1. Earth's mantle
     Structure    Rheological structure    Seismic structure    Mineralogical structure    Composition    Temperature and pressure    Movement    Exploration  

2. Other planetary mantles

3. Moons with mantles

4. Asteroids with mantles

5. See also

6. References

7. Further reading

8. External links

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets (including Earth), a number of asteroids, and some planetary moons have mantles.

Earth's mantle

The Earth's mantle is a layer of silicate rock between the crust and the outer core. Its mass of 4.01 × 10²⁴ kg is 67% the mass of the Earth.^[1] It has a thickness of {{convert|2900|km|mi}}^[1] making up about 84% of Earth's volume. It is predominantly solid but in geological time it behaves as a viscous fluid. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.^[2]

Structure

Rheological structure

The Earth's mantle is divided into two major rheological layers: the rigid lithosphere comprising the uppermost mantle, and the more viscous asthenosphere, separated by the lithosphere-asthenosphere boundary. Lithosphere underlying ocean crust has a thickness of around 100 km, whereas lithosphere underlying continental crust generally has a thickness of 150-200 km.^[3] The lithosphere and overlying crust make up tectonic plates, which move over the asthenosphere.

Seismic structure

The Earth's mantle is divided into three major layers defined by sudden changes in seismic velocity:

• the upper mantle (starting at the Moho, or base of the crust around {{convert|7|to|35|km|abbr=on}} downward to {{convert|410|km|abbr=on}})^[4]
• the transition zone (approximately {{convert|410|–|660|km|abbr=on|disp=or}})
• the lower mantle (approximately {{convert|660|–|2891|km|abbr=on|disp=or}})

The lower ~200 km of the lower mantle constitutes the D" (D-double-prime) layer, a region with anomalous seismic properties. This region also contains LLSVPs and ULVZs.

Mineralogical structure

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 Mohorovičić discontinuity or "Moho".^[5][6]

The upper mantle is dominantly peridotite, composed primarily of variable proportions of the minerals olivine, clinopyroxene, orthopyroxene, and an aluminous phase. The aluminous phase is plagioclase in the uppermost mantle, then spinel, and then garnet below ~100 km. Gradually through the upper mantle, pyroxenes become less stable and transform into majoritic garnet.

At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite. Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This has led to the hypothesis that the transition zone may host a large quantity of water.^[7] At the base of the transition zone, ringwoodite decomposes into bridgmanite (formerly called magnesium silicate perovskite), and ferropericlase. Garnet also becomes unstable at or slightly below the base of the transition zone.

The lower mantle is composed primarily of bridgmanite and ferropericlase, with minor amounts of calcium perovskite, calcium-ferrite structured oxide, and stishovite. In the lowermost ~200 km of the mantle, bridgmanite isochemically transforms into post-perovskite.

Composition

The chemical composition of the mantle is difficult to determine with a high degree of certainty because it is largely inaccessible. Rare exposures of mantle rocks occur in ophiolites, where sections of oceanic lithosphere have been obducted onto a continent. Mantle rocks are also sampled as xenoliths within basalts or kimberlites.

Most estimates of the mantle composition are based on rocks that sample only the uppermost mantle. There is debate as to whether the rest of the mantle, especially the lower mantle, has the same bulk composition.^[10] The mantle's composition has changed through the Earth's history due to the extraction of magma that solidified to form oceanic crust and continental crust.

Temperature and pressure

In the mantle, temperatures range from approximately {{convert|200|C|F||}} at the upper boundary with the crust to approximately {{convert|4000|°C|°F|abbr=on}} at the core-mantle boundary.^[11] The geothermal gradient of the mantle increases rapidly in the thermal boundary layers at the top and bottom of the mantle, and increases gradually through the interior of the mantle.^[12] 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.^[13] The enormous lithostatic pressure exerted on the mantle prevents melting, because the temperature at which melting begins (the solidus) increases with pressure.

The pressure in the mantle increases from a few kbar at the Moho to 1390 kbar (139 GPa) at the core-mantle boundary^[11].

Movement

Because of 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.^[14] Hot material upwells, while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. Locations on the surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism. The volcanism often attributed to deep mantle plumes is alternatively explained by passive extension of the crust, permitting magma to leak to the surface (the "Plate" hypothesis).^[15]

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 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 an essential component of 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.

