding fractures of the NW

volcanic field.

Ch. 6 - D

iscussion and conclusions

154

Table 6.3 M

orphometrical param

eter of the cones of Pantelleria (cones 1-26 N

W volcanic

field; cones 27-30 offshore Scauri, cones 31-34 offshore P

.ta Limarsi).

ID X Y Zs Zb |Zb-Zs| h co hm hco,v dmin dmax db WRc Pc Ac Vcom Vcof ARc ARc bsr εm

εf (°) azimuth

m m m m m m m m m m m m m km2 km3 km3 Favalli Seattle

interval 1 222624 408812

1 -212 -690 478 314.1 365.3 186.5 2270 3282 2975 2776 9342 5.91 0.368 0.619 0.106 0.123 1.45 24 15.48 -

2 224448 4087504

-190 -590 400 346.0 331.0 222.3 1490 2800 2317 2145 7275 3.25 0.241 0.375 0.149 0.143 1.88 34 24.92 N37°-46°W 3 226225 408800

3 -260 -598 338 220.5 234.6 150.7 1860 2793 2457 2327 7715 4.08 0.205 0.300 0.090 0.095 1.50 21 13.35 -

4 222374 4087009

-328 -575 247 121.0 159.6 111.3 720 868 780 794 2450 0.45 0.017 0.018 0.155 0.205 1.21 29 18.59 N77°E 5 223030 408670

0 -272 -575 303 222.0 242.4 100.3 1020 1480 1533 1250 4815 1.13 0.038 0.084 0.145 0.158 1.45 28 23.54 N51°-58°W

6 225769 4085686

-216 -470 254 102.4 164.5 129.3 1358 1371 1425 1365 4476 1.43 0.062 0.049 0.072 0.115 1.01 33 8.58 N17°-41°W 7 227909 408509

1 -218 -585 367 176.0 243.7 134.9 1280 1678 1427 1479 4480 1.59 0.072 0.093 0.123 0.171 1.31 36 15.38 N47°W

8 220605 4085332

-450 -675 225 161.5 148.9 163.2 870 1360 1203 1115 3776 0.93 0.051 0.050 0.134 0.124 1.56 37 20.38 N43°-N4E 9 222721 408390

4 -167 -435 268 64.6 144.6 111.6 820 825 779 823 2446 0.45 0.017 0.010 0.083 0.186 1.01 29 8.65 N53°-64°W

10 220747 4083200

-222 -685 463 199.7 302.7 149.2 2260 2521 2492 2391 7825 3.88 0.193 0.258 0.080 0.121 1.12 27 10.02 - 11 222109 408367

2 -202 -505 303 185.5 222.5 131.1 865 1048 982 957 3082 0.69 0.030 0.042 0.189 0.227 1.21 35 23.23 N63°E

12 222802 4082892

-169 -345 176 71.6 99.3 97.2 680 718 744 699 2335 0.39 0.013 0.009 0.096 0.133 1.06 31 11.90 N°77°W 13 222240 408268

6 -263 -328 65 66.9 65.4 60.4 200 799 558 500 1753 0.12 0.002 0.003 0.120 0.117 4.00 31 33.78 N°71°W

14 220910 4081742

-165 -685 520 291.8 361.0 209.6 1800 3515 3095 2658 9719 4.60 0.322 0.448 0.094 0.117 1.95 32 17.97 multiple 15 222066 408211

7 -200 -445 245 151.0 172.8 116.5 600 845 728 723 2285 0.35 0.014 0.018 0.208 0.237 1.41 36 26.73 N38°E

16 222797 4082171

-260 -348 88 49.0 60.2 47.1 216 251 242 234 760 0.04 0.001 0.001 0.202 0.249 1.16 35 24.42 N60°E 17 221937 408075

2 -497 -560 63 41.0 33.4 35.7 290 500 404 395 1267 0.10 0.001 0.001 0.102 0.083 1.72 30 15.80 N72°E

18 221652 4081123

-449 -555 106 62.7 62.7 59.7 390 548 469 469 1474 0.14 0.003 0.003 0.133 0.133 1.41 37 17.82 N34°W 19 219987 408864

4 -614 -680 66 68.7 68.7 64.7 256 300 279 278 876 0.06 0.001 0.001 0.246 0.246 1.17 37 28.24 -

20 221099 4086737

-531 -630 99 68.7 77.2 80.2 368 567 489 468 1537 0.16 0.004 0.004 0.140 0.158 1.54 33 20.48 N76°E 21 222582 407917

9 -316 -665 349 100.5 172.9 142.0 770 2070 1580 1420 4961 1.11 0.052 0.055 0.064 0.109 2.69 42 14.64 N58°E

22 222865 4078000

-486 -670 184 60.7 98.6 69.5 450 1142 859 796 2698 0.32 0.007 0.006 0.071 0.115 2.54 39 15.11 N55°-78°E 23 223867 407773

6 -333 -498 165 57.4 94.7 67.7 320 470 365 395 1145 0.09 0.002 0.002 0.157 0.260 1.47 41 19.73 N46°E

24 223309 4077501

-491 -635 144 61.8 70.5 64.0 484 528 476 506 1494 0.15 0.003 0.003 0.130 0.148 1.09 38 14.33 N40°E 25 222501 407589

8 -586 -728 142 99.7 109.2 142.0 436 500 476 468 1496 0.17 0.008 0.006 0.209 0.229 1.15 40 24.59 N90°E

26 223309 4075768

-504 -730 226 95.9 146.2 143.7 436 1258 987 847 3098 0.45 0.021 0.014 0.097 0.148 2.89 36 23.75 N51°-78°E 27 229296 407012

2 -238 -530 292 126.4 186.4 124.6 680 778 681 729 2137 0.35 0.014 0.015 0.186 0.274 1.14 36 20.40 N23°E

28 229261 4069406

-360 -600 240 137.0 161.8 225.3 500 630 585 565 1836 0.23 0.017 0.010 0.234 0.277 1.26 37 28.74 N39°E 29 229450 406901

2 -357 -695 338 213.6 186.6 260.9 696 1057 918 877 2883 0.56 0.049 0.040 0.233 0.203 1.52 40 31.56 E-W

30 228817 4068210

-600 -840 240 150.6 161.6 181.8 800 864 801 832 2516 0.48 0.029 0.024 0.188 0.202 1.08 37 20.64 N66°W 31 239196 406718

8 -330 -720 390 242.0

0 267.0 416.7 1386 1784 1585 1585 4977 1.53 0.213 0.123 0.153 0.168 1.29 38 19.26 N54°W

32 238525 4066302

-450 -760 300 210.00

290.0 196.5 912 960 1200 936 3769 0.88 0.058 0.062 0.305 0.332 1.05 38 30.27 N68°E 33 237972 406624

4 -455 -740 285 142.0

0 166.0 282.6 933 1153 1013 1043 3180 0.70 0.066 0.033 0.140 0.164 1.24 38 16.94 N59°W

34 237106 4066619

-485 -770 285 140.00

168.0 190.1 923 1253 1123 1088 3525 0.78 0.049 0.036 0.125 0.150 1.36 38 16.89 N70°W


155

6.3.2 Geometry of the volcanic centers

Using the abovementioned selective criteria, 34 volcanic cones have been

identified around Pantelleria and analyzed in detail for their morphological and

morphometric parameters. Most of them (i.e., cones 1-26) are located in the NW

volcanic field (Fig. 6.15), four (i.e., cones 27-30) are located offshore Scauri, and

four (i.e., cones 31-34) offshore P.ta Limarsi (Fig.6.9).

This study is mainly concerned with the growth and distribution of the cones in

the NW volcanic field, while other cones will be presented as a comparison.

Fig. 6.15 Slope aspect (0°-40°) maps draped over ba thymetry of the NW volcanic field. Top right: map of the area, in red: cone outlines and peaks. An ID number is given for each cone.

