1

Extrusive Igneous Rocks, Part 2

RHYOLITES, LATITE, TRACHYTE AND GLASSY

EXTRUSIVE ROCKS

2

IUGS Extrusive Igneous Rock Chart

3

Silica Content Classification

% S iO 2 D esigna t ion % D arkM ine rals

D esigna t ion Exam p lerocks

> 66 A c id < 40 Fe ls i c G ran i te ,Rhyo li te

52 - 66 In term ed ia te 40 - 70 In term ed ia te D io r i te ,G abbro

45 - 52 B as ic 70 - 90 M afic G abbro ,basa l t

< 45 U l trabas ic > 90 U l tram afic D un i te ,P er ido t i te

4

Naming aphanitic rocks

• In naming aphanetic extrusive rocks, the name is often based only on a few phenocrysts. This is why a mafic name prefix is often used, for example hypersthene andesite. A better name is hypersthene phenoandesite.

• Since the felsic minerals are invisible, the rock could actually be a hypersthene dacite, or hypersthene rhyodacite.

5

Naming aphanitic rocks, cont.• In formal writing, it is often wise to indicate the

origin of the name you assign. Examples: 1. The field name of this rock is hypersthene andesite. 2. Based on a careful petrographic examination in hand

specimen and thin-section, this rock is a hypersthene dacite. 3. It is understood that field names, particularly of

aphanetic extrusives, may be in error. Names based on thin section examination should be accurate.

6

Silica-rich magma• When a magma is rich in silica, it is more viscous than if it is

poor in silica• Within the liquid, the individual SiO4 tetrahedra may link to

form chains• As these chains lengthen, they twist around each other, and

make it more difficult for the magma to flow• Such magmas often crystallize at depth• Occasionally, silica-rich magmas do reach the surface and

may erupt explosively• Silica-rich magmas also crystallize at lower temperatures

than mafic magmas• Granitic/rhyolitic magmas typically exist to as low as 800°C,

sometimes lower

7

Wet magmas

• In addition, high silica magmas are generally “wet”

• At depth, they contain a dissolved aqueous phase

• If they get close to the surface, this phase may separate, with a concurrent large increase in volume

• This can lead to explosive eruptions

8

Tephra

• A general term for all pyroclastics of a volcano

• A pyroclast is an individual particle ejected during a volcanic eruption, and is usually classified according to size

9

Volcanic ash and tuff

• Ash = Fine pyroclastic material (under 2.0mm diameter; under 0.063mm diameter for fine ash)

• Term refers to the unconsolidated material

• Tuff is used for its consolidated counterpart

10

Tuff• Tuff is a general term for all consolidated

pyroclastic rocks, especially those who fragments are less than 2 mm across

• The IUGS definition is a pyroclastic rock whose average pyroclast size is less than 2 mm.

• Course tuff is 1/16 mm to 2 mm • Fine (or dust) tuff is less than 1/16 mm

11

Tuff texture

• Tuffs are explosively erupted volcanic material that is consolidated and lithified after deposition

• Tuffs may contain lithic fragments, glass shards, and/or broken mineral grains and have pyroclastic texture

12

Tuff photomicrographs• The photos at left (CN above,

PP below) show lithic crystal tuffs containing twinned, broken plagioclase clasts, and altered lithic clasts (right side of photos), in a matrix of very fine-grained material

13

Welded Tuff

• Extrusive igneous

• Siliceous tuff indurated by the welding together of glass particles under the combined action of heat retained by the particles, the weight of overlying materials, and hot gases

• Welded tuffs are often banded or streaked

14

Welded tuff photomicrographs• The pictures at left show above a

CN view showing extinct glass matrix, lithic clasts, and broken plagioclase grains and (below) a PP view of a highly welded tuff with numerous glass shards and broken plagioclase grains

15

Welded Tuff Viewing• Welded tuffs are formed during violent volcanic

eruptions; gas-charged material is expelled and deposited while still partially molten, so that the droplets become attached, or "welded" to one another

• Typically, the weight of overlying tuff flattens pumice fragments and produces aligned grains and lithic fragments

• Tuffaceous textures are commonly best viewed in plane polarized light, particularly if the matrix is glassy, so that grain outlines can be seen

