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Oceanic crust is the uppermost layer of the oceanic portion of the Plate tectonics. It is composed of the upper oceanic crust, with and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic rock cumulate rock.Gillis et al. (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208 The crust lies above the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.
Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.
The uppermost crust is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. As the lavas cool they are, in most instances, modified chemically by seawater.H. Elderfield (2006). The Oceans and Marine Geochemistry. Elsevier. pp. 182–. . These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma Crystallization at depth, within the lower oceanic crust. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.Lissenberg, C. J., MacLeod, C. J., Horward, K. A., and Godard, M. (2013). Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean). Earth Planet. Sci. Lett. 361, 436–447.
Prior to the Neoproterozoic 1000 million years ago, the world's oceanic crust was more mafic than the current crust. The more mafic nature of the crust meant that higher amounts of water molecules (Hydroxy group) could be stored in the altered parts of the crust. At subduction zones this mafic crust was prone to metamorphose into greenschist instead of blueschist at ordinary blueschist facies.
As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr−1 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean. Thicker than average crust is found above Mantle plume as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km.C.M.R. Fowler (2005) The Solid Earth (2nd Ed.), Cambridge University Press
The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it. The oceanic lithosphere subduction at what are known as convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd. The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson Cycle.
The oldest large-scale oceanic crust is in the west Pacific Ocean and north-west Atlantic Ocean — both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea could be remnants of the much older Tethys Ocean, at about 270 and up to 340 million years old.
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