Peridotite - Distribution and Location

Distribution and Location

Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones. However, pieces can be emplaced into or overthrust on continental crust by a process called obduction, rather than carried down into the mantle; the emplacement may occur during orogenies, as during collisions of one continent with another or with an island arc. The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses of peridotite have been emplaced into mountain belts as solid masses but do not appear to be related to ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites." Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite. Certain volcanic rocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can be termed peridotite. Small pieces of peridotite have even been found in lunar breccias.

The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite, a process in which the pyroxenes and olivines are converted to green serpentine. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in the serpentine group, chrysotile, is a type of asbestos.