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Mantle (geology)

The mantle is a layer inside a terrestrial planet and someother rocky planetary bodies. For a mantle to form,the planetary body must be large enough to have under-gone the process of planetary differentiation by density.The mantle lies between the core below and the crustabove. The terrestrial planets (Earth, Venus, Mars andMercury), the Moon, two of Jupiter's moons (Io andEuropa) and the asteroid Vesta each have a mantle madeof silicaterock.[1][2][3][4][5][6][7][8]Interpretation of space-craftdata suggests that at least two other moons of Jupiter

(Ganymede and Callisto), as well as Titan and Tritoneach have a mantle made of ice or other solid volatilesubstances.[9][10][11][12]

1 Earth’s mantle

The internal structure of Earth

The interior of Earth, similar to the other terrestrial plan-ets, is chemically divided into layers. The mantle is alayer between the crust and the outer core. Earth’s man-

tle is a silicate rocky shell with an average thickness of2,886 kilometres (1,793 mi).[13] The mantle makes upabout 84%of Earth’s volume.[14] It is predominantly solidbut in geological time it behaves as a very viscous uid.The mantle encloses the hot core rich in iron and nickel,which makes up about 15% of Earth’s volume.[14] Pastepisodes of melting and volcanism at the shallower levelsof the mantle have produced a thin crust of crystallizedmelt products near thesurface, upon whichwelive.[15] In-formation about structure and composition of the mantleeither result from geophysical investigation or from directgeoscientic analyses on Earth mantle derived xenoliths

and on mantle exposed by mid-oceanic ridge spreading.Two main zones are distinguished in the upper man-tle: the inner asthenosphere composed of plastic owing

rock of varying thickness, on average about 200 km (120mi) thick,[16] and the lowermost part of the lithospherecomposed of rigid rock about 50 to 120 km (31 to 75mi) thick.[17] A thin crust, the upper part of the litho-sphere, surrounds the mantle and is about 5 to 75 km(3.1 to 46.6 mi) thick.[18] Recent analysis of hydrousringwoodite from the mantle suggests that there is be-tween one[19] and three[20] times as much water in thetransition zone between the lower and upper mantle thanin all the world’s oceans combined.

In some places under the ocean the mantle is actually ex-posed on the surface of Earth.[21] There are also a fewplaces on land where mantle rock has been pushed to thesurface by tectonic activity, most notably the Tablelandsregion of Gros Morne National Park in the Canadianprovince of Newfoundland and Labrador and St. John’sIsland, Egypt or Zabargad in the Red Sea.

1.1 Structure

The mantle is divided into sections which are based uponresults from seismology. These layers (and their thick-nesses/depths) are the following: the upper mantle (start-ing 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)),[22] thetransition zone (410–660 km or 250–410 mi), the lowermantle (660–2,891 km or 410–1,796 mi), and anoma-lous core–mantle boundary with a variable thickness (onaverage ~200 km (120 mi) thick).[15][23][24][25]

The top of the mantle is dened by a sudden increase inseismic velocity, which was rst noted by Andrija Mo-horovičić in 1909; this boundary is now referred to asthe Mohorovičić discontinuity or “Moho”.[23][26] The up-permost mantle plus overlying crust are relatively rigidand form the lithosphere, an irregular layer with a maxi-mum thickness of perhaps 200 km (120 mi). Below thelithosphere the upper mantle becomes notably more plas-tic. In some regions below the lithosphere, the seismicshear velocity is reduced; this so-called low-velocity zone(LVZ) extends down to a depth of several hundred km.IngeLehmanndiscovereda seismic discontinuity at about220 km (140 mi) depth;[27] although this discontinuityhas been found in other studies, it is not known whetherthe discontinuity is ubiquitous. The transition zone is anarea of great complexity; it physically separates the upperand lower mantle.[25] Very little is known about the lower

mantle apart from that it appears to be relatively seis-mically homogeneous. The D” layer at the core–mantleboundary separates the mantle from the core.[15][23] In

1

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2 1 EARTH’S MANTLE

2015, research using gravitational data from GRACEsatellites and the long wavelength nonhydrostatic geoidindicated viscosity[28] increases by a factor of ten to 150about 1,000 kilometres (620 mi) below earth’s surface;separate research also indicates sinking tectonic platesstall at this depth, leading Robert van der Hilst to spec-ulate “In term’s of structure and dynamics, 1,000 kilo-meters could be more important” (than the currently ac-cepted 660 km depth upper—lower division).[29]

