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Encyclopedia > Mantle (geology)
Earth cutaway from core to exosphere.
Earth cutaway from core to exosphere.

Earth's mantle is a ~2,900 km thick 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. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... This article is about Earth as a planet. ... The planetary core consists of the innermost layer(s) of a planet. ... [fAgot png|thumb|200px|right|Atmosphere diagram showing the exosphere and other layers. ... This article is about Earth as a planet. ... For other uses, see Solid (disambiguation). ... This article is about volcanoes in geology. ... Earth cutaway from core to exosphere. ... “Air” redirects here. ...

Contents

Structure

The mantle is divided into sections based upon changes in its elastic properties with depth. These layers (and their depths) are: the upper mantle (33-410 km), the transition zone (410-670 km), the lower mantle (670-2798 km), and D" (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 referrered to as the "Moho". At about 50 km depth the upper mantle becomes notably more plastic in its rheology, as a result the seismic velocity is reduced, this - so-called - low velocity zone (LVZ) extends down to a depth of about 200 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 ubiquitous. The transition zone is an area of great complexity, it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be seismically homogeneous. D" is arguably the most complex region in the Earth. This is the layer which separates the mantle from the core. Andrija Mohorovičić (c. ... Ordovician ophiolite in Gros Morne National Park, Newfoundland. ... Rheology is the study of the deformation and flow of matter under the influence of an applied stress. ...


Characteristics

The mantle differs substantially from the crust in its mechanical characteristics and its chemical composition. 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. This behavior is also predicted by experiments that partly melt rocks thought to be representative of Earth's mantle. A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance as coarse powder, dark grey with bluish tinge Standard atomic weight 28. ... General Name, symbol, number aluminium, Al, 13 Chemical series poor metals Group, period, block 13, 3, p Appearance silvery Standard atomic weight 26. ...

Mapping the interior of the Earth with earthquake waves.
Mapping the interior of the Earth with earthquake waves.

Mantle rock shallower than about 400 km depth consists mostly of olivine, pyroxenes, spinel, and garnet: typical rock types are thought to be peridotite, dunite (olivine-rich peridotite), and eclogite. Between about 400 km and 650 km depth, olivine is not stable and is replaced by high pressure polymorphs with approximately the same composition: one polymorph is wadsleyite (also called beta-spinel type), and the other is ringwoodite (a mineral with the gamma-spinel structure). Below about 650 km, all of the minerals of the upper mantle begin to become unstable; the most abundant minerals present have structures (but not compositions) like that of the mineral, perovskite. The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior, and like the moho are readily detected using seismic waves. These changes in mineralogy may influence mantle convection, as they result in density changes and they may absorb or release latent heat as well as depress or elevate the depth of the polymorphic phase transitions for regions of different temperatures. The changes in mineralogy with depth have been investigated by laboratory experiments that duplicate high mantle pressures, such as those using the diamond anvil. Cross section of the whole Earth, showing the complexity of paths of earthquake waves. ... Cross section of the whole Earth, showing the complexity of paths of earthquake waves. ... This article is about Earth as a planet. ... An earthquake is the result of a sudden release of stored energy in the Earths crust that creates seismic waves. ... The mineral olivine (also called chrysolite and, when gem-quality, peridot) is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... Peridotite xenolith from San Carlos, southwestern United States. ... Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse grained or phaneritic texture. ... Eclogite is a coarse-grained, mafic-to-ultramafic grouping of metamorphic rocks of special interest on account of the variety of minerals they contain and their microscopic structures and geological relationships. ... Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Perovskite (calcium titanium oxide, CaTiO3) is a relatively rare mineral occurring in orthorhombic (pseudocubic) crystals. ... Mantle convection is the slow creeping motion of Earths rocky mantle in response to perpetual gravitationally unstable variations in its density. ... A diamond anvil, more properly a diamond anvil cell (DAC), is a device used by physicists to exert extreme pressures on a material. ...

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 is lower than the mantle silicates. The inner core is solid due to the overwhelming pressure found at the center of the planet. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance as coarse powder, dark grey with bluish tinge Standard atomic weight 28. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... Magnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium. ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... Wüstite (FeO) is a rare mineral form of iron(II) oxide found with meteorites and native iron. ... General Name, symbol, number aluminium, Al, 13 Chemical series poor metals Group, period, block 13, 3, p Appearance silvery Standard atomic weight 26. ... Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula Al2O3. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Standard atomic weight 40. ... Calcium oxide (CaO), commonly known as lime, quicklime or burnt lime, is a widely used chemical compound. ... For sodium in the diet, see Edible salt. ... Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... Potassium oxide is a compound of potassium and oxygen used mainly as a intermediate in inorganic synthesis. ...


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. Although the higher temperatures far exceed the melting points of the mantle rocks at the surface (about 1200°C for representative peridotite), the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting, because the temperature at which melting begins (the solidus) increases with pressure. Earth cutaway from core to exosphere. ... The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. ... Peridotite xenolith from San Carlos, southwestern United States. ... Overburden, or lithostatic pressure, is a term used in geology to denote the pressure imposed on a stratigraphic layer by the weight of overlying layers of material. ... In physics, melting is the process of heating a solid substance to a point (called the melting point) where it turns into a liquid. ... In chemistry, materials science, and physics, the solidus is a line on a phase diagram below which a given substance is stable in the solid phase. ...