A

B

C

13


156

Fig. 6.16 Top: slope aspect (0°-40°) map draped ove r bathymetry of the cones offshore

Scauri and P.ta Limarsi. Bottom: shaded relief map. In red: cone outlines and peaks. An ID number is given for each cone.

The volcanic centers show different shapes mainly consisting of pointed (e.g.,

cone 27-30 in Fig. 6.16), elongate (e.g. cone 2 in Fig. 6.15A), and composite

cones (e.g. cone 14 in Fig. 6.15B). A few of them (i.e., cones 1, 3, 6, 10, & 14)

have summit craters and a secondary small cone within the crater (see Sec.

6.1.3.5). Elongate shapes seem to be the result of the juxtaposition of a few

circular or sub-circular cones along an axial direction, which indicates an eruptive

fissure.. Composite shapes are formed by the coalescence or grouping of a few

cones that build up a larger structure. Among all the 34 detected cones in the


157

three areas, 44% of them were classified as pointed, 32% elongated, and 24% as

composite cones (Table 6.4).

ID CLASS n°peaks ID CLASS n°peaks 1 pointed 1 c 18 composite 2 2 elongated 16 19 pointed 1 3 pointed 1 c 20 elongated 1 4 pointed 1 21 elongated 8 5 elongated 5 22 elongated 1 6 composite 4 t/c 23 pointed 2 7 pointed 6 24 pointed 1 8 elongated 3 t 25 pointed 1 9 elongated 2 26 elongated 4

10 pointed 1 c 27 pointed 2 11 elongated 4 28 pointed 2 12 pointed 2 29 pointed 2 13 elongated 4 30 pointed 3 14 composite 17 c 31 composite 1 15 composite 2 32 composite 2 16 pointed 2 33 composite 3 17 elongated 1 34 composite 3

Table 6.4 Morphological classification of the submarine cones offshore Pantelleria

based on their shape. The nature of each cone's profile is related to the number of peaks, which is determined to be a single or multi-peak profile. Moreover, some of them show a truncated (t) profile or have summit craters (c).

The cones occur at depths (Zb) ranging between 328 m to 840 m, while the

summits (Zs) range between -614 m and -167 m (i.e., Cone 9). The difference in

elevation (i.e., Zb-Zs) between the basal surface depth (Zb) and the summit water

depth (Zs) of individual cones range from 63 m to 520 m, whereas the maximum

height (hco) range from 41 m to 346 m. Basal maximum diameter (dmax) varies

between 0.2 km to 3.5 km. The basal areas are between 0.04 km2 and 6 km2, and

the volumes between 7x10-4–0.37 km3. The aspect ratio (ARc = h/db) of the cones

ranges from 0.06 to 0.33 [average for 34 cones is 0.15 with Favalli method, and

0.17 with Settle method, with a standard deviation (σ) of 0.06)].

Each cone's profile, whether symmetrical, peaked, or flat, was also noted, and

all cones, with the exception of few truncated ones (i.e., cones 6 and 8), have a

single or multi-peak profile (Table 6.4). Most of the cones show bilateral symmetry

both in their shape and profile. However, symmetry decreases when the cone lies

on a slope for simple geometrical reasons. Considering the profile and shape,

three “end members” can be defined. The first is a small population characterized

by single-peak profiles and is seen in some of the pointed and elliptical cones and

also in two elongated ones (i.e., cones 20 & 22); the second population is


158

characterized by a multi-peak profile and is recurring in elongated and composite

cones; and the third includes the two truncated cones.

The analyzed cones had slope angles at the summit ranging from 21° to 42°.

The low slope angles are peculiar for some of the higher cones that usually result

in being pointed (i.e., cones 1 and 3), whereas the highest slope angles are more

frequent in the elongated and narrow cones.

Other distinctive attributes of the cone shapes was whether their flank get

steeper toward their summits so that their flank are concave upward. It was noted

that slope angles are uniform in pointed and elliptical cones, whereas in elongated

and composite cones, slope angles increase clearly toward the summit.

The majority of cones are elongated or display a certain orientation, mostly

resulting from the alignment of multiple peaks. In the eastern sector of the NW

volcanic field, azimuths of the eruptive fissures and of alignments between peaks

of nearby cones vary from N35°W to N51°W (from the E toward the W) (Fig. 6.17).

The western sector is mainly characterized by radial alignments with respect to the

shelf, whereas other orientations (i.e., NNE-SSW) are also present but extremely

subordinated (Fig. 6.17).

.

Fig. 6.17 Shaded relief map of the NW volcanic field of Pantelleria. Vent alignments and other orientation of other lineaments are shown.


159

The cluster of cones offshore Scauri are aligned NNE-SSW, perpendicular to

the coast. The cones offshore P.ta Limarsi form a semi-circular structure with

major alignment along NW-SE and NE-SW directions (Fig. 6.16).

Some of the cones (i.e., cones 1, 3, 6, 10, & 14) have been affected by summit

collapses, which decreased their height. The resolution of the data allows for the

reconstruction of their sub-conical pre-collapse shape. This allowed for an

estimate of the volume that collapsed as well as the volume of the new cones that

were emplaced within the scar left by the collapse.

6.3.3 Morphological parameters of the volcanic centers

A morphological analysis of the Pantelleria cones was performed, taking into

account the methods of previous studies of oceanic seamount and submarine

cones (e.g., Scheirer and Macdonald, 1995; Smith, 1996; Rappaport et al., 1997;

Ruiz et al., 2000; Clague et al., 2000b; Stretch et al., 2006) and their results for the

interpretation.

The cross correlations between some of the parameters described above were

computed for the Pantelleria NW volcanic field (Table 6.5).

Zb-Zs hco Zb Zs Ac Pc dmin dmax db WRc n°peak εm Vcom Vcof

Zb-Zs - hco 0.85 - Zb 0.34 0.32 - Zs -0.64 -0.52 0.51 - Ac 0.86 0.86 0.35 -0.49 - Pc 0.93 0.88 0.38 -0.54 0.96 -

dmin 0.90 0.83 0.30 -0.58 0.94 0.94 - dmax 0.91 0.87 0.40 -0.51 0.95 0.99 0.90 - db 0.93 0.88 0.38 -0.54 0.96 1.00 0.94 0.99 -

WRc 0.93 0.88 0.37 -0.55 0.97 1.00 0.96 0.98 1.00 - n°peaks 0.56 0.61 0.13 -0.40 0.41 0.55 0.35 0.60 0.55 0.51 -

εm -0.37 -0.44 0.12 0.45 -0.59 -0.52 -0.61 -0.46 -0.52 -0.53 0.09 - Vcom 0.82 0.87 0.35 -0.47 0.99 0.95 0.90 0.94 0.95 0.95 0.49 -0.55 - Vcof 0.80 0.88 0.34 -0.45 0.97 0.91 0.86 0.91 0.91 0.91 0.47 -0.54 0.99 -

Table 6.5 Pearson correlation coefficients (r). In grey, coefficients showing little to no correlation. See Table 6.2 for symbol definitions.


160

A total of 14 parameters were used; 10 (the geometric ones) show a good

Pearson among them, whilst the other 4 (water depth related parameters, number

of peaks, and flank slope) do not (Table 6.5). Positive linear relations between the

different variables are indicated by moderate to strong correlation ratios (0.5-0.99),

whilst negative relationships have moderate ratios (between -0.5 and -0.64) (Table

6.5). Some of the ratios useful in the characterization of the cones will be

illustrated in more detail (i.e., (dmax/dmin), hco /db, hco /Vco, & db /Vco).

