Hi I’m Geologist John Swanson for About.com and today I am going to talk about types of igneous rocks.Igneous rocks are fascinating geologic forms. Millions of years ago, the solidification of molten rocks formed the different types of igneous rock. There are two types of igneous rocks: intrusive, which are found underneath the earth’s surface and extrusive, which are found on or above the earth’s surface.Andesite: an extrusive igneous rock that is fine-grained.Basalt: another extrusive igneous rock that is also fine-grained.Diorite: this is our first intrusive igneous rock on the list and is a course-grained rock.Gabbro: an intrusive, course-grained rock.Granite: yet another intrusive, course-grained rock. Are you seeing a pattern between grain size and formational environment?Obsidian: Is quite different than the rocks above. It is volcanic glass with a very dark color. It is formed when molten rock is rapidly cooled; in fact it cools so quickly, crystals do not form.Pegmatite: an intrusive igneous rock that is light in color.Peridotite: nearly completely composed of olivine, Peridotite is a course-grained intrusive igneous rock.Pumice: is a vesicular igneous rock that is light in coloration.Rhyolite: is an extrusive igneous rock and is light in color and fine grained.Scoria: like pumice, scoria is vesicular extrusive rock. But unlike pumice, scoria is dark-colored.Tuff: is an extrusive rock that is formed when volcano-ejected materials fall to earth and are then lithified into rock.Hopefully you now understand the different types of igneous rock. Thanks for watching. If you’d like to learn more, please find us on the web at About.com.