1. Identify a Rock
Classifying Rocks

2. What characteristics are used to identify rocks?
When studying a rock sample, geologists observe the rock’s
Color
Texture
Mineral composition

3. Texture helps geologists classify rocks
Texture helps geologists classify rocks. Looking at the rocks below, describe the characteristics of a rock that help you define what a rock’s “grain” is.

4. Texture: Grain Size Texture is described with the terms based on:
Fine-grained
Coarse-grained
Grain shape
Smooth
Jagged
Grain pattern
banded
nonbanded

5. What are the three major groups of rocks?
Igneous rock
Forms from the cooling of molten rock – either magma below the surface or lava at the surface
Sedimentary rock
Forms when particles of other rocks or the remains of plants and animals are pressed and cemented together
Forms below the surface
Metamorphic rock
Forms when an existing rock is changed by heat, pressure, or chemical reactions
Most metamorphic rock forms deep underground

6. What tests do geologists use to identify minerals in rocks?
Scratch Test (Mohs)
Acid Test--Use acid to determine whether the rock contains the mineral calcite
Magnet test

7. Scratch Test: The harder rock/mineral/material will generally scratch the softer rock/mineral/material

8. Acid Test: A rock/mineral with calcium or other reactive substance will bubble when hydrochloric acid is placed on it: Limestone, Calcite, and some Sandstones may do this.

9. Magnet Test: A rock that is high in iron content may be magnetic: Magnetite is a good example.
Many meteorites are also magnetic!

10. ID Key:
Many Geologists use an ID key to determine they type and common name of a rock.

11. Example of a Rocks ID Chart:
Name:
Type: Ig/Sed/Met
Color
Texture
Minerals Present
Scratch Test: Mohs Hardness
Acid Test:
Granite
Igneous
White with black specks
Medium (smooth or rough)
Quartz, mica, feldspar
7-scratches steel
N/A