Download 3.3 * Classifying Rocks

3.3 – Classifying Rocks
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Essential Question:
1.
How Do Geologists Classify Rocks?
How Do Geologists Classify Rocks?
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Rocks are made of minerals.
To study a rock sample, geologists observe the
rock’s mineral composition, color, and texture.
Mineral Composition and Color
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Rock-forming minerals are the approximately 20
minerals that make up most of the rocks of Earth’s
crust.
Color alone is not enough to classify a rock.
Silica Level
High
Color
Light
Rock Type
Felsic
Medium
Gray(ish) Intermediate
Low
Dark
Mafic
Almost NONE Green(ish) Ultramafic
Texture
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Most rocks are made up of particles or minerals or
other rocks, which geologists call grains.
Grains give the rock its texture, the look and feel of
the rock’s surface.
 Grain
size
 Grain shape
 Grain pattern
Texture – Grain Size
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Rocks with large grains are coarse grained.
Rocks with small grains are fine grains.
Texture – Grain Shape
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Grains can either be rounded or angular.
Conglomerate
Breccia
Texture – Grain Pattern
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Grains can be banded under pressure.
Origin
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Geologists classify rocks into three major groups:
 Igneous
 Metamorphic
 Sedimentary
Origin - Igneous
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Igneous rocks form from the cooling of molten
material called magma or lava.
 Cooling
magma = Intrusive = Larger Grains
 Cooling Lava = Extrusive = Smaller Grains
Origin - Sedimentary
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Sedimentary rocks form when small particles of
rock and/or mineral and/or organic material are
cemented together.
Origin - Metamorphic
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Metamorphic rocks form when a rock is changed
by heat and/or pressure and/or chemical reactions.