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Mantle (geology)

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The mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are either 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. The mantle is bounded on the bottom by the planetary core and on top by the crust. In addition to the Earth, the other terrestrial planets, a number of asteroids, and moons have mantles.

Earth's mantle

The internal structure of Earth

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 2,900 kilometres (1,800 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.

Structure

Rheological structure

The Earth's mantle is divided into two major rheological layers: the rigid lithosphere comprising the uppermost mantle, and the less 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[2]. 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 7 to 35 km (4.3 to 21.7 mi) downward to 410 km (250 mi))[3]
  • the transition zone (approximately 410–660 km or 250–410 mi)
  • the lower mantle (approximately 660–2,891 km or 410–1,796 mi)

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".[4][5]

The upper mantle is dominantly peridotite, comprised 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[6]. 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 comprised 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 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.

Composition of the Earth's mantle[7]
Compound Mass percent
SiO2 44.71
Al2O3 3.98
FeO 8.18
MnO 0.13
MgO 38.73
CaO 3.17
Na2O 0.13
Cr2O3 0.57
TiO2 0.13
NiO 0.24
K2O 0.006
P2O5 0.019

However, 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[8]. The mantle composition has changed throughout Earth history due to the extraction of oceanic and continental crust.

Temperature and pressure

In the mantle, temperatures range from approximately 200 °C (392 °F) at the upper boundary with the crust to approximately 4,000 °C (7,230 °F) at the core-mantle boundary[9]. 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.[10] 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.[11] 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[9].

Movement

This figure is a snapshot of one time-step in a model of mantle convection. Colors closer to red are hot areas and colors closer to blue are cold areas. In this figure, heat received at the core–mantle boundary results in thermal expansion of the material at the bottom of the model, reducing its density and causing it to send plumes of hot material upwards. Likewise, cooling of material at the surface results in its sinking.

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.[12] 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).[13]

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.[14] The mantle within about 200 km (120 mi) 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.[15] 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 stick-slip faulting; however, below about 50 km (31 mi) 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 670 km (420 mi). 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 400 km (250 mi) and 670 km (420 mi).

The pressure at the bottom of the mantle is ~136 GPa (1.4 million atm).[16] 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 comprising the mantle. Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pa·s, depending on depth,[14] 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 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 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.[17][18] A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007.[19] The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to 7,000 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 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.[20] 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 100 km (62 mi) in a few decades beneath both oceanic and continental lithosphere.[21]

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.[22]

Other planetary mantles

Mercury has a silicate mantle approximately 490 km thick, comprising 28% of its mass[1]. Venus's silicate mantle is approximately 2800 km thick, comprising around 70% of its mass[1]. Mars's silicate mantle is approximately 1600 km thick, comprising ~74-88% of its mass[1], and may be represented by chassignite meteorites[23]. Jupiter's moons Io, Europa, and Ganymede have silicate mantles; Io's ~1100 km silicate mantle is overlain by a volcanic crust, Ganymede's ~1315 km thick silicate mantle is overlain by ~835 km of ice, and Europa's ~1165 km silicate mantle is overlain by ~85 km of ice and possibly liquid water[1].

The silicate mantle of the Earth's moon is approximately 1300-1400 km thick, and is the source of mare basalts[24]. The lunar mantle might possibly be exposed in the South Pole-Aitken basin and/or the Crisium basin[24]. The lunar mantle contains a seismic discontinuity at ~500 km depth, most likely related to a change in composition[24].

Titan and Triton each have a mantle made of ice or other solid volatile substances[25][26]. Vesta and numerous meteorite parent bodies have silicate mantles.

