Bull Volcanol (2005) 67:695–716DOI 10.1007/s00445-004-0402-6

R E S E A R C H A R T I C L E

P. D. Cole · E. Fernandez · E. Duarte · A. M. Duncan

Explosive activity and generation mechanisms of pyroclastic flowsat Arenal volcano, Costa Rica between 1987 and 2001

Received: 3 September 2004 / Accepted: 1 November 2004 / Published online: 16 February 2005� Springer-Verlag 2005

Abstract Explosive activity at Arenal and associatedtephra fall that has occurred over the 14-year period from1987–2001 is described. Explosions have been notablyvariable in both frequency and size. A marked decrease inboth frequency and quantity of tephra fallout occurred inearly 1998 until the end of 2001. Grainsize distributionsof cumulative tephra samples collected once a month aretypically bimodal. Aggregation causing premature falloutof fine ash and possibly fallout from ash plumes producedby pyroclastic flows are considered responsible for thebimodality of fallout. Scanning electron microscopy ofthe glass component of tephra from single explosionsshow predominantly blocky and blocky/fluidal clasttypes, interpreted as being the product of vulcanian typeexplosions. Fragmentation of a mainly rigid, degassedmagma body, and a minor molten component is inferredfor these explosions. Pyroclastic flows were producedeither associated with the larger explosions by a mecha-nism of column collapse (1987–1990), or unrelated toexplosions by partial collapse of the crater wall (1993,1998, 2000, 2001). Pyroclastic flow activity has migratedfrom west to north during the period reported. Pyroclasticflow deposits are variable in the quantity of juvenilematerial and any associated surge component. Large ju-venile blocks were partially molten on emplacement and

many have a typical cauliform texture. Blocks with bothjuvenile and lithic textures indicate that at the summitmagma was in intimate contact with the pre-existing ed-ifice, rather than as a simple open crater or lava pool.Crater wall collapse may have been promoted by the re-duction in explosive activity, which has increased the lavaaccumulation at the summit and in turn increased insta-bility of the summit region. Thus although explosive ac-tivity has waned, if the lava output is maintained, thehazard of pyroclastic flows is likely to continue.

Keywords Arenal · Vulcanian explosions · Tephra fall ·Pyroclastic flows · Pyroclastic surges

Introduction

Arenal is one of six historically active volcanoes in CostaRica. It is located 70 km south of the border with Ni-caragua and 90 km northwest of the capital, San Jose.Arenal is a steep-sided, 1,657-m-high strato-volcano(Fig. 1), that has a basal diameter of around 8 km, and hasbeen in continual activity since its reawakening in 1968(Melson and Saenz 1973). It is a young volcano, pre-dominantly <4,000 years old, whose products are mainlybasaltic andesite in composition (Melson 1984, 1994;Borgia et al. 1988). Tephra deposits that crop out mainlyto the west of Arenal record at least 10 major explosiveeruptions of either Plinian or subplinian magnitude since3,700 BP (Melson 1982, 1984). The older inactive Chatocomplex that lies 2.5 km to the southeast was formed bymainly basaltic andesite and andesite lava flows (Borgiaet al. 1988).

Activity since 1968 has consisted of near continuousextrusion of basaltic andesite lava flows (Reagan et al.1987; Streck et al. 2002). In addition, since the mid1980s, spasmodic explosions take place typically severaltimes a day (Fig. 2). These explosions have been de-scribed as being strombolian (Alvarado and Soto 2002)or, rarely, vulcanian (e.g. SEAN 1988b). Lava flows andtephra erupted in the last 32 years have built a steep-sided

Editorial responsibility: R. Cioni

P. D. Cole ())Geography Subject Group, School of Science and Environment,University of Coventry,Priory Street, Coventry, CV1 5FB, UKe-mail: p.cole@coventry.ac.uk

E. Fernandez · E. DuarteObservatorio Vulcanologico y Sismologico de Costa Rica(OVSICORI),Universidad Nacional,Apartado, 2346–3000 Heredia, Costa Rica

A. M. DuncanInstitute for Research in Applied Natural Sciences,University of Luton,Park Square, Luton, LU1 3JU, UK

cone, the summit of which is termed crater “C” (Melsonand Saenz 1973), a few hundred metres to the west of theoriginal, historically inactive pre-1968 crater “D” (Fig. 1).

Blocky lava flows extend up to 4 km from the summit.Since 1974 these have been extruded from crater “C“(Fig. 1). The lavas have been the focus of research mainlywith regard to volumetric output to enable calculation ofthe magma budget of the volcano (Wadge 1983) and therheology and emplacement of blocky lava flows (Borgiaet al. 1983; Borgia and Linneman 1990).

Sporadic pyroclastic flows have been formed manytimes since 1968 (Alvarado and Soto 2002). We use ob-servations of explosions and the nature of the tephra falldeposits formed at Arenal over the 14-year period from1987 to 2001 to analyse the pattern and nature of thiseruptive behaviour. In addition, we describe the pyro-clastic flows formed and their resulting deposits andcomment on the mechanisms of their generation andemplacement.

The 1968 eruption

Prior to 1968, Arenal was thought to be extinct and was asimple conical stratocone. However, increased springwater temperatures and minor fumarolic activity werenoted in the months prior to the eruption in 1968 (Al-varado 1993). The eruption of 1968 began on 29 July, themajor part of which lasted 3 days. Activity took placefrom three vents which opened on the west flank (cratersA, B and C on Fig. 1). The largest and most active ofthese, crater “A”, was �1 km west of the original sum-mit—crater “D” (Melson and Saenz 1968). Initial activityinvolved major vulcanian explosions that produced nu-merous ballistic blocks and a directed blast to the west,which probably caused most of the 80 fatalities (Melsonand Saenz 1973; Saenz and Melson 1977). A subplinianeruption column was generated, from which extensivefallout occurred at least as far as the Nicoya Peninsula150 km to the west (Saenz 1977; Melson 1984). Fol-

Fig. 1 Map of Arenal volcanoshowing the extent of lavasformed since 1968 and pyro-clastic flows formed at varioustimes since 1987. Location ofdetailed maps shown inFigs. 13, 14, 15 are also shown.Letters A, B, C and D are thelocation of craters referred to inthe text. Casetta “C” and Salidadel Bosque are the sites of te-phra fall collectors

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lowing a period of 2 days quiescence, an explosion on the31 July produced an energetic pyroclastic flow, whichmoved north-west down the Tabacon valley. Pyroclasticsurges associated with this flow caused extensive fellingof trees in this region.

