Rock altered in structure, composition, and texture by pressure or heat after its original formation. (Rock that actually melts under heat is called
igneous rock upon cooling.) For example, limestone can be metamorphosed by heat into marble, and shale by pressure into slate. The term was coined in 1833 by Scottish geologist Charles
Lyell. Metamorphism is part of the
rock cycle, the gradual formation, change, and re-formation of rocks over millions of years.
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