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There are three basic rock types: igneous, sedimentary, and metamorphic. These three different types have physical characteristics that make them unique from each other. Rocks are classified into one of these three types based on how they are formed.
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Overview
Watch Geology: Rock'em to get an overview of the three types of rocks and how they are formed.
Igneous
Igneous rocks are formed when molten rock (magma) cools. There are two basic types of igneous rocks:
- extrusive and
- intrusive.
View An Example of Igneous Rocks to see different examples of igneous rocks. Extrusive igneous rock. Agathla Peak in Monument Valley was originally formed below the earth's surface as intrusive rock. Over the years, weathering has exposed the hard igneous rock.
Sedimentary
Sedimentary rocks are formed when sediments small fragments of soil, sand, rock, and plant and animal material that is formed from weathering and erosion accumulate through the process known as deposition.
After sediments are deposited and accumulate, they undergo a process known as lithification or cementation. During this process, the sediment material becomes solidified into a rock. These rocks form on Earth's surface which can be on the land or in the water and in the Earth's crust. Fossils the remains or imprints of a pre-historic organisms that have been preserved in rock are normally found only in sedimentary rock.
Examples of sedimentary rock include:
- Shale - made from sediments of clay minerals mixed with tiny grains of quartz; often has natural gas and oil trapped under it.
- Limestone - has calcium carbonate and is often used in making cement for construction purposes.
View Sedimentary Rock Examples to see different examples of sedimentary rocks, how they are classified, and the two basic types of sedimentary rocks.
Metamorphic
Metamorphic rocks are formed when sedimentary and igneous rocks are exposed to high temperatures and pressure causing them to undergo a change or metamorphosis. The heat does not melt the rocks but instead changes them into a different rock.
The original sedimentary or igneous rock that is transformed into a metamorphic rock is referred to as a protolith. Because of the high temperature and pressure needed to cause the protoliths to change into metamorphic rock, this process takes place deep within the Earth's crust.
View Metamorphic Rocks: Transformation at Work to read more about how metamorphic rocks are formed, examples of metamorphic rocks, and how metamorphic rocks are classified. Make sure to click on the Classification link as well as the Examples link at the bottom of the page.
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