Metamorphism Most mountains are forming today in tectonically active regions where the movements of plates deform the rocks of the Earth's lithosphere. The tremendous energy that's expended in the mountain building process often has a profound effect on the rocks. The geologic events that accompany mountain building, such as the collisions between plates, deep subsidence of portions of the Earth's crust, moving masses of magma, and the displacement of rock bodies along fault zones focus heat and pressure on the rocks. As the result, these rocks are changed dramatically. This process of change by the effects of heat and pressure is called "metamorphism", a term derived from the Greek words "meta," which means "change" and "morph" meaning "form." Metamorphism changes the appearance of rocks, their mineral composition, and even their age as measured by radiometric dating. During metamorphism, atoms within the rock can dislodge themselves from mineral lattices and move about freely. This atomic reshuffling causes the existing minerals to recrystallize and new minerals to form. This process also resets the radioactive clock within the rock to the time of metamorphism. Source: U.S. Geological Survey