Essential Questions
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What are the different types and textures of igneous rocks?
•
How do cooling rates affect the grain sizes in igneous rocks?
•
What are some of the uses of igneous rocks?
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Classification of Igneous Rocks
Vocabulary
Review
New
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fractional crystallization
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intrusive rock
extrusive rock
basaltic rock
granitic rock
texture
porphyritic texture
vesicular texture
pegmatite
kimberlite
Classification of Igneous Rocks
Mineral Composition of Igneous Rocks
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When magma cools and crystallizes below Earth’s surface, intrusive
rocks form.
Magma that cools and crystallizes on Earth’s surface forms extrusive
rocks.
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Classification of Igneous Rocks
Mineral Composition of Igneous Rocks
•
Igneous rocks are classified by their mineral compositions.
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Basaltic rocks, also called mafic rocks, are dark-colored, have lower silica
contents, and contain mostly plagioclase and pyroxene.
•
Granitic rocks, also called felsic rocks, are light-colored, have high silica
contents, and contain mostly quartz and feldspar.
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Classification of Igneous Rocks
Mineral Composition of Igneous Rocks
•
Rocks that have a composition of minerals that is somewhere between
basaltic and granitic are called intermediate rocks.
•
A fourth category, called ultrabasic, or ultramafic, contains rocks with only
iron-rich minerals such as olivine and pyroxene and are always dark.
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Classification of Igneous Rocks
Mineral Composition of Igneous Rocks
•
Rock type can be determined by estimating relative percentages of minerals
in the rocks.
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Classification of Igneous Rocks
Texture
•
Texture refers to the size, shape, and distribution of the crystals or grains
that make up a rock.
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Classification of Igneous Rocks
Texture
Crystal size and cooling rates
•
When lava flows on Earth’s surface, it cools quickly and there is not enough
time for large crystals to form. The resulting extrusive igneous rocks have
crystals so small that they cannot be seen without magnification.
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Classification of Igneous Rocks
Texture
Crystal size and
cooling rates
•
When lava cools so
quickly that crystals
do not form, volcanic
glass is the result.
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When magma cools
slowly below Earth’s
surface, there is
sufficient time for
large crystals to
form. Thus, intrusive
rocks can have
crystals larger than 1
cm.
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Classification of Igneous Rocks
Texture
Porphyritic rocks
•
A porphyritic texture is characterized by large, well-formed crystals
surrounded by finer-grained crystals of the same mineral or different
minerals.
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Classification of Igneous Rocks
Texture
Vesicular rocks
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The spongy appearance that results from gas bubbles trapped in lava is
called vesicular texture.
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Classification of Igneous Rocks
Thin Sections
•
Scientists use a special microscope, called a petrographic microscope, to
help them identify minerals by examining the mineral grains in sections of
rock thin enough for light to pass through.
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Classification of Igneous Rocks
Igneous Rocks as Resources
Veins
•
Valuable ore deposits often occur within igneous intrusions. At other times,
they may occur as veins in the rocks surrounding intrusions.
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Classification of Igneous Rocks
Igneous Rocks as
Resources
Pegmatites
•
Veins of extremely largegrained minerals that
can contain ores are
called pegmatites.
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Classification of Igneous Rocks
Igneous Rocks as Resources
Kimberlites
•
Diamonds and other minerals that can form only under very high pressure
are found in rare, ultrabasic rocks known as kimberlites.
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Classification of Igneous Rocks
Igneous Rocks as Resources
Igneous rocks in construction
•
The interlocking grain textures and the resistance to weathering of many
of the minerals present in igneous rocks make them especially useful as
building materials.
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Classification of Igneous Rocks
Review
Essential Questions
•
What are the different types and textures of igneous rocks?
•
How do cooling rates affect the grain sizes in igneous rocks?
•
What are some of the uses of igneous rocks?
Vocabulary
• intrusive rock
• extrusive rock
• basaltic rock
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• granitic rock
• texture
• porphyritic texture
• vesicular texture
• pegmatite
• kimberlite
Classification of Igneous Rocks