igneous

Igneous rocks are formed by the cooling and solidification of molten rock. If the melt solidifies underground it is called magma; if it erupts onto the surface it is lava. The source of the melt is typically partial melting of existing crustal or mantle rocks. The cooling rate controls texture: slow cooling yields coarse-grained rocks with visible crystals, while rapid cooling produces fine-grained rocks. Very rapid cooling can result in glassy textures.

Igneous rocks are commonly classified by texture as intrusive (plutonic) or extrusive (volcanic). Intrusive rocks crystallize below the surface and are usually phaneritic, for example granite, diorite, and gabbro. Extrusive rocks erupt or crystallize at the surface and are typically aphanitic or glassy, such as rhyolite, andesite, basalt, obsidian, and pumice. Some rocks have porphyritic textures, with large crystals in a finer matrix.

They are also classified by composition into felsic, intermediate, mafic, and ultramafic groups. Felsic rocks are rich in light-colored minerals (feldspars, quartz) and silica, and tend to be light in color; mafic rocks contain more magnesium and iron and are usually darker. Ultramafic rocks are very rich in ferromagnesian minerals and are rare at the surface. Common examples include granite (felsic intrusive), diorite (intermediate intrusive), rhyolite (felsic extrusive), and basalt (mafic extrusive); gabbro and diabase are their coarse-grained equivalents.

Igneous processes such as magmatic differentiation, fractional crystallization, and assimilation drive diversity in composition and mineralogy, and igneous rocks form a central part of the rock cycle linked to tectonic settings.