Movement

Due to the temperature difference between the Earth's surface and outer core, and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material ascends as a plutonic diapir (somewhat akin to a lava lamp), perhaps from the border with the outer core (see mantle plume), 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 both adiabatically and by conduction into surrounding cooler mantle. The temperature of the material falls with the pressure relief connected with the ascent, and its heat distributes itself over a larger volume. Because the temperature at which melting initiates decreases more rapidly with height than does a rising hot plume, partial melting may occur just beneath the lithosphere and causing volcanism and plutonism. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... A lava lamp illustrates the basic principle of diapirism. ... A lava lamp illustrates the basic concept of a mantle plume. ... This article covers adiabatic processes in thermodynamics. ... This article is about volcanoes in geology. ...


The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of 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 coupled since descending lithosphere is the dominant driving force for convection in the mantle. The observed continental drift is a complicated relationship between the forces causing oceanic lithosphere to sink and the movements within Earth's mantle. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... A plot of the Lorenz attractor for values r = 28, σ = 10, b = 8/3 In mathematics and physics, chaos theory describes the behavior of certain nonlinear dynamical systems that under specific conditions exhibit dynamics that are sensitive to initial conditions (popularly referred to as the butterfly effect). ... Plates in the crust of the earth, according to the plate tectonics theory Continental drift refers to the movement of the Earths continents relative to each other. ...


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]. 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. The core-mantle boundary lies between the Earths silicate mantle and its iron-nickel core. ... Bullen is a surname, and may refer to Arthur Henry Bullen Keith Edward Bullen Lee Bullen Luke Bullen Marc Bullen Nicholas Bullen William Bullen Anne Bullen, better known as Anne Boleyn This page or section lists people with the surname Bullen. ... The core-mantle boundary lies between the Earths silicate mantle and its iron-nickel core. ... Post-perovskite is a high-pressure phase of MgSiO3 perovskite (which persists as enstatite at upper mantle pressures), and is likely to occur in cooler regions of the lowermost few hundred km of Earths mantle. ...


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. An earthquake is the result of a sudden release of stored energy in the Earths crust that creates seismic waves. ...


The pressure at the bottom of the mantle is ~136 GPa (1.4 million atm). 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. The entire mantle, however, is still thought to deform like a fluid on long timescales, with permanent plastic deformation accommodated by the movement of point, line, and/or planar defects through the solid crystals comprising the mantle. Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pa·s, depending on depth [1], temperature, composition, state of stress, and numerous other factors. Thus, the upper mantle can only flow very slowly. However, when large forces are applied to the uppermost mantle it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries. The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... Diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure at any point in the Earths atmosphere. ... The pascal second (symbol Pa·s) is the SI unit of dynamic viscosity. ...


Exploration

Exploration of the mantle is generally conducted at the seabed rather than on land due to the relative thinness of the crust.


The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration was approximately 180m (590 ft). CUSS I CUSS I Underseas beacons for positioning by sonar. ...


In 2005 the third-deepest oceanic borehole hole reached 1416 meters (4,644 feet) below the sea floor from the ocean drilling vessel JOIDES Resolution. Categories: Possible copyright violations ...


On March 5, 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, mid-way between the Cape Verde Islands and the Caribbean Sea. The exposed site lies approximately three kilometres beneath the ocean surface and covers thousands of square kilometres.[2][3] The RRS James Cook is a research ship belonging to the British Natural Environment Research Council. ... Map of Central America and the Caribbean Caribbean Sea from space (top left). ...


A relatively difficult attempt to retrieve samples from the Earth's mantle is scheduled for later in 2007 [4]. As part of the Chikyu Hakken mission, it will use the Japanese vessel 'Chikyu' to drill up to 7000m (23,000 ft) below the seabed. This is nearly three times as deep as preceding oceanic drillings. The DV Chikyu. ...


References

  1. ^ a b Mantle Viscosity and the Thickness of the Convective Downwellings retrieved on December 17, 2005
  2. ^ http://www.msnbc.msn.com/id/17407745/
  3. ^ http://www.sciencedaily.com/releases/2007/03/070301103112.htm
  4. ^ Physorg.com article of 2005/12/15 retrieved on December 17, 2005
  • Don L. Anderson, Theory of the Earth, Blackwell (1989), a textbook dealing with the Earth's interior, is now available on the web at

http://resolver.caltech.edu/CaltechBOOK:1989.001

External links

  • The Biggest Dig : Japan builds a ship to drill to the earth's mantle - Scientific American Magazine (September 2005)
  • Information on the Mohole Project

  Results from FactBites:
 
Mantle (geology) - Wikipedia, the free encyclopedia (1369 words)
Typical mantle rocks have a higher portion of iron and magnesium, a higher magnesium to iron ratio, and a smaller portion of silicon and aluminium than the crust.
The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of the motion of plates.
Mantle Viscosity and the Thickness of the Convective Downwellings retrieved on December 17, 2005
  More results at FactBites »

 
 

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