What are metamorphic rocks? American Geosciences Institute

Metamorphic rocks are rocks that have become changed by intense heat or pressure while forming. In the very hot and pressured conditions deep inside the Earth’s crust, both sedimentary and igneous rocks can be changed into metamorphic rock. In certain conditions these rocks cool and crystallize usually into bands of crystals. Later they can become exposed on Earth’s surface. One way to tell if a rock sample is metamorphic is to see if the crystals within it are arranged in bands.

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Glossary of Selected Geologic Terms

Colorado Geology - Colorado Geological Survey

Metamorphic Rocks

An absentee note for ancient blueschist: Lack of metamorphic rock does not date onset of plate tectonics

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Metamorphic rock - Wikipedia

Metamorphic rock - Wikipedia

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