Metamorphic rocks are a major class of crystalline rocks formed when pre-existing igneous, sedimentary, or earlier metamorphic rocks undergo solid-state mineralogical, textural, and sometimes chemical changes in response to variations in pressure, temperature, and fluid regime within Earth’s crust and upper mantle. Metamorphism produces characteristic mineral assemblages and fabrics (e.g., foliation, lineation) diagnostic of specific pressure–temperature conditions and metamorphic facies. These rocks record tectonothermal events such as orogenesis and crustal thickening, and are central to studies of crustal evolution, rheology, and geothermobarometry, enabling quantitative reconstruction of metamorphic paths (P–T–t histories) and regional geodynamic processes.
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