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Igneous Rocks
Text ref. Ch5 (pg. 98)
Contents
What are igneous rocks?
Classifying igneous rocks.
What are igneous rocks?
Text ref. ch.5.1 (pg 99-106)
Objectives
Compare and contrast intrusive and
extrusive igneous rocks
Describe the composition of magma
Discuss the factors that effect how rocks
melt and crystallize
Igneous Rocks
Formed from the
crystallization of magma
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Lava is magma the flows
out onto the Earth’s surface.
Extrusive igneous rocks are
fine grained and cool
quickly on Earth’s surface
(magma)
Intrusive igneous rocks are
coarse grained and cool
slowly beneath the Earth’s
surface. (lava flows)
Composition of Magma
Magma is often a
slushy mix of molten
rock, gases, and
mineral crystals.
Magma is classified
on its SiO2 content
Group
SiO2
content
Rhyolitic
70 percent
Andesitic
60 percent
Basaltic
50 percent
Factors that Affect Magma
Formation
Temperature
Pressure
Water content
Mineral composition
Temperature
Temperature gradient
compares the internal
temperature of the
Earth with depth.
Pressure
Pressure also increases with depth.
In general, as pressure on a rock
increases, so does its melting
temperature.
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Ex. Rock sample may melt at 1000°C at
Earth’s surface, but 1400°C at a depth of
100m.
Water content
Also affects magma formation.
As water content increases, melting
decreases
Mineral content
Magma containing minerals such as
olivine, calcium feldspar, and pyroxene
melt at higher temperatures
In general oceanic crust tends to melt at
higher temperatures than continental
crust, due to its Fe content.
How Rocks Melt
Rocks melt depending on their
composition
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Partial Melting
Fractional Crystallization
Partial Melting
The process whereby some minerals melt
at low temperatures while other minerals
remain solid is called partial melting.
Fractional Crystallization
The process wherein different minerals
form at different temperatures is called
fractional crystallization.
Iron-rich minerals
Left branch of Bowen’s reaction series
represents iron-rich minerals.
Undergo abrupt changes during fractional
crystallization.
Layered Intrusions
Minerals sometimes form in layers
according to Bowen’s reaction series.
Known a layered intrusion
Important formations in mining: largest
sources of platinum in U.S. found in
layered intrusions in Montana
Classifying Igneous
Rocks
Text ref. Ch.5.2, 107-113
Objectives
Classify different types and textures of
igneous rocks.
Recognize the effects of cooling rates on
the grain sizes of igneous rocks.
Describe some uses of igneous rocks.
Mineral Composition
Felsic rocks such
as granite are lightcolored, have high
silica contents, and
contain quartz and
the feldspars
orthoclase and
plagioclase.
Mineral Composition
Intermediate rocks such as diorite contain
moderate amounts of biotite, amphibole,
and pyroxene
Mineral Composition
Mafic rocks such as
gabbro are darkcolored, have lower
silica contents, and
are rich in iron and
magnesium.
Mineral Composition
Ultramafic rocks
are unusual,
containing
peridotite, dunite,
have low silica
contents and very
high levels of iron
and magnesium.
Grain Size
Igneous rocks differ in the size of the
grains that compose them
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Cooling rates will affect grain size
Fast cooling rocks will form smaller grains
Slow cooling rocks will form larger grains
Texture
Distinct crystal shapes caused by
fractional crystallization can be used to
indentify igneous rocks
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Porphyritic texture is characterized by large
crystals surrounded by smaller crystals of the
same or other minerals
Igneous Rocks as Resources
Used as building materials
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Granite
Ore Deposits
Veins of gold, platinum, silver, and other
precious metals can be found in igneous
rock deposits.
Pegmatites are veins
that contain extremely
large-grained crystals
Pegmatite containing Amethyst
Kimberlites
Diamond is found
naturally occurring
ultramafic rocks
known as
kimberlites.
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Named after
Kimberly, South
Africa where they
were first discovered
Form 150-300 km;
intense pressures
required for
kimberlites to form