Pore fluid present between mineral grains is an important medium through which atoms are exchanged. Rock can be transformed without melting because heat causes atomic bonds to break, freeing the atoms to move and form new bonds with other atoms. However, metamorphism can take place without metasomatism (isochemical metamorphism) or at depths of just a few hundred meters where pressures are relatively low (for example, in contact metamorphism). French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. Hall found that this produced a material strongly resembling marble, rather than the usual quicklime produced by heating of chalk in the open air. This hypothesis was tested by his friend, James Hall, who sealed chalk into a makeshift pressure vessel constructed from a cannon barrel and heated it in an iron foundry furnace. Hutton also speculated that pressure was important in metamorphism. Hutton wrote in 1795 that some rock beds of the Scottish Highlands had originally been sedimentary rock, but had been transformed by great heat. The importance of heating in the formation of metamorphic rock was first recognized by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology. Metamorphism is the set of processes by which existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree. (Right) Grains align orthogonal to the applied stress if a rock is subjected to stress during metamorphism Metamorphic processes (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 ☌ (300 ☏), 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 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. This reaction takes place in nature when a mafic rock goes from amphibolite facies to greenschist facies. Two minerals represented in the figure do not participate in the reaction, they can be quartz and K-feldspar. Abbreviations of minerals: act = actinolite chl = chlorite ep = epidote gt = garnet hbl = hornblende plag = plagioclase. Schematic representation of a metamorphic reaction. For other uses, see Metamorphism (disambiguation).
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