Identify a Rock

Classifying Rocks

What characteristics are used to identify rocks?

• When studying a rock sample, geologists observe the rock’s– Color– Texture– Mineral composition

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.

Texture: Grain Size

• Texture is described with the terms based on:

• Grain size– Fine-grained– Coarse-grained

• Grain shape– Smooth – Jagged

• Grain pattern– banded– nonbanded

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

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

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

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.

Magnet Test: A rock that is high in iron content may be magnetic: Magnetite is a good example.

Many meteorites are also magnetic!

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

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

Example of a Rocks ID Chart: