Mantle (geology) Totally Explained

Everything about Earth S Mantle totally explained

Definition

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 a ~2,900 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.

Structure

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 Mohorovičić 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 more plastic in its rheology. In some regions below the lithosphere, the seismic velocity is reduced; this – so-called – low velocity zone (LVZ) extends down to a depth of several hundred km. Inge Lehmann discovered a seismic discontinuity at about 220 km depth; although this discontinuity has been found in other studies it isn't known whether the discontinuity is ubiquitous. The transition zone is an area of great complexity; it physically separates the upper and lower mantle., pyroxenes, spinel, and garnet; typical rock types are thought to be peridotite,. Composition of Earth's mantle in weight percent>

Element	Amount	Compound	Amount
O	44.8
Si	21.5	SiO₂	46
Mg	22.8	MgO	37.8
Fe	5.8	FeO	7.5
Al	2.2	Al₂O₃	4.2
Ca	2.3	CaO	3.2
Na	0.3	Na₂O	0.4
K	0.03	K₂O	0.04
Sum	99.7	Sum	99.1

Why is the inner core solid, the outer core liquid, and the mantle solid/plastic? The answer depends both on the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and on the increase in temperature and pressure as one moves deeper into the Earth. At the surface both nickel-iron alloys and silicates are sufficiently cool to be solid. In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity. In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle. The metallic nickel-iron outer core is liquid despite the enormous pressure as it has a melting point that's lower than the mantle silicates. The inner core is solid due to the overwhelming pressure found at the center of the planet.

Temperature

In the mantle, temperatures range between 500 °C–900 °C (932 °F–1,652 °F) at the upper boundary with the crust to over 4,000 °C (7,200 °F) at the boundary with the core. The mantle within about 200 km above the core-mantle boundary appears to have distinctly different seismic properties than the mantle at slightly shallower depths; this unusual mantle region just above the core is called D″ ("D double-prime" or "D prime prime"), a nomenclature introduced over 50 years ago by the geophysicist Bullen. D″ may consist of material from subducted slabs that descended and came to rest at the core-mantle boundary and/or from a new mineral polymorph discovered in perovskite called post-perovskite.
   Due to the relatively 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 where cool material from the surface sinks downward, 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 ~136 GPa (1.4 million atm).
   A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007. As part of the Chikyu Hakken mission, was to use the Japanese vessel 'Chikyu' to drill up to 7000 m (23,000 ft) below the seabed. This is nearly three times as deep as preceding oceanic drillings.
   A novel method of exploring the uppermost hundreds km of the Earth was recently analysed using a small, dense, heat-generating probe which melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks. The probe consists of an outer sphere of tungsten ~ 1 m in diameter inside which is a 60Co radioactive heat source. It was calculated that such a probe will reach the oceanic Moho in less than 6 months and attain minimum depths of well over 100 km in a few decades beneath both oceanic and continental lithosphere.

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