4.5 Rocks

How many different rock types are in this photo?

A beach or river bed is a good place to see a lot of different rock types since the rocks there represent the entire drainage system. How could you tell how many different rock types were in the photo? What characteristics would you look for?

What Are Rocks?

A rock is a naturally formed, non-living Earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (Figure below).

How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger (Figure below).

Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Are the two samples in Figure below the same rock type? Do they have the same minerals? The same texture?

As seen in Table above, these two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has very tiny or invisible grains. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth’s surface.

A few rocks are not made of minerals because the material they are made of does not fit the definition of a mineral. Coal, for example, is made of organic material, which is not a mineral. Can you think of other rocks that are not made of minerals?

Summary

• Nearly all rocks are made of minerals. A few are made of materials that do not fit the definition of minerals.
• Rocks are typically identified by the minerals they contain and their textures.
• The texture of a rock describes the size, shape, and arrangement of mineral grains and is a reflection of how the rock formed.

Review

1. Name a rock type that is not made of minerals and state how a rock could not be made of minerals.
2. Can a rock be made of only one type of mineral, or do rocks need to be made of at least two minerals?
3. Why is texture so important in classifying rock types?