Rock Types

There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are rocks that have cooled (frozen or crystallized) from a melt or magma. Although they usually form at or near plate boundaries, a few are made at other places called hot spots (such as Yellowstone or Hawaii) where deeply sourced plumes of rising hot mantle rock occur.

The two most common igneous rocks are basalt and granite. Basaltis a fine-grained, dark rock that usually forms from magma or lava derived from partial melting of the mantle. It makes up the entire floor of the deep oceans and is also the most common fine-grained igneous rock on the continents.Granite is the most common coarse-grained igneous rock. It is pink to gray in color (from the color of the common feldspars) and is found only on the continents. Granite usually forms from the melting of sedimentary and metamorphic rocks or from andesitic rocks that themselves formed from partial melting of mantle rock above subduction zones. Granite rocks are often associated with the process of mountain building.