Craters “A” and “B” are now covered with lava andare inactive. Activity migrated to crater “C” in the early1970s, which remains the currently active crater.

Changes in crater morphology 1987–2001

Knowledge of the detailed morphology of the activesummit complex (crater “C”) is limited due to problemsof access on account of safety considerations. However,those observations that have been made indicate that theactive summit complex has changed significantly overtime.

In 1987, crater “C” was visibly lower than crater “D”by around 50 m. Since 1987 crater “C” has increased inheight. By 1996 the two craters were approximately thesame height (Barquero 1997), and by September 2000crater “C” was around 20 m higher than crater “D”.

Prior to the onset of more frequent explosive activityin 1982, crater “C” was 60 m in diameter and was oc-cupied by an active lava pool which directly fed lavaflows that moved down the flanks of the volcano (Cigoliniet al. 1984).

Aerial photographs taken in 1988 reveal that thesummit complex was �250 m from north to south and�170 m from east to west (Fig. 3a). The southern part ofcrater “C” was composed of a single, broadly circular lavapool �120 m in diameter. This was flanked to the northby a flatter, plateau area �150 m wide from east to westand �100 m wide form north to south. Strong fumarolicactivity was taking place from this plateau area. Achannel-fed lava flow issued from the north-west edge ofthis plateau area.

During 1993 the explosive activity shifted from thesouth to the north-west part of crater “C” (GVN Bulletin1993). In 1996 there were five different explosively activevents at crater “C” (GVN Bulletin 1996).

Oblique aerial photographs and observations madeduring March 2000 showed that crater “C” was approxi-mately 150 m in diameter and had a shallow amphitheatremorphology open to the west. An active crater <25 m in

Fig. 2 Photographs of typical explosions at Arenal volcano taken from the west flank in January and February 1996, showing tephraplumes rising up to �1 km above the crater

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diameter was the source of explosive activity and waslocated at the north-east margin of the amphitheatre. Thiscrater had dark drapes of fresh lava around its rim(Fig. 3b). A broad, inactive dome �50 m across sat withinthe western opening of the amphitheatre. In addition, atleast one lava flow was being extruded from the northernpart of the summit complex.

Oblique aerial photographs taken in May 2001 revealthat the summit complex (crater “C”) had a similarmorphology to that in March 2000 with a single activecrater <25 m across located on the north-east part of thesummit (Fig. 3c). An amphitheatre-shaped depression�75 m wide and �40 m deep had developed on thenorthern side of the summit complex and was probably

formed as the result of partial collapse of the summitassociated with the pyroclastic flows in March 2001. Atleast three lava flows were being extruded from thesummit complex; one to the north; one to north west andone from the active crater to the north east (Fig. 3c).

It is clear that in 2000 and 2001, and probably for afew years previous to this, there was no active lava pool atcrater “C”. Lava flows are and have been extruded bothfrom fissures round the margins of the summit area aswell as from the active crater.

Explosive activity at Arenal

Since 1968 explosive activity at Arenal has varied with anumber of periods of particularly intense activity.

Explosive activity on July 17, 1975 formed five py-roclastic flows. The first pyroclastic flow reached theconfluence of the Rio Tabacon and Rio Arenal 4 km northwest of the summit (Fig. 1). Each successive event formedflows that had progressively shorter run out distances.Deposits formed by the July 1975 pyroclastic flows arebetween 8 and 10 m thick (Alvarado and Soto 2002). TheTabacon hot springs bathing resort is built on the 1975pyroclastic flow deposits (Fig. 1). The 1975 pyroclasticflow deposits are similar to others formed between 1987and 2001. Van der Bilt et al. (1976) interpreted the July1975 pyroclastic flows to be generated by collapse of partof the crater, whereas Melson (1994) suggests a columncollapse origin for the flows.

Intense explosive activity also occurred in 1984 withaudible and visible explosions occurring at variable in-tervals between 30 and 10 min (SEAN 1984). No pyro-clastic flows were reported associated with this phase ofactivity.

Explosions occurring between 1987 and 1990 wereparticularly large and generated frequent pyroclasticflows (Fig. 4a). Reports of pyroclastic flows associatedwith explosions occurred in March, June and July 1987(SEAN 1987a, 1987b); in March and September 1988

Fig. 3 Sketch maps of the active summit crater of Arenal volcanoshown at three different times. The maps were generated from bothvertical and oblique aerial photographs. a 1988, b 2000, c 2001

Fig. 4 a Occurrence of different types of pyroclastic flows atArenal. Minor crater collapse pyroclastic flows shown travelled�1 km from the summit. b Documented explosions (either audibleor visual) at Arenal in the 14-year period from 1987 to 2001. Datapoints represent mean daily observed explosions during typicalobservation periods of 7–10 days. Error bars represent actualmaximum and minimum daily explosions during the observationperiod. Data from (SEAN 1987a, b 1988a, 1988b and 1988c, 1989;GVN Bulletins 1990a, 1990b and 1990c, 1991a, 1991b, 1991c, and1991d, 1992a, 1992b, 1993, 1994 and 1995) and author’s ownobservations. c Seismic events at Arenal recorded at seismic sta-tions: 4 km W of the summit 1987–1992, and 2.7 km NE of summit1993–2001. Data points represent mean daily frequency of seismicevents over 1-month periods. Error bars represent the maximumand minimum daily variation (where data is available). Breaks inthe data represent data gaps. Data from various SEAN and GVNBulletins 1987–2003. d Tephra mass per day per m3 versus time fortwo different collectors on the west flank of the volcano. For lo-cation of collectors see Fig. 1. Tephra was collected on broadlymonthly intervals. OSIVAM data is from various GVN bulletinsbetween 1993 and 1996

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(SEAN 1988a, 1988c); several times in April 1989 andonce in November 1989 (SEAN 1989). Detailed de-scription and discussion of one these explosions and as-sociated pyroclastic flows is given in a later section.

Explosive activity since 1990 has remained variablewith sporadic formation of large pyroclastic flows(Fig. 4a). Although rare, continuous or semi-continuouslava fountaining has been described several times be-tween 1989 and 1996 (e.g. SEAN 1989; GVN Bulletin1996). Eruption of lava flows and regular explosionsoccur simultaneously, but a trend of explosive activitybeing inversely proportional to lava output has been re-ported at Arenal (SEAN 1989). Data are scarce, althoughbetween 1992 and 1996 lava output is estimated at be-tween 0.25 and 0.39 m3/s (GVN Bulletin 1997).

