igneous rocks.

Igneous rocks are formed at high temperetures and are made of lava or magma that has cooled and solidified. Magma which cools on the surface forms extrusive or volcanic rocks. Magma often never makes it to the surface, the magma that cools under the surface is able to crystilize to a much greater extent and forms a intrusive igneous rock.

Most igneous rocks are formed of four or five minerals which determine the main characteristics of the rock. These minerals are referred to as essential minerals. Any other minerals that occur in igneous rocks are referred to as accessory minerals and play little role in rocks properties.

Igneous rocks are classified by which essential minerals are present and whether or not they are extrusive or intrusive. There are four categories determined by essential minerals, the chart below shows these categories