Felsiques

Felsiques is a term used in geology to describe light-colored, silica-rich igneous rocks. In English, this category is known as felsic. Felsiques are contrasted with mafic and ultramafic rocks, which are richer in magnesium and iron and typically darker in color. Felsiques crystallize from relatively low-temperature magmas and are dominated by light-colored minerals such as feldspars and quartz.

Mineralogy and composition: The essential minerals of felsique rocks are feldspars (primarily orthoclase and plagioclase) and quartz. Micas such as muscovite or biotite and minor amounts of amphibole or pyroxene may occur. They have high silica (SiO2) content, typically above about 65 percent, and relatively high aluminum; they are correspondingly low in magnesium and iron compared with mafic rocks.

Textures and representative rock types: Felsiques occur as both intrusive and volcanic rocks. Intrusive equivalents include granite and granodiorite, which crystallize as coarse-grained rocks within the crust. Extrusive felsic rocks include rhyolite, obsidian, and related tuffs, which record rapid cooling at or near the surface and often show porphyritic textures or glassy groundmass.

Formation and tectonics: Felsic magmas commonly form by partial melting of existing continental crust or by differentiation of mafic magmas. They are prevalent in continental crust and are commonly produced in subduction zones or continental collision settings. The high viscosity of felsic magmas tends to trap volatiles, often leading to explosive eruptions when erupted.

Geochemical significance and use: Felsic rocks contribute most of the continental crust's mass and density. Economically important granitoids include granite and granodiorite, extensively used as building stone. Their distinctive light color and mineralogy also aid geological mapping and dating studies.