3.1 The Rock Cycle

Is this what geologists mean by the rock cycle?

Okay, very punny. The rock cycle shows how any type of rock can become any other type of rock. Some rocks may stay the same type for a long time, for example, if they’re at the base of the crust, but other rocks may relatively rapidly change from one type to another.

The Rock Cycle

The rock cycle, illustrated in Figure below, depicts how the three major rock types – igneous, sedimentary, and metamorphic – convert from one to another. Arrows connecting the rock types represent the processes that accomplish these changes.

Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly. Rocks deep within the Earth are right now becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them. Even at the surface, we may not notice the changes. The rock cycle has no beginning or end.

The Rock Cycle ^[1]

The Rock Cycle.

Click here for an interactive rock cycle demonstration

The Three Rock Types

Rocks are classified into three major groups according to how they form. These three types will be described in more detail in other lessons in this concept, but here is an introduction.

Igneous rocks form from the cooling and hardening of molten magma in many different environments. The chemical composition of the magma and the rate at which it cools determine what rock forms. Igneous rocks can cool slowly beneath the surface or rapidly at the surface. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known.
Sedimentary rocks form by the compaction and cementing together ofsediments, broken pieces of rock-like gravel, sand, silt, or clay. Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure below the surface.

A simple explanation of the three rock types and how to identify them can be seen in this video: http://www.youtube.com/watch?v=tQUe9C40NEE&feature=fvw.

This video discusses how to identify igneous rocks:http://www.youtube.com/watch?v=Q0XtLjE3siE&feature=channel.

This video discusses how to identify a metamorphic rocks:http://www.youtube.com/watch?v=qs9x_bTCiew&feature=related.

The Processes of the Rock Cycle

Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism.

Crystallization

Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.

Erosion and Sedimentation

Weathering wears rocks at the Earth’s surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.

Metamorphism

When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. For that reason, a metamorphic rock may have a new mineral composition and/or texture.

Summary

The three main rock types are igneous, metamorphic and sedimentary.
The three processes that change one rock to another are crystallization, metamorphism, and erosion and sedimentation.
Any rock can transform into any other rock by passing through one or more of these processes. This creates the rock cycle.

Making Connections

Practice

Use these resources to answer the questions that follow.

This Science Made Fun video discusses the conditions under which the three main rock types form (3c): http://www.youtube.com/watch?v=G7AWGhQynTY&feature=related (3:41).

1. How do igneous rocks form?

2. What are the two types of igneous rocks and how do they differ?

3. What are metamorphic rocks?

4. How do metamorphic rocks form?

5. How do sedimentary rocks form?

6. List three examples of igneous rocks.

7. List three examples of sedimentary rocks.

8. What forms coal?

9. List three examples of metamorphic rocks.

10. Can an igneous rock become an igneous rock? Can a sedimentary rock become a sedimentary rock? Can a metamorphic rock become a metamorphic rock?

11. Draw an diagram of the rock cycle and include the processes that transform rocks from one type to another.

Review the rock cycle – click a rock to begin.

http://www.phschool.com/atschool/phsciexp/active_art/rock_cycle/index.html

Test your rock identification skills with this activity:

Name that Rock – http://library.thinkquest.org/J002289/rocks.html

Review

1. What processes must a metamorphic rock go through to become an igneous rock?

2. What processes must a sedimentary rock go through to become a metamorphic rock?

3. What types of rocks can become sedimentary rocks and how does that happen?

Vocabulary Words

Rock Cycle – depicts how the three major rock types – igneous, sedimentary, and metamorphic – convert from one to another

Igneous rocks – form from the cooling and hardening of molten magma in many different environments. The chemical composition of the magma and the rate at which it cools determine what rock forms

Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay. Those sediments can be formed from the weathering and erosion of preexisting rocks

Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure below the surface.

Metamorphism – Changes the mineral composition and the texture of the rock