Jump to content

Upper mantle

From Wikipedia, the free encyclopedia
(Redirected fromUpper mantle (Earth))

Theupper mantleofEarthis a very thick layer of rock inside the planet, which begins just beneath thecrust(at about 10 km (6.2 mi) under the oceans and about 35 km (22 mi) under the continents) and ends at the top of thelower mantleat 670 km (420 mi). Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 1,200 K (930 °C; 1,700 °F) at the boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55%olivine,35%pyroxene,and 5 to 10% ofcalcium oxideandaluminum oxideminerals such asplagioclase,spinel,orgarnet,depending upon depth.

Seismic structure[edit]

1 = continental crust, 2 = oceanic crust, 3 = upper mantle, 4 = lower mantle, 5+6 = core, A = crust-mantle boundary (Mohorovičić discontinuity)

The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of the rock at increased depths. Abrupt changes in density occur where the material composition changes.[1]

The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move.

Crust andmantleare distinguished by composition, while thelithosphereandasthenosphereare defined by a change in mechanical properties.[2]

The top of the mantle is defined by a sudden increase in the speed of seismic waves, whichAndrija Mohorovičićfirst noted in 1909; this boundary is now referred to as theMohorovičić discontinuityor "Moho."[3]

The Moho defines the base of the crust and varies from 10 km (6.2 mi) to 70 km (43 mi) below the surface of the Earth.Oceanic crustis thinner thancontinental crustand is generally less than 10 km (6.2 mi) thick. Continental crust is about 35 km (22 mi) thick, but the large crustal root under theTibetan Plateauis approximately 70 km (43 mi) thick.[4]

The thickness of the upper mantle is about 640 km (400 mi). The entire mantle is about 2,900 km (1,800 mi) thick, which means the upper mantle is only about 20% of the total mantle thickness.[4]

Cross-section of the Earth, showing the paths of earthquake waves. The paths curve because the different rock types found at different depths change the waves' speed. S waves do not travel through the core

The boundary between the upper and lower mantle is a 670 km (420 mi) discontinuity.[2]Earthquakes at shallow depths result fromstrike-slip faulting;however, below about 50 km (31 mi), the hot, high-pressure conditions inhibit further seismicity. The mantle is viscous and incapable offaulting.However, insubduction zones,earthquakes are observed down to 670 km (420 mi).[1]

Lehmann discontinuity[edit]

The Lehmann discontinuity is an abrupt increase ofP-waveandS-wavevelocities at a depth of 220 km (140 mi)[5](Note that this is a different "Lehmann discontinuity" than the one between the Earth's inner and outer cores labeled in the image on the right.)

Transition zone[edit]

Thetransition zoneis located between the upper mantle and thelower mantlebetween a depth of 410 km (250 mi) and 670 km (420 mi).

This is thought to occur as a result of the rearrangement of grains in olivine to form a denser crystal structure as a result of the increase in pressure with increasing depth.[6]Below a depth of 670 km (420 mi), due to pressure changes, ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen usingbody wavesfromearthquakes,which are converted, reflected, or refracted at the boundary, and predicted frommineral physics,as the phase changes are temperature and density-dependent and hence depth-dependent.[6]

410 km discontinuity[edit]

A single peak is seen in all seismological data at 410 km (250 mi), which is predicted by the single transition from α- to β- Mg2SiO4(olivine towadsleyite). From theClapeyron slopethis discontinuity is expected to be shallower in cold regions, such assubductingslabs, and deeper in warmer regions, such asmantle plumes.[6]

670 km discontinuity[edit]

This is the most complex discontinuity and marks the boundary between the upper and lower mantle. It appears in PP precursors (a wave that reflects off the discontinuity once) only in certain regions but is always apparent in SS precursors.[6]It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640–720 km, or 397–447 mi). The Clapeyron slope predicts a deeper discontinuity in colder regions and a shallower discontinuity in hotter regions.[6]This discontinuity is generally linked to the transition fromringwooditetobridgmaniteandpericlase.[7]This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models.[8]

Other discontinuities[edit]

