Download - Catalyst

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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.
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.
Viscosity
The viscosity (resistance
to flow) of a magma is
controlled by its silica
content and its
temperature.
Why do you think
viscosity units are
graphed using a
logarithmic scale (each
unit change is a 10-fold
increase)?
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.
Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity
(remember viscosity related to high SiO2 content and low temperature).
Basaltic composition magma forms at four different tectonic settings. Basaltic
magma is always derived from a partial melt of the asthenosphere.
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 intersects the
melting temperature curve for upper
mantle rock (garnet peridotite).
km
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 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. Note that
the temperature of the rising magma is
~200° C higher than its melting
temperature at the surface (more liquid).
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 mineral
content (largely pyroxene and Ca-rich plagioclase).
Aa Flow (Think about what
you would say if you had to
walk on this aa flow (ah, ah).
Pahoehoe Flow
(Smooth word, smooth
flow).
Pahoehoe (ropey textured) basalt flows have a lower viscosity than aa (blocky
textured) flows, which have degassed and cooled.
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).
km
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 3545 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.
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.
As collisional tectonic mountain
ranges are uplifted the overlying
marine sedimentary and
metamorphic rocks are eroded
exposing the underlying granitic
plutons.
The granitic rocks of New
Hampshire and Vermont represent
old granitic plutons that were
intruded when the Appalachian
Mountains formed 300 million years
ago as North American continent
colided with proto-European
continent.
Granitic rock excavated from a quarry in Barre, Vermont formed as plutons
beneath the Appalachian Mountains when North Africa collided with
eastern North America 300 million years ago.
Granitic composition magma reaches to the surface in Yellowstone Park because the
continental crust is being heated closer to the surface (5-10 km) by upwelling basaltic
magma generated from a asthenosphere hotspot.
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. Why is explosivity of a volcanic eruption related to viscosity and gas content of the
magma?
Intermediate composition magma can
crystallize below the surface beneath
subduction zones and create large
plutonic bodies composed of coarsegrained 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 in origin to
the plutonic bodies forming under the
modern Cascades.
Grano-diorite
rock from the
Sierra Nevada
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, which rises and partially melts the overlying crust. In
an ocean-continental convergent margin it may mix with partially melted continental
crust, increasing the magma’s silica content (becomes more felsic). Mount St. Helens
dacites are more silica rich than Mt. Rainier andesite, likely due to continental source.
Mt. St. Helens is composed of intermediate composition dacitic flows. Dacite is
slightly more felsic (has greater silica content) than andesite, but more mafic
(higher Fe and Mg content) than rhyolite.
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.
Continuous reaction series occur when Ca atoms continually exchange with Na atoms
when the melt and solid phases are not separated You can have any proportionality of
Na/Ca plagioclase (feldspar) minerals. The composition of the plagioclase mineral in a
crystallizing magma changed continuously (even though the crystal structure remains
unchanged. The zoned feldspar crystal (shown on left) is composed of Ca-rich feldspar
in the center and Na-rich towards the outer rim. Zones in between are intermediate
compostion (50/50 Ca:Na).
early formed
olivine crystals
(shown as
brightly colored
crystals under
polarized light)
later formed
pyroxene crystals
(shown as gray
color under
polarized light).
Discontinuous reaction series occur when early formed crystals form entirely new
minerals through reaction with the melt. For example, olivine will crystallize first
(highest melting/freezing temperature) leaving a magma that is slightly more silica-rich
and possessing a greater proportion of other elements (Ca) that were not present in the
olivine structure. Olivine will react with the magma to produce and entirely new mineral
pyroxene. The discontinuous reaction series will occur as long as the melt and crystal
phases can react.
Differentiation of magma can occur from fractional crystallization of magma, where the
solid phase is separated from the melt phase. The solid phase will have a composition
that is relatively more mafic than the residual more silica-rich melt phase. The reverse
reaction process occurs when rock is subjected to partial melting, where the melt phase
is separated from the residual solid phase.
Garnet-peridotite
upper mantle
(asthenosphter).
Basalt/gabbro
composition magma
Andesite/diorite
composition magma
Granite/rhyolite
composition magma
As earlier formed minerals are removed from the magma by fractional crystallization, a
greater proportion of the denser elements (i.e., FeO & MgO) are removed leaving a
residual melt that is more enriched in SiO2 and lighter elements. Minerals and rocks that
form later will have a greater proportion of lighter elements (i.e., SiO2, Na2O and K2O).
A version of this diagram is
always on my midterm!
You should understand how
to use it to determine
igneous rock classification
and mineral content
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 (phaneritic) igneous rocks form underground in intrusive
complexes. To determine mineral composition of a given igneous, project a line
vertically downward and read the percentage numbers along the Y-axis of the chart.
For example, rhyolite/granite (beneath the white dashed lines) is composed of 20%
quartz, 20% potassium feldspar, 40% Na-plagioclase, <10% biotite and <15%
Gabbro
Diorite
Granite
Coarse grained igneous rocks crystallize slowly underground. Composition of
the rock depends upon the source of the magma and its cooling history.
Grano-diorite is intermediate composition between granite and diorite. Note the
quartz, plagioclase and biotite crystals.
A pink granite is dominated by potassium feldspar (pink crystals), quartz (gray
glassy appearance), plagioclase (porcelain white mineral) and biotite (black
sheets).
Basalt is a fine-grained igneous rock that is erupted along diverse tectonic plate settings.
Its black color and hardness is distinct. Do not try this unless you are trying to collect on
your life insurance policy or a big proponent of Darwin’s natural selection theory!
A close-up image of basalt. The Ca-rich plagioclase crystals appear gray.
Small olivine phenocrysts appear green. Which minerals formed first?
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. Which mineral scrystallized
first, which crystallized last?
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.
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 conchoidal (rounded)
fracture diagnostic of silica.
Porous Textured Igneous
Rocks
Pumice (felsic composition) or
scoria (intermediate or mafic
composition) form when gas
bubbles are trapped in rapidly
cooling pyroclasts (air fall).
Gas bubbles can also be trapped
in solidifying lava flows such as
this vesicular basalt shown on
the left. Note the presence of
green olivine phenocrysts.
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.
Volcanic ash (tephra) derived from the Mount Mazama (Crater Lake,
Oregon) eruption 6800 years ago.
Volcanic tuff is comprised of welded ash and fine-grained volcanic lithic and
pumaceous fragments.
This photo was taken of the Bishop Ash in eastern California.
Volcanic breccia forms from a welded, mixture of large, angular volcanic clasts
within a matrix of fine ash. This photo was takenon Lipari Island, Italy by
Raymond Coveney.