1.2 Characteristics

The mantle differs substantially from the crust in itsmechanical properties which is the direct consequenceof chemical composition change (expressed as differentmineralogy). The distinction between crust and mantleis based on chemistry, rock types, rheology and seismic

characteristics. The crust is a solidication product ofmantle derived melts, expressed as various degrees ofpartial melting products during geologic time. Partialmelting of mantle material is believed to cause incompat-ible elements to separate from the mantle, with less densematerial oating upward through pore spaces, cracks,or ssures, that would subsequently cool and solidify atthe surface. Typical mantle rocks have a higher magne-sium to iron ratio and a smaller proportion of silicon andaluminium than the crust. This behavior is also predictedby experiments that partly melt rocks thought to be rep-resentative of Earth’s mantle.

In nercore

Outer core

Lower mantle Upp er mant le

Crust

Focus ofearthquake

Kilometers

0 10,000

S

SS SS

SSKS

SKP

SP

P

P

P

P

PP

PKPPPP

P

P

K

K

Mapping the interior of the Earth with earthquake waves.

Mantle rocks shallower than about 410 km (250 mi)depth consist mostly of olivine, pyroxenes, spinel-structure minerals, and garnet;[25] typical rock types arethought to be peridotite,[25] dunite (olivine-rich peri-dotite), and eclogite. Between about 400 km (250 mi)and 650 km (400 mi) depth, olivine is not stableand is re-placed by high pressure polymorphs with approximately

the same composition: one polymorph is wadsleyite (alsocalled beta-spinel type), and the other is ringwoodite (amineral with the gamma- spinel structure). Below about

650 km (400 mi), all of the minerals of the upper man-tle begin to become unstable. The most abundant miner-als present, the silicate perovskites, have structures (butnot compositions) like that of the mineral perovskite fol-lowed by the magnesium/iron oxide ferropericlase.[30]

The changes in mineralogy at about 400 and 650 km (250and400mi)yielddistinctive signatures in seismic recordsof the Earth’s interior, and like the moho, are readily de-tected using seismic waves. These changes in mineralogymay inuence mantle convection, as they result in den-sity changes and they may absorb or release latent heat aswell as depress or elevate the depth of the polymorphicphase transitions for regions of different temperatures.The changes in mineralogy with depth have been investi-gated by laboratory experiments that duplicate high man-tle pressures, such as those using the diamond anvil.[31]

The inner core is solid, the outer core is liquid, and themantle solid/plastic. This is because of the relative melt-ing points of the different layers (nickel–iron core, sil-icate crust and mantle) and the increase in temperatureand pressure as depth increases. At the surface bothnickel–iron alloys and silicates are sufficiently cool to besolid. In the upper mantle, the silicates are generally solid(localised regionswithsmall amounts of melt exist); how-ever, as the upper mantle is both hot and under relativelylittle pressure, the rock in the upper mantle has a rela-tively low viscosity. In contrast, the lower mantle is undertremendous pressure and therefore has a higher viscositythan the upper mantle. The metallic nickel–iron outercore is liquid because of the high temperature, despite

the high pressure. As the pressure increases, the nickel–iron inner core becomes solid because the melting pointof iron increases dramatically at these high pressures.[34]

1.3 Temperature

In the mantle, temperatures range between 500 to 900°C (932 to 1,652 °F) at the upper boundary with thecrust; to over 4,000 °C (7,230 °F) at the boundary withthe core.[34] Although the higher temperatures far ex-ceed the melting points of the mantle rocks at the surface(about 1200 °C for representative peridotite), the mantleis almost exclusively solid.[34] The enormous lithostaticpressure exerted on the mantle prevents melting, becausethe temperature at which melting begins (the solidus) in-creases with pressure.

1.4 Movement

Main article: Mantle convectionBecause of the temperature difference between the

Earth’s surface and outer core and the ability of the crys-talline rocks at high pressure and temperature to un-

dergo slow, creeping, viscous-like deformation over mil-lions of years, there is a convective material circulationin the mantle.[23] Hot material upwells, while cooler (and

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1.5 Exploration 3

This gure is a snapshot of one time-step in a model of mantleconvection. Colors closer to red are hot areas and colors closer to blue are cold areas. In this gure, heat received at the core– mantle boundary results in thermal expansion of the material at thebottom ofthe model, reducing its densityand causingit to send plumes of hot material upwards. Likewise, cooling of material at the surface results in its sinking.

