Greenschist is a type of metamorphic rock that forms under moderate temperature and pressure conditions. It is primarily composed of minerals such as chlorite, biotite, and plagioclase, which give it a characteristic green color. This rock typically originates from the metamorphism of basalt or other mafic igneous rocks. The formation of greenschist occurs in environments like subduction zones or mid-ocean ridges, where tectonic activity leads to the alteration of existing rocks. Greenschist is often associated with the Greenschist facies, a set of metamorphic conditions that indicate specific temperature and pressure ranges during rock formation.