What Causes Rocks To Metamorphose

Marble is an expensive metamorphic rock sometimes used in home design.

Metamorphic rock is rock that has been greatly changed due to its surrounding environment. The word comes from the Greek words meta, meaning change, and morph, meaning form. Marble, quartzite and slate are a few examples of metamorphic rock. Marble is formed from limestone, slate is formed from shale, and quartzite is formed from sandstone. The metamorphic process often happens deep underground and the rock is usually quarried or resurfaces due to erosion.

Temperature

One of the biggest causes of rock metamorphose is heat. Rock buried deep underground is subjected to heat coming from the hot magma at the center of the earth. Different levels of heat cause different grades of metamorphose. The deeper the rock is buried the more heat it is exposed to. A low-grade metamorphose happens when a rock is subjected to low pressure and heat between 200 to 320 degrees Celsius, according to the Tulane University website. Low-grade metamorphic rock tend to have a lot of minerals that contain water, such as clay and chlorite. High grade metamorphism happens at temperatures of 320 degrees Celsius or higher. This type of rock tends to have minerals that don’t contain a lot of water, such as muscovite and garnet.

Pressure

Pressure combined with heat helps to merge the rock together. Often when rocks began to metamorphose, minerals like mica found in the rock become compressed and flattened by the pressure. The result is called foliation. Different depths and various directions of pressure create different metamorphic rock. Pressure from being buried deep underground and pressure caused by tectonic plates colliding head-on underground often causes regional metamorphism, where rocks over a wide area are metamorphosed. Pressure along fault lines where tectonic plates slide past each other in opposite directions causes dynamic metamorphism. Dynamic metamorphism is when localized rocks are changed by deformation, usually as the result of high pressure but low heat.

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Fluid Content

As rock begins to compact, fluid in the rocks is slowly pushed out of the rock. The more fluid the rock loses the more it metamorphoses and changes. This can also affect rock that is reversing its metamorphic process, known as retrograde metamorphose. Retrograde metamorphosis happens when pressure and heat are reduced due to events like erosion. As the rock begins to surface and the heat and pressure lessen the rock transformation reverses. The more fluid the rock lost during the original metamorphose process the less likely the metamorphose process will reverse. Although even if a rock still contains a lot of its original fluid content, it is not possible for a metamorphic rock to completely reverse back the original rock, according to the James Madison University website.

Time

Chemical reactions that cause certain minerals and crystals to appear in rock during the metamorphic process take a long time. According to the Tulane University website, the longer a rock goes through the metamorphosis process the bigger the mineral crystals are. Metamorphic rock that contains coarse grained minerals have often undergone a long metamorphic process, possibly millions of years.