Greenschist

Greenschist is a green, foliated metamorphic rock rich in minerals such as chlorite, actinolite, or epidote. It commonly forms when basaltic rocks undergo low-grade metamorphism.

Chlorite: A group of green sheet silicate minerals common in low-grade metamorphic rocks. Chlorite gives many greenschists their green color and records low-temperature alteration.
Greenschist facies: A metamorphic facies characterized by low- to moderate-temperature mineral assemblages such as chlorite and actinolite. Greenschist rock commonly records the conditions named by this facies.
Protolith: The original rock from which a metamorphic rock forms. Identifying a greenschist's protolith explains its inherited chemistry.
Blueschist: A metamorphic rock formed at high pressure and relatively low temperature, commonly containing glaucophane. Blueschist records colder, higher-pressure conditions than typical greenschist.
Actinolite: A green calcium-bearing amphibole mineral found in metamorphic rocks. Its presence helps characterize the mineral assemblage of greenschist.
Metamorphism: The solid-state transformation of rock through changes in temperature, pressure, or fluid chemistry. Greenschist forms through metamorphic reactions rather than melting.
Greenstone belt: An ancient, deformed sequence of metamorphosed volcanic and sedimentary rocks, often dominated by greenstone. Greenschist-grade rocks are widespread within many greenstone belts.
Amphibolite: A medium- to high-grade metamorphic rock dominated by amphibole and plagioclase. Amphibolite generally records hotter conditions and a different mineral assemblage.
Epidote: A group of calcium-aluminium-iron silicate minerals common in metamorphic rocks. Epidote is another green mineral that can contribute to greenschist's appearance and composition.
Subduction zone: A region where one tectonic plate descends beneath another into the mantle. Subduction can bury and metamorphose basaltic crust into greenschist.

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