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Oceanic-continental convergence and creation of accretionary wedge Stages of accretion through time with accretionary wedge and volcanic island arc. In geology, accretion is a process by which material is added to a tectonic plate at a subduction zone, frequently on the edge of existing continental landmasses.
The thickness of Earth's crust (km). The continental crust consists of various layers, with a bulk composition that is intermediate (SiO 2 wt% = 60.6). [5] The average density of the continental crust is about, 2.83 g/cm 3 (0.102 lb/cu in), [6] less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm 3 (0.12 lb/cu in).
Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading. Upwelling beneath the spreading centers is a shallow, rising component of mantle convection and in most cases not directly linked to the global mantle upwelling.
In geochemistry, the primitive mantle (also known as the bulk silicate Earth) is the chemical composition of the Earth's mantle during the developmental stage between core-mantle differentiation and the formation of early continental crust. The chemical composition of the primitive mantle contains characteristics of both the crust and the mantle.
The small sections of oceanic crust that are thrust over the overriding plate are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolites, are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and ...
Crustal growth rates can be used to calculate estimates for the age of the continental crust. This can be done through analysis of igneous rocks with the same isotopic composition as initial mantle rock. These igneous rocks are dated and assumed to be direct evidence of new continental crust formation. [22]
The mantle's composition has changed through the Earth's history due to the extraction of magma that solidified to form oceanic crust and continental crust. It has also been proposed in a 2018 study that an exotic form of water known as ice VII can form from supercritical water in the mantle when diamonds containing pressurized water bubbles ...
The silicate mantle of the Earth's moon is approximately 1300–1400 km thick, and is the source of mare basalts. [4] The lunar mantle might be exposed in the South Pole-Aitken basin or the Crisium basin. [4] The lunar mantle contains a seismic discontinuity at ~500 kilometers (310 miles) depth, most likely related to a change in composition. [4]