Metamorphic grade

Metamorphic grade describes the pressure and temperature conditions a rock experienced during metamorphism, commonly inferred from its mineral assemblage and texture.

Metamorphic facies: A set of metamorphic mineral assemblages characteristic of a particular range of pressure and temperature. Facies classify rocks by the conditions that grade describes.
Metamorphism: The solid-state transformation of rocks through changes in mineralogy, texture, or chemical composition. Metamorphic grade measures conditions within this broader transformation.
Metamorphic zone: A region in which rocks share a characteristic metamorphic mineral assemblage. Mapped zones show spatial changes in grade across metamorphic belts.
Metamorphic intensity: The degree of mineralogical or textural change a rock has undergone during metamorphism. Intensity concerns the extent of change, while grade concerns pressure and temperature.
Index mineral: A mineral whose presence or absence helps constrain the conditions under which a rock formed. Index minerals provide practical evidence for assigning metamorphic grade.
Protolith: The original rock from which a metamorphic rock formed. Protolith composition affects which minerals can indicate a given grade.
Isograd: A line on a map marking the first appearance of a particular metamorphic index mineral. Isograds outline boundaries between areas of different metamorphic grade.
Metamorphic facies series: An ordered sequence of metamorphic facies linked to a characteristic pressure–temperature regime. A facies series describes a regional sequence rather than one rock's grade.
Barrovian metamorphism: Regional metamorphism characterized by a sequence of mineral zones associated with increasing temperature. Its mineral zones show how grade changes across a metamorphic terrain.
Metamorphic rock: Rock altered in the solid state by heat, pressure, or chemically active fluids. Metamorphic rocks preserve the mineral and textural evidence used to estimate grade.

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