Classifying Rocks Chapter 2.2

When studying a rock sample, geologists observe three things: The rock’s mineral composition The color The texture

Mineral Composition: Some rocks are made up of a single mineral Some rocks are made of mixtures of minerals and other materials Look at figure 10 and note that Granite is made up of several minerals About 20 minerals make up most of the rocks of Earth’s crust. These minerals are known as rock-forming minerals.

Color: A rock’s color provides clues to the rock’s mineral composition. Granite is one example. It is generally light in color and has high silica content. Basalt is another example. It is dark in color and is low in silica.

Texture: Most rocks are made up of particles of minerals and other rocks, which geologists call grains. The grains give the rock its texture. To a geologist, the texture is the look and feel of the rock’s surface. Look at figure 12 pg. 56 and see the types of textures geologists look for in a rock.

How Rocks Form: Using color, texture, and mineral composition, geologists can classify a rock according it its origin. An origin is how a rock is formed. Geologists classify rocks into three major groups: Igneous rock Sedimentary rock Metamorphic rock

Igneous Rock: Igneous rock forms from the cooling of magma or lava.

Sedimentary Rock: Most sedimentary rock forms when particles of other rocks or the remains of plants and animals are pressed and cemented together. Sedimentary rock forms in layers that are buried below the surface.

Metamorphic Rock: Metamorphic rock forms when an existing rock is changed by heat, pressure, or chemical reactions. Most metamorphic rock is formed deep underground.

ASSIGNMENT! In complete sentences, answer the questions 1-2 at the bottom of page 57.