metamorphic

Metamorphic refers to rocks that have undergone metamorphism, a solid-state transformation of pre-existing rocks due to changes in temperature, pressure, and chemically active fluids. This process alters mineralogy and texture, and sometimes chemistry, without melting the rock. The term derives from the Greek meta- meaning change and morphe meaning form.

Metamorphism is driven by heat, pressure, and fluids. Heat from adjacent magmatic bodies or deeper crustal sources promotes new mineral growth through recrystallization. Pressure, especially directed or differential stress, reorients minerals and can produce foliated textures. Fluids such as water and carbon dioxide facilitate chemical reactions that drive mineral transformations and metasomatism. Metamorphism can occur over broad areas or in localized settings, and may involve varying combinations of these agents.

Common types of metamorphism include regional metamorphism, which affects large areas at convergent plate margins and is associated with deep burial and tectonic stresses; contact metamorphism, caused by heat from intruding magma that alters surrounding rocks in a localized aureole; and dynamic metamorphism, related to deformation and shearing along faults. Subduction zones can produce high-pressure, low-temperature metamorphism, yielding blueschist facies.

Metamorphic rocks are typically classified as foliated or non-foliated. Foliated rocks (such as slate, phyllite, schist, and gneiss) display aligned minerals due to directed stress. Non-foliated rocks (such as marble, quartzite, and hornfels) lack preferred orientation and result from more uniform conditions. Metamorphic facies, including greenschist, amphibolite, and granulite, reflect characteristic pressure–temperature conditions and associated index minerals like chlorite, biotite, garnet, or kyanite. Metamorphic rocks record the thermal and tectonic history of the crust and are widely used to study Earth's geologic evolution.