Download Ch. 3: “Rocks”

Ch. 3: “Rocks”
3.2: “Igneous Rocks”
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Formation of Igneous Rocks
• The word igneous comes from the
Latin word ignis, which means “fire.”
• Igneous rock forms from volcanic
eruptions at the earth’s surface as
well as deep beneath the Earth’s
surface.
• Classified into two groups
according to where the magma
cools:
– Intrusive: magma hardens beneath
Earth’s surface
– Extrusive: lava hardens above Earth’s
surface
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Intrusive Igneous Rocks
• Hardens slowly
• Forms large crystalline grains
– Ex: granite – major rock of continental
crust
– When cooling occurs slowly ions are
allowed to move large distances in
magma.
• Forms coarse grains or coarse texture.
• Some cool slowly at first, then cools
rapidly as it nears the surface.
– Forms rocks with large crystals
embedded in small crystals.
– Called porphyry (porphyritic texture)
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Extrusive Igneous Rocks
• When magma reaches Earth’s
surface, it is called lava.
• When lava hardens, the rocks that
form are called extrusive igneous
rocks.
• Hardens rapidly.
• When magma comes to surface,
gases escape.
• Produces small grains.
– Fine-grained texture.
– Ex: rhyolite
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Classification of Igneous Rocks
• Coarse-Grained Texture
– Magma cools slowly-few
centers of crystal growth
develop.
– Slow cooling allows
charged atoms, or ions, to
move large distances in the
magma.
– Results in the formation of
large crystals.
– This yields a coarsegrained texture.
• Fine-Grained Texture
– Magma cools rapidly, the
ions in the melted material
lose their motion & quickly
combine.
– Large # of tiny crystals that
all compete for the
available ions.
– Results in rocks with small,
interconnected mineral
grains.
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Classification of Igneous Rocks
• Porphyritic Texture
• Glassy Texture
– Extremely fast cooling
occurs & ions are randomly
distributed.
– Such rocks have a glassy
texture.
– Ex: obsidian & pumice
– Some minerals within a rock do
not crystallize at the same time
or rate.
– The resulting rock can have
large crystals surrounded by
fine-grained minerals.
– These rocks experienced
different rates of cooling.
– Ex: andesite
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Classification of Igneous Rocks
• Granitic Composition
– Have light colored silicate
minerals quartz & feldspar.
– Most contain about 10%
dark silicate minerals,
which are often biotite mica
& amphibole.
– Granitic rocks contain
about 70% silica & are the
major rocks of the
continental crust.
• Basaltic Composition
– Have dark colored silicates.
– These are rich in the
elements magnesium &
iron.
– Typically darker & denser
than granitic rocks.
– Ex: basalt
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Other Compositional Groups
• Rocks with a composition
between granitic & basaltic rocks
have an andesitic composition.
– Contain at least 25% dark silicate
minerals
– Ex: andesite
• Rocks that are composed almost
entirely of dark silicate minerals is
called ultramafic.
– Composed mostly of iron and
magnesium-rich minerals.
– Rare at Earth’s surface.
– Ex: peridotite
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Classification of Igneous Rocks
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