Classifying Rocks
• Rocks form the earth’s crust
– Mountains
– Hills
– Valleys
– Beaches
– Ocean floor
• When studying a rock sample, observe the
rock’s mineral composition, color and
texture
Classifying Rocks
Mineral Composition and Color
• Rocks are made of mixtures
of minerals and other
materials. Some rocks
contain only one mineral,
others more than one.
– Granite is made of quartz,
feldspar, hornblende and mica.
• About 20 minerals make up
the earth’s crust.
• These are known as rockforming minerals
Classifying Rocks
Mineral Composition and Color
• Color provides clues to rock’s
mineral composition.
– Granite is generally light colored that
is high in silica.
– Basalt is dark-colored that is low in
silica
• In identifying rocks, like minerals,
color isn’t the best way, and tests
done on minerals are also done on
rocks.
• Acids often is used to see if it
contains compounds called
carbonates.
Classifying Rocks
Texture
• Texture is useful in identifying. Rocks are
made of particles of minerals or other
rocks called grains. Grains give a rock
texture.
• Geologists use terms based on size,
shape and pattern of the grain to describe.
Classifying Rocks
Texture
• GRAIN SIZE
• Grain size can be
large and easy to see
or so small a
microscope is
necessary.
• Fine grained like slate
• Coarse grain like
diorite
• No visible grain like
flint.
Classifying Rocks
Texture
• GRAIN SHAPE
• Shape vary widely. Some look like tiny
particles of sand. Others small seeds,
and still others exploding stars.
• Grains results from the shapes of the
crystals the form the rock.
• In others, grain shape results from
fragments of several rocks.
• Rounded grains- conglomerate
• Jagged grains- Breccia
Classifying Rocks
Texture
• GRAIN PATTERN
• Grains in rock often form patterns.
Some lie in flat layers like stacks
of pancakes. Others swirl in
patterns. Some have bands of
different colors like gneiss.
• Others are randomly
• Non-Banded-Quartzite
• Banded-Gneiss
How rocks are formed
• Three major rock groups based on color,
texture, and mineral composition.
• IGNEOUS
• SEDIMENTARY
• METAMORPHIC
How rocks are formed
• IGNEOUS
– Forms from the cooling of
magma or lava.
• SEDIMENTARY
– Forms when particles of other
rocks or the remains of plants
and animals are pressed and
cemented together
Sedimentary rock forms I layers
that are buried below the
surface
How rocks are formed
• METAMORPHIC
– Forms when an existing rock is changed by
heat, pressure or chemical reactions. Most
metamorphic rock forms deep underground.
IGNEOUS ROCKS
• 4.6 billion years ago much of the earth
was covered in hot magma. As it cooled a
rocky crust formed. This crust then sank
because it was denser. More lava erupted
over the surface and hardened to form
rock.
Igneous rocks are called fire rocks and
are formed either underground or
above ground. Underground, they are
formed when the melted rock, called
magma, deep within the earth becomes
trapped in small pockets. As these
pockets of magma cool slowly
underground, the magma becomes
igneous rocks.
Igneous rocks are also formed when
volcanoes erupt, causing the magma to
rise above the earth's surface. When
magma appears above the earth, it is
called lava. Igneous rocks are formed
as the lava cools above ground.
Classifying Igneous Rock
• Ignis means fire in Latin
• Igneous rock is any rock
formed from magma or
lava.
• Igneous rocks are classified
according to their origin,
texture and mineral
composition.
• Igneous may form on or
beneath earth’s surface.
Classifying Igneous Rock
• Extrusive rock is igneous rock
formed from lava that erupts
onto Earth’s surface.
• Basalt is the most common
extrusive rock.
– Basalt forms much of the crust,
– including the oceanic crust,
– shield volcanoes and
– lava plateaus
Classifying Igneous Rock
• Igneous rock formed when magma hardened
beneath Earth’s surface is called intrusive rock.
Igneous Rock
• Igneous rock examples and uses.
• Click on the link and list the 4 examples of igneous rocks
and examples of their use in complete sentences.
• 1.___________________________________________
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• 2. ___________________________________________
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• 3. __________________________________________
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• 4. __________________________________________
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QUESTIONS
• 1. How are igneous rocks classified?
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• 2. What are extrusive rocks and intrusive rocks?
Define each and explain the difference.
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QUESTIONS
• What are three common uses of igneous
rocks?____________________________
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