The basal ratio (dmax/dmin) varied between 1 and 4 (Table 6.3), and highlights

that the cone shapes are irregular and strongly affected by elongation (i.e., cones

2, 13, 14, 21, 22, & 26). Diameters measured manually rather than calculated

automatically from the perimeter can differ by several hundred meters, especially if

cone’s shape is elongated, although when plotted against each other they show a

strong positive correlation (Tab.6.5). The mean diameter of cones using the

measured diameters is 1098 m, with a standard deviation (σ) of 770 m. The mean

diameter of the cones using the calculated diameters is 1165 m (σ =852 m).

The distribution of the cone diameters shows a modal class between 200-400 m

and low kurtosis (A in Fig. 6.18). The cone height histogram (B in Fig. 6.18) shows

a positively skewed distribution, indicating the abundance of small cones (hco

between 40 m to 70 m) respect to the others. The mean height is 133 m (σ=88),

with 65% of cones less than 130 m in height. Slope angles range between 8.6° to

34° and shows a slight bi-modal distribution with t wo modal classes between 15–

16° and 23-25° (C in Fig. 6.18). The mean slope ang le of the cones is 19° ( σ=6.3).

Fig. 6.18 Histograms of NW field cone dimensions: (a) histogram of cone diameter; (b) histogram of cone height; (c) histogram of the average slope angle of the cones.

0

1

2

3

4

5

6

7

200 800 1400 2000 2600

cone diameter (m)

coun

t

0

1

2

3

4

5

8 10 12 14 16 18 20 22 24 26 28 30 32 34

slope angle (degree)

0

2

4

6

8

10

12

40 100 160 220 280 340

h co (m)

A B C


161

Fig. 6.19 Scatter plot showing aspect values (cone height versus diameter) of the NW

field cones [cones 1, 3, 6, 10, & 14 were plotted with both present-day values (considering that summit-collapses have occurred) and the reconstructed pre-collapse values]; Scauri and P.ta Limarsi submarine cones, and some of the “cuddie” of the NW portion of the island (i.e., Cuddie Rosse, Cuddia del Monte, Cuddia del Cat, M.te Gelkhamar, & Cuddia Bruciata). The positive correlation for the NW field values improves from R2=0.76 to R2=0.91 using the heights inferred from pre-collapses.

A strong positive linear correlation is found (R2= 0.76 in Fig. 6.19) when plotting

the cone height (hco) versus the average diameter (db) of the cones of the NW

volcanic field. Moreover, the plot shows that the aspect value decreases with

higher diameter values and some of the cones characterized by wider diameters

(i.e., cones 1, 3, 6, 10, & 14, shown as empty squares) do not follow the linear

trend because they have a lower height compared to the others. As the

morphology shows, this might be due to a failure affecting the flank (i.e., cones 1,

3, 10, & 14) or erosional processes (i.e., cone 6). On the basis that the flank

slopes are not affected by those processes, the profiles of the cones have been

geometrically re-created with the aim to estimate their former heights. In Fig. 6.19,

diameter versus height is plotted using both measured and reconstructed values

for those cones whose summits are partially collapsed. Considering the

reconstructed values the correlation between height and diameter improves

(R2=0.9). This improvement in the correlation for the calculated former cone

heights gives confidence in the assumption that the breached and truncated cones

y = 0.1258xR2 = 0.9086

y = 0.108xR2 = 0.7619

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500 3000 3500

db (m)

Hco

(m

)

NW field values not affectedby collapse

NW field present-day values

Scauri

P.ta Limarsi

cuddie

pre-collapse linear trend

collapse linear trend


162

were previously higher and likely grew following the same trend as the other

cones. The lower heights for these cones can be concluded to be a result of

failures and erosional processes.

Furthermore, the scatter plot shows that cones developed self-similarly and

mostly aligned along the same trend ((h/db=0.13). Also, the larger cones followed

this trend before being affected by failure and renewing of activity, which is

testified to by the occurrence of narrow cones inside the scar. Likewise, the shape

of a few cones having a different trend suggest that out-range values might

indicate a construction mechanism that took place in successive phases, where

also erosion and instability had played a role.

In Fig. 6.19, height versus diameter is shown also for the cones offshore Scauri,

P.ta Limarsi, and for some the “cuddie” of the NW portion of the island (i.e.,

Cuddie Rosse, Cuddia del Monte, Cuddia del Cat, M.te Gelkhamar, Cuddia

Bruciata). The cones offshore Scauri and P.ta Limarsi show a similar trend to the

NW cones, whereas the values for the cuddie are scattered and lower compared

to the others. The average aspect ratio of the cuddie is 0.08 (σ= 0.11), significantly

lower than the 0.15 found for the submerged cones.

For the same diameter, a higher height was preserved and seems characteristic

of most of the submerged cones, while is lower for on land cones. The low value

found on land seems the result of a different evolution process than offshore. This

might be due to a different mechanism of emplacement in the subaerial

environment, to weathering and erosion, or a combination of the two factors.

As a whole, the morphometric analysis of the submerged cones of Pantelleria

shows that their aspect ratio is mainly related to constructive processes, as most

of the cones were not affect by erosion and instability processes. Conversely, the

low aspect values for some of the submerged cones might be the result of both

different constructive processes and a longer and more complex history, as it has

been shown by analysis of the on land cones. A comparison between the products

of the submerged and emerged cones is required to better understand differences

in how they grew. Above all, such a comparison should be useful in showing how

the marine setting influenced these morphologies.

Moreover, the aspect ratio calculated for the submerged cones is in agreement

with the value of the pointed cones offshore Hawaii (Clague et al., 2000b), which

have an aspect ratio between 0.11 to 0.25 (average for 19 cones is 0.183). Similar


163

to those pointed cones, and differently from subaerial scoria cones, Pantelleria

submerged ones also do not have summit craters. This might be due to the fallout

and piling up of fragments closer to the vent compared to what generally occurred

on land. However, a detailed morphological analysis of the cuddie of Pantelleria is

lacking, and was not possible to make a better comparison.

At Pantelleria, the cones occur at water depths between -300 m to -840 m

(Table 6.5). From the statistical analysis (Table 6.5), the cone basal depth (Zb) is

not correlated with any of the other parameters (e.g., height, diameter, volume,

slope, area). This is also evident from the similarity between shallower and deeper

cones, in terms of shape, slope, and size. These observations suggest that, in the

depth range where the cones were identified, the growth of the cones is related to

a common eruption mechanism, which produced similar morphologies.

Fig.6.20 Scatter plot showing volume versus radius (A)and height (B) of the cones of the NW field, offshore Scauri, P.ta Limarsi, and some of the cuddie of the NW portion of the island.

A cross-correlation was found between volume and cone radius (A in Fig. 6.20).

This correlation is clearly consistent with a power law model (degree 2; R2 =0.96)

and supports the quality of the 3D method used for the definition of the volume of

the cones. However, the volume versus height (B in Fig. 6.20) is less correlated,

as the volume is measured using the 3D method (and variable height) instead of

the volume formula for a regular cone.

As a whole, 45.5% of the total volume estimated (2.24 km3) for the 34 cones

offshore Pantelleria is made up by cones that range between 1x10-3–7x10-2 km3,

while 54.5% is made up by the 6 largest edifices (i.e., cones 1, 2, 3, 10, 14, & 31),

R2 = 0.9594

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 500 1000 1500db/2 (m)

volu

me

(km

3)

NW field

Scauri

P.ta Limarsi

cuddie

Potenza

R2 = 0.8341

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 50 100 150 200 250 300 350hco (m)

A B


164

each having a volume >0.1 km3. The largest cones are located at the edges of the

NW volcanic field, probably in relation to lower stress regime and the greater

possibility for growth.

The scatter-plot diagram and the histogram of Fig. 6.21 show the summit depth

(Zs) of the 26 cones of the NW field with depths varying between -165 m and -614

m. Empty depth ranges (i.e., between -330 and -450 m) are also present. It should

be taken into account that if geometries were re-built for cones affected by

collapses, the resulting summit depths would be shallower (i.e., between -125 m

and -190 m, empty triangles in Fig. 6.21 and blue value in the histogram).