Since 1968 Arenal has erupted lava and tephra of arather uniform basaltic andesite composition with phe-nocrysts of plagioclase, clinopyroxene and orthopyroxene(Reagan et al. 1987). Though homogeneous in terms oftheir bulk composition, Streck et al. (2002) demonstratethat the lavas are petrographically complex reflecting along magmatic history prior to eruption. In particular,Streck et al. (2002) describe clinopyroxene crystalsshowing different zonation patterns. Some clinopyroxenesdisplay multiple high-Cr bands indicating that they werein contact with basaltic magma several times during theirgrowth history. They argue that this reflects repeatedmagma mixing and that Arenal can be best explained as aquasi-steady-state system with periodic replenishment bybasaltic magma into a small magma reservoir. It is evi-dent that such episodes of replenishment have played acrucial role in sustaining the current eruptive phase thathas lasted since 1968. The magma is homogenised beforeeruption explaining the uniform composition of theerupted products. This is supported by the geochemistryof samples of juvenile blocks from the 1992, 1993, 1998and 2000 pyroclastic flow deposits which all show SiO2within the range 54.8–55.5 wt% and MgO 4.4–4.9 wt%.

This paper describes activity occurring between 1987and 2001, as tephra derived from explosions was col-lected on a regular basis and there is a relatively completerecord of eruptive behaviour during this period.

Nature and frequency of explosive activitybetween 1987 and 2001

Typical regular explosions at Arenal generated tephracolumns that rose to between 0.3 and 1 km above thesummit of the volcano (Fig. 2), although many explosionsproduced smaller tephra columns. Numerous ballisticblocks and bombs associated with these explosions wereejected to distances of a few hundred metres from thecrater, although in exceptional cases up to 1 km. Suchballistics were often incandescent at night. The ash frac-tion generated by the these explosions was generallycarried up to a distance of 5 km west of the crater by thetrade winds that blow from east to west. Samples werecollected from the deposits of this type of explosion.

Most explosions were accompanied by loud “jet-en-gine” type degassing sounds, however, reports indicatethat some explosions were mute with little or no soundassociated with them (SEAN 1989; GVN Bulletin 1997).The sounds of explosions at Arenal occurring between1987 and 1994 were characterised by Melson (1995) intothree types: “Chugs”—rhythmic gas emission with orwithout tephra emission; “Whooshes”—jet engine soundsaccompanying sustained tephra emission and “Explo-sions”—which lasted <2 s. “Explosions” were often fol-lowed by “whooshes” and “chugs”, which Melson (1995)interpreted as reflecting successive opening of the ventfacilitating the degassing events.

Visual observations indicate that explosions occurringover this 14-year period showed considerable variationsin frequency. Periods of continuous 24 h-a-day visualobservation (typically for 10 consecutive days) between1987 and 1993 showed strong variability in explosionfrequency (Fig. 4b) (SEAN 1988a, 1988b, 1989; GVNBulletin 1990a, 1990b, 1990c). There have been severalperiods without explosions, lasting for 5 days in 1989 andup to 3 weeks in mid 1990. These were interspersed withperiods of explosive activity that ranged from 3 explo-sions day�1 to 94 explosions day�1 in 1988 (SEAN 1989;GVN Bulletin 1990c).

Visual observations in December 1995 indicated thatexplosions typically occurred 30 min apart with up to 60explosions day�1 reported (GVN Bulletin 1996). How-ever, in early 1998 there was a marked decrease in ex-plosive activity. Our visual observations in February1998, January 1999 and 2000 recorded explosions oc-curring at between <1 and 5 day�1 (Fig. 4b). To sum-marise, although there has been a large variation in thefrequency of explosions a marked decrease in explosiveactivity took place in early 1998.

Seismicity at Arenal also shows large variations. Fig-ure 4c plots the monthly average of seismic ‘events’where reported. In addition, daily variations (whereavailable) in seismic events are also represented (Errorbars on Fig. 4c). Impulsive seismic “events” at Arenal arefrequently reported as “explosions” (GVN Bulletin 2003).Periods of actual observed explosions are rare (Fig. 4b)making any comparison between the two datasets diffi-cult. However, correlation between actual observed ex-plosions and ‘explosion’ events is not straightforward.Furthermore explosions without significant seismic sig-nals did occur, albeit rarely, and may correlate with themute events described earlier.

Tephra fall 1987–2001

Two sources generated tephra fall at Arenal volcano. Thefirst was fallout from plumes generated by explosions andthe second was from ash clouds produced by pyroclasticflows. Although large pyroclastic flows were rare, smallerflows that travelled �500 m from the summit occurredfrequently, typically every month. A significant contri-

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bution to ashfall by plumes from pyroclastic flows istherefore likely.

Tephra fall in the downwind zone typically began afew minutes following a typical explosion. Measurementsmade during this study showed that at distances between 2and 3 km west of the vent, tephra fall was initiallydominated by coarser particles (0.25–2 mm diameter).Finer material that fell as small ash aggregates 1–5 mm indiameter occurred less than 1 min after initial fallout.

Tephra collectors (small buckets) have been main-tained at a range of locations on the west flank of Arenalvolcano by OVSICORI since 1987. These collectors wereemptied on a monthly basis and the mass of tephra per m2

calculated. Rainfall at Arenal is high (380 mm/month)such that there is generally water within the collector,which minimised selective reworking of tephra by wind.Monthly tephra masses from two collectors, one 2 kmwest (Casetta C) and another 2.4 km west-south-west(Salida del Bosque-for location of sites see Fig. 1) of theactive crater are plotted (recalculated as mean dailymasses) to show the variation in tephra production withtime (Fig. 4d).

The mass of tephra (gm�2) per day at particular sitesplotted against time shows that tephra masses varied butwere highest in 1989 and 1990, with values of>250 gm�2 day�1 recorded in late 1989 (Fig. 4d). Tephramasses gradually waned to <10 gm�2 day�1 until 1993 and1994, after which they increased reaching a maximum of60 gm�2 day�1 by mid 1996. Tephra masses then de-creased markedly in late 1997. Between mid-1998 and2001 monthly tephra masses were <1 gm�2 day�1. Ex-plosive activity clearly diminished at the end of 1997 andremained low between 1998 and 2001. Between 1998 and2001 observed explosions were relatively small, and in-frequent rarely sending tephra columns >200 m above thesummit. The reduced tephra masses therefore probablyrepresent both fewer and smaller explosions.

