Extrusive Igneous Rocks, part 1
BASALTS, DIABASES,
ANDESITES, DACITES and
RELATED ROCKS
1
Types of Igneous Rock
2
Igneous Rock Classification
• Igneous rocks may be classified on the basis
of what minerals they contain
• A distinction is made between essential
minerals (those that must be present) and
accessory minerals (often present, but not
necessary)
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Igneous Rock Classification
• Example: Granite
 Essential: Feldspar, Quartz
 Accessory: Biotite, Hornblende
• Accessory minerals may become part of the
rock name: Hornblende andesite, because
hornblende is not an essential mineral
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IUGS Extrusive Igneous Rock
Chart
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Basalt
• A general term for dark-colored mafic igneous rocks,
commonly extrusive but locally intrusive (e.g. as
dikes), composed chiefly of calcic plagioclase and
clinopyroxene; the fine-grained equivalent of gabbro.
Nepheline, olivine, orthopyroxene, and quartz may
be present in the CIPW norm, but not all
simultaneously: nepheline and olivine can occur
together, as can olivine and orthopyroxene, and
orthopyroxene and quartz, but nepheline does not
coexist with orthopyroxene or quartz, nor quartz with
nepheline or olivine.
6
Basalt
• Often in lava flows, shield volcanoes associated with MOR eruptions to create
oceanic crust (MORB)
• May be hypabyssal in dikes or sills (compare
with diabase - columnar basalt)
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Basalt Occurrence and Mineralogy
• Basalt is the most abundant volcanic rock; it
underlies ocean floors and forms
voluminous outpourings (flood basalts) on
several continents; it is also a major
constituent of island arcs
• Principal mineral constituents: calcic
plagioclase and pyroxene (augite) and/or
olivine
• Basalts are commonly porphyritic
• Color dark gray to black (Fe content)
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Basalt
• Vesicular basalt
• Gas in magma is
trapped by rapid
cooling, leaving
vesicles
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Flood Basalts
• Twickenham flows of
the Colombia River
Basalt Group,
Washington State
• Note columnar basalt
near top of picture
• Photo: M.L. Bevier
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Lava flow
• Basaltic lava flow
• Location probably near Raton, New
Mexico
11
Columnar basalts
• Dark, fine grained volcanic
rocks, chiefly basalt and
andesite, are exposed at the
northern edge of the Fraser
Valley. These rocks formed
as lavas, shallow intrusions,
and volcanic ash deposits.
Most volcanic rocks are
resistant to erosion and form
Basalt with columnar jointing,
prominent hills in the Fraser
near Whistler, British Columbia
Valley
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Pillow basalts
• Pillow basalts form
during underwater
eruptions
• This basalt was found
in Marin County,
California
13
Pillow basalt
• Pillow lava forms when
eruptions are underwater
or when lava flows enter a
body of water
• Abundance of pillows and
pillow deltas indicates
rivers and lakes were
common features during
the formation of the
Columbia River Flood
Basalt Province
14
Pillow deltas
• Pillows can be up to 5
m long and about 0.5
to 1 m thick
• They are elongate in
the direction of flow
and dip 20-30 degrees
• Dips decrease higher
up in the sequence
15
Scoriaceous Basalt
Photomicrograph
• The black oval features in
this scoriaceous basalt are
vesicles
• Acicular, white
plagioclase laths
throughout
• Euhedral, white olivine
phenocryst at the lower
right
16
Plagioclase
• This slide showcases one of plagioclase's very
common features: its polysynthetic twinning.
17
Olivine
• Almost all of the grains in this rock are olivine
• Note the high order interference colors and the
minor secondary calcite
18
Basalt Photomicrographs
• Both photographs are under crossesd
nicols
• (Above) Gray and white plagioclase
and brightly colored , tiny
clinopyroxene grains are in a matrix of
dark glass
• Below, plagioclase laths and more
birefringent orthopyroxene are the
principal phenocrysts
19
Clinopyroxene photomicrographs
• Clinopyroxene crystal in basalt:
crossed nicols above, plane
polarized below - field of view 1.5
mm.
