Contact Metamorphism

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Metamorphism

The two most common types of the metamorphic processes-contact(thermal)metamorphism, and regional(dynothermal) metamorphism occur within different sets of circumstances, as well as form different rocks as a result of each. The main difference between contact metamorphism and regional metamorphism is that in one case(contact metamorphism) the main factor and cause is temperature, whereas in regional, both temperature and pressure are involved. High temperatures during contact metamorphism occur at relatively shallow depths beneath the surface of the earth and are from hot mantle heating a “contact” between it and a pluton, which is also known as an “aureole”. The aureole usually will occur in a small space between 1 and 100 meters wide. The resultant rocks formed by contact metamorphism are most commonly non-foliated, such as marble, limestone, and quartzite.

The case of regional(dynothermal)metamorphism occurs at great depths beneath the surface of the earth as well covering very wide areas, often covering thousands of square kilometers, as well as convergent plate margins. The high pressure and temperatures are due to the process taking place at depths greater than 5 kilometers. Regional metamorphic rocks are almost always foliated and are formed under various degrees of differential stress caused by the tectonic forces and plate motions in combination with various settings of high pressures and temperatures. Basalt metamorphosed at low temperatures will result in crystals of green-schist. At higher temperatures and pressures the same basalt will result in an amphibolite,which is a rock composed of hornblende, plagioclase feldspar, and possibly garnet.

Metamorhic facies refer to classifications scientists gave to various metamorphic rocks that contain the same set of pressure or temperature sensitive materials, or to imply that they formed under the broadly similar pressure and temperatuer conditons. A zone on a chart with different...
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