Metamorphism

Metamorphism is the solid-state transformation of rock as temperature, pressure, or fluid conditions change. It alters minerals and textures without melting the rock completely.

Metamorphic grade: A measure of the intensity of metamorphism, commonly inferred from mineral assemblages and reaction progress. Grade tracks how strongly changing conditions have transformed a rock.
Mineral assemblage: A group of minerals occurring together in a rock or geological setting. The minerals preserved together help reveal the conditions of metamorphism.
Index mineral: A mineral whose presence indicates a characteristic range of metamorphic pressure and temperature. Index minerals let geologists estimate metamorphic conditions from exposed rocks.
Blueschist: A metamorphic rock containing blue amphibole, typically formed under high-pressure, low-temperature conditions. Blueschist records the unusual conditions common in subduction zones.
Metamorphic facies: A set of mineral assemblages characteristic of a particular range of pressure and temperature. Facies connect the minerals formed during metamorphism to the conditions that produced them.
Pressure–temperature diagram: A graph showing how pressure and temperature vary, often used to represent geological conditions or processes. Metamorphic stability fields are commonly interpreted on pressure–temperature diagrams.
Geothermobarometry: Methods that estimate the temperature and pressure at which mineral compositions equilibrated. Mineral chemistry can quantify the conditions reached during metamorphism.
Eclogite: A dense metamorphic rock dominated by garnet and omphacitic pyroxene, usually formed at high pressure. Eclogite shows how crustal rocks transform at depths associated with subduction.
Metamorphic reaction: A solid-state chemical reaction that changes a rock’s minerals as pressure, temperature, or fluid composition changes. Reactions explain how new minerals replace earlier ones without requiring complete melting.
Foliation: A planar fabric in rock produced by aligned minerals, compositional layers, or flattened grains. Directed stress during metamorphism can organize minerals into planar fabric.

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