Rocks

Rocks are naturally occurring solid aggregates of one or more minerals or mineraloids. They are classified by the processes that formed them, which can be igneous, sedimentary, or metamorphic.

Formation

Igneous rocks are formed when magma cools and solidifies. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

Sedimentary rocks are formed by the accumulation of sediments. There are three types, clastic, organic, and chemical. Clastic sedimentary rocks are formed from the accumulation of clasts: small pieces of broken rocks. Organic sedimentary rocks are formed from the accumulation of any animal or plant debris such as shells and bones. Chemical sedimentary rocks are formed by the precipitation of minerals from water.

Metamorphic rocks are formed by subjecting any rock type—sedimentary rock, igneous rock or another older metamorphic rock—to different temperature and pressure conditions than those in which the original rock was formed.

Classification

Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks.

Economic importance

Rocks have been used by humans for millions of years, from early tools and weapons through to various construction materials. There are three major types of rock: igneous, sedimentary, and metamorphic. The rock cycle illustrates the relationships among them.

See also

• Geology
• Petrology
• Rock cycle
• List of rock types