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Chapter 4
Classifying rocks
Rocks are made up of two or minerals.
 About 20 minerals make up most of the
earth’s rocks.
 When studying rocks, geologists
observe the mineral composition, color
and texture of the rock.

This rock
contains
three
minerals.
Rock color
A rock’s color can indicate the rock’s
mineral composition. Certain minerals
are light in color while other minerals are
dark in color.
 Granites are usually light
 Basalts are usually dark

Rock Texture
 Texture
1)
2)
3)
involves 3
components:
Grain size
Grain shape
Grain pattern
Fine grained
Coarse grained
Rounded grains
Jagged grains
Nonbanded
banded
Three Groups of Rocks



Igneous – form from
the cooling of
magma or lava
Sedimentary – form
when particles of
rocks are pressed &
cemented together
Metamorphic –
when existing rocks
are changed by
heat, pressure, or
chemical reactions.
This is NOT the correct answer!
IGNEOUS ROCKS
Igneous rocks are
classified according
to their origin,
texture, and mineral
composition.
 ORIGIN:
Extrusive – formed
from lava that has
erupted.
Intrusive – formed
when magma
hardens under the
earth’s surface.

Most common
Extrusive rock
Basalt
Most common
Intrusive rock
Granite
Extrusive vs. Intrusive
 Extrusive



Rapid cooling
Small crystal size;
fine grained
Can have a glassy
texture
 Intrusive


Slow cooling
Larger crystals;
coarse grained
Types of Magma/Lava


Different
compositions of lava
or magma determine
the types of igneous
rocks formed.
Magma that
contains SILICA will
form light colored
rocks such as
granite.

If the magma is low
in silica will form
dark colored rocks
such as basalt.
Uses of Igneous Rock

Igneous rock has
been used for
thousands of years
for buildings and
statues. Granite is
the most widely
used igneous rock.
SEDIMENTARY ROCKS

Sediments are small bits & pieces of
rocks, shells, sand, bones, or parts of
plants.
This is
called silt
Can you see
the shells?
Sediments can be carried
away by water to be
deposited somewhere else.
From Sediment to Rock
• Particles are
carried away
by wind or
water
• Particles are
deposited
• DEPOSITION
Erosion
Compaction
• Particles are
squeezed
together by
pressure
• Particles
are glued
together as
mineral
solutions
harden
Cementation
Sedimentary rock formation
Formation under water
Three Major Types of
Sedimentary Rocks
Clastic
Organic
• Rock formed by rock fragments that are cemented together;
sometimes visible large fragments, sometimes small, fine
fragments
• Two common types are conglomerates, sandstone, breccias
• Rocks formed by the remains of plants/animals that are
cemented together.
• Two common types are coal and limestone
• Formed when minerals dissolved in water are
crystallized when the water evaporates away
Chemical • Calcite and Halite can form this way.
CLASTIC SEDIMENTARY ROCKS
This is a breccia.
Notice the jagged
edges.
These are conglomerates
This is sandstone.
The individual fragments
Are not easily seen.
This is shale, splits
easily into thin sheets.
ORGANIC SEDIMENTARY ROCKS
Limestone – forms in the ocean Coquina- a special type of limestone
from shells, corals, calcite skeletons In which shells can be easily seen
being compacted
Fossils are found mainly
In sedimentary rocks.
Coal is one of the most
important organic rocks.
The formation of COAL
Coal is made from once
living plants & animals.
Limestone is formed from corals,
and other sea animals.
These shells and animal
remains get crushed and
cemented together over
time to form limestone.
Limestone formations
These are called
pancake rocks!
These are
located in
Vietnam.
More limestone
formations
Here are some
hot springs in
limestone.
Many cave
formations are
made of
limestone.
Chemical Sedimentary Rocks
Gypsum and Halite are two of the most
common chemical sedimentary rocks.
 They can also be minerals!
 They formed by dissolved chemicals
crystallizing as water evaporates.
 For this reason they are sometimes
called EVAPORITES.

This is GYPSUM in a cave in
New Mexico. Gypsum is
Calcium Sulfate. It is used
In plaster of paris, drywall,
and Cement.
This is a chemical
form of
limestone. It is
called TUFA.
Coral Reefs are responsible for
much of the limestone on earth.

Coral is an animal much like a sea
anenome. They have tentacles and eat
plankton. To protect their soft bodies
they secrete a hard substance called
CALCITE. When they die, these calcite
shells remain.
Ancient Coral Reef vs. Living
Coral Reef
METAMORPHIC ROCKS
Heat & pressure deep in the earth can change
any rock into a metamorphic rock. They can
form from igneous, sedimentary and other
metamorphic rocks.
 Extremely high temps and pressures deep in
the earth can actually change a rocks mineral
composition.

Two Types of Metamorphic Rocks
FOLIATED
NONFOLIATED
Rocks grains are
arranged in parallel
bands or layers
Common examples:
Slate, gneiss, schist
Grains are arranged
randomly.
Common examples:
Marble, quartzite
Increasing pressure causes greater foliation
Increasing Pressure
SHALE
SLATE
SCHIST
GNEISS
With pressure, granite becomes
gneiss. Igneous rock is changed
to a metamorphic rock.
Nonfoliated Metamorphic Rocks
USES OF METAMORPHIC ROCK
World’s Largest Marble Quarry
Tate, GA
Uses of SLATE
The Rock Cycle
It is a series of processes
in the earth’s crust, mantle
and on the earth’s surface
that SLOWLY change
rocks from one kind to
another.