Geology And Geophysics

Types of Metamorphism

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There are six general types of metamorphism established by geologists. Metamorphic rocks are created when sedimentary, igneous, or other metamorphic rocks are exposed to extreme heat, pressure, and occasionally chemical solutions. The types of metamorphism are distinguished by the factors involved and also the location that the rocks were formed.

Contact metamorphism 

Non-foliated, or granular, metamorphic rocks are often created by contact metamorphism, in which pressure is not a significant factor. This type of metamorphism occurs adjacent to magma that has moved up into the earth’s crust. The invading magma creates a relatively small area where its surroundings are heated, called a metamorphic or contact aureole. Beyond that area, rock remains unaffected by the heat. The closer to the aureole, the hotter the rock, and the more intense the metamorphism. Hornfels is an example of rock created by the baking process of contact metamorphism.

Regional metamorphism

Unlike contact metamorphism, regional metamorphism affects rocks over a large area that is not strongly influenced by magma. The main factor involved in this type of metamorphism is pressure, creating foliated rocks like slates, schists, and gneisses. Regional metamorphosed rock is found in eroded mountain ranges where land masses previously collided, thrusting the compressed rock to the surface as well as deeper into the earth, subjecting it to more pressure.

Cataclystic metamorphism

Fault zones sometimes create a unique environment when one body of rock slides past another. The friction of the rock sliding creates heat and also crushes and breaks other rocks. This type of metamorphism is not common and is limited to a narrow area along fault zones. 

Hydrothermal metamorphism

Rocks and minerals being submitted to high pressure and high temperatures in the presence of hot, mineral-rich fluids, experience hydrothermal metamorphism. This happens a lot in basalt and similar rocks. The presence of the hydrothermal fluids alters the rocks by adding new minerals during the chemical reactions taking place under the heat and pressure. This often results in rich ore deposits being found in this type of metamorphic rock.

Burial metamorphism

At pressures slightly less than that of regional metamorphism, a type of metamorphism occurs called burial metamorphism. Sedimentary rocks are buried hundreds of meters in the earth, exposed to high temperatures, but they do not undergo metamorphism. As greater pressures and temperatures come into play because of the differential stress of factors like tectonic plates, this slight metamorphism occurs. 

Shock metamorphism

Ultrahigh pressures result in minerals only stable at other high pressures. Coesite and stishovite are examples. A meteorite impacting the earth or a large volcanic explosion can create the environments necessary for these rocks. Shock metamorphism also results in unique texture patterns known as shock lamellae. The rock at the site of the impact will also display a shatter cone pattern.

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