2.2 Classifying Rocks

To study a rock sample, geologist observe the
rock’s mineral composition, color, and texture.

Rocks are made of mixtures of minerals and
other materials.
Some rocks contain only one mineral. Other
rocks contain several minerals.
 A rock’s color provides clues to the rock’s
mineral composition. However, the color alone
is not enough evidence to identify a rock.


grains – the particles in rocks made up of
minerals and other rocks

texture – the look and feel of a rock’s surface

To describe the texture of a rock, geologists
use terms that are based on the size, shape,
and pattern of the grains.

3 types of grain sizes:
1.
coarse grain – rocks with grains that are large
and easy to see.

2.
fine grain – rocks with grains that are so small
they can only be seem with a microscope

3.
for example: granite, diorite
for example: basalt, slate
no visible grain

for example: flint
2
1.
types of grain shape:
rounded grain – round edges

2.
for example: conglomerate
jagged grain – jagged or sharp edges

for example: breccia
2
1.
types of grain pattern:
banded – grains lie in a pattern of flat layers or
can form swirls or colored bands

2.
for example: gneiss
nonbanded – gains do not have any visible
pattern

for example: quartzite


Using the characteristics of color, texture, and
mineral composition, geologist can classify a
rock according to its origin (how the rock
formed).
Geologist classify rocks into 3 major groups:
1.
2.
3.
Igneous rock – forms from the cooling of magma
or lava
Sedimentary rock – small particles of rocks or
the remains of plants and animals are pressed and
cemented together, forming layers
Metamorphic rock – rock that is changed by heat
and pressure, or by chemical reactions
What type of rock are these?
What type of rock are these?