Pressure, Temperature, and Transformation
When rocks are buried by tectonic forces, rising pressure and temperature drive solid-state mineral reactions that transform their mineralogy and texture without melting. Metamorphic petrology maps these changes onto pressure–temperature diagrams, revealing the conditions deep within mountain belts, subduction zones, and contact aureoles around magma intrusions.
The Facies Concept
Pentti Eskola's metamorphic facies concept groups rocks by the mineral assemblages they contain, regardless of their original composition. A rock in greenschist facies is defined by the stability of chlorite and albite; amphibolite facies by hornblende and plagioclase. This framework allows geologists to read temperature and pressure directly from a thin section, reconstructing conditions that existed millions of years ago kilometres underground.
P-T Paths and Tectonic Setting
Different tectonic environments produce characteristic pressure–temperature trajectories. Subduction zones follow cold, high-pressure paths through blueschist and eclogite facies. Continental collision zones trace moderate Barrovian paths through greenschist and amphibolite facies. Contact metamorphism near plutons creates steep high-temperature paths at low pressure. The P-T path recorded in a single rock's mineral zoning tells the story of its tectonic journey.
Index Minerals and Grade
Barrow's zones — chlorite, biotite, garnet, staurolite, kyanite, sillimanite — define a progression of metamorphic grade in pelitic rocks. Each index mineral first appears at a specific temperature, providing a natural thermometer for field geologists mapping metamorphic terranes. The aluminium silicate triple point, where kyanite, sillimanite, and andalusite coexist, is one of the most precisely calibrated P-T markers in geology.