Lesson 2.2: Classifying Rocks
Study Guide
Rock-forming material
Vocabulary
Any of the common minerals that make up most of the rocks of Earth’s crust
Granite
Basalt
Grain
A usually light-colored igneous rock that is found in continental crust
A dark, dense igneous rock with a fine texture, found in oceanic crust
The particles of minerals or other rocks that give a rock its texture
Texture
The look and feel of a rock’s surface, determined by the size, shape, and
pattern of a rock’s grains
A type of rock that forms from the cooling of molten rock at or below the
surface
Igneous rock
Sedimentary rock
Metamorphic rock
A type of rock that forms when particles from other rocks or the remains of
plants and animals are pressed and cemented together
A type of rock that forms from an existing rock that is changed by heat,
pressure, or chemical reactions
Key Idea: To study a rock sample, geologists observe the rock’s mineral composition, color, and
texture.
Mineral Composition
 Rocks are made from mixtures of minerals and other materials.
 Rock-forming minerals are 20 minerals that make up most of the rocks
in Earth’s crust.
Granite
 Granite is made up of quartz, feldspar, mica, and horneblend.
Color
*Color alone is not enough
information to identify a
rock.
Basalt
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Granite is a light-colored rock with high silica content.
Granite has larger crystals than basalt.
Basalt is a dark-colored rock with lower silica content.
Basalt is made up of small mineral crystals that cannot be seen with the
naked eye.
Texture
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Grains (particles of minerals) make up the texture.
Describes the look and feel of the rock using terms that are based on
the size, shape, and pattern of the grains.
o Size- coarse (large), fine (small)
o Shape- rounded, jagged
o Pattern- banded (pattern of layers), nonbanded (no pattern)
Key Idea: Geologists have classified rocks into three major groups based on how they were formed.
Igneous Rocks
 Form from the cooling of magma or lava
Sedimentary Rocks
 Form when small particles of rocks or the remains of plants and animals are
pressed and cemented together
 Form in layers that are buried under the surface
Metamorphic Rocks
 Forms when a rock is changed by heat or pressure, or by chemical reactions
 Forms deep underground