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.^[16] The mantle within about {{convert|200|km|abbr=on}} 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"), a nomenclature introduced over 50 years ago by the geophysicist Keith Bullen.^[17] 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.

Earthquakes at shallow depths are a result of strike-slip faulting; however, below about {{convert|50|km|abbr=on}} the hot, high pressure conditions ought to inhibit further seismicity. The mantle is considered to be viscous and incapable of brittle faulting. However, in subduction zones, earthquakes are observed down to {{convert|670|km|abbr=on}}. A number of mechanisms have been proposed to explain this phenomenon, including dehydration, thermal runaway, and phase change.

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 {{convert|400|km|abbr=on}} and {{convert|670|km|abbr=on}}.

The pressure at the bottom of the mantle is ~136 GPa (1.4 million atm).^[18] Pressure increases as depth increases, since the material beneath has to support the weight of all the material above it. The entire mantle, however, is 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 composing the mantle. Estimates for the viscosity of the upper mantle range between 10¹⁹ and 10²⁴ Pa·s, depending on depth,^[16] 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.

Exploration

Exploration of the mantle is generally conducted at the seabed rather than on land because of the relative thinness of the oceanic crust as compared to the significantly thicker continental 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 {{convert|180|m|ft|abbr=on}}. In 2005 an oceanic borehole reached {{convert|1416|m|ft}} below the sea floor from the ocean drilling vessel JOIDES Resolution.

On 5 March 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, midway 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.^[19][20]

A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007.^[21] The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to {{convert|7000|m|ft|abbr=on}} below the seabed. This is nearly three times as deep as preceding oceanic drillings.

A novel method of exploring the uppermost few hundred kilometres of the Earth was proposed in 2005, consisting of 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.^[22] The probe consists of an outer sphere of tungsten about one metre in diameter with a cobalt-60 interior acting as a 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 {{convert|100|km|abbr=on}} in a few decades beneath both oceanic and continental lithosphere.^[23]

Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago.^[24]

Other planetary mantles

Moons with mantles

The silicate mantle of the Earth's moon is approximately 1300-1400 km thick, and is the source of mare basalts.^[26] The lunar mantle might possibly be exposed in the South Pole-Aitken basin and/or the Crisium basin.^[26] The lunar mantle contains a seismic discontinuity at ~500 km depth, most likely related to a change in composition.^[26]

Asteroids with mantles

Some of the largest asteroids have mantles;^[29] for example, Vesta has a silicate mantle similar in composition to diogenite meteorites.^[30]

See also

• Core–mantle boundary
• Earth's internal heat budget
• Lehmann discontinuity
• Mantle xenoliths
• Mantle convection
• Mesosphere (mantle)
• Mohorovičić discontinuity
• Numerical modeling (geology)
• Post-perovskite phase transition
• Primitive mantle

References

Further reading

• Don L. Anderson, Theory of the Earth, Blackwell (1989), is a textbook dealing with the Earth's interior and is now available on the web. Retrieved 2007-12-23.
• {{cite book

• Nixon, Peter H. (1987). Mantle xenoliths: J. Wiley & Sons, 844p., ({{ISBN|0-471-91209-3}}).
• Donald L. Turcotte and Gerald Schubert, Geodynamics, Cambridge University Press, Third Edition (2014), {{ISBN|978-1-107-00653-9}} (Hardback) {{ISBN|978-0-521-18623-0}} (Paperback)

External links

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

• House at 2437 Fifteenth Street, NW
• House at 2537 Fifteenth St., NW
• House at 305 E. Ashley
• House at 3325 Via de la Reiva
• House at 36 Forest Street
• House at 379 West State Street
• House at 380 Chiltern Drive
• House at 380 Chiltern Drive-Deerpath Hill Estates
• House at 380 Chiltern Drive–Deerpath Hill Estates
• House at 380 W. Del Mar Blvd.
• House at 51 Market Street
• House at 530 S. Marengo Avenue
• House at 574 Bellefontaine St.
• House at 5910 Amboy Road
• House at 674 Elliot Drive
• House at 712 N. Mill Street
• House at 965 Castlegate Court
• House at 965 Castlegate Court-Deerpath Hill Estates
• House at 965 Castlegate Court–Deerpath Hill Estates
• House at Big Hill
• House at Caulfield
• House at Lake Connewarre
• House at L'estaque
• House at Pireus
• House at Springdell