See also

References

  1. ^ a b c d e f Katharina., Lodders, (1998). The planetary scientist's companion. Fegley, Bruce. New York: Oxford University Press. ISBN 1423759834. OCLC 65171709.
  2. ^ Stephen, Marshak (2015). Earth: Portrait of a Planet (5th ed.). New York: W. W. Norton & Company. ISBN 9780393937503. OCLC 897946590.
  3. ^ The location of the base of the crust varies from approximately 10 to 70 kilometers. Oceanic crust is generally less than 10 kilometers thick. "Standard" continental crust is around 35 kilometers thick, and the large crustal root under the Tibetan Plateau is approximately 70 kilometers thick.
  4. ^ Alden, Andrew (2007). "Today's Mantle: a guided tour". About.com. Retrieved 2007-12-25.
  5. ^ "Istria on the Internet – Prominent Istrians – Andrija Mohorovicic". 2007. Retrieved 2007-12-25.
  6. ^ Bercovici, David; Karato, Shun-ichiro (2003-09). "Whole-mantle convection and the transition-zone water filter". Nature. 425 (6953): 39–44. doi:10.1038/nature01918. ISSN 0028-0836. Check date values in: |date= (help)
  7. ^ Workman, Rhea K.; Hart, Stanley R. (2005-02). "Major and trace element composition of the depleted MORB mantle (DMM)". Earth and Planetary Science Letters. 231 (1–2): 53–72. doi:10.1016/j.epsl.2004.12.005. ISSN 0012-821X. Check date values in: |date= (help)
  8. ^ Murakami, Motohiko; Ohishi, Yasuo; Hirao, Naohisa; Hirose, Kei (2012-05). "A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data". Nature. 485 (7396): 90–94. doi:10.1038/nature11004. ISSN 0028-0836. Check date values in: |date= (help)
  9. ^ a b Katharina., Lodders, (1998). The planetary scientist's companion. Fegley, Bruce. New York: Oxford University Press. ISBN 1423759834. OCLC 65171709.
  10. ^ Turcotte, DL; Schubert, G (2002). "4". Geodynamics (2nd ed.). Cambridge, England, UK: Cambridge University Press. pp. 136–7. ISBN 978-0-521-66624-4.
  11. ^ Louie, J. (1996). "Earth's Interior". University of Nevada, Reno. Retrieved 2007-12-24.
  12. ^ Alden, Andrew (2007). "Today's Mantle: a guided tour". About.com. Retrieved 2007-12-25.
  13. ^ Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0.
  14. ^ a b Walzer, Uwe; Hendel, Roland and Baumgardner, John. Mantle Viscosity and the Thickness of the Convective Downwellings. igw.uni-jena.de
  15. ^ Alden, Andrew. "The End of D-Double-Prime Time?". About.com. Retrieved 2007-12-25.
  16. ^ Burns, Roger George (1993). Mineralogical Applications of Crystal Field Theory. Cambridge University Press. p. 354. ISBN 0-521-43077-1. Retrieved 2007-12-26.
  17. ^ Than, Ker (2007-03-01). "Scientists to study gash on Atlantic seafloor". Msnbc.com. Retrieved 2008-03-16. A team of scientists will embark on a voyage next week to study an “open wound” on the Atlantic seafloor where the Earth’s deep interior lies exposed without any crust covering.
  18. ^ "Earth's Crust Missing In Mid-Atlantic". Science Daily. 2007-03-02. Retrieved 2008-03-16. Cardiff University scientists will shortly set sail (March 5) to investigate a startling discovery in the depths of the Atlantic.
  19. ^ "Japan hopes to predict 'Big One' with journey to center of Earth". PhysOrg.com. 2005-12-15. Archived from the original on 2005-12-19. Retrieved 2008-03-16. An ambitious Japanese-led project to dig deeper into the Earth's surface than ever before will be a breakthrough in detecting earthquakes including Tokyo's dreaded "Big One," officials said Thursday.
  20. ^ Ojovan M.I., Gibb F.G.F., Poluektov P.P., Emets E.P. 2005. Probing of the interior layers of the Earth with self-sinking capsules. Atomic Energy, 99, 556–562
  21. ^ Ojovan M.I., Gibb F.G.F. "Exploring the Earth’s Crust and Mantle Using Self-Descending, Radiation-Heated, Probes and Acoustic Emission Monitoring". Chapter 7. In: Nuclear Waste Research: Siting, Technology and Treatment, ISBN 978-1-60456-184-5, Editor: Arnold P. Lattefer, Nova Science Publishers, Inc. 2008
  22. ^ University of California – Davis (2009-06-15). Super-computer Provides First Glimpse Of Earth's Early Magma Interior. ScienceDaily. Retrieved on 2009-06-16.
  23. ^ Swindle, T. D. (2002-01-01). "Martian Noble Gases". Reviews in Mineralogy and Geochemistry. 47 (1): 171–190. doi:10.2138/rmg.2002.47.6. ISSN 1529-6466.
  24. ^ a b c Wieczorek, M. A. (2006-01-01). "The Constitution and Structure of the Lunar Interior". Reviews in Mineralogy and Geochemistry. 60 (1): 221–364. doi:10.2138/rmg.2006.60.3. ISSN 1529-6466.
  25. ^ "Layers of Titan". NASA. 23 February 2012. Retrieved 7 October 2015.
  26. ^ "Triton: In Depth". NASA. Retrieved 16 October 2015.

Further reading

External links