Fig.6.21 Top left: Shaded relief map of the NW volcanic field of Pantelleria and cone IDs. Top right: Scatter plot of the summit depths of the 26 cones of the NW field. Summit depths from the re-built cones affected by collapses are shown by empty triangles. Colors indicate different depth ranges. Bottom left: Histogram of NW field cone summit peak heights (Zs), considering both pre-collapse (in grey) and re-built (in blue).

-700

-600

-500

-400

-300

-200

-100

0

ID

pea

k d

epth

(m

) .

14 9 12 2 15 11 1 6 7 10 3 16 13 5 21 4 23 18 8 22 24 17 26 20 25 19

0

1

2

3

4

5

6

7

100 150 200 250 300 350 400 450 500 550 600

Zs

coun

t


165

6.3.4 Distribution of volcanic cones in the NW field

Azimuths along the eruptive fissures and between the peaks of nearby cones

display preferred directions suggesting that they are not random and may

therefore provide insight into their development.

Almost all of the cones identified in the NW field can be grouped based on their

spatial distribution as clustered or aligned, with only a few isolated ones. Some of

the clustered cones have coalesced to form a single composite edifice (e.g., cone

14 in Fig. 6.15); however the orientation of each individual cone can still be clearly

recognized.

The alignment of cones seems related to the occurrence and distribution of

eruptive fissures. In general, the development of eruptive fissures represents a

surface expression of dike emplacement and a response to crustal cracking with

associated magma intrusions (Chadwick Jr. and Howard, 1991). Therefore, the

structures might result from the magma ascending along dikes feeding fissure

eruptions (Fig. 6.22). The resulting cone morphologies range from elongated

structures, if volcanic activity is diffuse along the eruptive fissure, and central

vents, if the magma concentrates in a single conduit along the fissure (Fig. 6.22).

Fig.6.22 Growth mechanism of aligned cones via eruptive fissures.


166

Fig.6.23 Left: Two different minor scale settings for aligned peaks: (A) -OOO- array; (B) -O-O-O- array. Right: Types of arrays for aligned peaks: parallel (a), segmented (b), crossed (c), and branched (d).

On a minor scale, the peaks of the cones are aligned in two different patterns:

a) the peaks of the cones are close and continuous resulting in an -OOO- array (A

in Fig. 6.23); b) the peaks of the cones are clearly spaced and separated forming

an -O-O-O- array (B in Fig. 6.23). Moreover, in both types of arrays, the peaks of

the cones are often not perfectly aligned along the same line, which results in a

disarticulated path and four types of peak alignments (parallel, segmented,

crossed, and branched) have been observed (a-d in Fig. 6.23). As a whole,

considering that the number of peaks, hence their distribution, is not correlated

with other parameters of the cones (Table 6.5), how these arrays are related with

cone growth is not clear. However, similar patterns have been observed for some

seamounts offshore Tenerife (Ruiz et al., 2000).

Overall, the majority of the cones from the NW field display preferred

morphological orientations (Fig. 6.17) in response to a variety of fault geometries.

These orientations are mainly NW-SE and WNW-ESE and are consistent with the

Sicily Channel Rift Zone (SCRZ) regional fault system (Colantoni, 1975; Cello et

al., 1985; Civile et al., 2008), indicating that the tectonic framework of the area has

a significant influence on their development. However, even if the main orientation

is parallel to regional structures, conjugate NNE-SSW trends, which deviate some

60° from the main orientation, are also present and appear to be more developed

in the western area of the field, which is also characterized by radial alignments of

the vents with respect to the shelf. This might have occurred in response of a local

fracture pattern due to a different response to the stress field. In addition, it was

shown by Catalano et al. (2009) that on land the distribution of eruptive fissures,

dikes, and eruptive centers is , as a whole, aligned along NNE–SSW belts thus


167

suggesting that crustal cracking has occurred with a similar trend in response to

an ESE-striking extension. Therefore, the orientation of the vents corresponding to

the NNE trend might be due to: 1) a local response to the regional stress field, or

2) a regional Late Quaternary ESE–WNW oriented extension (Catalano et al.,

2009).

On the whole, fissure eruptions seem to be the primary volcanic process acting in

the NW area, creating linear arrangements of cones mainly consistent with the

tectonic strike of the SCRZ and radial respect to the shelf (Fig. 6.17). Other

orientations are also present but subordinated and might be related to a different

stress field. Detailed geophysical or gravimetric studies need to be undertaken for

a better definition of the structural framework of the area.

6.3.5 Summit collapses

Some of the cones identified in the area (i.e., cones 1, 3, 10, 14, 6,) have been

affected by instability phenomena and dismantling processes that changed their

profile, decreasing their former height. These cones are characterized by a summit

collapse, a horseshoe shaped scar and a narrow cone (or two) within it, indicating

concurrent or successive volcanic activity.

Fig. 6.24 Reconstruction of the pre-collapse morphologies for cones 1 and 3. The profiles show present-day, pre-collapse, and basal surfaces considered for the estimated volumes. The present-day surface shows concave upwards morphology of the slopes due to the failure (2× vertical exaggeration).


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Moreover, it was possible, on the basis of the contour across the cone, to

geometrically re-construct the sub-conical pre-collapse shape of some of these

cones which were affected by collapse (i.e., cones 1, 3, & 10). This procedure is

similar to that performed for measuring cone volumes and is shown in Fig. 6.24 for

cones 1 and 3. Surface modeling of the pre-, post-collapse, and present-day

constitutes the basis for estimating the volume that collapsed (Table 6.6). Overall,

the volume that collapsed is calculated as the difference between the pre-collapse

volume (Vpc) and the present-day volume (Vcom), with the volume of the narrow

cone (Vic) subtracted.

ID Vpc (km3) Vcom Vic Vcom-Vic Vc Vc/Vpc

1 0.394 0.368 0.006 0.361 0.033 8%

3 0.226 0.205 0.006 0.199 0.027 12%

10 0.197 0.193 0.005 0.188 0.009 5%

6 - 0.062 0.001 0.061 -

14 - 0.322 0.004 0.318 - Table 6.6 Summary of estimated volumes in km3 (Vcom= estimated present-day cone

volume, Vinco= narrow inner cone, and Vpc=pre-collapse). It was possible to estimate the volume that collapsed (Vc = Vpc-Vcom+Vic) for cones 1, 3, and 10.

Fig.6.25 Left: Shaded relief and gradient map showing the minor-scale features of

cones 1 and 3 including horseshoe shaped scars, rims, and pointed narrow cones. The ID identifies the volcanic edifice. Dashed lines indicate inferred tectonic alignments. Contour every 50 m. Top right: cross-section of the two cones.


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In order to investigate the summit collapses, the specific cones are hereafter

described and interpreted.

In the case of both cones 1 and 3, as shown in relief and gradient maps in Fig.

6.25, collapse events have produced evident horseshoe-shaped scars,

characteristic concave-convex flank profiles (Fig. 6.25), and lower aprons due to

the transport of volcaniclastic material downslope.

Cone 1 (Fig. 6.25) is characterized by a sub-conical shape (diameter ~3 km)

with a near constant 18°–21° slope that is interpre ted as angles of repose of the

volcaniclastic deposits generated at the summit. The flank is affected by a failure

scar open to WSW and the material removed is estimated to be about 0.03 km3.

The headwall rim has low relief (<2m) and an crenellate sub-circular edge, which

might indicate the occurrence of repeated collapses. A narrow cone (diameter: 500

m) was built up inside the scar after the flank collapsed. The cone has a conical

shape, a vertical relief of 95 m, and an estimate volume of about 6x106 m3, and

lies on a basal surface sloping as much as 15°.