16

Welded Tuff Photomicrogaphs• The photos at left show welded

tuffs with irregularly shaped, light gray glass shards and mashed pumice clasts in a glassy matrix

17

Rhyolite• A group of extrusive igneous rocks, typically porphyritic

and commonly exhibiting flow texture, with phenocrysts of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass; also, any rock in that group; the extrusive equivalent of granite

• Rhyolite grades into rhyodacite with decreasing alkali feldspar content and into trachyte with a decrease in quartz

• Term coined in 1860 by Baron von Richthofen (grandfather of the World War I aviator). Etymol: Greek rhyo-, from rhyax, "stream of lava"

18

Rhyolite, cont.• Extrusive igneous , sometimes hypabyssal• Aphanitic, light-colored rock • Color ranges from white or yellow to pink or reddish• Often porphyritic, usually containing phenocrysts of alkali

feldspar (sanidine, anorthoclase, or orthoclase) - other phenocrysts include quartz, biotite, rare hornblende or pyroxene

• Groundmass may be almost completely glassy; although in older rhyolites devitrification may have occurred, resulting in a fine-grained mixture of quartz, feldspar, and other minerals

19

Rhyolite, cont.

• The alkali feldspars phenocrysts formed at high temperature, followed by quick chilling - resulting feldspars are often metastable high temperature forms such as sanidine or anorthoclase.

• Smoky quartz, resembling a mafic, may be present

20

Rhyolite

• Apanitic rhyolite

• “Mafic” grains are probably smoky quartz

21

Rhyolite Photomicrograph• The light gray

phenocrysts at the top of the photo are sanidine; quartz grain near the bottom edge of the photo

• Flow banding is clearly visible in the glassy matrix

• Rhyolites are a volumetrically minor component of some volcanic arcs and occur as vast ash flows in some areas affected by tensional tectonics

22

Rhyolite Photomicrograph• Rhyolite porphyry -

photomicrograph showing euhedral quartz and albite-twinned plagioclase phenocrysts with a glassy groundmass (Cross nicols; Field of view=5mm)

• Photo by S. McKinley• Location : Seneca property,

Harrison Mills, SW British Columbia

23

Rhyolite Domes

• Obsidian domes, Long Valley Caldera, California• Photo M.L. Bevier

24

Rhyodacite

• Extrusive igneous

• Older term for rhyolites whose feldspar content is one-third to two-thirds plagioclase

25

Latite• A porphyritic extrusive rock having phenocrysts of

plagioclase and potassium feldspar (probably mostly sanidine) in nearly equal amounts, little or no quartz, and a finely crystalline to glassy groundmass, which may contain obscure potassium feldspar; the extrusive equivalent of monzonite

• Named by Ransome in 1898, is derived from Latium, Italy.

26

Latite

• Latite grades into trachyte with an increase in the alkali feldspar content, and into andesite or basalt, depending on the presence of sodic or calcic plagioclase, as the alkali feldspar content decreases

• It is usually considered synonymous with trachyandesite and trachybasalt, depending on the color

27

Latite texture and mineralogy• Textures may be glassy, felsitic, porphyritic, or

vitrophyric • The mineralogy consists principally of sub-equal

amounts of orthoclase and plagioclase• IUGS classification is 35-65 P/(A+P), 0-10 Q • Mafics include hornblende, biotite, or augite• Similar to trachytes but the feldspar phenocrysts range

from alkali feldspar to intermediate plagioclase

28

Quartz latite

• Extrusive igneous

• Similar to latite except that quartz is an essential mineral (>10% Q)

• Usually porphyritic

29

Mammoth Mountain, California

• Mammoth Mountain is one of the eruptive centers that developed late in the evolutionary cycle of the Long Valley caldera complex

• Located in the eastern Sierra Nevada about 250 miles north of Los Angles - Photo by R. Forrest Hopson

Composite volcano (11,050 feet) is composed of about 12 rhyodacite and quartz latite domes extruded along the southwest rim of Long Valley caldera from 200 to 50 thousand years ago

30

Trachyte• A group of fine-grained, generally porphyritic,

extrusive rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase; also, any member of that group

• Extrusive equivalent of syenite

31

Trachyte, cont.

