metamorphism

Metamorphism refers to the alteration of preexisting rocks (the protolith) in response to changes in temperature, pressure, and chemically active fluids, producing new mineral assemblages and textures without melting. It occurs mainly within the crust and is driven by heat from intrusions, burial of sediments, and tectonic forces that generate directed pressure and fluid flow.

Regional metamorphism affects large geographic areas during mountain-building events when rocks are heated and compressed by tectonic deformation. Contact metamorphism is localized around intruding magma bodies, where high temperatures dominate. Hydrothermal metamorphism involves hot, chemically active fluids circulating through rocks, often along fractures.

Metamorphic rocks are grouped by texture: foliated rocks show aligned minerals formed under directed stress (slate, phyllite, schist, gneiss). Nonfoliated rocks lack foliation and crystallize under more uniform conditions (quartzite from sandstone, marble from limestone, hornfels from various protoliths).

Metamorphism is characterized by mineral assemblages that record pressure-temperature conditions, known as metamorphic facies (greenschist, amphibolite, granulite, blueschist, eclogite). Graded increases in temperature and pressure lead to progressively higher-grade rocks as platy mineral growth and recrystallization produce distinct textures.

In some settings, partial melting produces migmatites and, eventually, igneous bodies. Metamorphism ceases when rocks adjust to new stable conditions at the surface or during uplift. The study of metamorphism helps reconstruct tectonic histories and crustal evolution.