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Mantle (geology)

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The Earth's mantle is the layer in the structure of the Earth that lies directly under the Earth's crust and above the Earth's outer core. The term is also applied to the structure of other planets. Earth's mantle lies roughly between 30 and 2,900 km below the surface.

The boundary between the crust and the mantle is the Mohorovičić 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 cold and therefore rigid mantle 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 100C at the upper boundary to over 3,500C 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. It some regions of the earth, this subregion of the mantle is associated with a region of the mantle that passes seismic waves more slowly. This region is called the low-velocity zone. The cause of this low velocity zone is still debated. Currently theories include the influence of temperature and pressure or the existance of a small amount of partial melt.

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 outer core, while cooler (and heavier) material sinks downward. This is often in the form of large-scale lithospheric downwellings at plate boundaries called subduction zones. 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 the motion of plates. Plate motion should not be confused with the older term continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are thereby partially decoupled, since due to the rigidity of the lithosphere, 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, in subduction zones, the geothermal gradient can be lowered, increasing the strength of the surrounding mantle, and allowing earthquakes to occur down to a depth of 400 km and 670 km.

The pressure at the bottom of the mantle is ~140 GPa (1.4 Matm). As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 1021 and 1024 Pas, depending on depth [2]. Thus, the upper mantle can only flow very slowly.

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.

Last updated: 06-02-2005 04:17:05
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