Metamorphic rock consists of minerals which were originally
sedimentary rock or
igneous rock and which have been chemically altered by
heat,
pressure or the introduction of other chemical substances, but which have not been remelted. This process is called
metamorphism or
metamorphosis.
Metamorphic rocks are sometimes revealed when plates collide (see obduction and subduction) and mountains are created.
See also: List of minerals, rock (geology)