4.6: Rocks

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Will this rock be there forever?

Rocks may seem permanent, but they're not. Over time a rock will change into another type of rock. How this happens is known as the rock cycle. There are three main types of rocks. There are several processes that can change one type into another. First we need to learn what the rocks types and processes are.

What is a Rock?

A rock is a naturally formed, non-living Earth material. Rocks are made of minerals. The minerals may be so tiny that you can only see them with a microscope. The minerals may be really large. A rock may be made of only one type of mineral. More often rocks are made of a mixture of different minerals. A few types of rocks are made from materials that are not minerals. For example, coal is organic so it is not a mineral, nor is it made of minerals. Yet coal is a rock.

Picture of granite, basalt, coal, and jade

(A) Granite has large crystals because it cools slowly. (B) Basalt has very small crystals because it cools quickly. (C) Coal is made up of organic material. (D) Jade is made of the mineral jadeite.

Rocks are named for the combinations of minerals they are made of and the ways those minerals came together. Remember that different minerals form under different environmental conditions. So the minerals in a rock contain clues about the conditions in which the rock formed (Figure above).

Rock Types

Geologists group rocks based on how they form. There are three major rock types. Each will be described in more detail in the coming concepts.

Igneous rocks form when magma cools below Earth’s surface or lava cools at the surface.

Sedimentary rocks form when sediments are compacted and cemented together. Sediments are pieces of rock. They may be gravel, sand, silt, or clay. Some sedimentary rocks form the solid minerals left behind after a liquid evaporates.

Metamorphic rocks form when an existing rock is changed by heat or pressure. The minerals in the rock change but do not melt. The rock experiences these changes within the Earth.

Rocks can change from one type to another. The rock cycle describes how this happens.

Summary

Nearly all rocks are made of minerals. A few are made of materials that do not fit the definition of minerals.
Igneous rocks form from cooled magma or lava.
Metamorphic rocks form as an existing rock is altered by high temperature or pressure.
Sedimentary rocks form from sediments that are cemented and compacted.

Review

Name a rock type that is not made of minerals. How can a rock be made of material that is not minerals?
What is an igneous rock?
What is a metamorphic rock?
What is a sedimentary rock?

Explore More

Use the resource below to answer the questions that follow.

How do igneous rocks form?
Why do lessons about rocks always begin with igneous rocks?
What directly correlates with crystal size in an igneous rock?
How do sedimentary rocks form?
What two things related to sedimentary rocks do you see at the Grand Canyon?
What are metamorphic rocks?
How do metamorphic rocks form?
Can an igneous rock become an igneous rock? Can a sedimentary rock become a sedimentary rock? Can a metamorphic rock become a metamorphic rock?
Draw an diagram of the rock cycle. Include the processes that transform rocks from one type to another.