Learner Description: This page was created for middle school students in an Earth Science class. Students will be able to explain what the three types of rock are and how to identify the rock types when looking at a rock sample.

Rock Classification

Concept map of rock types including subcategories igneous, sedimentary, metamorphic External link to pictures and explanations of a variety of igneous rocks External link to pictures and explanations of a variety of sedimentary rocks External link to pictures and examplanations of a variety of metamorphic rocks

When identifying a type of rock you need to consider how the rock formed and the rock texture. There are three classifications of rocks; igneous, sedimentary, and metamorphic. Each of these three classifications have special characteristics that help to identify where they were formed.

Igneous

Igneous rocks are rocks formed from the cooling of molten rock. If the rock is formed from magma it is called an intrusive igneous rock. If the rock is formed from lava it is called an extrusive igneous rock.

Intrusive igneous rocks have cooled slowly below Earth's crust. Since they have cooled slowly mineral crystals have formed and you can see the individual crystals within the rock.

Click on any of the above images to view pictures.

Extrusive igneous rocks have cooled from lava erupting out of a volcano. These rocks have the same mineral composition as intrusive rocks, however, they cool too quickly for the crystals to form within the structure of the rock. These rocks can have a bubbly or glassy appearance depending on how quickly they cooled.

Sedimentary Rocks

Sedimentary rocks are formed when sediment compacts together and the particles are cemented together. There are three types of sedimentary rocks; clastic, organic, and chemical.

Clastic rocks form when rock fragments are squeezed and cemented together. Examples of clastic rocks are shale, sandstone, conglomerate, and breccia.

Organic rocks form when the remains of plant or animals are deposited in thick layers and become cemented together. Coal and limestone are both examples of organic rocks.

Chemical rocks are formed when minerals that are dissolved in solution crystallize. Rock salt is an example of a chemical rock.

Metamorphic Rocks

Metamorphic rocks are formed when heat and pressure below Earth's surface change a rock. Metamorphic rocks can be either foliated or nonfoliated.

Foliated rocks are metamorphic rocks that have their grains arranged in parallel layers or bands. Slate is an example of a foliated rock. Before becoming a metamorphic rock slate's parent rock is shale.

Nonfoliated metamorphic rocks are rocks that the grains are arranged randomly. Marble is an example of a nonfoliated metamorphic rock.