heavier) material sinks downward. Downward motionof material occurs at convergent plate boundaries calledsubduction zones. Locations on the surface that lie overplumes are predicted to have high elevation (because ofthebuoyancy of thehotter, less-denseplume beneath)andto exhibit hot spot volcanism. The volcanism often at-tributed to deep mantle plumes is alternatively explainedby passive extension of the crust, permitting magma toleak to the surface (the “Plate” hypothesis).[35]

The convection of the Earth’s mantle is a chaotic pro-cess (in the sense of uid dynamics), which is thoughtto be an integral part of the motion of plates. Plate mo-tion should not be confused with continental drift whichapplies purely to the movement of the crustal compo-

nents of the continents. The movements of the litho-sphere and the underlying mantle are coupled since de-scending lithosphere is an essential component of con-vection in the mantle. The observed continental driftis a complicated relationship between the forces caus-ing oceanic lithosphere to sink and the movements withinEarth’s mantle.Although there is a tendency to larger viscosity at greaterdepth, this relation is far from linear and shows layerswith dramatically decreased viscosity, in particular inthe upper mantle and at the boundary with the core.[36]

The mantle within about 200 km (120 mi) above the

core–mantle boundary appears to have distinctly differ-ent seismic properties than the mantle at slightly shal-lower depths; this unusual mantle region just above thecore is called D″ (“D double-prime”), a nomenclatureintroduced over 50 years ago by the geophysicist KeithBullen.[37] D″ may consist of material from subductedslabs that descended and came to rest at the core–mantleboundary and/or from a new mineral polymorph discov-ered in perovskite called post-perovskite.Earthquakes at shallow depths are a result of stick-slipfaulting; however, below about 50 km (31 mi) the hot,high pressure conditions ought to inhibit further seismic-

ity. The mantle is considered to be viscous and incapableof brittle faulting. However, in subduction zones, earth-quakesareobserveddownto 670km(420 mi). A number

of mechanisms have been proposed to explain this phe-nomenon, including dehydration, thermal runaway, andphase change. The geothermal gradient can be loweredwhere cool material from the surface sinks downward, in-creasing the strength of the surrounding mantle, and al-lowing earthquakes to occur down to a depth of 400 km(250 mi) and 670 km (420 mi).The pressureat the bottomof the mantle is~136GPa(1.4million atm).[25] Pressure increases as depth increases,since the material beneath has to support the weight ofall the material above it. The entire mantle, however, isthought to deform like a uid on long timescales, withpermanent plastic deformation accommodated by themovement of point, line, and/or planar defects throughthe solid crystals comprising the mantle. Estimates forthe viscosity of the upper mantle range between 1019 and1024 Pa·s, depending ondepth,[36] temperature, composi-tion, state of stress, andnumerousother factors. Thus, theupper mantle can only ow very slowly. However, whenlarge forcesareapplied to theuppermostmantleit canbe-come weaker, and this effect is thought to be importantin allowing the formation of tectonic plate boundaries.

1.5 Exploration

Exploration of the mantle is generally conducted at theseabed rather than on land because of the relative thin-ness of the oceanic crust as compared to the signicantlythicker continental crust.

The rst attempt at mantle exploration, known as ProjectMohole, was abandoned in 1966 after repeated failuresand cost over-runs. The deepest penetration was approx-imately 180 m (590 ft). In 2005 an oceanic boreholereached 1,416 metres (4,646 ft) below the sea oor fromthe ocean drilling vessel JOIDES Resolution .On 5 March 2007, a team of scientists on board theRRS James Cook embarked on a voyage to an area ofthe Atlantic seaoor where the mantle lies exposed with-out any crust covering, mid-way between the Cape VerdeIslands and the Caribbean Sea. The exposed site liesapproximately three kilometres beneath the ocean sur-

face and covers thousands of square kilometres.[38][39] Arelatively difficult attempt to retrieve samples from theEarth’s mantle was scheduled for later in 2007.[40] TheChikyu Hakken mission attempted to use the Japanesevessel Chikyū to drill up to 7,000 m (23,000 ft) below theseabed. This is nearly three times as deep as precedingoceanic drillings.A novel method of exploring the uppermost few hundredkilometres of the Earth was recently proposed, consistingof a small, dense, heat-generating probe which melts itsway down through the crust and mantle while its positionand progress are tracked by acoustic signals generated in

the rocks.[41] The probe consists of an outer sphere oftungsten about one metre in diameter with a cobalt-60interior acting as a radioactive heat source. It was calcu-

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4 3 REFERENCES

lated that sucha probewill reach theoceanicMohoin lessthan 6 months and attain minimum depths of well over100 km (62 mi) in a few decades beneath both oceanicand continental lithosphere.[42]

