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The minerals that gave the original basalt its black colour have been metamorphosed into green minerals. Intense heat or great pressure transforms basalt into its metamorphic rock equivalents. Depending on the temperature and pressure of metamorphism, these may include greenschist , amphibolite , or eclogite .
Where subduction is taking place, the basalt of the subducting slab is metamorphosed to high-pressure metamorphic facies. It initially undergoes low-grade metamorphism to metabasalt of the zeolite and prehnite-pumpellyite facies, but as the basalt subducts to greater depths, it is metamorphosed to the blueschist facies and then the eclogite facies.
Blueschist on Île de Groix, France Photomicrograph of a thin section of blueschist facies metamorphosed basalt, from Sivrihisar, Turkey. Blueschist (/ ˈ b l uː ʃ ɪ s t /), also called glaucophane schist, is a metavolcanic rock [1] that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (200–500 °C (392–932 °F ...
In between, the metamorphosed siltstone is visible as both the dark layer (~5 cm thick) and the pale layer below it. Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock.
Metasomatism (from the Greek μετά metá "change" and σῶμα sôma "body") is the chemical alteration of a rock by hydrothermal and other fluids. [1] It is traditionally defined as metamorphism which involves a change in the chemical composition, excluding volatile components. [2]
Basalt is the most common extrusive igneous rock [9] and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns. The Giant's Causeway in Antrim, Northern Ireland is an example. The molten rock, which typically contains suspended crystals and dissolved gases, is called magma. [10]
Metamorphosed basalt (metabasalt) creates ortho-amphibolite and other chemically appropriate lithologies create para-amphibolite. Although tremolite is a metamorphic amphibole, it is most commonly derived from highly metamorphosed ultramafic rocks , and thus tremolite-talc schist is not generally considered a variety of amphibolite.
The Vishnu Schist is believed to be highly metamorphosed igneous rocks and shale, from basalt, mud and clay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well.