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CLASSIFYING ROCKS
Coulter
 When
studying a rock sample, geologists observe the
rocks minerals composition, color, and texture
 Rocks
are made of mixtures of minerals and other
materials.
 About
20 minerals make up most of the rocks of
Earth’s crust, these minerals are known as rockforming minerals
 Geologist
observe the shape and color of crystals in a
rock to identify the minerals that the rock contains.
MINERAL COMPOSITION AND COLOR

Texture is the look and feel of the rock’s surface

Grain size:



often grains in a rock are large and easy to see, these are
called coarse-grained.

Grains sizes are so small that they can only be seen by
microscope, fine-grained.

Some have no visible grain even under a microscope.
Grain shape:

Some grains look like tiny particles of sand, small seeds, or
exploding stars.

In some rocks grain shape results from fragments, they can be
smooth and rounded or they can be jagged
Grain pattern:

Some grains lie in flat layers that look like a stack of pancakes

Others make a swirling pattern

Others the grains occur randomly throughout.
TEXTURE
 Geologists
classify rocks into three major groups
 Igneous
rock: forms from cooling of magma or lava
 Sedimentary
rock: forms when particles of other
rocks or the remains of plants and animals are
pressed and cemented together.
 Metamorphic
rock: forms when an existing rock is
changed by heat, pressure, or chemical reaction.
HOW ROCKS FORM
 Igneous
rocks are classified according to their origin,
texture, and mineral composition.
 Origin:
 Igneous
surface
rock may form on or beneath earth’s
 Extrusive
rock is igneous rock formed from lava that
erupted onto Earth’s surface.
 Intrusive
rock is igneous rock that formed when
magma hardened beneath Earth’s surface.
CLASSIFYING IGNEOUS ROCKS

Texture:
 Texture of an igneous rock depends on the size and
shape of its mineral crystals.
 Rapidly cooling lava forms fine-grained igneous rocks
with small crystals.
 Slowly cooling magma forms coarse-grained rocks with
large crystals.
 Intrusive and extrusive rocks usually have different
textures
 Intrusive rocks have larger crystals than extrusive rock.
 Extrusive rocks have a fine-grained or glassy texture.
Obsidian is an extrusive rock that cooled very rapidly
without forming crystals.
IGNEOUS ROCK CLASSIFICATION:
TEXTURE
 Most
of Earth’s minerals contain silica, a material
formed from oxygen and silica.
 The
amount of silica in magma and lava affect the
type of rock they form.
 Lava
that is low in silica usually forms dark-colored
rocks.
 Magma
rocks.
that is high in silica usually forms light-colored
IGNEOUS ROCK CLASSIFICATION:
MINERAL COMP.

Most sedimentary rocks are formed through a series of processes:
erosion, deposition, compaction, and cementation.

Erosion: running water, wind or ice loosen and carry away fragments
of rocks.

Deposition: is the process by which sediment settles out of the water
or wind carrying it.

Compaction: process that press sediment together.

Cementation: is the process in which dissolved minerals crystallize
and glue particles of sediment together.
SEDIMENTARY ROCKS

There are three major groups of sedimentary rocks: clastic rocks,
organic rocks, and chemical rocks.
TYPES OF SEDIMENTARY ROCK

Is a sedimentary rock that forms when rock fragments are
squeezed together.

These fragments can range in size from clay that are to small to
be seen with a microscope to large boulders that are too heavy
for you to lift.

Clastic rocks are grouped by the size of fragments of which they
are made.
CLASTIC ROCK

Organic rocks forms where the remains of plants and animals are
deposited in thick layers.

Organic means once part of a living thing.

Two important organic sediment is coal and limestone
ORGANIC ROCK

Limestone can form when calcite that is dissolved in lakes, seas,
or underground water comes out of solution and forms crystals.
This kind of limestone is considered a chemical rock.

Chemical rocks can also form from mineral deposits left when
seas or lakes evaporate.
CHEMICAL ROCK

Heat and pressure deep beneath Earth’s surface can change
any rock into metamorphic rock.

Types of metamorphic rock: geologists classify metamorphic
rocks according to the arrangement of the grains that make up
the rocks.
METAMORPHIC ROCK

Metamorphic rocks that have their grains arranged in parallel
layers or bands are said to be foliated.

One common foliated rock is slate. Heat and pressure change
the sedimentary rock shale into slate
FOLIATED ROCKS

Mineral grains in these rocks are arranged randomly.

Metamorphic rocks that are nonfoliated do not split into layers.
NON-FOLIATED
ROCK CYCLE