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3.8: Rocks and Processes of the Rock Cycle

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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

The Rock Cycle.

The Three Rock Types

Rocks are classified into three major groups according to how they form. These three types are described in more detail in other concepts in this chapter, but here is a summary.

  • 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 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. 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 .

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

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

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

Explore More

Use these resources to answer the questions that follow.

https://www.youtube.com/watch?v=pm6cCg_Do6k

  1. What are the three major types of rocks?
  2. What does the rock cycle mean?
  3. What do wind and water do to rocks at the surface?
  4. How do sedimentary rocks formed?
  5. What must happen to a rock for it to become a metamorphic rock?
  6. When a rock becomes metamorphic does it look the same as it did originally?
  7. What must happen for an igneous rock to form?
  8. What are the two places that a magma can cool?
  9. Is there only one path through the rock cycle?

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

crystallization

crystallization

The formation of mineral grains from cooling magma.
erosion

erosion

The transport of weathered materials and sediments by water, wind, ice, or gravity.
igneous rock

igneous rock

A rock formed from cooled magma.
metamorphic rock

metamorphic rock

A rock that forms from a previous rock that is exposed to heat and/or pressure.
metamorphism

metamorphism

A solid state change in an existing rock due to high temperature and/or pressure that creates a metamorphic rock.
precipitate

precipitate

Solid substance that separates out of a liquid to form a solid, usually when the liquid evaporates.
rock cycle

rock cycle

The never-ending cycle in which one rock type changes into another rock type.
sediment

sediment

Small particle of soil or rock deposited by wind or water.
sedimentary rock

sedimentary rock

A rock that forms from the compaction of sediments or the precipitation of material from a liquid
sedimentation

sedimentation

Sediments are laid down in a deposit.
weathering

weathering

The chemical or physical breakdown of rocks, soils, or minerals at Earth's surface.

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Difficulty Level:

At Grade

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Date Created:

Feb 24, 2012

Last Modified:

Oct 08, 2014
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