The Earth’s Mantle

December 31, 2008 by  


tufail

The mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer directly under the crust, and above the outer core. Earth’s mantle is ~2,890 km thick (1,800 miles) rocky shell comprising approximately 70% of Earth’s volume. It is predominantly solid and overlies the Earth’s iron-rich core, which occupies about 30% of Earth’s volume. Past episodes of melting and volcanism at the shallower levels of the mantle have produced a very thin crust of crystallized melt products near the surface, upon which we live. The gases evolved during the melting of Earth’s mantle have a large effect on the composition and abundance of Earth’s atmosphere.

The mantle is divided into sections based upon results from seismology. These layers (and their depths) are the following: the upper mantle (33–410 km) (20 to 254 miles), the transition zone (410–670 km), the lower mantle (670–2798 km), and the D” layer (2798–2998 km).

The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovic(ic’ in 1909; this boundary is now referred to as the “Moho”. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km. Below the lithosphere the upper mantle becomes notably softer. In some regions below the lithosphere, the seismic velocity is reduced; this “low velocity zone” (LVZ) extends down several hundred km. Inge Lehmann discovered a seismic discontinuity at about 220 km depth; although this discontinuity has been found in other studies it is not known whether the discontinuity is everywhere. The transition zone is an area of great complexity; it physically separates the upper and lower mantle.

The mantle is different from the crust. The distinction between crust and mantle is based on chemistry, rock types, rheology and seismic characteristics. The crust is, in fact, a product of mantle melting. Partial melting of mantle material is believed to cause incompatible elements to separate from the mantle rock, with less dense material floating upward through pore spaces, cracks, or fissures, to cool and freeze at the surface. Typical mantle rocks have a higher magnesium to iron ratio, and a smaller portion of silicon and aluminium than the crust.

The pressure at the bottom of the mantle is 1.4 million atmospheres. There exists increasing pressure as one travels deeper into the mantle, since the material beneath has to support the weight of all the material above it.      

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