Igneous and metamorphic rocks form within the earth at high temperatures. Sedimentary rocks form at the earth's surface. We will spend most of this meeting learning their distinguishing characteristics.
Igneous rocks form by cooling and crystallization from a molten state.
Therefore, they consist of interlocking crystals. Intrusive igneous rocks cool and crystallize slowly within the earth and so have coarse grains. Igneous rocks are further classified by their chemical composition. The types of minerals in the rock reflect this chemistry. Generally, the richer in silica and oxygen, the lighter colored the rock.