Download Lab 5 - Rhyolite, Latite, Trachyte, Glassy

Extrusive Igneous Rocks, Part 2
RHYOLITES, LATITE,
TRACHYTE AND GLASSY
EXTRUSIVE ROCKS
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IUGS Extrusive Igneous Rock
Chart
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Silica Content Classification
% SiO2
Designation
% Dark
Minerals
Designation
Example
rocks
> 66
Acid
< 40
Felsic
Granite,
Rhyolite
52 - 66
Intermediate
40 - 70
Intermediate
Diorite,
Gabbro
45 - 52
Basic
70 - 90
Mafic
Gabbro,
basalt
< 45
Ultrabasic
> 90
Ultramafic
Dunite,
Peridotite
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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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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"
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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
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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
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Rhyolite
• Apanitic rhyolite
• “Mafic” grains
are probably
smoky quartz
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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
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Rhyolite Photomicrograph
• Rhyolite porphyry photomicrograph showing
euhedral quartz and albitetwinned plagioclase
phenocrysts with a glassy
groundmass (Cross nicols;
Field of view=5mm)
• Photo by S. McKinley
• Location : Seneca property,
Harrison Mills, SW British
Columbia
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Rhyolite Domes
• Obsidian domes, Long Valley Caldera, California
• Photo M.L. Bevier
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Rhyodacite
• Extrusive igneous
• Older term for rhyolites whose feldspar
content is one-third to two-thirds
plagioclase
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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.
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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
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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
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Quartz latite
• Extrusive igneous
• Similar to latite except that quartz is an
essential mineral (>10% Q)
• Usually porphyritic
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Mammoth Mountain, California
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
• 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
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Angles - Photo by R. Forrest Hopson
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
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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 calcalkali
• Calc-alkali trachyte is more common and is
sometimes referred to as normal trachyte
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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
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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
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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
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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
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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.
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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
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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
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Pitchstone
• Extrusive igneous - exhibits a pitchlike
luster
• Volcanic glass chemically similar to
rhyolite except for higher water content, 410%.
• Color may be black, gray, olive green,
brown, or red
• Fracture is conchoidal
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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
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Pumice Photo
• Pumice is a glassy,
vesicular rock that is
very light, often light
enough to float
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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
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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
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