The amount of silica (silicon dioxide) in magma determines how easily it moves. For example, magma that forms basalt has about 50% silica and is very fluid when it erupts. Rhyolite magma, with about 75% silica, is very sticky and does not flow easily. Andesite is about 60% silica.

Most igneous rocks contain minerals from six groups– quartz, feldspar, mica, amphibole, pyroxene and olivine. These occur in various combinations, depending on the magma composition, and are used to classify and name igneous rocks. The composition of igneus rocks provides clues to where magma forms in the Earth. Many igneous rocks contain small amounts of unusual minerals that can have useful properties.