The distribution of tephra collectors allowed samplingof a large part of the fallout area and enabled isomassmaps to be drawn for the ash fraction of tephra fall, i.e.<2 mm (Fig. 5). Although the tephra collectors are posi-tioned mainly on the western flank, the consistent tradewinds from east to west should minimise any samplingbias. These isomass maps indicate that the mean dispersalaxis of tephra varies slightly between west and west-south-west. Isomass maps were calculated for a number ofmonths across the reporting period. Assuming eruptionfrequencies of 20–40 explosions day�1 for the majority ofthe reporting period, a mean explosion volume (fraction<2 mm only) can be estimated. These data yield totaltephra masses between 1 and 10 m3 per explosion. Thesevolumes do not, however, consider the coarser tephrafraction, including ballistics, which undoubtedly form asignificant proportion of material ejected during theseexplosions. Crude estimates of total volumes of individualexplosions including ballistics are probably in the regionof 10–50 m3.

Grainsize of tephra fall

Tephra fall from single explosions, cumulative falloutsampled on a monthly basis and fallout derived frompyroclastic flows were subject to grainsize analyses. Thefraction >63 mm was dry sieved at 1-phi intervals and thefine ash fraction <63 mm was analysed using a laserparticle sizer.

Tephra sampled from collectors on a monthly basisrepresents cumulative fallout between 50 and 1,000 ex-plosions. These cumulative monthly samples are of no-

Fig. 5 Selected isomass maps for three periods a June 1990, bJanuary 1997, and c September 1997. The isomass maps representcumulative fallout from explosions over one month. Values aregrams per m2 per month

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tably regular grainsize with modes and median diametersbeing relatively constant at between +1.7 and +2.5 phi forthose collected at the same location (Fig. 6). The rela-tively similar grainsize may reflect the averaging effectfrom the large number of explosions. The grainsize of thecumulative monthly tephra varies systematically relativeto the location within the dispersal axis. Samples frommore proximal sites 2 km west of the vent (Casetta C) areconsistently coarser grained and better sorted than thosefrom more a distal and peripheral site 2.6 km west-southwest of the vent (Salida del Bosque, Figs. 7 and 8). Inaddition, 85% of the samples from the Salida del Bosquesite show bimodality with one strong mode at +2 phi andanother weaker mode at +6 phi. Less than 30% of thesamples from Casetta C show such bimodality. Bi-modality is particularly marked for sites that are off thedispersal axis to the south (Fig. 8, e.g. Segundo Claro andVuelta).

Tephra fall was collected from single explosions usinglarge (>1 m2) sheets of plastic spread on the ground.Sufficient tephra was collected from only four singleexplosions for grainsize analysis. Single explosions aremore variable in grainsize, median diameters range from+0.25 to +1.75, but generally coarser than the cumulativesamples (Fig. 6). This is probably partially a product ofthe variable range of locations from which single explo-sions were sampled and that the sampled explosions werelarger than normal.

Ash plumes formed by pyroclastic flows producedashfall that was collected in the days immediately fol-lowing the events. This ash has median diameters be-tween +5 and +6 phi, and is considerably finer grainedthan that formed by typical explosions (+0.25 to +1.75).

One of the single explosions sampled was associated witha pyroclastic flow and it also has a slight minor mode at+6 phi (Fig. 9). One possibility is that the smaller secondfiner-grained mode present in many of the monthly cu-mulative tephra samples is a product of fallout from ashplumes derived from pyroclastic flows. The bimodalitymay therefore be a product of tephra fall from twosources. The coarser +2 phi mode represents fallout di-rectly from the explosions whereas the finer +6 phi moderepresents fallout from ash plumes derived from pyro-clastic flows.

Bimodality of tephra fall has also been identified atother volcanoes (Carey and Sigurdsson 1982; Bonadonnaet al. 2002). At Soufriere Hills volcano, Montserrat, bi-modality of tephra fall was attributed to a number offactors including: (i) Multi source tephra fall, i.e. frompyroclastic flows and explosion plumes, (ii) aggregationcausing premature tephra fallout and (iii) multiple si-multaneous sources of tephra fallout from plumes at dif-fering heights (Bonadonna et al. 2002). As ash plumes atArenal were restricted to relatively low level <5 km itseems unlikely that multiple plumes at different altitudesis responsible for the bimodality at Arenal. In fact weobserved fine ash aggregates during fallout at Arenal. Asimilar aggregation process probably accounts for theincrease in the fine-grained mode of fallout in the off axisregion. Therefore, the bimodality of these deposits is in-terpreted to be a product of ash aggregation and falloutfrom ash plumes generated by pyroclastic flows.

Components of tephra fall

Tephra, both from cumulative monthly samples and sin-gle explosions, was separated into crystals, lithics (alteredgrains), juvenile glass fractions and fragments that werecomposed of approximately equal proportions of crystalsand glass. Tephra <1 mm in size is composed of >50%glass particles, whereas crystals and particles of glasswith crystals form equal proportions of the remainder.Lithic fragments were rare forming �1–2% of particles(Fig. 9). Glass fragments of the most abundant fractions(+1 and +2 phi) were examined with a scanning electronmicroscope to determine the range in clast morphologies(Fig. 10) and allow comment on the nature of the frag-mentation mechanisms involved in their generation.

Scanning electron microscopy allowed subdivision ofthe glassy fraction into four types: vesicular, fluidal,blocky and clasts that showed both blocky and fluidalportions (Figs. 9 and 10). The blocky type forms the mostabundant clast morphology constituting between 50 and71%; part fluidal and part blocky types form between 21and 40%; wholly fluidal types form up to 6% and vesic-ular clasts up to 5% (Fig. 9). Proportions of different clasttypes for single explosions were similar to that of cu-mulative monthly samples.

