Controlling Factors of Metamorphic Rocks

The world of geology is not a static world. While the rocks we see around us seem stable, everything is undergoing change. Most of the rocks we see are attacked by the forces of weathering, slowly broken down into their mineral components to be formed into new sedimentary rocks. Weathering is one of the geological processes that help to create new rocks. While weathering works to break down rocks, there is another geologic process that is responsible for creating new rock from existing rock, without the original rock having to be broken down by weathering. These new rocks are created through increases in heat and/or pressure as the parent rock is buried in the earth. This process is known as metamorphism and creates metamorphic rocks.

As we look at the Rock Cycle we can see that metamorphism involves heat and pressure to change igneous and sedimentary rocks (and other metamorphic rocks) into metamorphic rocks. Metamorphism is defined as the change in the characteristics of a rock in response to changes in temperature, pressure, or fluid content. Usually these changes do not change the chemical composition of the rock (the minerals remain the same), but the crystallization of new mineral phases. A rock that has been altered through changes in temperature or pressure is called a metamorphic rock. The formation of metamorphic rocks is determined by different controlling factors: composition, temperature and pressure, and foliation.

The composition of a metamorphic rock is determined by the mineral content of the parent rock. Only in very rare instances will a metamorphic rock have a different mineral composition than its parent rock. Knowing that the composition doesn't change allows a geologist to determine the type of parent rock. Knowing the parent rock allows the geologist to infer much about the origin of the material, allowing for deduction to be made concerning the presence of ancient mountain ranges, coastlines, and seas, even after the sedimentary and igneous rocks have been buried and altered through intense heat and pressure.

Two of the most important controlling factors are temperature and pressure. Though all of the controlling factors work in concert to create metamorphic rocks, temperature and pressure help to create much of the variation. Temperature affects the changes in a minerals composition, as the temperature increases the mineral is altered. Temperature changes occur as rocks are subducted into the earth, or as hot magma rises from the earth's interior. Minerals are only stable over a specific range of temperatures that are unique for the mineral. Geologists have done laboratory studies to determine the range of temperatures at which specific minerals are stable. By using this information a geologist can then study a metamorphic rock and determine the temperature of metamorphism at which the rock was formed.

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