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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”.
The Rock Cycle
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
volatiles, dissolved
gases, that escape at
the surface and so
are not present in
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
•both consist of melt
and possibly solid
crystals
Rate of cooling and crystallization
A 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
A 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
If lava cools too rapidly,
the orderly repeating
crystalline structure,
associated with
minerals, does not
have time to form.
This obsidian is not
crystalline, instead, it is
a considered a glass.
Igneous Rock Classification Criteria #1:
Texture
Overall appearance of rock based on size and
arrangement of mineral crystals
– Texture indicates the environment in which the rock
crystallized since crystal size is determined by the
cooling environment
– Extrusive (volcanic) rocks cooled quickly at the
surface from lava
– Intrusive (plutonic) rocks cooled slowlyh at
depth from magma
– Includes secondary factors such as vesicles (cavities
left by escaping gas)
Aphanitic (fine-grained)
• Rapid rate of surface
cooling results in
microscopic crystals
• The top sample is
rhyolite, which has the
same compostion as
granite
• Aphanitic rocks may
exhibit a secondary
vesicular texture, like
this basalt, as gas
escaped from the lava.
Phaneritic (coarse-grained)
• Slow cooling in
Earth’s interior
results in visible
crystals, like this
granite
• Crystals
approximately the
same size.
Pegmatitic (very coarse-grained)
• Composed of
crystals >1 cm in
diameter
• They form late in the
crystallization stage
of a magma.
Porphyritic - large crystals embedded in
a fine-grained groundmass
• Crystallized in (at
least) 2 different
environments
• Large crystals,
phenocrysts,
crystallize first
• Matrix of smaller
crystals, the
groundmass,
crystallize last
• See sample #10.
Glassy
• When molten rock
cools too quickly, the
resulting rock lacks
crystalline structure
and is called a glass,
like the obsidian (top).
• 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 ash-sized
and the resulting rock
is called welded tuff.
– Often appear similar
to sedimentary rocks
– Larger fragments
form
Pyroclastic
Larger-sized
fragments form
other types of
pyroclastic rocks,
such as this
vesicular scoria
(see sample #7),
made from cindersized fragments.
Decide if each igneous texture below indicates an
extrusive (volcanic), or intrusive (plutonic) origin, based
on its texture:
A and C are extrusive and B is intrusive
D probably started out in an intrusive environment and
ended up in an extrusive environment
Igneous Rock Classification Criteria #2:
Mineral 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.
Ultramafic - Peridotite
• The lowest silica (SiO2)
content of the igneous
rocks
• Composed entirely of
ferromagnesian
silicates, with a
relatively high iron and
magnesium content.
• Rarely found in crust;
main constituent of the
upper mantle.
Intermediate ( andesitic) composition
• Contains at least 25 percent dark silicate minerals.
• Andesite, named after the Andes Mountains in
South America, is associated with explosive
volcanic activity.
• None in our collection 
Felsic (Granitic) composition
•Composed of primarily of light silicates
•Granite not shown here, is a common example.
•Contains up to 70% silica (SiO2).
•Major constituents of continental crust.
•Sample #10 is a felsic porphyry called trachyte.
Rhyolite
Trachyte porphyry
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 (see animation on
mantle melting)
– 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.
– See animation on mantle melting addition
of volatiles is called wet melting
Addition of volatiles (water) lowers the
melting point of subducting plate
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 of the solid rock on the left
results in a more mafic solid portion and a
more felsic magma that is free to rise.
Formation of Mafic Magmas
• Mafic (basaltic) magmas
originate from partial
melting of ultramafic rock
in the mantle.
• These magmas form at
mid-ocean ridges by
decompression melting
and at hot spots.
• Much of the oceanic crust
is formed by partial melting
of ultramafic material.
Formation of Andesitic and Felsic Magmas
These magmas originate from:
• partial melting of subducting mafic ocean crust which has
had water added to it.
• assimilation of pieces of continental crust, which is usually
felsic.
• What is the composition of the Andes of South America?