"Metamorphic rocks provide a record of the processes that occurred inside earth as the rock was subjected to changing physical and chemical conditions. This gives the geologist literally "inside information" on what occurs within the earth during such processes as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into hot oceanic crust. Metamorphic rocks are like probes that have gone down into the earth and come back, bringing an record of the conditions they encountered on their journey in the depths of the earth. "
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