There is another major phase transition predicted at 520 km (320 mi) for the transition of olivine (β to γ) andgarnetin thepyrolitemantle.[9]This one has only sporadically been observed in seismological data.[10]

Other non-global phase transitions have been suggested at a range of depths.[6][11]

Temperature and pressure[edit]

Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,930 °C; 7,100 °F) at the core-mantle boundary.[12]The highest temperature of the upper mantle is 1,200 K (930 °C; 1,700 °F).[13]Although the high temperature far exceeds themelting pointsof the mantle rocks at the surface, the mantle is almost exclusively solid.[14]

The enormouslithostatic pressureexerted on the mantle preventsmeltingbecause the temperature at which melting begins (thesolidus) increases with pressure.[15]Pressure increases as depth increases since the material beneath has to support the weight of all the material above it. The entire mantle is thought to deform like a fluid on long timescales, with permanent plastic deformation.

The highest pressure of the upper mantle is 24.0 GPa (237,000 atm)[13]compared to the bottom of the mantle, which is 136 GPa (1,340,000 atm).[12][16]

Estimates for the viscosity of the upper mantle range between 1019and 1024Pa·s,depending on depth,[17]temperature, composition, state of stress, and numerous other factors. 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 oftectonic plateboundaries.

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.[17]

Movement[edit]

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 aconvectivematerial circulation in the mantle.[3]

Hot materialupwells,while cooler (and heavier) material sinks downward. Downward motion of material occurs atconvergent plate boundariescalledsubduction zones.Locations on the surface that lie over plumes are predicted to havehigh elevation(because of the buoyancy of the hotter, less-dense plume beneath) and to exhibithot spotvolcanism.

Mineral composition[edit]

The seismic data is not sufficient to determine the composition of the mantle. Observations of rocks exposed on the surface and other evidence reveal that the upper mantle ismaficminerals olivine and pyroxene, and it has a density of about 3.33 g/cm3(0.120 lb/cu in)[1]

Upper mantle material that has come up onto the surface comprises about 55% olivine and 35% pyroxene, and 5 to 10% ofcalcium oxideandaluminum oxide.[1]The upper mantle is dominantlyperidotite,composed primarily of variable proportions of the minerals olivine,clinopyroxene,orthopyroxene,and an aluminous phase.[1]The aluminous phase is plagioclase in the uppermost mantle, then spinel, and then garnet below about 100 kilometres (62 mi).[1]Gradually through the upper mantle, pyroxenes become less stable and transform intomajoritic garnet.

Experiments on olivines and pyroxenes show that these minerals change the structure as pressure increases at greater depth, which explains why the density curves are not perfectly smooth. When there is a conversion to a more dense mineral structure, the seismic velocity rises abruptly and creates a discontinuity.[1]

At the top of the transition zone, olivine undergoes isochemical phase transitions towadsleyiteandringwoodite.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.[18]

In Earth's interior, olivine occurs in the upper mantle at depths less than 410 kilometres (250 mi), and ringwoodite is inferred within thetransition zonefrom about 520 to 670 kilometres (320 to 420 mi) depth.Seismicactivity discontinuities at about 410 kilometres (250 mi), 520 kilometres (320 mi), and 670 kilometres (420 mi) depth have been attributed tophase changesinvolving olivine and itspolymorphs.

At the base of the transition zone,ringwooditedecomposes intobridgmanite(formerly called magnesium silicate perovskite), andferropericlase.Garnet also becomes unstable at or slightly below the base of the transition zone.

Kimberlitesexplode from the earth's interior and sometimes carry rock fragments. Some of thesexenolithicfragments are diamonds that can only come from the higher pressures below the crust. The rocks that come with this areultramaficnodules and peridotite.[1]

Chemical composition[edit]

The composition seems to be very similar to the crust. One difference is that rocks and minerals of the mantle tend to have more magnesium and less silicon and aluminum than the crust. The first four most abundant elements in the upper mantle are oxygen, magnesium, silicon, and iron.

