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General Metamorphism
Metamorphic processes require heat, pressure or a combination of both. Chemically, nothing is added to the rock or lost. Existing elements are simply forced to recombine into new mineral arrangements. This rearrangement is caused by temperature and pressure variations that differ from the conditions under which the rock was originally formed. Metamorphic changes, as opposed to sedimentary morphing, occurs at temperatures greater than 392 degrees Fahrenheit and pressures greater than 300 Mega Pascals, or about 3,000 atmospheres of pressure. However, the process must proceed without the rock melting, known as solid-state metamorphism, or the outcome is igneous rock, rather than metamorphic rock.
Cataclistic Process
The cataclistic process is caused by mechanical deformation of the rock. This process occurs along transform boundaries, represented by faults. This process both crushes the rock and heats it as a result of friction, producing metamorphic changes. This process is not very common, limited to faults such as the San Andreas strike-slip fault in California. Cataclistic metamorphism is considered a low-temperature, high-pressure process.
Burial Process
The burial process occurs when sedimentary rock is buried to depths greater than around 1,000 feet. Temperatures can rise above the 392-degree threshold, resulting in metamorphic changes. These changes include the growth of new minerals, such as zeolites. Burial metamorphism does not involve mechanical deformation; rather, the heat is generated strictly by pressure due to depth.
Contact Process
The contact process is caused by exposure to hot magma, known as igneous intrusions. As this magma contacts the surrounding rock, it is heated above the 392-degree threshold. The result is a thin envelope of metamorphic rock, called a metamorphic aureole, which surrounds the magma field. This contact process typically occurs at shallow depths and is considered a high-temperature, low-pressure metamorphic process.
Regional Process
The regional process is caused by extreme differentiated stress, where rock layers are horizontally compressed. This compression produces a foliated, or folded, structure. Regional metamorphism occurs over large areas, typically in convergent zones where continental plates are colliding. The folds force rock upwards, forming mountains, and downwards, thickening the crust. The rock that is forced downward is then exposed to higher temperatures and pressures, where further metamorphism occurs. An example of regional metamorphism is the Himalayan Mountains, where the Indian plate is colliding with the Eurasian plate.