Igneous rocks are formed when volcanoes erupt, or when any magma rises and cools. Intrusive igneous rock does not exit the earth’s crust. Extrusive igneous rock does exit the crust; this is what you would see as lava pours out of a volcano.

Clastic sedimentary rock is formed from the fragments of other rocks. Chemical sedimentary rock is formed when ions dissolve in water.

Metamorphic rocks are rocks that were, at some point, sedimentary or igneous. When rocks change from recrystallization in the solid state, they become metamorphic. This can happen when temperature goes up, pressure increases, or the composition of pore fluids changes. A new rock texture develops, and the rock physically looks different. Metamorphic rock can be found in the cores of eroded mountain ranges, such as the Appalachian.

The two major groups of metamorphic rock are foliated and nonfoliated. Foliated metamorphic rocks have aligned platy minerals. Nonfoliated metamorphic rocks lack platy materials. Also, a rare third group, called mylontic are very fine grained examples of metamorphic rock.