The different clast morphologies are thought to re-present the nature of the state of the magma on frag-mentation. Vesicular clasts form from vesicular, degas-

Fig. 6 Grainsize median diameter versus sorting plot for fallouttephra at Arenal. Both single explosion and cumulative monthlysamples are shown. Pyroclastic flow and surge deposits are alsoshown for comparison

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sing magma. Fluidal clasts represent wholly moltenfragments of magma. Blocky clasts are interpreted asbeing cool, rigid parts of the magma that had degassed,and part blocky-part fluidal clasts represent fluidal, mol-ten magma that was fragmented as the clast cooled duringeruption. This interpretation indicates that the fragmen-tation process was sequential rather than instantaneous.Therefore, although these are described as explosions,fragmentation of tephra occurred over a period of timeprobably several seconds.

Pyroclastic flow forming events

Pyroclastic flows have been generated at Arenal both bycolumn collapse and by collapse of the crater wall. Be-tween 1987 and 1989 pyroclastic flows were generatednumerous times by column collapse, whereas in August

1993 (see Alvarado and Soto 2002), May 1998, August2000 and March 2001 they formed by crater wall collapse(Fig. 4a) or lava flow front collapse in February 1992. Inthis section, we describe major pyroclastic flow-formingevents that are well constrained and documented. Severalsmaller pyroclastic flows have been produced at othertimes, such as October 1999 (shown as minor cratercollapse on Fig. 4a) for example (GVN Bulletin 2000),but the events were not well documented are not con-sidered further here.

1987–1989 column collapse events

Several times between 1987 and 1989 pyroclastic flowswere formed immediately following explosions. Notableevents occurred on March 18, June 29, July 13 and 17,1987 and on March 11, June 16 and September 17, 1988,

Fig. 7 a Grainsize histogramsfor cumulative monthly tephrafall deposits. Pairs of his-tograms compare the cumula-tive monthly tephra fall forCasetta “C” (black) and Salidadel bosque (grey) sampled atthe same time. b Mdj versussorting for cumulative monthlytephra samples. Tie lines con-nect samples collected fromdifferent sites at the same time.c Grainsize histograms oftephra fall generated by ashplumes from pyroclastic flows

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several times in April 1989, and once in November 1989(SEAN 1989). All these pyroclastic flows were associatedwith marked explosions and photographs demonstrate thatthey formed by fall back or collapse of tephra onto theupper flanks from short explosion columns.

We use the well-documented pyroclastic flows formedon March 11, 1988 to illustrate this generation mechanism(Fig. 11). These pyroclastic flows can clearly be seen tobe associated with an explosion. Initial views show aneruptive column rise to �300 m above the crater. Materialfalls from this column onto the summit region (arrow inFig. 11a), developing pyroclastic flows that divide intotwo distinct lobes, a larger one moving to the south and asmaller lobe to the west (Fig. 11b). The two pyroclasticflows continue to move down the flanks (Fig. 11c). Thefact that many of these events formed simultaneous py-roclastic flows moving in more than one direction cor-roborates the column collapse mechanism of these flows(see also SEAN 1987b). Generally, these pyroclasticflows descended the southern and southwestern flanks ofthe volcano and had run outs between 1 and 2 km. Thelocation of the active crater at this time in the southernpart of the summit is likely to have controlled this pre-ferred distribution to the south and southwest.

Crater-wall collapse events

Several crater wall collapse events have generated sig-nificant pyroclastic flows. Here we describe important

events related to each of these such as precursory activityand observations of the pyroclastic flows. A notablefeature of crater wall collapse pyroclastic flows are that incontrast with column collapse events they always propa-gate in a single direction emerging from a specific regionon the crater wall.

August 28, 1993 events

In early August 1993, OVSICORI measurements of theEDM reflector on SW flank of Arenal showed 15 cm ofshortening, and a reflector on the southern flank length-ened by 8 cm. This was equivalent to between 4 and 5years of deformation in 1 month and indicates that therewas significant increased localised deformation in thesummit region of the volcano in the weeks prior to theformation of the pyroclastic flows.

Around 10 days of elevated tremor occurred prior tothe 28 August. In addition, at least one small pyroclasticflow was formed on 26 August, two days prior to the mainevent, and on 28 August at around 16:00 h three smallpyroclastic flows were generated.

May 5, 1998 events

For several months prior to this event, explosive activityand lava output were low (Fig. 4a, d). No precursoryseismicity or tremor was recorded, although typical ex-

Fig. 8 Grainsize histograms for tephra fall collected from a range of different tephra collectors on the west flank of Arenal volcano. Thedeposits represent approximately one month of tephra fallout prior to February 20, 1995

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plosions had taken place earlier that day. Alvarado andSoto (2002) describe an “A” type earthquake swarm 3months prior to this eruption, which they consider pre-cursory to this eruption, however we consider this toolong a period of time to be confidently associated withthese pyroclastic flows.

Between 12:54 and 18:00 on May 5, 1998, 23 pyro-clastic flows were generated which travelled down thenorth-west flank. Photographic evidence shows ashclouds developing from the pyroclastic flows as theydescended the flanks and associated explosion columns at

the volcano’s summit are absent (Fig. 12a). Initial flowswere nearly continuous with one another and later oneswere separated by pauses of several minutes.

August 23, 2000 events

On August 23, 2001 a series of 27 seismically distinctpyroclastic flows travelled down the northern flank of thevolcano in two main batches. One began at 9:30 lastingfor 30 min, and a second began at 13:36 and continued for

Fig. 9 Grainsize and compo-nent histograms for tephra col-lected from six single explo-sions over a 9-year period. Piecharts show the different pro-portion of glass morphologiesexamined under the scanningelectron microscope illustratedin Fig. 10

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Fig. 10 Scanning electron mi-crographs of the different clasttypes identified in fallout de-posits. a Vesicular clast, b Flu-idal clast c Blocky clast, and dBlocky/fluidal clast. The whitebar in each photo is 100 mi-crons long

Fig. 11 Sequence of pho-tographs by Jorge Barquero ofpyroclastic flows formed onMarch 11, 1988 at 09:35 am.The main flow had a maximumrunout of �1 km. Arrow in (a)indicates material falling ontoNW flank

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74 min. This second batch produced longer run-out andmore voluminous flows than the first.

The pyroclastic flows travelled a maximum of 2.5 kmreaching to within 100 m of a small lake and campsite.Three people who were hiking on the flanks of the vol-cano were impacted by the surge component of the firstbatch of pyroclastic flows, two of whom later died.

