A large amount of metamorphism over a broad geographic area is called regional metamorphism. Movements of the Earth’s tectonic plates are the primary cause of regional metamorphism. As continental plates collide, the increased pressure, deep within developing mountain ranges, squishes rocks. At plate boundaries where one plate is subducted below another, the subducted rocks are altered by the increased pressure and temperature deep underground. Regional metamorphism can also happen to rock that is buried over time as overlying rocks are deposited.
Typically, regional metamorphism occurs at depths of 5-40 km, where both the pressure from the weight of the overlying rocks and the temperature is high. Because of the radioactive decay of elements, temperature increases as you go deeper within the Earth about 20-30 degrees Celsius for every kilometer of depth within the crust.
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