Metamorphic rock

Metamorphic rock forms when preexisting rock is altered by heat, pressure, or chemically active fluids without melting completely. Its minerals and textures record conditions during transformation.

Metamorphism: The solid-state transformation of rock through changes in temperature, pressure, or chemical environment. This broader process produces metamorphic rock without requiring complete melting.
Protolith: The preexisting rock from which a metamorphic rock forms. Its identity helps explain which minerals and structures metamorphic change inherited.
Regional metamorphism: Metamorphism affecting broad regions, commonly during tectonic burial, mountain building, or crustal thickening. It produces extensive belts of metamorphic rock during large-scale tectonic events.
Slate: A fine-grained foliated metamorphic rock formed mainly from shale or mudstone at low metamorphic grade. Slate demonstrates how low-grade metamorphism creates a strong, closely spaced cleavage.
Metamorphic facies: A set of mineral assemblages characteristic of a particular range of pressure and temperature. Facies link the minerals in metamorphic rock to conditions of formation.
Index mineral: A mineral whose presence indicates a particular range of metamorphic conditions. Index minerals provide evidence for the pressure and temperature recorded by a rock.
Contact metamorphism: Metamorphism caused mainly by heat from nearby magma, typically over a limited area. It transforms surrounding rock near intrusions, often with limited directed pressure.
Schist: A medium- to coarse-grained foliated metamorphic rock with abundant visible platy or elongate minerals. Its conspicuous mineral alignment illustrates schistosity and medium-grade change.
Metamorphic grade: A measure of the intensity of metamorphism, commonly inferred from mineral assemblages and reactions. Grade summarizes how strongly a rock’s minerals and textures were altered.
Porphyroblast: A large mineral crystal that grew within a finer-grained metamorphic rock. Porphyroblasts can preserve evidence of mineral growth during metamorphism.

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