Pyroclastic flow deposits

Deposits formed by several crater wall collapse pyro-clastic flows were mapped in detail. Deposits of columncollapse pyroclastic flows were not described. The mapsof the pyroclastic flow deposits show that they are typi-cally valley confined and generally lobate with positiverelief lobes typically <10 m wide and up to 100 m long(Figs. 13–15 and Table 1). The numerous discrete flowstypically formed by each event have produced stacks offlow lobes. Many lobes show a paucity in fine-ash in theirupper and outermost parts. Super elevation effects, suchas on the outside of bends, were rare and only locallydeveloped (e.g. February 1992 deposits). Upper surfacesof deposits are typically inclined at angles between 10 and20 degrees. Analysis of digital elevation models—DEM(e.g., Shuttle Radar Topography Mission—SRTM—30 mcell size) demonstrates that pyroclastic flow deposits areemplaced on slopes <20 degrees.

Similar pyroclastic flow deposits have been describedat Nguarhoe, New Zealand, (Nairn and Self 1978), Aza-ma, Japan (Aramaki 1957) although most are formed byeruption column collapse, and Colima, Mexico (Ro-driguez-Elizarraras et al. 1991; Saucedo et al. 2002).

We refer to these deposits as pyroclastic flow depositsrather than block-and-ash flow deposits, as blocks withinthem show a range of different textures with quite vari-able vesicularity (Fig. 16). Block-and-ash flows comprise

Fig. 12 a Photographs of pyroclastic flows formed on May 5, 1998viewed from the north. Note the absence of explosion columns atthe summit (crater “C”). The inactive crater “D” is to the left. bPhotograph immediately following the May 5, 1998 pyroclasticflows. Note deep canyon carved in proximal northwest flank of thevolcano

Table 1 Features of pyroclastic flows and their deposits discussed in the text. (a) Data modified from Alvarado and Soto (2002)

Date of event 24th Feb1992

28th Aug1993

5th May1998

23rd August2000

24–26th March2001

Number of flows 1 1 23 27 24Maximum runout 2 km 3.2 km 2 km 2.5 km 2 kmTotal volume (a) 1�104 2�106 5�105 2�106 2�105

Dimensions of lobes <20 m wide 10 m wide <20 m wide <20 m wide 10 m wide> 100 m long<30 m long 50 m long >100 m long >100 m long

<2 m thickLevees Some 1 m high Not observed None observed None observed None observedMax size and proportionof juvenile blocks(Size > 50 cm)

<30 m <6 m <20 m <30 m <40 m<40% <90% <5% <30% <40%

Nature of juvenile blocks Cauli-scoria Cauli-scoria Scoriaceous Cauli-glassy Cauli-scoriaDeposit surfaceinclination

11� mean 12� mean 10� mean, up to 22� 8� to 20� <20�

Proximal erosion Not known Canyon 50 m Deep<100 m wide

Canyon 20 m deep,50–100 m wide

Canyon up to1.5 km from vent.Erosion 200 m wideat 1.5 km

Associated ash fall Not known Broken tree branches3 km from volcano

0.5 cm thick3 km west

Extensive to west Extensiveto west

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clasts that are typically non-vesicular and monolithologic(Cas and Wright 1987, p. 111; Cole et al. 2002)

Lithology

Blocks of juvenile material >20 cm in size are distinctivein that they are formed by dark bulbous, variably vesic-ular basaltic andesite. Density studies from four differentcrater collapse pyroclastic flow deposits show that thejuvenile material generally ranges from 1,600 to2,200 kg m�3 (Fig. 16). The bulbous nature of the juvenilematerial typically produces a “cauliflower-like” surfacetexture, which we term cauliform (Fig. 18a). We prefer toavoid the term “bomb” used by Alvarado and Soto (2002)

to describe these blocks as “bomb” implies an explosiveballistic origin when clearly many of these pyroclasticflows were not associated with explosive activity oreruption columns. Generally the cauliform-type juvenileclasts are most abundant in the August 1993 depositslocally forming up to 90% of blocks >0.5 m in size in themost distal part of the deposit. Within deposits that haveless abundant cauliform types the juvenile blocks eitheroccur as a blocky, glassy, plastically deformed variety(August 2000) or a more rigid scoriaceous type (e.g., May1998 pyroclastic flow deposits).

The bulbous exterior of cauliform blocks is coveredwith a network of fine cracks possibly related to the rapidcooling of the exterior. In addition, many of the cauliformjuvenile blocks possess abundant slickenside impact

Fig. 13 a Maps of pyroclastic flows formed on February 1992 and b 28 August 1993. For location see Fig. 1. Cross sections verticalexaggeration �2

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marks, similar to those described from Soufriere HillsVolcano, Montserrat (Grunewald et al. 1999). Over 20separate marks, some up to 50 cm long and 20 cm wideoccur on several individual blocks. Cross sections throughcauliform type blocks reveal that the outer 1–2 cm isslightly more vesicular than the interior with a mean

density of 1,650 kg m�3. Typically, the remainder of theinterior is massive with a mean density of 1,900 kg m�3.Only rare examples of blocks with breadcrust textures arepresent with more vesicular, lower density interiors(1,500 kg m�3).

Fig. 14 Map of pyroclasticflow deposits formed on May 5,1998. For location see Fig. 1

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Large juvenile blocks were distinctive in that manyhad an oblate form, being typically <1 m high and up to6 m wide. This suggests that many of the juvenile blocksbehaved in a plastic manner with deformation of theblocks occurring on or after emplacement. Examples oc-cur where juvenile blocks have broken open the morerigid crust, exposing the hotter more fluid interior, whichflowed for �1 m in the manner of a lava flow breakout.Within the 1993 deposits some of the larger juvenileblocks, 1–3 m in diameter, form a crude imbricatestacking structure (Fig. 13b). One juvenile block, whichbecame stranded on a small, 2-m-wide pre-existing ridge,had been torn in two. The two halves of the block were

still partially connected by a characteristically low-den-sity (1,400 kg m�3) filamentous type of scoria (Fig. 17b).

Generally, the maximum block size decreased withdistance from source in the deposits studied. In theproximal region around 1.5 km from the crater, blocksbetween 5 and 10 m across were abundant. In exceptionalcases, (e.g. March 2001, Fig. 16) large juvenile blocks orslabs up to 40 m across were emplaced in more proximalregions; some were locally stranded on proximal, erodedinterfluves devoid of any surrounding deposits (e.g.,August 2000, Fig. 17c). At the distal terminus of thepyroclastic flow deposits blocks >1 m in size were rare(e.g., August 2000 and March 2001).

