Magma (or lava if erupted to the surface) is composed of liquid, solid (mineral crystals) and gas. Its composition is largely controlled by its source. The image shown above is a pahoehoe basalt flow.

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Magma (or lava if erupted to the surface) is
composed of liquid, solid (mineral crystals) and
gas. Its composition is largely controlled by
its source. The image shown above is a pahoehoe
basalt flow.
2
Magmas (lavas) are subdivided largely by silica
content. As silica (SiO2) content increases iron
and magnesium content (FeO and MgO) decreases.
Note that lighter elements, such as sodium (Na2O)
and potassium (K2O) content follow the silica
trends. Elemental composition of magmas and
rocks are described in terms of oxide composition
because of their common bonds with oxygen.
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The viscosity (resistance to flow) of a magma is
controlled by its silica content and its
temperature. High silica content and low
temperature magmas tend to have higher
viscosities. The image above is an aa basaltic
lava flow that has cooled and degassed.
5
Rhyolite/dacite flows will retain steep slope
fronts because of high viscosity.
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Basaltic composition magma forms at different
tectonic settings. Basaltic magma is always
derived from a partial melt of the asthenosphere.
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Basaltic magma forms from a partial melt of the
asthenosphere. Partial melting of the
asthenosphere occurs at a depth (100-350 km)
where the geothermal gradient (actual temperature
curve) intersects the melting temperature curve
for upper mantle rock (garnet peridotite). It is
important to note that the geothermal gradient is
dependent upon pressure (depth), while the
melting temperature curve is dependent upon
pressure (depth) and composition of the
substance. Basaltic magma is composed of a dry
melt (little dissolved water) and its melting
temperature decreases with decreasing pressure
(as the magma rises). As basaltic magma further
melts it density decreases causing it to continue
to rise until it reaches the surface.
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Basaltic composition magma (lava) has a
relatively low viscosity and will flow great
distances from its vent. It is dark colored
because of its mafic content (largely pyroxene
and Ca-rich plagioclase).
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Aa Flow
Pahoehoe Flow
Pahoehoe (ropey textured, shown below) basalt
flows have a lower viscosity than aa (block
textured, shown above) flows, which have degassed
and cooled.
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Granitic magma forms from a partial melt of
continental crust, which contains dissolved
water. Dissolved water content in a magma reduces
its melting temperature with increasing pressure
(water molecules will inhibit the silicate
tetrahedra from forming bonds). Note that the
melting temperature curve for a wet granitic melt
increases with decreasing pressure (opposite of
basaltic dry melt). Melting occurs at a depth of
35-45 km within continental crust. As granitic
magma rises it solidifies (point X) as its
melting temperature increases while the
geothermal gradient (actual temperature)
decreases. Granitic composition magmas rarely
reach the surface as volcanic rhyolite flows
because of the high water content and
corresponding increase in melting temperature as
it rises towards the surface.
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Granitic composition magma is produced at
continental collision margins. As the
continental crust thickens it begins to partially
melt at depth. Igneous intrusions (plutons) form
below the mountain belts. Volcanism is rare in
continental collision boundaries.
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Granitic composition magma reaches to the surface
in Yellowstone Park because the continental crust
is being heated by upwelling magma generated from
a asthenospheric hotspot.
13
The Yellowstone Caldera (Wyoming) formed
following a very large eruption 600,000 years
ago. The rhyolite flows are very viscous and
internal gas pressures can be very high.
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Intermediate composition magma can crystallize
below the surface beneath subduction zones and
create large plutonic bodies composed of
coarse-grained igneous rock. Compositons can
range from granite to diorite. El Capitan shown
on the left is part of the Sierra Nevada
intrusive complex that formed over 90 million
years ago when a subduction zone existed along
the margin of California. The plutonic bodies
comprising the Sierra Nevada are similar to the
plutonic bodies forming under the modern Cascades.
Grano-diorite rock from the Sierra Nevada
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Andesitic magma is produced from a partial melt
of oceanic crust along subduction zones.
Introduction of water forced out of the
subducting plate lowers the melting temperature
of the upper mantle rised and may mix with
overlying continental crust.
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Mt. St. Helens is composed of intermediate
composition dacitic flows. Dacite is slightly
more felsic (has greater silica content) than
andesite.
