Mantle (geology)

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The mantle is the layer in the structure of the Earth that lies directly under the Earth's crust. It lies between 30 and 2,900 km below the surface.

The boundary between the crust and the mantle is the Mohorovicic discontinuity, named for its discoverer, and is usually called the Moho. The Moho is a boundary at which there is a sudden change in the speed of seismic waves. At one time some thought that the Moho was the structure at which the earth's rigid crust moved relative to the mantle. Current research places this zone of movement within the mantle, from 70 km (43 mi) below the ocean crust to 150 km (93 mi) below the continental crust. The mantle just below the crust is composed of rigid, solidified, basaltic rock fused to the crust but at the same time separated from it by the Moho. This rigid layer of crust and the upper mantle forms the lithosphere.

The mantle differs substantially from the crust in its mechanical characteristics and its chemical composition. It is chiefly the difference of chemistry on which the distinction between crust and mantle is based. Mantle rock consists of olivines, different pyroxenes and other mafic minerals. Typified by peridotite, dunite, and eclogite, mantle rocks also possesses a higher portion of iron and magnesium and a smaller portion of silicon and aluminium than the crust. In the mantle, temperatures range between 100°C at the upper boundary to over 3,500°C at the boundary with the core. Although these temperatures far exceed the melting points of the mantle rocks, particularly in deeper ranges, they are almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents them from melting.

The subregion of the mantle extending about 250 km (155 mi) below the lithosphere is called the asthenosphere. This region of the mantle passes seismic waves more slowly, leading this region to also be called the low-velocity zone. The conditions of temperature and pressure here make this material semi-molten, or plastic.

Due to the temperature difference between the Earth's crust and outer core there is a convective material circulation in the asthenosphere. Hot material ascends as a plutonic diapir from the border with the core, while cooler (and heavier) material sinks downward. During the ascent the material of the mantle cools down adiabatically. The temperature of the material falls with the pressure relief connected with the ascent, and its heat distributes itself over a larger volume. Near the lithosphere the pressure relief can lead to partial melting of the diapir, leading to volcanism and plutonism.

The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to drive continental drift. The movements of the continents and the Earth's mantle are thereby partially decoupled, since due to the rigidity of the crust, a tectonic plate can only move as a whole. Continental drift is therefore only a diffuse image of the movements at the upper limit of the Earth's mantle. The convection of the mantle is not yet clarified in detail. There are different theories, according to which the Earth's mantle is divided into different floors of separate convection.

Although there is a tendency to larger viscosity at greater depth, this relation is far from linear, and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core [1].

Due to the low viscosity in the upper mantle one could reason that there should be no earthquakes below approximately 300 km depth. However, deep earthquakes with foci between 400 km and 670 km under the earth's surface have been registered.

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