Metamorphism is the isochemical process by which rocks are changed by heat or pressure or both. Chemical composition of the parent rock will be the same as the metamorphic rock produced.
Rock undergoes slow process of solid-state recrystallisation without melting.
Different temperaturs and pressures cause new minerals to grow in rocks that have the same composition. The minerals are produced directly related to pressure and temperature conditions. The lower temperature limit for metamorphism is between 200 and 150 degrees. Below these temperatures, changes are part of diagenesis. There is no lower pressure limit. The upper temperature limit is where melting occurs, this happens at around 800 degrees.
The process of metamorphism may result in:

destruction of fossils, beds and sedimentary structures
hardening of the rock
change in colour
alignment of minerals
growth of new metamorphic minerals

Temperature

high temperatures occur near to igneous intrusions, where the magma heats the surrounding rocks
temperature also increases with depth, due to the geothermal gradient

As temperature increase, the rate of metamorphic reactions also increases. This is because many of the chemical reactions require heat to take place. Higher temperatures increase the rate at which ions diffuse between minerals, though it is still a slow process because the ions have to move through solid rock during metamorphism. The whole process is greatly speeded up by water, which allows ions to diffuse rapidly.

Pressure

pore pressure in the pressure exerted by fluids between the grains in a porous rock. The presence of water speeds up reaction by acting as a catalyst and increasing the rate and ease of ion exchange.
load pressure is the weight of the overlying rocks and physically brings minerals into contact with each other over very long periods of time
tectonic stress or pressure is caused as the rocks undergo folding or faulting and very high pressures are exerted , but usually over relatively short periods of time.

Higher the pressure, the greater the degree of metamorphism.

Types of Metamorphism
Contact metamorphism
Contact metamorphism occurs adjacent to igneous intrusoins which increase the temperature in the surrounding country rock. The metamorphism is important on a local scale, producing a metamorphic aureole. Temperature is generally high but pressure is low - so minerals are not aligned

Burial Metamorphism
Medium to high pressure and relatively low temperatures. Affects rocks deeply buried by overlying rocks, also occurs at subduction zones where sea floor sediments and basalts are buried. Rocks buried at the deepest levels almost always contact the blue mineral glaucophane. They are called blueschists.

Regional Metamorphism
Caused by low to high temperature and low to high pressure at convergent plate margins. Can result from subduction or continental collision. Pressure is significant so minerals have a preferred alignment. Regionally metamorphosed rocks occur in the cores of fold mountain belts where mountain where mountain ranges have been eroded.

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Undergraduate Geology - Part 2 (Metamorphic Processes and Products) Notiz am Temperature, Pressure and Metamorphism, erstellt von siobhan.quirk am 19/05/2013.