Metamorphic rocks are sedimentary or igneous rocks that have been altered by heat and/or pressure. As they are derived from previously existing igneous, sedimentary or even metamorphic rock, their appearance is variable. They are identified by the types of minerals they contain and their texture. Thermally metamorphosed rocks occur bordering igneous intrusions, which altered the surrounding rock originally because of their intense hear, resulting also in the formation of new minerals such as andalusite and garnet. Regionally metamorphosed rocks occur in the roots of mountain ranges, where intense pressures and high temperatures formed platy minerals (e.g. mica) and high-pressure minerals (e.g. staurolite).
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