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Because minerals crystallize at specific
temperatures certain minerals will be compatible
and form together in igneous rocks (e.g.,
olivine, pyroxene, and Ca-rich plagioclase). The
crystallization temperature is highest for
olivine and becomes progressively lower until
quartz forms last from the residual SiO2 melt.
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Differentiation of magma can occur from
fractional crystallization in the magma chamber
by partial melting. The solid phase will have a
composition that is relatively more mafic than
the residual melt phase.
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aphanitic/ porphyritic
phaneritic
Volume Mineral Composition
Igneous rocks are classified based on texture and
composition. Fine-grained (aphanitic) and
porphyritic igneous rocks form at the surface of
the earth in volcanic settings. Coarse-grained
igneous rocks form underground in intrusive
complexes. To determine mineral composition of a
given igneous, project the line vertically
downward and read the percentage numbers along
the Y-axis of the chart. For example,
rhyolite/granite (beneath the white dashed line)
is composed of 30 quartz, 45 potassium
feldspar, 15 Na-plagioclase, 10 muscovite,
biotite and amphibole. Note that porphyritic
(two-crystal sizes) igneous rocks are volcanic
and crystallize first underground in the magma
chamber and then are erupted to the surface.
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Gabbro
Diorite
Granite
Coarse grained igneous rocks crystallize slowly
underground. Composition of the rock depends
upon the source of the magma and subsequent
cooling history.
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Grano-diorite is intermediate composition between
granite and diorite. Note the quartz,
plagioclase and biotite crystals.
22
A pink granite is dominated by potassium feldspar
(pink crystals), quartz (gray glassy appearance),
plagioclase (porcelain white mineral) and biotite
(black sheets).
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Basalt is a fine-grained igneous rock that is
erupted along diverse tectonic plate settings.
Its black color and hardness is distinct.
24
A close-up image of basalt. The Ca-rich
plagioclase crystals appear gray. Small olivine
phenocrysts appear green. The olivine
phenocrysts formed first.
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Andesite is a porphyritic rock that forms at
subduction zones. The large phenocrysts are
plagioclase and the small phenocrysts are
amphibole. The gray ground mass is composed of
biotite, potassium feldspar and plagioclase.
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Rhyolite forms from very viscous silica-rich
lava. It is the fine-grained equivalent of
granite. Eruptions are typically very explosive
because of the high silica content coupled with
high gas content.
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Obsidian forms from the residual melt of a
fractionated felsic magma body and is composed
almost exclusively of silica. It is an amorphous
glass in that it does not have a crystalline
structure. The dark coloration is due to the
presence of small amounts of magnetite. Note the
characteristic concoidal fracture diagnostic of
silica.
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Pumice (felsic composition) or scoria
(intermediate or mafic composition) form when gas
bubbles are trapped in rapidly cooling pyroclasts
(air fall).
29
Gas bubbles can also be trapped in solidifying
lava flows such as this vesicular basalt shown
above. Note the presence of green olivine
phenocrysts.
30
A large pyroclastic eruption of Mount Pinatubo in
the Philippines (1992). The ash and other
volcanic derived clasts can become welded
together to form fine-grained tuff or
coarse-grained volcanic breccia.
31
Volcanic ash (tephra) derived from the Mount
Mazama (Crater Lake, Oregon) eruption 6800 years
ago.
32
Runout from mudflows (lahars) generated from the
Mount Pinatubo eruption.
33
Volcanic breccia forms from a mixture of welded
volcanic clasts, ash, and mud..
34
The morphology of a volcano is strongly
controlled by the viscosity of the eruptive
product. The Hawaiian shield volcano shown above
is composed of interlayered basalt flows. Slope
angles typcally range between 7-10 for shield
volcanoes.
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The slope angle of the of strato (composite)
volcanoes is controlled by the angle of repose
for unconsolidated pyroclasts and the viscosity
properties of the silica-rich andesite and dacite
flows. The volcanoes shown above are part of the
Aleutian Island, Alaska.
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OCEANIC CRUST
MANTLE
Shield vocanoes are composed of interlayered
basalt flows. Basaltic lava is erupted along
linear vents lying above asthenosphere (upper
mantle) hot spots.