Fig. 15 Map of pyroclasticflow deposits formed on August23, 2000 and March 26 and 27,2001. For location see Fig. 1

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More massive, pale-grey, non-vesicular basaltic an-desite blocks that range from 2,300–2,800 kg m�3 indensity (Fig. 16) were also present and are considered tobe mainly lithic. Such massive lava blocks showed onlyrare slickensides compared to the juvenile blocks.

Transitional blocks that often contained both the ju-venile and the lithic facies with intimate relations betweenthe two occurred in a number of forms in all the pyro-clastic flow deposits studied. Some had notably sharpcontacts between the massive and more scoriaceous parts

(Fig. 18a) and one block showed the more fluidal lithol-ogy following an intricate fracture. Others transitionalblocks showed more gradational boundaries (Fig. 18b).Some blocks contained fragments of massive, lithicblocks enclosed with cauliform juvenile material. There is

Fig. 16 Histograms showing the range in density of clasts >20 cmwithin the different pyroclastic flow deposits

Fig. 17 a Juvenile blocks within the August 1993 deposits showingthe characteristic cauliform texture. Scale bar is in 1 cm intervals. bJuvenile block in the August 1993 pyroclastic flow deposits. Theblock was emplaced on a pre-existing ridge (marked) and hasbroken in two. Characteristically filamentous scoria (f) connects thetwo pieces (marked–“B”). Day-pack for scale. c Isolated large(�30 m diameter) juvenile blocks (j) stranded on eroded prehistoricdeposits (e). Location of block is marked with a “B” on Fig. 15. Theblock was partially cauliform in nature and had filamentous mar-gins, parts of the block had fallen away and rests at the base of thesteep slope. Note person for scale

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abundant evidence that the juvenile and lithic materialwere in intimate contact within the source region fromwhich these pyroclastic flows were derived.

Erosion

Most of the larger pyroclastic flows eroded significantcanyons in the proximal region of the volcano, up to1.5 km from the crater (Table 1 and Fig. 12b). Analysis ofthe SRTM derived DEM indicates that the pyroclasticflows erode mainly on proximal slopes >20 degrees. TheAugust 2000 pyroclastic flows, however, eroded a 250-m-wide swath of interfluve of prehistoric pyroclastic rocksand recent lavas, removing all vegetation and soil(Figs. 15 and 17c).

A number of situations also exist where erosion oc-curred in more distal regions of the pyroclastic flows, forinstance where flows overspill the confines of the valleyand became unconfined. The May 1998 pyroclastic flowsscoured areas where they travelled over locally steepground (Fig. 14). The March 2001 pyroclastic flowsproduced strong scouring where the flows passed down anarrow valley and the 30-m-high walls of the valley have

been extensively eroded (Figs. 15 and 19a). These ex-amples indicate that marked erosion occurs where thepyroclastic flows developed strong turbulence. This oc-curred both on the steep proximal regions, where veloc-ities were high and also due to topographic variationssuch as valley constrictions.

In most cases after the generation of the pyroclasticflows in the days following lava issued over the notchformed by the collapsed crater wall and down the canyoncarved by the pyroclastic flows (see Alvarado and Soto(2002) for more information).

Temperatures of the deposits

A number of juvenile clasts in the flows showed evidencefor having been locally partially molten on emplacementand must have had temperatures approaching 1,000�C.Abundant evidence, however, indicates that the sur-rounding deposits were clearly much cooler. This is par-ticularly clear in the August 2000 pyroclastic flow de-posits that showed large variations in the extent of car-bonisation of wood. Some trees that had been incorpo-

Fig. 18 a Transitional block within 1993 deposits showing sharp,intimate contacts between juvenile material (J) and more lithic parts(L). The more fluid textured material appears to have filled afracture within the lithic part. b Transitional block showing gra-dational contacts between the juvenile (J) and lithic parts (L)

Fig. 19 a View looking toward the volcano up the March 26, 2001pyroclastic flow deposits. Note the confined, narrow valley thewalls of which are heavily scoured (marked “S” on Fig. 15). Treefelling caused by pyroclastic surges pinches out as the valleywidens towards the camera. b Tree felling at the margins of theAugust 2000 pyroclastic flow deposits. Note the variable orienta-tion of breakage directions

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rated into the pyroclastic flows were extensively car-bonised while others, <10 m away showed no carboni-sation even on the outer surface. This indicates large localtemperature variations between 200 and 400�C within theflows and the resulting deposits. Wide temperature dif-ferences are likely to be a function of the variation in therelative abundance of cold lithic blocks and hot juvenileblocks in this heterogeneous deposit.

Pyroclastic surges

Many of these pyroclastic flows travelled over and wereemplaced on recent lava flows with little or no vegetation,e.g. February 1992 and August 1993. Consequently, forthese pyroclastic flows it is difficult to assess the extent ofpyroclastic surges associated with them. However, pyro-clastic flows formed in 1998, 2000 and 2001 impactedprimary rainforest and their effects on the vegetation al-lows assessment of the extent and nature of the associatedthe pyroclastic surge component (Figs. 14 and 15).

May 5, 1998

The vegetation immediately adjacent to these pyroclasticflow deposits was largely unaffected (Fig. 14). Flatteningof vegetation and tree felling in a zone a few metres widefrom associated pyroclastic surges occurred only locallyin some areas. This indicates that pyroclastic surges wereessentially absent from the distal margins of these pyro-clastic flows (final 0.5 km). Partial scorching of vegeta-tion along the margins of the pyroclastic flow was evidentand is attributed to the hot, vertically lofting ash cloudsthat were generated from these pyroclastic flows.

August 23, 2000

Extensive felling of trees at the margins of these pyro-clastic flow deposits demonstrates that associated pyro-clastic surges were abundant (Figs. 15 and 19b). At thelateral margins of the pyroclastic flow deposit between 50and 100% of trees were felled in zones up to 100 m wide.The orientation of felled trees at flow margins was par-allel or slightly oblique to the flow direction (Fig. 15).Exceptions occur where the surges encountered distinctrelief, such as abrupt scarps. At these locations the ori-entation of tree felling became locally quite variable(Fig. 15, point “x”). Locally, tree felling by pyroclasticsurges extended 200 m beyond the pyroclastic flow ter-minus where <50% of trees were felled. The orientationof felled trees beyond the distal limit of the flow was alsoconsiderably more variable. Single trees often hadbranches broken in several directions (Fig. 20b). Beyondthe flow terminus trees were often broken off �2 m abovethe ground surface and trees were not scoured and lichenand bark were totally unaffected by the affects of heat.

