Igneous rocks are classified by geologists using various schemes. One of the several schemes based on chemical composition divides igneous rocks into four categories according to silica (silicon dioxide, SiO₂) content: (1) Rocks containing more than 66% silica are silicic. (2) Rocks containing 52–66% silica are classified as intermediate. (3) Rocks containing 45–52% silica are mafic. (4) Rocks containing less than 45% silica are ultramafic. The term acidic is sometimes used as a synonym for silicic and the terms basic and ferromagnesian as synonyms for mafic. Mafic is an invented adjective based on the chemical symbols for magnesium (Ma) and iron (Fe): Ma-Fe-ic, mafic. Mafic is sometimes used as a synonym for "darkcolored" when discussing the appearance of minerals.

Some mafic and ultramafic rocks are found on Earth's surface. However, because magnesium and iron are denser than silica, mafic rocks are denser than silicic rocks and tend to sink below them. This density difference explains the...