37
Recent pahoehoe basalt flows erupted on the Big
Island of Hawaii. Mauna Loa is shown in the
background.
38
The Hawaiian Islands and Emperor Seamount chains
formed over a mantle hotspot. As the Pacific
Plated moved to the northwest, new islands form
above the hotspot. The age of the islands
becomes progressively older to the northwest.
Note that the plate changed directions 38
million years ago from a north-south direction
to northwest-southeast direction.
39
Hawaiian basalt flows can travel great distances
from their linear vent because of the low
viscosity of the flow.
40
As the oceanic plate moves away from the hotspot
the crust begins to cool causing its density to
increase. The crust subsides causing the island
to submerge relative to sea level. The volcanic
island is subject to constant wave erosion as
well. In tropical oceans limestone reefs can
form around the island where the water is
shallow. Eventually the volcano can become
completely eroded leaving a fringing reef island
defined as an atoll. Over extended periods of
time the island can become completely submerged
and form a seamount (or guyot).
41
Many hotspots exist within the Pacific Ocean
basin. Note that the linear trend of the
respective island chains reflects the direction
of plate motion (shown by black arrow). Note
that Yellowstone National Park is situated above
a hotspot.
42
Strato (composite) volcanoes form along
subduction zones where partially melted ocean
crust, marine sediments and water-enriched mantle
rock rise to the surface. The image shown above
is a cross-section of the Japanese subduction
zone.
43
Strato (composite) volcanoes form from
interlayered pyroclastic and intermediate
(andesite or dacite) lava flows. Strato
volcanoes can become larger over time with
subsequent eruptions.
44
Strato (composite) volcanoes become large with
upbuilding. Eruptions along the flank can occur
where extensional cracks can develop as magma
upwells within the magma chamber. Over time
magma can become more silica-rich through
fractionation and the volcanos life cycle can
end with a cataclysmic eruption and emptying of
the magma chamber.
45
The Cascade volcanoes are typical strato
volcanoes with slope angles between 25-35.
Mount Rainier is shown in the foreground with Mt.
St. Helens lying to upper right.
46
Crater Lake, Oregon occupies a collapsed caldera
of Mt. Mazama which had a cataclysmic eruption
6800 years ago. The lake is over 2000 feet
deep. Wizard Island is small dacite dome that
formed shortly after the main eruption.
47
1
4
Crater Lake, Oregon formed from the collapse of
Mt. Mazama following a cataclysmic eruption 6800
years ago.
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Pyroclastic cones form largely from erupted
pyroclasts (cinders). Pyroclasts solidify in the
air and fall to the ground as air-fall.
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High gas content is a key component of
pyroclastic eruptions. Pyroclastic cones can be
composed of felsic (pumice) to mafic (scoria)
compositions.
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Pyroclastic cones may collapse to form maars
following an explosive eruptive phase that
empties the underlying magma chamber.
51
Porous texture
Pyroclasts, such as the pumice lapilli shown
above, consist of porous textures derived from
gas bubbles preserved in the rock (see inset
image).
52
Pyroclastic cones attain slope angles 35, which
is the angle of repose for unconsolidated lapilli
(cinders).
53
Plateau basalts form where basalt flows are
erupted along linear rifts in terrestrial
settings, such as continental rift zones (e.g.,
East Africa) or back are basins (e.g., Columbia
Plateau).
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The East African rift zone is composed of
basaltic lava plateaus. Steam from a recent
eruption is seen in the middle of the image.
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Basaltic lava is being actively erupted along the
East African rift zone. A large extensional
fault is shown in the foreground of the image.
56
Miocene basalt flows comprise the Columbia
Plateau. The low viscosity flows were erupted
from linear rifts located near the border of
Washington, Oregon and Idaho. Some of these
flows made it to the Pacific Ocean near the
modern Columbia Gorge. The deep canyon of
Frenchmans Coulee was eroded recently by
floodwaters from the breeched ice dam occupying
glacial Lake Missoula 15,000 years ago.
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Pillow basalts on ocean floor.
palagonite
Pillow basalts form when basalt is erupted into
water, such as along a seafloor spreading margin
or into a lake. Palagonite is reddish-orange
clay mineral that forms from rapidly weathered
basaltic glass. It often surrounds pillow
structures.