On the north-west margin of the deposits a pre-existingtrack extended away from the flow terminus parallel toflow direction north west through the forest. Trees im-mediately either side of this track were felled generallyperpendicular to the flow direction (Fig. 20a). Treeswithin the surrounding forest were totally unaffected.Felled trees are not scoured and lichen and bark weretotally unaffected. In addition, there were no significantsurge deposits in this area.

These varied tree blow-down directions at the beyondthe flow terminus are best explained as being related tohelical vortices that extended beyond the flows. In at leastone instance the vortex was not associated with any sig-nificant surge (i.e., one that produced a deposit or burnedvegetation) and extended away from the flow terminusdirected down a forest track. The direction of tree blow-down is not related to the density of trees as both areas ofsparse tree cover and dense rainforest show variableblow-down direction. In other places there were apparent

Fig. 20 a Trees felled at the margins of a track (marked “T” onFig. 15) that extends NW from the distal limit of the August 2000pyroclastic flow deposits. b Tree beyond the NE terminus of theAugust 2000 pyroclastic flow deposits. Note branches are brokenwithout a preferred orientation

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Fig. 21 Cartoon summarising the main features discussed in thetext. Insets show: pyroclastic flow generation mechanisms; Ex-plosion mechanism; Transition between erosion and deposition in

pyroclastic flows; Airpush at front of some pyroclastic flows.Sketches are not to scale and are based on observations

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localised down drafts or vortices that caused the treeblow-down without a preferred orientation.

March 24–26, 2001

Pyroclastic surges occurred in many places at the marginsof these flow deposits (Fig. 15). Within one section theflow descended a narrow, steep-sided valley 200 m long.The steep valley sides are heavily scoured and at themargins tree felling by pyroclastic surges is abundant.Beyond the steep-sided valley evidence for pyroclasticsurges, such as tree felling rapidly pinches out (Figs. 15and 19a). In the final 200 m runout of the pyroclastic flowthe slope gradient steepens and the valley turns to thenorthwest. At this point pyroclastic surges that felled treeshave extended >100 m beyond the terminus of the denseflow (Fig. 15).

Generally these pyroclastic surges were derived fromthe dense pyroclastic flows in the manner of ash cloudsurges. However, these surges were limited in distal ex-tent and tree felling beyond the flow terminus appears tobe related to vortices within an air push that preceded thedense pyroclastic flow (Fig. 21).

Discussion and conclusions

Explosive activity at Arenal volcano has changed con-siderably over the 14-year period that this study covers.Explosive activity at the beginning of this period between1987 and 1989 involved relatively large explosions andcollapse of some of these explosion columns or fountainsgenerated pyroclastic flows (Fig. 21). The quantity oftephra fall waned in the early 1990s. As the frequency ofexplosions was maintained until 1994, we interpret thisdecrease in tephra fall to be due to a reduction in explo-sion size. The nature of the glass component of the tephrafall produced by the explosions indicates that the explo-sive activity at Arenal has been typically vulcanian wherethe majority of the juvenile component was derived froma rigid degassed magma body with only a minor portionof tephra showing evidence for having been molten oneruption (Fig. 21).

Deposits formed by all crater wall collapse pyroclasticflows are similar morphologically and were emplacedonly on slopes <20 degrees (Fig. 21). All the flows de-velop deposits that are distinctly lobate with flat orslightly convex upper surfaces with steep sides and flowfronts. Levees are generally not well developed. Theproportion of juvenile to lithic material varies consider-ably both within individual flows, and between crater wallcollapse flow deposits formed at different times. Thedeformed nature of many blocks clearly shows that thejuvenile material behaved in a plastic manner duringtransport within the crater collapse pyroclastic flows.Cauliform blocks were clearly fragile and have beenbroken or deformed either during or immediately afteremplacement. The hot, partially molten nature of many of

the juvenile blocks promoted the formation of slickensidefriction marks, as demonstrated by their abundance onjuvenile blocks compared to lithic blocks that only showrare impact marks.

Transitional blocks containing parts of both juvenileand non-juvenile material provide important informationabout the nature of the source region of crater wall col-lapse pyroclastic flows. These transitional blocks suggestthat there was an intimate combination of new magmaalong fractures and fissures within the existing edifice inthe summit region. Magma may breach the surface any-where within the active summit region of the summit andfeed lava flows. Rather than a simple open lava pool, itseems that with the waning of explosive activity, crater“C” has become more complex within lava flows beingfed from a broad “active” region in the northern part ofthe summit (Fig. 21).

Although explosive activity waned during the report-ing period lava output has been maintained. Pyroclasticflows have continued to be generated by a process ofcrater wall collapse. Instability of the crater region mayhave been the result of lava flow accumulation in thesummit region or due to internal magma pressurizationeffects, e.g. in 1993 when ground deformation of thesummit occurred prior to pyroclastic flow formation. Onepossibility is that the summit is becoming less stablesimply due to continued growth and the accumulation oflava in that region. In the late 1980s and early 1990s,more frequent explosive activity may have removed lavaat the summit, inhibiting excessive accumulation. Thereduction in explosive activity since 1998 may have al-lowed lava to accumulate at the summit relatively un-checked, except for the partial collapses that generate therecent pyroclastic flows. As a consequence, the hazardfrom explosions may have diminished, however the haz-ard associated with pyroclastic flows has remained. Therehas also been a shift in the direction of pyroclastic flowsfrom a southerly or south westerly direction for many ofthe explosion related pyroclastic flows formed in the late1980s, to west in 1993, north west in 1998, and north in2000 and 2001. This has important hazard implications, asfuture pyroclastic flows are likely to continue in anortherly direction, which is toward the more populatedareas on the volcanoes northern flanks.

Acknowledgments The University of Luton is thanked for fundingPDC and AMD for fieldwork. The staff at OVSICORI are thankedfor their openness in sharing data, ideas and also their assistancewith field logistics. Stuart Gill helped in the drafting of some fig-ures. Guido Giordano, Raffaello Cioni and an anonymous reviewerare thanked for comments that improved the manuscript

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