Cone 3 (Fig. 6.25) is characterized by a sub-conical-elongated shape (diameter

~2.5 km) and a quasi-bilateral symmetry with respect to a NNE-SSW axis. Its

flanks have a near constant 16°–21° slope and are c haracterized by a failure

surface facing NNE (estimated material removed is some 0.03 km3). Actually, the

cone shows two different nested rims that suggest the occurrence of at least two

major collapse events. The outer headwall scarp has about 20 m vertical relief and

is related to the older event. The inner headwall scarp is related to the younger

collapse and has about 30 m vertical relief. The scar is 1.4 km long and 0.7 km

wide, and is bounded by two steep escarpments downslope. Inside the inner

sector, a narrow cone (diameter: 480 m) lies on a slope of 15°. The cone is ~100

m in height and has an estimate volume of some 6x106 m3.

Cone 10 (Fig. 6.26) has a sub-conical shape near constant 18°–21° slope and

shows a relatively small summit collapse, which likely lowered its height from

about -130 m to m -220 m. There is a remnant of a sub-circular rim, but no

headwall scarp is present. The collapse seems to have affected only a small

portion of the edifice, and the material removed is estimated to be about 0.009

km3. A post-collapse cone (diameter: 300 m) occupies the summit (Fig. 6.26). It

has a height of about 75 m and a volume of 5x106 m3.


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The composite cone 14 (Fig. 6.26) shows two narrow cones at the summit of

the NE edifice, which is one of the shallowest of the area (summit peak -165 m).

The cones have heights of about 40 m and 100 m, and a total volume of some

3.6x106 m3. The identification of a sub-circular rim indicates the occurrence of a

summit collapse. However, post-collapse cones fill the slide scar and completely

veil the previous morphology.

Fig. 6.26 Left: Gradient map draped over shaded relief. The ID identifies the volcanic

edifice. Arrows point to the rim. Section profiles are shown on the right for cone 10 and the summit area of cone 14. Contour every 50 m.

Fig. 6.27 Left: Gradient map draped over shaded relief of cone 6. Right: 3D view of the

same cone from the north. Features identified in this view include a) a truncated summit, b) a cone, and c) peaks. The arrows point to minor scars.


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Between the cones which show a summit collapse, Cone 6 (Fig. 6.27), although

has a sub-conical shape, has a more articulated morphology. The summit of the

cone is partially truncated (a in Fig.6.27), but the flat-top morphology does not

continue laterally, as other peaks with an “-OOO-“ array are present (c in Fig.

6.27). A collapse likely affected also this cone, lowering its summit from about –

170 m to – 215 m. Features associated with the collapse are barely discernible in

the relict horseshoe-shaped scar facing NE. A narrow cone (diameter: ~300 m; h=

25 m) occurs on the upper portion of scar (b in Fig. 6.27). The cone has a volume

of some 0.6x 106 m3, but seems partly dismantled. The occurrence of minor scars

on the NE flank (arrows in Fig. 6.27), as well as its highly degraded morphology,

led to consider that erosion was the main active process after the cone was

affected by collapse.

As a whole, the collapse of cone 1 and cone 3 is comparable in size and

volume of material removed, which was estimated to be about 0.03 km3 (Table

6.6). For cone 10, only 9x10-3 km3 was removed. These results show that volume

involved in the collapse represents between the 5% and 12% of the pre-collapse

cone volume (Table 6.6). Moreover, some of the newly re-constructed cones (i.e.,

cones 1, 3, 6) seems concentric respect to the edifice, while the others are not.

The concentric ones indicate that the collapse likely occurred during the activity of

the cone (i.e., syn-eruptive), whereas the others might indicate the superposition

of a second structure over time, as observed in Cone 6, or, less likely, a shift in the

summit vent.

Analysis of cone height versus diameter (Fig. 6.19) showed that the cones

affected by collapse are amongst the largest (diameter >1600 m) of the NW

volcanic field. Initially, these collapses were thought to be related to the slope of

the flanks. However, comparing the slope of the cones affected by collapse with

the other cones offshore Pantelleria, their slope is similar (or lower), and no critical

height for cones seems to exist, as one of the highest (i.e., cone 2 in Fig. 6.19)

was not affected by summit collapse. This indicate that oversteepening of the flank

was not the main factor that triggered these failures. These failures might have

been triggered by magma bulging beneath the cone, as is inferred by the

occurrence of a newly re-constructed cone inside the scar indicating a concurrent

or post-collapse volcanic activity. Moreover, the diagram highlights that these

cones have the shallowest summits, while plotting their re-built heights. This might


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indicate a relationship between summit depth, collapse and activity. Moreover,

these summits would have been even shallower if the activity of the cones

occurred during the last low stand period.

Since some of the newly re-constructed cones indicate their collapse was due

to a syn-eruptive process, and the cones seem to be monogenetic, a change in

the stability condition during the eruption, while the cone approaches shallower

depths, seems the main cause of instability. The change in stability condition might

be related to an increase of explosivity which could have triggered the failure.

Similar to these cones, Monowai submarine volcano (Wright et al., 2008),

between 1998-2004 experienced a collapse and subsequent re-growth of a 90 m-

high cone within the scar. The volume of the collapse was measured to be ~0.085

km3. Two surveys (pre and post-collapse) were able to detect the changes in

morphologies and show that the summit depth at Monowai deepened from ~45 to

~130 m b.s.l. Based on the amplitude of hydroacoustic T wave (i.e., the tertiary

wave arrival from an tremor that travels through the sea), the collapse was

interpreted as related to the exposure of magma which probably produced an

eruption phase, and indeed edifice inflation associated with nascent volcanism

could have initiated the collapse, defined as a ‘‘hot landsliding’’ (Wright et al.,

2008).

The Pantelleria and Monowai summit collapse cones show not only similar

morphologies, but are both characterized by the occurrence of the peak at shallow

depth, corroborating the hypotheses of the relationship between depth, activity

and collapse.

Overall, at Pantelleria, the summit collapse, hence the removing of load

pressure, was not the cause of a reactivation of the volcanic activity. On the

contrary, the volcanic activity of the monogenetic cone, and its relation with

shallow depth, seems to be the main cause of the collapses.

Furthermore, in cones 1, 3, and 6, the directions of the sector collapse are

perpendicular both to the vent alignments and other lineaments (Fig. 6.17), while

the directions of the collapses of cones 10 and 14 affected only the summit and

are parallel to the peaks of the newly reconstructed cones.


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Fig.6.28 Models of regional tectonic stress and volcano loading, showing the

relationship of stress and direction of sector collapse development. A) Model of Moriya (1980) showing crater opening perpendicular to dike orientation and the σHmax. B) Model of Francis and Self (1987) showing crater opening perpendicular to the strike of normal faults and the σHmax (after Lagmay and Valdivia, 2006).

As it was observed for several volcanic edifices on land, sector collapses might

often open in the direction related to the regional tectonic stress and volcano

loading (Lagmay et al., 2000; Lagmay and Valdivia, 2006). This suggests that also

at Pantelleria, the directions of breaching were related to the volcano-tectonic

setting of the area. Therefore, one of these two models might be inferred for the

cones 1, 3, and 6 (Fig. 6.28).

6.3.6 Summary of the geo-morphometric analysis of the cones

The analysis of morphological and morphometric data has been used to define

the submarine cones at Pantelleria with the aim of summarizing the most

significant geo-morphological parameters. This allow a comparison between the

NW volcanic field cones and others cones offshore and onshore Pantelleria Island.

In addition, in this work, an innovative method to compute the volume parameters

has been introduced.

From the data analyzed and discussed in this section several points emerged

and may be summarized as follows:

1. compared to onshore “cuddie”, the submarine cones show higher slope

gradient, probably related to a different mechanism of emplacement;

2. submarine cones seems that have developed self-similarity, since there is a

constant relationship between cone height and diameter;

3. collapse occurred at the shallowest cones and likely indicate a strong

relationship between failure, depth and activity;

4. the alignment of the cones seems to reflect the tectonic fabric of the region.


