mafic rocks are the most common type of rock in the earth's oceanic crust.
mafic rocks are the most common type of rock in the earth's oceanic crust.
the most common type of rock in the earth's oceanic crust is?
mafic rocks
arc magmas are produced by partial melting of metasomatic domains in the mantle wedge, which was reacted with liquid phases derived from dehydration melting of minerals contained in the subducting oceanic crust formed at mid-ocean ridges. the subducting oceanic crust consists of a four major units. the topmost unit is a thin cap of pelagic sediments up to 0.3 km thick composed of siliceous and calcareous shells, meteoric dusts, and variable amounts of volcanic ash. the next unit is composed of 0.3–0.7 km thick pillow basalts, formed by the quenching of basaltic magma as it erupts into ocean water. under the pillow basalts is a basaltic sheeted dike complex, that represent cooled magma conduits. the bottom units represent the crystallized magma chamber, feeding the mid-ocean ridge at which the crust was formed. it is composed of 1–5 km thick layered gabbro atop <7 km thick layer of ultramafic rocks (e.g. wehrlite, harzburgite, dunite, and chromite). oceanic crust is referred to as a metabasite.
subduction zone metamorphism
as explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both. for example, the african plate includes the continent and parts of the floor of the atlantic and indian oceans. the distinction between oceanic crust and continental crust is based on their modes of formation. oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. oceanic crust is also denser than continental crust owing to their different compositions. oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic"). as a result of this density stratification, oceanic crust generally lies below sea level (for example most of the pacific plate), while continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).
plate tectonics