Exploration can also be aided through computer sim-

ulations of the evolution of the mantle. In 2009, asupercomputer application provided new insight into thedistribution of mineral deposits, especially isotopes ofiron, from when the mantle developed 4.5 billion yearsago.[43]

2 See also

• Core–mantle boundary

• Lehmann discontinuity

•

Mantle xenoliths• Mantle convection

• Mesosphere (mantle)

• Mohorovičić discontinuity

• Post-perovskite phase transition

• Primitive mantle

• Earth’s internal heat budget

3 References[1] “Earth Structure”. Trinity University, Texas. Retrieved

16 October 2015.

[2] Zharkov, V. N. and Zasurskii, I. Ia. (1981). “Distributionof the shearing stresses in the silicate mantle of Venus”.Astronomicheskii Vestnik 15: 11–16.

[3] Longhi, John; et al. (1992). “The bulk composition,mineralogy and internal structure of Mars”. Mars (A93-27852 09-91) . University of Arizona Press, Tucson. pp.184–208. Retrieved 16 October 2015.

[4] “MESSENGER Provides NewLook at Mercury’ssurpris-ing core and landscape curiosities”. NASA. 21 March2012. Retrieved 16 October 2015.

[5] “Moon ABCs Fact Sheet” (PDF). NASA. Retrieved 16October 2015.

[6] NASA (6 October 2000). “Scientists Show Jovian MoonIo’s Mantle is Similar to Earth”. NASA. Retrieved 7 Oc-tober 2015.

[7] “Frequently Asked Questions about Europa”. NASA. Re-trieved 16 October 2015.

[8] Neumann, W.; et al. (2014). “Differentiation ofVesta: Implications for a shallow magma ocean”.Earth and Planetary Science Letters 395: 267–280.doi:10.1016/j.epsl.2014.03.033.

[9] “Ganymede: In Depth”. NASA. Retrieved 16 October2015.

[10] “Callisto: In Depth”. NASA. Retrieved 16 October 2015.

[11] “Layers of Titan”. NASA. 23 February 2012. Retrieved7 October 2015.

[12] “Triton: In Depth”. NASA. Retrieved 16 October 2015.

[13] Sorokhtin, O.G.; Chilingarian, G.V.; Sorokhtin, N.O.(2011). Evolution of Earth and its climate birth, life and death of Earth . Amsterdam: Elsevier Science Ltd. p.137. ISBN 9780444537584. Retrieved 29 May 2015.

[14] Robertson, Eugene (2007). “The interior of the earth”.USGS. Retrieved 2009-01-06.

[15] “The structure of the Earth”. Moorland School. 2005.Retrieved 2007-12-26.

[16] Thompson, Graham R.; Turk, Jonathan(2007). Earth sci-ence and the environment (4th ed., International studentedition. ed.). Australia: Thomson Brooks/Cole. pp. 133–134. ISBN 9780495112877. Retrieved 29 May 2015.

[17] Lithosphere: Schlumberger Oileld Glossary. Glos-sary.oileld.slb.com. Retrieved on 2013-05-11.

[18] Crust: Schlumberger Oileld Glossary. Glos-sary.oileld.slb.com. Retrieved on 2013-05-11.

[19] “Rare Diamond conrms that Earth’s mantle holds anocean’s worth of water”. Scientic American . March 12,2014. Retrieved March 13, 2014.

[20] Schmandt, Brandon; Jacobsen, Steven D.; Becker,Thorsten W.; Liu, Zhenxian; Dueker, Kenneth G.(13 June 2014). “Dehydration melting at the top ofthe lower mantle”. Science 344 (6189): 1265–1268.doi:10.1126/science.1253358. Retrieved 13 June 2014.

[21] Mission to Study Earth’s Gaping 'Open Wound'. Live-Science. Retrieved on 2013-05-11.

[22] The location of the base of the crust varies from approxi-mately 10 to 70 kilometers. Oceanic crustis generally lessthan 10 kilometers thick. “Standard” continental crust isaround 35 kilometers thick, and the large crustal root un-der the Tibetan Plateau is approximately 70 kilometers

thick.[23] Alden, Andrew (2007). “Today’s Mantle: a guided tour”.

About.com. Retrieved 2007-12-25.

[24] The Mantle. mediatheek.thinkquest.nl (2000)

[25] Burns, Roger George (1993). Mineralogical Applications of Crystal Field Theory . Cambridge University Press. p.354. ISBN 0-521-43077-1. Retrieved 2007-12-26.

