Blueschist

A metavolcanic rock, also known as glaucophane schist, produced by the metamorphism of basalt under an unusual combination of relatively low temperature, between about 200 and 500 degrees Celsius, and very high pressure corresponding to depths of 15 to 30 kilometers, conditions that occur only during the subduction of one tectonic plate beneath another. Its characteristic blue color comes from minerals such as glaucophane and lawsonite, and its mineral assemblage typically includes glaucophane together with lawsonite or epidote, and may also include jadeite, albite, chlorite, garnet or muscovite, all within an overall basaltic composition; platy minerals such as chlorite and mica give the rock an elongated crystal structure. Blueschist is geologically significant because it provides direct evidence of subduction zone processes, forming only under the low temperature, high pressure conditions found where an oceanic plate is descending into the mantle. Because such rock must be exhumed quickly to reach the surface before it heats up and transforms into a different rock type, blueschist is rare at the surface but scientifically valuable for understanding subduction and the Earth's internal dynamics.