Igneous Rocks
Igneous rocks are formed from solidification of molten (melted) rock. They generally fall into two categories: extrusive or intrusive. Extrusive igneous rocks form at or near the surface. These rocks cool very quickly and have fine crystals (tiny, almost invisible grains) or are sometimes glassy. Intrusive igneous rocks form from magma that cools deep beneath the Earth’s surface. These rocks cool slowly and therefore form large crystals (grains are very visible).