• Trachyte grades into latite as the alkali feldspar content decreases, and into rhyolite with an increase in quartz

• Etymology: Greek trachys, ''rough'', in reference to the fact that rocks of this group are commonly rough to the touch

• There are two types of trachyte, alkali and calc-alkali

• Calc-alkali trachyte is more common and is sometimes referred to as normal trachyte

32

Trachyte Mineralogy• The mineralogy consists of perthitic orthoclase or

microcline, oligoclase (or, rarely, andesine), diopside or diopsidic augite, and accessory quartz

• Alkali feldspar phenocrysts being sanidine, anorthoclase, or soda-rich plagioclase such as oligoclase or albite

• Feldspathoid-bearing trachytes contain accessory feldspathoids

33

Alkali Trachyte Mineralogy

• Alkali trachyte is rich in sodium - alkali feldspars are strongly perthitic or anorthoclase, and plagioclase is albite or albite-oligoclase

• Mafics are iron-rich, ranging from Fe-rich biotite, amphiboles including hastingsite, arfvedsonite, or riebeckite, and pyroxenes including aegirine-augite and aegirine

34

Trachyte Lava Flow, Hawaii• Puu Waawaa is on the north rift of

Hualalai volcano - Puu Waawaa cone is the single largest volume (5.5 km3) eruption on the Island of Hawaii

• Trachyte lava flow is also associated with the cone (left margin of photo).

• Flow is as thick as 900 feet (275 m) and highway 190 climbs its edge

• Cone and flow are about 105,000 years old

• Photograph courtesy of the U.S. Geological Survey

35

Glassy Extrusive Rocks

• If magma is quickly chilled, it will not have time to crystallize - glass will form

• Glass is less stable than crystalline material of the same composition

• Over time, devitrification may occur - conversion of glass to fine crystals

36

Obsidian

• Extrusive igneous• Volcanic glass with the composition of rhyolite

that breaks with a fine conchoidal fracture and is black, gray, dark brown, red, or (rarely) green

• Often flow banded• Nearly free of water, in contrast to pitchstone. • Name from the Latin obsidianus, a rock

discovered in Ethiopia by Obsidus, ca. 280 B.C.

37

Obsidian Photo

• When obsidian breaks it fractures with a distinct conchoidal fracture

• Ancient people throughout the world used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds• Today obsidian is used as a scalpel by doctors in very sensitive eye operations

38

Pearlite

• Extrusive igneous - Also spelled perlite• Volcanic glass having the composition of rhyolite,

and a higher water content (to 4%) than obsidian• Colorless, gray, blue, green, red, or brown• Often shows numerous concentric cracks, with

onionlike partings, which impart a pearly luster• May have been derived from obsidian through

hydration by meteoritic water

39

Pitchstone

• Extrusive igneous - exhibits a pitchlike luster

• Volcanic glass chemically similar to rhyolite except for higher water content, 4-10%.

• Color may be black, gray, olive green, brown, or red

• Fracture is conchoidal

40

Pumice• Extrusive igneous• A frothy, light-colored rock with a composition near rhyolite• It may form as crusts on more compact lava, or may occur in

volcanic ejectamenta• Glassy material is filled with so many small air bubbles that

pore space may be much greater than the glass volume• Rock has low specific gravity, often floating on water• Name from the Latin, pumicis, perhaps from spuma, foam

41

Pumice Photo• Pumice is a glassy,

vesicular rock that is very light, often light enough to float

42

Pumice Photomicrograph

• Pumice fragments from an air fall tephra layer intercalated with Miocene Chilcotin Group basalt flows

• Near Deadman River B.C.• Photo M.L. Bevier

43

Vitrophyre• Extrusive igneous - Volcanic glass containing phenocrysts• Composition is rhyolitic• Phenocrysts include quartz, sanidine, minor plagioclase

(usually clear), and lesser amounts of augite, hornblende, or biotite

• The most common variety is pitchstone vitrophyre• Obsidian, perlite, or pumice vitrophyres are also possible• Name from the Latin, vitrum, glass and porphyry