Ultra-High-Pressure Metamorphism

Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of silica, meaning at least about 2.7 gigapascals. It is recognized by diagnostic minerals such as coesite or diamond, or by mineral assemblages such as magnesite with aragonite, and the evidence is usually preserved in eclogite. The formation of many ultra-high-pressure terrains has been attributed to the subduction of microcontinents or continental margins, and their exhumation to the buoyancy of low-density continental crust. Their discovery in 1984 revolutionized understanding of plate tectonics, because it showed that continental rocks can reach such pressures. More than twenty localities are known around the globe, most of them in Eurasia.