Composition of the Earth's upper mantle (depletedMORB)[19][20]
Compound Mass percent
SiO2 44.71
MgO 38.73
FeO 8.18
Al2O3 3.98
CaO 3.17
Cr2O3 0.57
NiO 0.24
MnO 0.13
Na2O 0.13
TiO2 0.13
P2O5 0.019
K2O 0.006

Exploration[edit]

Chikyu drilling ship

Exploration of the mantle is generally conducted at the seabed rather than on land because of the oceanic crust's relative thinness as compared to the significantly thicker continental crust.

The first attempt at mantle exploration, known asProject Mohole,was abandoned in 1966 after repeated failures and cost overruns. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below the seafloor from the ocean drilling vesselJOIDES Resolution.

On 5 March 2007, a team of scientists on board theRRSJames Cookembarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between theCape Verde Islandsand theCaribbean Sea.The exposed site lies approximately 3 kilometres (1.9 mi) beneath the ocean surface and covers thousands of square kilometers.[21][22] [23]

The Chikyu Hakken mission attempted to use the Japanese vesselChikyūto drill up to 7,000 m (23,000 ft) below the seabed. On 27 April 2012,Chikyūdrilled to a depth of 7,740 metres (25,390 ft) below sea level, setting a new world record for deep-sea drilling. This record has since been surpassed by the ill-fatedDeepwater Horizonmobile offshore drilling unit, operating on the Tiber prospect in the Mississippi Canyon Field, United States Gulf of Mexico, when it achieved a world record for total length for a vertical drilling string of 10,062 m (33,011 ft).[24]The previous record was held by the U.S. vesselGlomar Challenger,which in 1978 drilled to 7,049.5 meters (23,130 feet) below sea level in theMariana Trench.[25]On 6 September 2012, Scientific deep-sea drilling vesselChikyūset a new world record by drilling down and obtaining rock samples from deeper than 2,111 metres (6,926 ft) below the seafloor off the Shimokita Peninsula of Japan in the northwest Pacific Ocean.

A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe that melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks.[26]The probe consists of an outer sphere oftungstenabout 1 metre (3 ft 3 in) in diameter with acobalt-60interior acting as a radioactive heat source. This should take half a year to reach the oceanicMoho.[27]

Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, asupercomputerapplication provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago.[28]

In 2023 JOIDES Resolution recovered cores of what appeared to be rock from the upper mantle after drilling only a few hundred meters into theAtlantis Massif.The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarilyperidotite.There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However, the samples offer a much closer analogue to mantle rock than magmaticxenolithsas the sampled rock never melted into magma or recrystallized.[29]

References[edit]

