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The Earth's mantle is the layer in the structure of the Earth, which lies directly under the Earth's crust. It lies between 30-35 km and 2,900 km below the surface.
The mantle is just as firm as the Earth's crust, but differs substantially from this in its mechanical characteristics and its chemical composition. Mantle rock consists of olivines, different pyroxenes and other one mafic minerals. Mantle rocks possesses therefore a higher portion of iron and magnesium and a smaller portion of silicon and aluminium than the crust. The distinction between the crust and the mantle is essentially based on the different chemism. In the Earth's mantle temperatures range between some 100°C at the upper boundary and over 3,500°C at the border with the core. Although these temperatures, in particular in deeper ranges, exceed by far the melting point of the mantle rocks, they are almost exclusively solid. The enormous lithostatic pressure in the Earth's mantle prevents the rocks from melting, since solid rock occupies a smaller volume than its liquid state.
The conditions of temperature and pressure in the Earth's mantle make the mantle material viscious. The viscosity ranges between 1021 and 1024 Pa·s, depending on the depth (larger viscosity means higher stiffness). As the pressure increases as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic and semi-molten. 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 (Asthenosphere) and at the boundary with the core [1] (http://www2.uni-jena.de/chemie/geowiss/geodyn/poster2.html).
Due to the low viscosity in the upper mantle one could assume that there were no more earthquakes below approx. 300 km depth. Deep earthquakes between 400 km and 670 km under the earth's surface were nevertheless registered.
The border between the plastic Earth's mantle and the outside, brittle shell (called lithosphere), does not coincide with the (chemically defined) border between the Earth's crust and the Earth's mantle, but rather runs within the Earth's mantle. Besides the Earth's crust the lithosphere contains also the (likewise brittle) outermost ranges of the Earth's mantle. The border between the lithosphere and the Earth's mantle is a thin, some 10 km thick layer, which is characterised by a relatively high portion of melted material (about 3%). This region is called the asthenosphere or, because of remarkably low speeds of seismic waves, as the low velocity zone.
Due to the temperature difference between the earth's crust and the outer earth's core there is a convective material circulation in the Earth's mantle. 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, since due to the pressure relief connected with the ascent, its amount of 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 the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which drives continental drift. The movements of the continents and the Earth's mantle are thereby partially decoupled, since due to the rigidity of the Earth's 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 Earth's 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.
de:Erdmantel
nl:Samenstelling van de aarde
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