Geologists divide rocks into three main types: metamorphic, sedimentary, and igneous. Igneous rocks, the primary rocks from which all other rocks are made, are divided into extrusive rocks that form on the Earth's surface, and plutonic rocks that form deep underground. "Igneous" means made by fire, and all of the igneous rocks are made from very hot liquid rock. In this section of Exploring our Dynamic Earth we'll take a look at the igneous rocks what they are and how they form.
First, let's take a look at what rocks are made of. All rocks are made of materials called minerals. Hundreds of minerals in different amounts make up hundreds of different kinds of rocks. If you look closely at a rock you'll be able to see minerals like quartz, feldspar, or biotite. Minerals are made from elements like silicon, iron, aluminum, or calcium. Most of the sand grains on a beach are made of quartz, a mineral which is made from the elements silicon and oxygen. biotite, on the other hand, is a small shiny mineral that contains the elements aluminum, potassium, and silicon. Geologists name rocks according to the type of minerals they contain. Only six minerals: feldspar, quartz, pyroxene, amphibole, mica, and olivine form 95% of all igneous rocks. If you can recognize these minerals in a rock, you can usually name the rock.