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6.4 Samples from the NW volcanic field and relation

with the 1891 eruption

6.4.1 Case history of floating basaltic lava blocks

The 1891 eruption represents the last known activity at Pantelleria and a rare

case of an intermediate-water basaltic submarine eruption characterized by the

emission of floating lava bombs.

Floating basaltic lava blocks are a peculiar volcanic product. Their occurrence

has been reported associated with lava flows entering the sea at Hawaii (Tribble,

1991), or related with intermediate-water submarine eruptions. The floating

basaltic lava blocks are characterized by hissing, fracturing and bursting on the

surface, and finally sinking. Their formation when subaerial lava enters the sea

results from the incorporation of seawater into the lava; whereas, their formation

underwater is still controversial and a comparison among similar eruptions is of

interest.

Similar historical eruptions akin to the last submarine activity of Pantelleria are

quite rare in the literature and poorly known. At present, three similar eruptions

occurred in: 1998-2001 at Serreta Ridge, Azores (Gaspar et al., 2003); 1993-1994

at Socorro Island, Mexico (Siebe et al., 1995); and 1877 at Mauna Loa, Hawaii

(Moore et al., 1985).

Of these three, only at Socorro and Serreta were the eruptions monitored and

documented with contextual surveys and their products directly collected during

the eruptions. In Socorro, the emission of floating lava blocks was accompanied by

floating reticulite. In Mauna Loa, detailed submarine surveys, submersible dive

and bathymetric mapping, and samplings on the emission area have been carried

out since 1975 (Moore et al., 1985; Wanless et al., 2006).

Table 6.7 summarizes both the historical reports and the preliminary studies

(Whitney, 1877; Riccò, 1892; Moore et al., 1985; Siebe et al., 1995; Gaspar et al.,

2003) of the eruptions and highlights the recurrent features and proprieties

identified for this kind of eruption.


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Big Island, Hawaii

Socorro, Mexico Serreta Ridge, W of Terceira Island, Azores

Pantelleria, Italy

Eruption duration 24 February 1877 29/1/1993-April 1994

18/12/1998-04/2001

17-24 October 1891

Single/multiple activity

Single Multiple & intermittent

Multiple Single

Distance offshore 1.6 km W from Kealakeakua Bay

2.4 km NW of P.ta Costa

10 km W of Terceira Is.

5 km NW from Pantelleria Village

Estimated depth 37 m to 110 m 30 m; 210 m 300 m to 1000 m (from nautical chart)

>320 m, not reliable

Dimension of interested area

1.6 km line 2 distinct areas of ~6000 m2 tot

~20000 m2 50x850m (42500 m2)

Alignment X NW-SE strip NE-SW strip

Max block dimension 0.60 m 3 m >2m 1.5 m

Bubble sizes (Ø) µm to 30 cm

Bursting X X X

Steam and blocks jet X X Yes, 15-20 m

Floating lava blocks X X X X

Reticulite X

Pele’s hair X

Ash at the surface X X

Drifting on the surface

X X X X

Incandescent outer surface

X No

Outer surface of the floating blocks

Glassy Ropy texture and cracker fusion crust for water quenching

Glassy skin and very vesicular

Sometimes glassy

Incandescent inner surface

X X X

Inner surface of the floating blocks

wrinkled with several furrows

Inner temperature evaluation

>900° >450°, <800°

Weight of measured sample

20 kg Half bomb= 12 kg

Specific gravity (g/cm3)

0.88±0.21 2.4 removing air & 1.5 with air

Roars and hisses X X

Presence of sulfur X X X

Steam emission X X X X

Associated seismicity X X Weak Weak

Associated fumaroles activity

X No

Seismic precursors X X X X

Table 6.7 Summary of the historical reports and preliminary studies for floating lava blocks eruptions (Whitney, 1877; Riccò, 1892; Moore et al., 1985; Siebe et al., 1995; Gaspar et al., 2003) (Empty fields are not given).


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6.4.2 Products and eruptive style of the NW volcanic field

• Re-location of the 1891 vent

At Pantelleria, the samples analyses (Sec. 5.1) were mainly focused on finding

and characterizing the 1891 eruption products, which were collected in the area

surrounding where the event occurred (Riccò, 1892).

Combining the historical reports and the detailed bathymetric map (Sec. 4.1.2.1)

with the sampling of the NW flank of Pantelleria (Sec. 5.1 & Fig. 5.1), most of the

scoriaceous block fragments were recovered near cone 16 (16 in Fig. 4.12). The

scoriaceous block fragments occurred at the seafloor and are not buried by

sediment. Lapilli and volcaniclastic sands occur in the surrounding area and

present a fine blackish volcaniclastic cover on the top (Fig. 5.1).

The block fragments are highly porous and contain large elongated vesicles

(Fig. 5.2). They display different layers corresponding with those described for

buoyant scoria not only at Pantelleria (Riccò, 1892), but also in other settings

(Siebe et al., 1995; Gaspar et al., 2003).

According to these data, the scoriaceous block fragments display the clearest

morphological record of 1891 volcanic activity on the seafloor and are inferred to

be debris produced during the emission of floating lava bombs.

Based on the proximity of the scoriaceous block fragments identified as from

the 1891 eruption products, Cone 16 (basal depth -350 m and summit peak at -

260 m) was the likely location of the vent and was built up during the eruption. As

the volume of the cone was measured from the bathymetry data (i.e., ~600,000

m3) and the emission of floating bombs lasted for 8 days, it is possible to roughly

estimate of the average eruption rate. Using the volume and length of eruption, an

eruption rate of about 0.9 m3/s was calculated. This value represents an average

rate, as from the historical accounts (Riccò, 1892) we know that the production of

floating lava bombs was considerable during the first days then decrease during

the following days, therefore in the initial rate might have been higher (up to 2 or 3

times) than what estimated. Moreover, the estimate does not take into account the

volume of distal products (non cone-forming) dispersed by currents in the nearby

area, as well as the possibility that the activity lasted for a longer time, during a

period without the formation of floating lava bombs at the sea surface. Even with


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these limiting assumptions, this value represents the only estimate available for

cone growth in the NW volcanic field at Pantelleria.

Moreover, the floating lava bomb eruption at Pantelleria represents the deepest

case when compared with similar known eruptions of this type (note that the depth

at Azores referred to a nautical chart).

• Eruption style of the 1891 eruption

The scoriaceous block fragments collected nearby the cone, indicated as the

1891 vent, were analyzed. Whole rock analyses indicate that the scoria samples

(i.e., PD36, PX4, PX7, & PX8) have a similar composition as expected and were

classified as basalt, according to the total alkali-silica diagram (TAS, Le Bas et al.,

1986; Fig. 5.13). Moreover, the samples are comparable in composition with the

historical analyses of the 1891 bombs given by Butler and Perry (1892),

reinforcing that they are products of this eruption.

In addition to the scoriaceous block fragments, the glassy fragments collected

northwestward of the cone were also considered. The analyses focused on the

upper core unit and box-corer layers (Sec. 5.1). Sedimentological, EMP, SEM

analyses were performed to characterize the glass fragments present in these

layers.

From the sedimentological analyses, the upper unit of the core is characterized

by a homogeneous volcaniclastic sand. The box-corers (with the exception of PX2,

which reached a lower pelitic unit) sampled only the upper part of a volcaniclastic

unit (Fig. 5.3), with PX4 and PX7 dominated by scoria and lapilli. This upper unit is

mostly constituted by a lower blackish sand-silt layer, which shows an increase in

the amount of pelite towards the upper portion, and by a cover of blackish fine

volcaniclastic sand, which shows different thicknesses (0.5-3.0 cm) both in the

same sample and amongst the others (Fig. 5.3).

