(Left) Randomly-orientated grains in a rock before metamorphism. Metamorphic petrologists rely heavily on statistical mechanics and experimental petrology to understand metamorphic processes. Metamorphic petrology is the study of metamorphism. Metamorphism occurring at increasing pressure and temperature conditions is known as prograde metamorphism, while decreasing temperature and pressure characterize retrograde metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface. Metamorphism takes place at temperatures in excess of 150 to 200 ☌ (300 to 400 ☏), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. The grey and white crystals are quartz and (limited) feldspar. The black (isotropic) crystal is garnet, the pink-orange-yellow colored strands are muscovite mica, and the brown crystals are biotite mica.
A cross-polarized thin section image of a garnet- mica- schist from Salangen, Norway showing the strong strain fabric of schists.
0 kommentar(er)
