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 dependence of Earth's composition on depth. Earth's core consists mostly of fairly pure metal (iron and nickel); surrounding the core is the mantle, a layer consisting mostly of ultramafic rock (metals mixed with silica). The outermost layer of the earth, the crust , consists of two basic types of crust, one primarily mafic (oceanic crust) and the other primarily silicic (continental crust). Oceanic crust, which is only about 4 miles (6 km) thick, consists mostly of basalt , a mafic rock. As oceanic crust inches away from its point of origin at a mid-ocean ridge, its underside cools the ultramafic mantle rocks over which it slides. These cooled mantle rocks stick to the underside of the oceanic crust, thickening it over time. The oceanic crust is thus weighed down by an increasingly thick undercoating of cooled ultramafic mantle rock as it ages. This cool undercoating is denser than the chemically identical but hotter mantle rocks below. Eventually it becomes heavy enough to drag the oceanic crust right down into the mantle, as occurs at a spontaneous subduction zone . The continents, in contrast, are silicic, and float permanently on the mantle. Mafic oceanic crust is spontaneously subducted into the mantle after at most 200 million years, while the continents have never been subducted in the three or four billion years since they were formed.
By weight, Earth consists mostly of mafic and ultra-mafic rocks, but silicic rocks are far more abundant on Earth's surface. Mafic rocks commonly found on the surface include basalt, pyroxene, and biotite; common ultramafic rocks are dunite and peridotite
See also Earth, interior structure; Felsic; Sea-floor spreading