From the chemical analyses, the glassy groundmass compositions of all

samples (including scoria) straddle the boundary between the hawaiite/basalt

fields (Fig. 5.13). The differences in this compositional range and those between

the whole rock analyses and the corresponding glass in scoria are due to the

significantly different phenocrysts and microlite crystallization. Moreover, the

variability in composition found in each layer prevented the identification of major


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differences between the two layers identified in the box-corers (i.e., the top cover

and lower one), adding to the difficulty in distinguishing if they are related to

different events or to other sedimentation processes.

Microscope and SEM observations of the upper volcaniclastic units of the core

and box-corers allowed for distinguishing and characterization of a wide range of

glass morphologies (Sec. 5.1.2). The most common glassy fragments of the upper

units are highly vesicular to scoriaceous and fluidal-shaped fragments. More

fragile glassy fragments (such as Pele’s hair or reticulite) are also present, but

subordinated. Moreover, the glassy fragments of the upper volcaniclastic units are

characterized by broken sharp surfaces, which indicate little to no reworking. In

general, similar morphologies were observed for hawaiian or strombolian type

eruptions (Clague et al., 2009a).

Based on the morphological distribution and characteristics of glassy fragments,

it was not possible to understand if these units sampled one or more eruptive

events. However, the micro scale morphology of the scoriaceous blocks, inferred

to be 1891 products, leads one to consider that part of the highly vesicular to

scoriaceous fragments from the core and box-corers might have resulted from the

breakage of scoriaceous blocks.

The presence of reticulite fragments, which have been traditionally interpreted

to form from the evolution of basaltic foams during subaerial hawaiian lava

fountains from low-viscosity volatile-rich magma (Mangan and Cashman, 1996), is

pertinent. In the submarine setting, their occurrence is rare and they were

described for the first and only time by Siebe et al. (1995) for the floating lava

bomb eruption at Socorro, where the reticulite occurred either as entirely

independent clasts, or as the cores of much larger fragments with a scoriaceous

carapace. At Pantelleria, fragile reticulite pyroclasts were identified in all samples

of the upper units. The analysis of selected reticulite fragments from the top of the

core (PC1 retic. 17-19 in Fig. 5.13) showed the same composition as the other

glass fragments of that layer, indicating that the reticulite likely belongs to the

same eruptive event.

In the fluidal-shaped fragments, strain-induced vesicle elongation is the most

peculiar characteristic. Such fluidal pyroclasts display many features in common

with subaerial lava fountains (fluidal shapes, ribbons, droplets, and strands).

Fountains are characteristic of hawaiian activity and imply abundant small vesicles


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in rapidly rising magma and can only form in volatile-rich magmas. Moreover, the

hypothesis of fountaining is also supported for the Socorro eruption with the

occurrence of reticulite fragments, as their formation is associated with the

supersaturation of volatiles (mostly CO2) (Siebe et al., 1995).

Combining all available data, it can be inferred that the upper units contain

products of the 1891 eruption, which likely formed the floating lava bombs as well

as other vesicle-rich glassy fragments, associated with a volatile-rich magma. The

high vesicle content in the scoriaceous blocks and glass fragments is related to

the extensive degassing from a volatile-rich magma (Schipper et al., 2010). The

occurrence of larger vesicles in the buoyant scoria was likely a consequence of

the shallow depth (i.e., low hydrostatic pressure) of the vent, which allowed the

exsolved volatiles to expand more efficiently. In fact, after the magma was

erupted, the rapid quenching of the outer layer of the bombs likely inhibits the loss

of volatiles by bubble escape, whereas the volatiles trapped in the interior of the

blocks, which remain longer at a molten state, likely coalesced and the vesicles

expanded. The production of buoyant scoria from submarine vents was inferred to

be related to intermittent fire fountaining at fixed vents (Siebe et al., 1995). A

different model for the formation of the “lava balloons” (Gaspar et al., 2003;

Kueppers et al., 2008; Kueppers et al., 2009) was proposed. In this model, the

exsolved volatiles are inferred to rise at the surface and accumulate under a melt

skin on a lava pond and create a growing bulge as a consequence of

decompression (Gaspar et al., 2003). This bulge may finally detach producing gas-

filled cavities of lava.. However, at Pantelleria, the formation of primary pyroclasts,

which include Pele's hair, fluidal fragments, and highly vesicular volcanic glasses,

support the hawaiian eruption style, and likely characterize the deposits found at

the seafloor to be a basaltic fire fountain type. Submarine fire fountains occurring

at undetermined water depths have been previously inferred from ancient deposits

(Simpson and McPhie, 2001). Recently, fire fountains and bubble-burst eruptive

activity were directly observed at West Mata Volcano (-1200 m). Sampling showed

these eruptions produced respectively small highly vesicular pyroclasts that

included Pele's hair and fluidal fragments, and Pele's hair, limu o Pele, spatter-like

lava blobs, and scoria (Clague et al., 2009b).

As a whole, the different volcaniclastic layers identified in the deposit might be

related to depositional processes of the pyroclasts related to the 1891 eruption,


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with the thin volcaniclastic cover representing a finer-distal deposit related to the

decantation of suspended particles or to a different event with similar eruptive

style, as the particles are characterized by similar morphologies and compositions.

However, the chemical analyses do not display clear distinctions between the

layers analyzed and the slightly different compositions found in the upper units

might indicate that 1) the eruption was not compositionally homogeneous and the

deposits are a mix of earlier and later materials of slightly different compositions or

2) the deposits results from the sedimentary reworking-mixing of pyroclasts of

different eruptions close in space and time, indicating that the 1891 vent was not

the only active vent at the time of the eruption.

• Evidence of earlier eruptions

The stratigraphy of the core collected in the NW offshore of Pantelleria testifies

to the occurrence of at least two different young volcanic events (Fig. 5.4). These

volcaniclastic units are separated by an interval during which eruptions did not

occur, allowing fine-grained sediment to deposit in a thick muddy sand or sand-

rich mud (Folk, 1954; Tortora, 1999) unit.

The strongly unsorted character of the upper volcaniclastic massive unit from

the core (PC1 15-83 cm from the top) suggests it was emplaced by a high density

flow. Whereas, the low amount of eroded carbonate material at the interface with

the lower unit suggests that the turbulence of the volcaniclastic flow was

suppressed shortly, without high incorporation of the sediment below.

The middle unit of the core is a muddy-sand with slight variation in grain size.

This unit is rich in biogenic and shelf-derived carbonate (between 47% and 56%,

see Table 5.2). The absence of volcaniclastic particles attests to a period

characterized by sedimentation without the occurrence of volcanic events or

pyroclast re-working. Sediment accumulation rates on Pantelleria basin were

estimated to be on the order of 0.02 cm/yr (Colantoni, 1975). No information is

present on the area close to the shelf break, where the sedimentation rate is

usually higher due to the efficient contribution from land. As a whole, considering

the basin sediment accumulation rate, it is possible to roughly estimate an interval

of some ~2500 yr without the occurrence of eruptions (or less if we assume a

higher sedimentation rate), however detail dating are needed.


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The lower volcaniclastic unit of the core is characterized by finer glass

fragments with respect to the upper unit. The finer pyroclasts might indicate that

the core sampled the finer distal products of an event (or more than one) that took

place further away than the one deposited in the upper unit, however more cores

in the area are needed to confirm this hypothesis. Moreover, this unit contains a

few trachytic or pantelleritic glasses and crystals, indicating that it incorporated

different units. Although other similar fragments were not identified in this area,

they might be related to sedimentary reworking that can mix pyroclasts from

different eruptions. In this regard, parallel laminations and cross-bedding

stratification, which characterized the lower unit of the core, support the hypothesis

of re-sedimentation processes. Re-sedimentation processes in this area might be

due to bottom current activity or to the occurrence of gravity flows from the shelf

and along the steep flanks of the nearby cones. However, the compositions of the

different layers analyzed from the lower unit of the core (Sec. 5.1.3) are almost

homogeneous (with the only exception of the few crystals mentioned above),

indicating that are likely relate to the same event.