  1. ^abcdefghLangmuir, Charles H.; Broecker, Wally (2012-07-22).How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind.pp. 179–183.ISBN9780691140063.
  2. ^abRothery, David A.; Gilmour, Iain; Sephton, Mark A. (March 2018).An Introduction to Astrobiology.p. 56.ISBN9781108430838.
  3. ^abAlden, Andrew (2007)."Today's Mantle: a guided tour".About.Retrieved2007-12-25.
  4. ^ab"Istria on the Internet – Prominent Istrians – Andrija Mohorovicic".2007.Retrieved2007-12-25.
  5. ^William Lowrie (1997).Fundamentals of geophysics.Cambridge University Press. p. 158.ISBN0-521-46728-4.
  6. ^abcdefFowler, C. M. R.; Fowler, Connie May (2005).The Solid Earth: An Introduction to Global Geophysics.ISBN978-0521893077.
  7. ^Ito, E; Takahashi, E (1989). "Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications".Journal of Geophysical Research: Solid Earth.94(B8): 10637–10646.Bibcode:1989JGR....9410637I.doi:10.1029/jb094ib08p10637.
  8. ^Fukao, Y.; Obayashi, M. (2013)."Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity".Journal of Geophysical Research: Solid Earth.118(11): 5920–5938.Bibcode:2013JGRB..118.5920F.doi:10.1002/2013jb010466.S2CID129872709.
  9. ^Deuss, Arwen; Woodhouse, John (2001-10-12). "Seismic Observations of Splitting of the Mid-Transition Zone Discontinuity in Earth's Mantle".Science.294(5541): 354–357.Bibcode:2001Sci...294..354D.doi:10.1126/science.1063524.ISSN0036-8075.PMID11598296.S2CID28563140.
  10. ^Egorkin, A. V. (1997-01-01). "Evidence for 520-Km Discontinuity". In Fuchs, Karl (ed.).Upper Mantle Heterogeneities from Active and Passive Seismology.NATO ASI Series. Springer Netherlands. pp. 51–61.doi:10.1007/978-94-015-8979-6_4.ISBN9789048149667.
  11. ^Khan, Amir; Deschamps, Frédéric (2015-04-28).The Earth's Heterogeneous Mantle: A Geophysical, Geodynamical, and Geochemical Perspective.Springer.ISBN9783319156279.
  12. ^abLodders, Katharina(1998).The planetary scientist's companion.Fegley, Bruce. New York: Oxford University Press.ISBN978-1423759836.OCLC65171709.
  13. ^ab"What Are Three Differences Between the Upper & Lower Mantle?".Sciencing.Retrieved14 June2019.
  14. ^Louie, J. (1996)."Earth's Interior".University of Nevada, Reno. Archived fromthe originalon 2011-07-20.Retrieved2007-12-24.
  15. ^Turcotte, DL; Schubert, G (2002). "4".Geodynamics(2nd ed.). Cambridge, England, UK: Cambridge University Press. pp.136–7.ISBN978-0-521-66624-4.
  16. ^Burns, Roger George (1993).Mineralogical Applications of Crystal Field Theory.Cambridge University Press. p. 354.ISBN978-0-521-43077-7.Retrieved2007-12-26.
  17. ^abWalzer, Uwe."Mantle Viscosity and the Thickness of the Convective Downwellings".Archived fromthe originalon 2007-06-11.
  18. ^Bercovici, David; Karato, Shun-ichiro (September 2003). "Whole-mantle convection and the transition-zone water filter".Nature.425(6953): 39–44.Bibcode:2003Natur.425...39B.doi:10.1038/nature01918.ISSN0028-0836.PMID12955133.S2CID4428456.
  19. ^Workman, Rhea K.; Hart, Stanley R. (February 2005). "Major and trace element composition of the depleted MORB mantle (DMM)".Earth and Planetary Science Letters.231(1–2): 53–72.Bibcode:2005E&PSL.231...53W.doi:10.1016/j.epsl.2004.12.005.ISSN0012-821X.
  20. ^Anderson, D.L. (2007).New Theory of the Earth.Cambridge University Press. p.301.ISBN9780521849593.
  21. ^Than, Ker (2007-03-01)."Scientists to study gash on Atlantic seafloor".NBC News.Retrieved2008-03-16.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.
  22. ^"Earth's Crust Missing In Mid-Atlantic".Science Daily.2007-03-02.Retrieved2008-03-16.Cardiff University scientists will shortly set sail (March 5) to investigate a startling discovery in the depths of the Atlantic.
  23. ^"Japan hopes to predict 'Big One' with journey to center of Earth".PhysOrg.2005-12-15. Archived fromthe originalon 2005-12-19.Retrieved2008-03-16.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.
  24. ^"- - Explore Records - Guinness World Records".Archived fromthe originalon 2011-10-17.
  25. ^"Japan deep-sea drilling probe sets world record".The Kansas City Star.Associated Press. 28 April 2012. Archived fromthe originalon 28 April 2012.Retrieved28 April2012.
  26. ^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
  27. ^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,ISBN978-1-60456-184-5,Editor: Arnold P. Lattefer,Nova Science Publishers, Inc.2008
  28. ^University of California – Davis (2009-06-15).Super-computer Provides First Glimpse Of Earth's Early Magma Interior.ScienceDaily.Retrieved on 2009-06-16.
  29. ^At long last, ocean drillers exhume a bounty of rocks from Earth’s mantle(Report). 2023-05-25.doi:10.1126/science.adi9181.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Upper_mantle&oldid=1209209592"