Metamorphic rock

Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat (greater than 150 degrees Celsius) and extreme pressure causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.
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Noun
1. rock altered by pressure and heat
(hypernym) rock, stone
(hyponym) amphibolite

A rock that has undergone chemical or structural changes. Heat, pressure, or a chemical reaction may cause such changes.