Earth Science
Chapter 3
Section 1
A. How Geologists Classify Rocks:
When studying a rock sample, geologists observe the rock’s color
and texture and determine its mineral composition.
Using these characteristics, geologists can classify a rock
according to its origin, or where and how it formed.
1. Texture:
Texture is the size, shape and pattern of the rocks minerals.
Some textures can be smooth and glassy, others rough or
chalky.
Rocks are made up of particles of minerals or other rocks,
which are called grains.
A rock’s grains give the rock its texture.
Rock Texture:
Grain Size:
-Coarse-grained like diorite
-Fine-grained like slate
-Microscopic grains like slate
Grain Shape:
Grain shape results from the shape of the crystals.
These crystal fragments can be smooth and rounded like in
conglomerate, or jagged like in Breccia.
Grain Pattern:
The grains in a rock form patterns.
Flat layers,
Wavy, swirling
Rows of multicolored grains
Random patterns
No visible grains:
Some rocks have no visible grains even when examined under a
microscope.
These rocks have no grains because when they formed, they
cooled quickly.
Flint and obsidian are examples.
2. Mineral Composition:
Mineral composition is determined by examining the rock to
identify minerals. Example: A microscope to view size and
shape of minerals, acid test to test for carbonates, magnets to
test for iron and nickel.
3. Origin:
There are three major groups of Rock:
1. Igneous Rock – Rock formed from the cooling of magma
or lava.
2. Sedimentary Rock – Rock formed from particles of other
rocks, or plant and animal remains that have been pressed
and cemented together. These rocks form in layers.
3. Metamorphic Rock – Rock formed from existing rock
that is changed by heat, pressure or chemical reactions.
These rocks form deep underground.