The morphologies of the glassy fragments from the two units are similar in

shape, however no reticulite pyroclasts were found in the lower unit. This might be

due to the more efficient fragment breakage of the pyroclasts or to a different

eruptive style with respect to the younger event.

From the chemical analyses, the upper and lower units from the core are

characterized by glassy fragments of hawaiitic-basaltic composition, however they

present some differences as the lower unit of the core, with respect to the upper

one, is characterized by slightly higher SiO2, Al2O3, and MgO contents, and lower

K2O, TiO2, and FeO contents, confirming that two separated events took place.

Moreover, the composition found for the submarine samples (i.e., scoria whole

rock analyses in Fig. 5.13) are compatible with other lavas erupted in the NW

portion of Pantelleria Island (Civetta et al., 1998; White et al., 2009; Gioncada and

Landi, 2010), even if they are shifted slightly towards a more evolved basaltic

composition, as indicated by the lower MgO content (TAS diagram, Le Bas et al.,

1986; Fig. 5.13).


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6.4.3 Concluding remarks

Several points emerge from this section and may be summarized as follows:

• on a vertical dm scale, the deposits of two main eruption events were

sampled; one seems related to the 1891 eruption, while the other is related to

an event (or more) completely unknown before this research;

• a rough estimate of the minimum eruption rate of the 1891 eruption is about

0.9 m3/s, based only on the volume of the cone thought to be monogenetic;

• by comparison with three similar known floating lava bomb eruptions, the one

at Pantelleria represents the deepest case;

• the morphologies of glass fragments (from the upper unit of core and box-

cores samples), indicate that vesiculation and fragmentation processes are

related to a submarine fire fountain. Therefore the debris and distal deposits

related to this kind of intermediate-water basaltic submarine eruption

represent one of only a few examples of basaltic submarine fire fountain

deposits investigated to date.

Moreover, the sampling of the monogenic cones of the NW field show repetitive

eruptions, separated by long periods with no volcanic activity. One of the most

straightforward applications of the volcaniclastic particle analysis is that they can

be used to establish stratigraphic sequence and correlation between eruptions in a

region. While other cores from this area have not yet been analyzed, these data

might prove useful for future correlations.


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6.5 Marine Geohazards along the Italian Coasts

The morphological mapping of the area offshore Pantelleria volcano, which

comprises parts of the northern Pantelleria Rift and the Adventure and Tunisian

slopes, will be included in the national project MaGIC (Marine Geohazards along

the Italian Coasts).

MaGIC (see also the project website: www.magicproject.it) is a 5-years project

(2007-2012) funded by the Italian Civil Protection Department designed to produce

a bathymetric database with an accurate description of superficial geology and

related geohazards on the most sensitive and hazard-prone areas. The long-term

goal of MaGIC is to produce a database that can be used for a wide range of

research and management issues. This is being achieved mainly through

extensive interpretative morpho-structural mapping of the continental margins

between –50 and –600 m water depth. These maps will form the “Map of the

elements of geohazards of the Italian Seas”, consisting of 72 Map Sheets at scale

1:50.000.

Fig. 6.29 Area included in the “Map Sheet 30” for the Magic project.

The applicative part of this thesis and the main contribution to the project is in

the area contained in “Map Sheet 30” (Fig. 6.29 & Appendix B1). As a part of the

study carried out during this research, the interpretation of the Pantelleria Island


184

flanks was extended to the whole surrounding area, for a total coverage of ~1600

km2.

The Geohazards Map Sheet 30 produced in this work is formatted with the map

key and legend proposed by the Scientific Committee of the Magic Project

(Appendix B2). A section on Pantelleria and the surrounding area (including the

Geohazards Map and interpretation) is included in a report written for the Italian

Civil Protection Department (in Italian- Appendix B3).

The map (Map Sheet 30), which was developed over the three years of this

thesis work, not only represents a directly applicable contribution to the evaluation

of hazards in the Pantelleria area, but also allows for the planning of further

studies.


185

6.6 Main conclusions

Knowledge of the submarine volcanism at Pantelleria is needed to determine

both its evolution and the existing volcanic hazards. Its spatial extension and

distribution with respect to emerged areas are particularly significant for this

purpose. The data presented here has led to the following conclusions:

1. The correlation with the onshore basaltic lava flows on the NW shelf, as well

as the finding of basaltic rocks in the NW volcanic field, extends the area and

volume of basaltic products of Pantelleria volcano and should be considered

when defining its mafic activity.

2. As a whole, major sector collapses and debris avalanches were not identified

in the area.

3. The swath bathymetry of the submarine flanks of Pantelleria revealed for the

first time several volcanic centers. The cones grew following the same trend,

and the ones affected by summit collapses led us to infer a relationship

between summit depth, collapse, and activity.

4. No indication about submarine eruptive activity has been reported apart from

for the 1891 eruptive vent. This vent, which is located in the NW volcanic

field, likely represents the most recent eruption that formed the cluster of

cones on the NW, although both the activity of the field and when it began to

form remain mostly unknown.

5. Considering on shore and offshore structures, Pantelleria appears to be a

single volcano complex controlled by regional tectonic structures.

6. A complex history of the southern sector can be envisaged from the

occurrence of overlapping processes that are related to volcanism (i.e.,

cones, dikes), structural control (i.e., escarpments), mass wasting and

differential erosion (i.e., erosional remnants of volcanic outcrops, channels,

and possible instability phenomena along the slope). No indication on the

timing of the activity along this flank is available, but its general setting points

out to an older age than the northern flank. More investigations are needed to

better constrain the evolution of this sector that may give hints to the earlier

geology of the island. The morphological difference between the NW and SE

flanks, as well as the occurrence of the historical eruption offshore the NW

slope, attests to the possibility that, over time, the focus of volcanic activity


186

has been propagating northwestward. The submarine evidence for this

migration is in agreement with the hypothesis derived by Mahood and

Hildreth (1986) based on subaerial volcanological data.

7. The main geohazard of the area is the resumption of volcanic activity in the

NW offshore of Pantelleria, and the possibility of associated instability

phenomena. In this regard, with the exception of the last eruption in 1891, no

volcanic activity has been directly observed in the past years. However, the

analysis of the gravity core indicated the occurrence of repetitive eruptions,

separated by periods with no volcanic activity. More investigations are

required to establish the recurrence and timing of the events that built up the

cones to better understand and foresee the related hazard. From this point of

view, it is worthwhile to note that the cones are located just offshore the two

main inhabited villages of the island, so that the volcanic centers may be a

threat for navigation.


187

6.7 Future work

The interpretation of the data acquired during this PhD work allowed for

characterization of the submerged flanks of Pantelleria. Nevertheless, the data

does not completely cover the total shelf area. Even where data is available,

some unresolved issues remain, mainly due to the paucity of samples and

seismic surveys. Therefore, new field-work is required to fill in the data gaps

and promote a better understanding of the various processes involved. To this

end, more samples are needed to better understand of the processes acting in

the area and to verify the conceptual model proposed for the NW shelf.

Petrography analyses and, in particular, dating and textural characteristics of

volcanic/volcaniclastic flow deposits are required to reconstruct the processes

that occurred at the subaerial–submarine transition and for identifying the

locations of ancient shorelines.

In the future, more sampling and direct observations are needed to better

support some of the assumptions of this work and to understand the lava flow

processes that occur at the subaerial–submarine transition on a steep slope, as

well as along the underwater flank where morphologies analogous to on land

ones have been identified.

188

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