Most intrusive rock is plutonic-that is, coarse-grained rock that solidified slowly at considerable depth. Most plutonic rock exposed at the earth's surface is in batholiths-large plutonic bodies with no particular shape. A smaller, irregular body is called a stock.

Silicic (or felsic) rocks are rich in silica, whereas mafic rocks are silica deficient. Most igneous rocks are named on the basis of their mineral content, which in turn reflects the chemical composition of the magmas from which they formed, and on grain sizes. Granite, diorite, and gabbro are the coarse-grained equivalents of rhyolite, andesite, and basalt, respectively. Ultramafic rocks are made entirely of ferromagnesian minerals and are mostly associated with the mantle.

Basalt and gabbro are strongly predominant in the oceanic crust. Granite strongly predominates in the continental crust. Younger granite batholiths occur mostly within younger mountain belts. Andesite is largely restricted to narrow zones along convergent plate boundaries.

The geothermal gradient is the increase in temperature with increase in depth. Hot mantle plumes, from the lower mantle, and magma at shallow depths in volcanic regions locally raise the geothermal gradient.

No single process can satisfactorily account for all igneous rocks. Several hypotheses adequately explain some igneous rocks. In the process of differentiation, based on Bowen's reaction series, a residual magma more silicic than the original mafic magma is created when the early-forming minerals separate out of the magma. In assimilation, a hot, original magma is contaminated by picking up and absorbing rock of a different composition. Magma mixing produces a magma whose composition is intermediate, between that of the two types of magma that were mixed.

Partial melting of the mantle usually produces basaltic magma whereas granitic magma is most likely produced by partial melting of the lower crust.

The theory of plate tectonics incorporates various parts of previous theories. Basalt is generated where hot mantle rock partially melts, most notably along divergent boundaries. The fluid magma rises easily through fissures, if present. The ferromagnesian portion that stays solid remains in the mantle as ultramafic rock. Granite and andesite are associated with subduction. Differentiation, assimilation, partial melting, and mixing of magmas may each play a part in creating the appropriate rocks.