1. Classifying Rocks

2. What are Rocks? Made of two or more different minerals that have been:
cemented together
squeezed and heated together
melted and cooled together.

3. Mineral Composition and Color
About 20 minerals make up most of the rocks of Earth’s Crust.
These minerals are known as rock-forming minerals.
A rock’s color provides clues to the rock’s mineral compostition.
Granite is generally a light colored rock that has a high silica content.
Basalt is a dark colored rock that is low in silica.
As with minerals color alone does not provide enough information to identify a rock.

4. Texture Very useful in identifying a rock
Look and feel of the rock’s surface
Most rocks are made up of particles of other rocks – called grains
Geologists use terms based on the size, shape, and pattern of the grains
The picture is Rhyolite.

5. Grain Size Coarse Grained – large easy to see
Fine Grained -- grains are so small they can only be seen under a microscope
Fine Grain
Coarse Grain
Fine Grained rock is Slate .
Coarse grained rock is quartz-diorite

6. Grain Shape Grains in a rock vary widely in shape
The grains can be round (Conglomerate), jagged (Breccia), star-shaped, look like small seeds, etc…
They can be formed from other rocks
Conglomerate
Breccia

7. Grain Pattern The grains in rocks often form patterns.
Flat layers like pancakes
Swirling patterns
Foliated – different colored bands
Random

8. How Rocks Form
Using color, texture, and mineral composition, geologists can classify rock according to its origin.
Igneous rock – forms from cooling 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.

9. Classifying Rocks Review
Where do rocks come from?
What are Rocks?
Mineral Composition
Color
Texture
Grain – size, shape, pattern
How Rocks Form?

10. The End