• Clinopyroxenes are abundant in
many intrusive and extrusive
igneous rocks, including diorite,
gabbro, the peridotite family, basalt,
and andesite - They also occur in
metamorphosed mafic rocks, and
jadeite is a constituent of some
blueschist facies rocks
20
Exsolution in clinopyroxenes
• The photos above show a
clinopyroxene grain in crossed nicols
(above) and plane polarized (below)
• Exsolution lamellae of orthopyroxene
are clearly visible in the upper photo
21
Orthopyroxene Photomicrograph
• Enstatite, hypersthene, and
ferrosilite are members of the
orthopyroxene solid solution series
• All have low birefringence (first
order red maximum), parallel
extinction, and pyroxene 87°
cleavage
• Pale green, pale red, or pale purple
pleochroism occurs in some grains
• Distinguished from clinopyroxene
by low order interference colors
and parallel extinction
• The photo shows one cleavage and
thin exsolution lamellae
22
Orthopyroxene Photomicrograph
• Photo shows fine exsolution
lamellae in orthopyroxene - field
of view 1.5 mm, crossed nicols
• Orthopyroxene occurs with
clinopyroxene in many igneous
rocks, such as gabbro, diorite,
basalt, andesite, and the
peridotite family
• It is an indicator mineral for the
granulite and pyroxene hornfels
facies of metamorphism
23
Olivine Basalt Photomicrographs
• Upper photo (crossed nichols) shows
brightly colored, equant olivine and
gray plagioclase phenocrysts in a
fine-grained matrix of glass and
clinopyroxene
• Lower photo is the same in plane
polarized light
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Diabase
• In the U.S., an intrusive rock whose main
components are labradorite and pyroxene and
which is characterized by ophitic texture
• As originally applied by Brongniart in 1807, the
term corresponded to what is now recognized as
diorite
• "The word has come to mean a pre-Tertiary
basalt in Germany, a decomposed basalt in
England, and a dike-rock with ophitic texture in
the United States and Canada"
25
Ophitic texture
• Said of the granular texture of an igneous
rock (especially diabase) in which lathshaped plagioclase crystals are partially or
completely included in pyroxene crystals,
typically augite
26
Diabase
• Hypabyssal - dikes or sills
• Generally fine to medium grained, usually
no glass
• Mineralogy similar to basalt, but pyroxene
may show zoning (especially in thin
section)
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Diabase
• Often found in dikes
or sills
• Hypabyssal - may
contain phenocrysts
• Chemically equivalent
ot basalt
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Diabase dike
• Vertical Triassic diabase
cross-cutting JuraTriassic sedimentary
strata
• Note dike offset
• Ramon Crater, Israel
• Photo Samuel Root,
College of Wooster
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Andesite
• A dark-colored, fine-grained extrusive rock
(sometimes hypabyssal)
• Microcrystalline to glassy textures - often
porphyritic (2 stage cooling history)
• Color - brown to reddish, may be gray to
greenish (not black)
• Often associated with stratovolcanoes
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Andesite Porphyry
• Contains phenocrysts composed primarily of zoned
sodic plagioclase (esp. andesine) and one or more of the
mafic minerals (e.g. biotite, hornblende, pyroxene)
• Groundmass composed generally of the same minerals
as the phenocrysts, although the plagioclase may be
more sodic and quartz is generally present
• Extrusive equivalent of diorite
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Andesite
• Andesite grades into latite with increasing
alkali feldspar content, and into dacite with
more alkali feldspar and quartz
• Named by Buch in 1826 from the Andes
Mountains, South America
32
Andesite Mineralogy
• Major minerals: plagioclase (andesine to
oligoclase - more Na than basalt)
• Mafic mineral hornblende, biotite, cpx
(diopside or augite), or opx (hypersthene Schiller luster)
• The micas and amphiboles are hydrous,
indicating a lower temperature of
crystallization
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Andesite
• Plagioclase
phenocrysts,
aphanitic
groundmass
34
Andesite Photomicrographs
• (Upper, CN; lower. PP)
• Zoned and twinned plagioclase
phenocrysts in a matrix of glass and
plagioclase microclites
• Plagioclase microlites are
subparallel, producing trachytic
texture
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Dacite
• A fine-grained extrusive rock with the same
general composition as andesite but having less
calcic plagioclase and more quartz
• According to many, it is the extrusive equivalent
of granodiorite
• Syn: quartz andesite
• The name, given by Stache in 1863, is from the
ancient Roman province of Dacia (now part of
Romania)
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Dacite Mineralogy
• Major minerals: similar to andesite, except
that quartz occurs as an essential mineral,
often as phenocrysts
• Mafic minerals are minor, usually biotite,
hornblende, or pyroxene
37
Dacite
• Quartz crystals should
be visible - this
separates dacite from
andesite in hand
specimen - hard to see
here
38
Dacitic to basaltic pumice
Location: British Columbia,
Canada
• Pumice blocks from
airfall deposits of the
Bridge River Fm.
associated with the
2350 B.P. eruption of
Mount Meager
• Photo shows one stage
of mechanical mixing
of basalt into dacite
39
Dacite boulder, Mt. Lassen
Location: Mt. Lassen National
Park, California
• Dacite boulder in volcanic
mudflow of the Devastated
Area mudflow, from the 1915
eruption of Lassen Peak
• Angular joint surfaces are
evidence that the boulder was
emplaced very hot but cooled
rapidly in place contracting to
form the angular joint
surfaces
• Hammer for scale
• Photograph by R. Forrest
Hopson
40
Dacite Photomicrograph
• Dacite is a quartz-rich
extrusive (volcanic) rock that
contains abundant sodic
plagioclase
• Dacite is a minor constituent
of many arc volcanoes
• Euhedral light gray
plagioclase (the bottom
crystal is beautifully zoned)
and rounded quartz
phenocrysts, in a very
fine-grained matrix of the
same minerals.
41
Pyroclastic eruptions
• High silica magmas typically contain a
dissolved aqueous phase
• Near the surface this aqueous phase
separates (exsolution) (forming a 2 stage
system) with a large increase in volume
• May create a bomb if pressure cannot
escape - Subaerial eruption will result
42
Pyroclastic eruptions
• Rock fragments (called pyroclasts or tephra)
are produced. These may be either:
 Lithic fragments: crystalline
 Vitric fragments: glassy
43
Pyroclastic size classification
• Fragment size Fragment name
• >265 mm
Block
• 64 - 264 mm Bomb
• 2 - 64 mm
Lapillus
•
< 2 mm
Ash
Rock name
Breccia
Agglomerate
Lapilli Tuff
Tuff
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Bombs and blocks
• Blocks are often angular to subangular they were solidified before the explosion
and simply represent shrapnel
• Bombs are rounded to sub-rounded - they
were viscous liquids when erupted, and
were rounded in flight
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