Rocks formed by the action of heat and/ or pressure on any pre-existing rock.
They can be recognised easily as they are crystalline, all crystals are interlocking, and are layered as the crystals have a preferred orientation. Fossils and sedimentary structures are rare.
image from http://gly1000-03.fa00.fsu.edu/lectures/metarocks_files/images006.jpg
Metamorphism occurs at temperatures over 200 degrees centrigrade. Metamorphic changes occur in the solid state, there is no melting.
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