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Igneous Rocks
The Rock Cycle
The continuous
and reversible
processes that
illustrates how
one rock changes
to another.
“One rock is the
raw material for
another”.
Rock Cycle Processes – Crystallization
Rock Cycle Processes - Weathering
Rock Cycle Processes - Lithification
Rock Cycle Processes Metamorphism
Magma and Lava
Differences
• magma is in the
interior of the earth;
lava is at the surface.
• magma contains
dissolved gases that
escape from lava.
• magma cools very
slowly; lava cool
relatively rapidly,
leading to differences
in crystal size.
Similarities
• Parent material of
igneous rocks
• Forms from partial
melting of rocks at
depth
Rate of cooling and crystallization
process
Slow cooling rate
promotes an
interlocking
mass of mineral
crystals, all
visible to the
naked eye. This
granite
crystallized from
slow-cooling
magma.
Rate of cooling and crystallization
process
Rapid rate of
cooling doesn’t
allow the mineral
crystals to grow
large enough to
see with the
naked eye. This
basalt crystallized
from lava.
Rate of cooling and crystallization
process
If lava cools too rapidly,
the regularly
repeating crystalline
structure does not
form. This obsidian is
not a crystalline solid,
it is a glass.
Igneous Rock Classification Criteria:
Texture and Mineral Composition
• Texture – overall appearance of rock based on
size and arrangement of mineral crystals
• Texture indicates the environment in which the
rock crystallized
• Crystal size primarily determined by rate of
cooling
– Extrusive (volcanic) rocks cooled at the
surface from lava
– Intrusive (plutonic) rocks cooled at depth from
magma
• Includes secondary factors such as vesicles (gas
bubbles) and volcanic inclusions (pyroclasts).
Aphanitic (fine-grained)
• Rapid rate of
cooling
• Microscopic
crystals
• May contain
vesicles (holes from
gas bubbles)
Phaneritic (Coarse-grained)
• Slow cooling
• Crystals can be
identified without a
microscope
• Crystals
approximately the
same size
Porphyritic – large crystals
embedded in a fine-grained matrix
• Minerals form at
different
temperatures as well
as differing rates
• Large crystals, called
phenocrysts, are
embedded in a
matrix of smaller
crystals, called the
groundmass.
Glassy
• Very rapid cooling of
molten rock
• Resulting rock lacks
crystalline structure
• A glassy-textured rock
can also have a
vesicular texture, like
the pumice shown in
the picture.
Pyroclastic
– Various-sized
fragments ejected
during a violent
volcanic eruption
– the most common
fragment is ashsized.
– Often appear more
similar to
sedimentary rocks
Decide if each igneous texture below indicates an
extrusive (volcanic), or intrusive (plutonic) origin, based
on its texture:
Igneous Composition
Igneous rocks are composed primarily of
silicate minerals
• Dark (ferromagnesian) silicates
–
–
–
–
Olivine Group
Pyroxene Group (Augite)
Amphibole Group (Hornblende)
Biotite Mica
• Light (nonferromagnesian) silicates
– Quartz
– Muscovite mica
– Feldspars
Mafic (Basaltic) composition
– Composed of
ferromagnesian
silicate minerals and
calcium-rich feldspar
(e.g. labradorite).
– Approximately 50%
silica (SiO2) content.
– More dense (heavy)
than granitic rocks
– Comprise the ocean
floor and many
volcanic islands,
although also found
on continental crust
as lava flows, and
intrusive bodies.
Felsic (Granitic) composition
•Composed of primarily of light
silicates.
•Contains up to 70% silica (SiO2).
•Major constituents of continental
crust.
Rhyolite
Granite
Intermediate ( andesitic) composition
• Contains at least 25 percent dark silicate
minerals.
• Andesite associated with explosive volcanic
activity.
Igneous compositions
• Ultramafic composition
– high in magnesium and iron.
– Composed entirely of ferromagnesian
silicates.
– Rarely found in crust; main constituent of
the upper mantle.
Mineralogy of igneous rocks
Origin of Magma
• Earth’s crust and upper mantle primarily
composed of solid rock.
• Earth’s outer core is considered molten,
but magma has the same composition as
mantle and crust, not iron core.
• Geologists conclude that magma
originates when solid rock of crust and
mantle melts.
• What causes this to happen?
Role of Heat: Geothermal gradient
• Rocks in lower
crust and
upper mantle
are already
near melting
points.
• Any additional
heat (e.g.
basaltic
magma
beneath silicarich rocks)
may induce
melting.
What causes rock to melt
• Role of pressure
– Melting point increases with depth due to
increased pressure, so rocks that would
melt on the surface remain solid at depth.
– Reducing the pressure lowers the melting
temperature; decompression melting
occurs.
– Occurs at divergent boundaries, where
rock is buoyant and ascending so pressure
is low.
Decompression melting
Figure 3.14
What causes rock to melt
• Role of volatiles (i.e. water and
dissolved gases)
– Volatiles (primarily water) cause rocks to
melt at lower temperatures.
– This is particularly important where wet
oceanic lithosphere descends into the
mantle.
Bowen’s Reaction Series: Systematic
crystallization of silicate minerals based on
their melting points
• High temperature silicates have high melting
points (up to 1200 degrees C):
– first minerals to crystallize from molten rock, last
to melt from solid rock.
– Includes ferromagnesian silicates and Ca-rich
plagioclase feldspar.
Bowen’s Reaction Series: Systematic
crystallization of silicate minerals based on
their melting points
• Low temperature silicates have
“low”melting points (as low as 750
degrees C
– Last to crystallize from molten rock, and
first to melt from solid rock.
– non-ferromagnesian silicates, Na-rich
plagioclase feldspar and K feldspar.
Partial Melting and Magma
Formation
Partial melting – incomplete melting of rocks
due to differences in mineral melting
temperature
• Low temperature minerals melt first and form a
magma which migrates upward.
• Low temperature minerals are the ones with a high
silica content (quartz, feldspars etc.)
• Therefore, the product magma or rock of partial
melting always is more silica-rich (i.e. granitic)
composition than the parent rock.
Partial Melting and Magma
Formation
• Formation of basaltic magmas
• Most originate from partial melting of ultramafic
rock in the mantle.
• Basaltic magmas form at mid-ocean ridges by
decompression melting or at subduction zones.
• Formation of andesitic or granitic
magmas
• Originate from partial melting of mafic or andesitic
material
• Assimilation of surrounding crust as magma rises