GSLM - The Rock Cycle or the Geologic Cycle (Lesson)

The Rock Cycle or the Geologic Cycles

Click here to watch the PBS Rocks and Minerals video before you begin. Links to an external site.

Introduction

There are three types of rocks: igneous, sedimentary, and metamorphic. Each of these types is part of the rock cycle. Through changes in conditions, one rock type can become another rock type. Or it can become a different rock of the same type.

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

How is a rock different from a mineral? Rocks are made of minerals. 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. The texture is a description of the size, shape, and arrangement of mineral grains.

• Are the two samples below the same rock type?
• Do they have the same minerals?
• The same texture?

 

Sample	Minerals	Texture	Formation	Rock Type
1	plagioclase, hornblende, pyroxene	Crystals, visible to the naked eye	Magma cooled slowly	Diorite
2	plagioclase, hornblende, pyroxene	Crystals are tiny or microscopic	Magma erupted and cooled quickly	Andesite

As seen in the 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 andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth's surface.

Three Main Categories of Rocks

Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied in outcrop, exposed rock formations that are attached to the ground, at the location where they are found.

Igneous Rocks

Igneous rocks form from cooling magma. Magma that erupts onto Earth's surface is lava. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.

 

Igneous Rock: Basalt

Sedimentary Rocks

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.

Sedimentary Rock: Shale

Metamorphic Rocks

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

 

Metamorphic Rock: Gneiss

The Rock Cycle 

Please watch the following video.

 

Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly; many take place below the Earth's surface, so we may not even notice the changes. Although we may not see the changes, the physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called the rock cycle.

The concept of the rock cycle was first developed by James Hutton, an eighteenth-century scientist often called the "Father of Geology." Hutton recognized that geologic processes have "no [sign] of a beginning and no prospect of an end." The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be "rock solid" and unchanging, but in the longer term, change is always taking place.

In the rock cycle, illustrated in the figure below, the three main rock types - igneous, sedimentary, and metamorphic - are shown. Arrows connecting the three rock types show the processes that change one rock type into another. The cycle has no beginning and no end. 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.

 

 

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 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.

Rock Cycle Summary

The rock cycle is a continuous process with no beginning or end. Here are a few clues to help you remember the three types and how they are formed:

• Igneous - actually means fire and these rocks form from the cooling of liquid rock above (extrusive) or below (intrusive) the surface of the Earth. These rocks have been made by fire.
• Metamorphic - breaking down the word helps in remembering how this type of rock is formed. Meta means after and morphic means shape change or transform. This rock type has changed from one type to another by heat and pressure.
• Sedimentary - perhaps the easiest type to remember, these rocks are formed from the collection of sediment after weathering and erosion, that is compressed and cemented over time.

What's up with Weathering?

Weathering is how rock is broken down, while erosion is how the rocky fragments move. Weathering types include physical (often called mechanical), chemical and biological.

Physical Weathering

Physical weathering occurs when rock is broken down by any of several processes that do not involve a chemical change in the rock. These processes include the freeze-thaw cycle, extrinsic crystal formation (such as salts), isolation, and pressure.

 

Chemical Weathering

Chemical weathering occurs in rock due to exposure and the chemical changes in the rock itself causing decay of the mineral matrix. Water plays a major role in chemical weathering. Examples of chemical processes include oxidation of iron-rich rocks, carbonation (limestone is very susceptible), hydrolysis, and hydration.

 

Biological Weathering

Biological weathering occurs through the interaction of living organisms and rocks. This includes plant roots, lichens growing on and chemically decomposing the rocky substrate, and the movement of burrowing animals.

 

Self Assessment: Rock Cycle Summary

Please download and complete the Rock Cycle Summary sheet below.

Click here to download the Rock Cycle Summary. Links to an external site.

 

RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.