[26] “Istria on the Internet – Prominent Istrians – Andrija Mo-horovicic”. 2007. Retrieved 2007-12-25.

[27] Carlowicz, Michael (2005). “Inge Lehmann biogra-phy”. American Geophysical Union, Washington, D.C.Archived from the original on 2007-09-30. Retrieved2007-12-25.

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5

[28] Rudoph, Maxwell (11 December 2015). “Viscosity jumpin Earth’s mid-mantle”. Science. Retrieved 16 January2016.

[29] Sumner, Thomas (10 December 2015). “Gooey rock inmantle thickens 1,000 kilometers down”. Science News .Retrieved 16 January 2016.

[30] Anderson, DonL. (2007)New Theory of theEarth. Cam-bridgeUniversityPress. ISBN978-0-521-84959-3, ISBN0-521-84959-4

[31] Alden, Andrew. “The Big Squeeze: Into the Mantle”.About.com. Retrieved 2007-12-25.

[32] mantle@Everything2.com. Retrieved 2007-12-26.

[33] Jackson, Ian (1998). The Earth’s Mantle - Composition,Structure, and Evolution . Cambridge UniversityPress. pp.311–378. ISBN 0-521-78566-9.

[34] Louie, J. (1996). “Earth’s Interior”. Universityof Nevada,

Reno. Retrieved 2007-12-24.[35] Foulger, G.R. (2010). Plates vs. Plumes: A Geological

Controversy . Wiley-Blackwell. ISBN 978-1-4051-6148-0.

[36] Walzer, Uwe; Hendel, Roland and Baumgardner, John.Mantle Viscosity and the Thickness of the ConvectiveDownwellings. igw.uni-jena.de

[37] Alden, Andrew. “The End of D-Double-Prime Time?".About.com. Retrieved 2007-12-25.

[38] Than, Ker (2007-03-01). “Scientists to study gash on At-lantic seaoor”. Msnbc.com . Retrieved 2008-03-16. Ateam of scientists will embark on a voyage next week tostudy an “open wound” on the Atlantic seaoor where theEarth’s deep interior lies exposed without any crust cov-ering.

[39] “Earth’s Crust Missing In Mid-Atlantic”. Science Daily .2007-03-02. Retrieved 2008-03-16. Cardiff Universityscientists will shortly set sail (March 5) to investigate astartling discovery in the depths of the Atlantic.

[40] “Japan hopes to predict 'Big One' with journey to centerof Earth”. PhysOrg.com . 2005-12-15. Archived from theoriginal on 2005-12-19. Retrieved 2008-03-16. An am-bitious Japanese-led project to dig deeper into the Earth’s

surface than ever before will be a breakthrough in detect-ing earthquakes including Tokyo’s dreaded “Big One,” of-cials said Thursday.

[41] Ojovan M.I., Gibb F.G.F., Poluektov P.P., Emets E.P.2005. Probing of the interior layers of theEarth with self-sinking capsules. Atomic Energy, 99, 556–562

[42] Ojovan M.I., Gibb F.G.F. “Exploring the Earth’s Crustand Mantle Using Self-Descending, Radiation-Heated,Probes and Acoustic Emission Monitoring”. Chapter 7.In: Nuclear Waste Research: Siting, Technology and Treat-ment , ISBN 978-1-60456-184-5, Editor: Arnold P. Lat-tefer, Nova Science Publishers, Inc. 2008

[43] University of California – Davis (2009-06-15). Super-computerProvides First Glimpse Of Earth’s Early MagmaInterior. ScienceDaily. Retrieved on 2009-06-16.

4 Further reading

• Don L. Anderson, Theory of the Earth , Blackwell(1989), is a textbook dealing with the Earth’s in-terior and is now available on the web. Retrieved2007-12-23.

• Jeanloz, Raymond (2000). “Mantle of the Earth”.In Haraldur Sigurdsson, Bruce Houghton, HazelRymer, John Stix, Steve McNutt. Encyclopedia of Volcanoes . San Diego: Academic Press. pp. 41–54.ISBN 978-0-12-643140-7. Retrieved 2010-05-17.

• Nixon, Peter H. (1987). Mantle xenoliths: J. Wiley& Sons, 844p., (ISBN 0-471-91209-3).

5 External links

• The Biggest Dig: Japan builds a ship to drill tothe earth’s mantle – Scientic American Magazine(September 2005)

• Information on the Mohole Project

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6 6 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

6 Text and image sources, contributors, and licenses

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