Structure of the Earth

Structure of the Earth

The interior structure of the Earth is layered in spheri-cal shells, like an onion. These layers can be defined by ei-ther their chemical or their rheological properties. Earthhas an outer silicate solid crust, a highly viscous mantle, aliquid outer core that is much less viscous than the man-tle, and a solid inner core. Scientific understanding ofthe internal structure of the Earth is based on observa-tions of topography and bathymetry, observations of rockin outcrop, samples brought to the surface from greaterdepths by volcanoes or volcanic activity, analysis of theseismic waves that pass through the Earth, measurementsof the gravitational and magnetic fields of the Earth, andexperiments with crystalline solids at pressures and tem-peratures characteristic of the Earth’s deep interior.

1 Mass

The force exerted by Earth’s gravity can be used to calcu-late its mass, and by estimating the volume of the Earth,its average density can be calculated. Astronomers canalso calculate Earth’s mass from its orbit and effects onnearby planetary bodies.

2 Structure

Earth’s radial density distribution according to the pre-liminary reference earth model (PREM).[1]

0 2 4 6 8 10 12 14radius in 1000 km

0

2

4

6

8

10

12

acce

lera

tion

in m

/s2

PREMLinear densityConstant density

Free-fall acceleration of EarthInnerCore

OuterCore

LowerMantle

UpperMantle

Space

Earth’s gravity according to the preliminary referenceearth model (PREM).[1] Comparison to approximationsusing constant and linear density for Earth’s interior.

Innercore

Outer core

Lower mantle Upper mantle

Crust

Focus of earthquake

Kilometers

0 10,000

S

SSSS

SSKS

SKP

SP

P

P

P

P

PP

PKPPPP

P

P

K

K

Mapping the interior of Earth with earthquake waves.

1

2 2 STRUCTURE

35 700 2885 51550 6371 km

A B C

12

3

4

5

6

2885 km

2270 km

1216 km

30-65 km

Schematic view of the interior of Earth. 1. continen-tal crust – 2. oceanic crust – 3. upper mantle – 4.lower mantle – 5. outer core – 6. inner core – A:Mohorovičić discontinuity – B: Gutenberg Discontinuity– C: Lehmann–Bullen discontinuity.

The structure of Earth can be defined in two ways:by mechanical properties such as rheology, or chemi-cally. Mechanically, it can be divided into lithosphere,asthenosphere, mesospheric mantle, outer core, and theinner core. The interior of Earth is divided into 5 im-portant layers. Chemically, Earth can be divided into thecrust, upper mantle, lower mantle, outer core, and innercore. The geologic component layers of Earth[2] are atthe following depths below the surface:The layering of Earth has been inferred indirectly usingthe time of travel of refracted and reflected seismic wavescreated by earthquakes. The core does not allow shearwaves to pass through it, while the speed of travel (seismicvelocity) is different in other layers. The changes in seis-mic velocity between different layers causes refractionowing to Snell’s law, like light bending as it passes througha prism. Likewise, reflections are caused by a large in-crease in seismic velocity and are similar to light reflect-ing from a mirror.

2.1 Core

Main articles: inner core and outer core

The average density of Earth is 5,515 kg/m3. Sincethe average density of surface material is only around3,000 kg/m3, we must conclude that denser materials ex-ist within Earth’s core. Seismic measurements show thatthe core is divided into two parts, a “solid” inner core witha radius of ~1,220 km[3] and a liquid outer core extend-ing beyond it to a radius of ~3,400 km. The densities are

between 9,900 and 12,200 kg/m3 in the outer core and12,600–13,000 kg/m3 in the inner core.[4]

The inner core was discovered in 1936 by Inge Lehmannand is generally believed to be composed primarily ofiron and some nickel. It is not necessarily a solid, but,because it is able to deflect seismic waves, it must behaveas a solid in some fashion. Experimental evidence has attimes been critical of crystal models of the core.[5] Otherexperimental studies show a discrepancy under high pres-sure: diamond anvil (static) studies at core pressures yieldmelting temperatures that are approximately 2000 K be-low those from shock laser (dynamic) studies.[6][7] Thelaser studies create plasma,[8] and the results are sugges-tive that constraining inner core conditions will dependon whether the inner core is a solid or is a plasma withthe density of a solid. This is an area of active research.In early stages of Earth’s formation about four and a halfbillion (4.5×109) years ago, melting would have causeddenser substances to sink toward the center in a processcalled planetary differentiation (see also the iron catas-trophe), while less-dense materials would have migratedto the crust. The core is thus believed to largely be com-posed of iron (80%), along with nickel and one or morelight elements, whereas other dense elements, such as leadand uranium, either are too rare to be significant or tendto bind to lighter elements and thus remain in the crust(see felsic materials). Some have argued that the innercore may be in the form of a single iron crystal.[9][10]

Under laboratory conditions a sample of iron nickel alloywas subjected to the corelike pressures by gripping it ina vise between 2 diamond tips (diamond anvil cell), andthen heating to approximately 4000 K. The sample wasobserved with x-rays, and strongly supported the theorythat Earth’s inner core was made of giant crystals runningnorth to south.[11][12]

The liquid outer core surrounds the inner core and is be-lieved to be composed of ironmixed with nickel and traceamounts of lighter elements.Recent speculation suggests that the innermost part of thecore is enriched in gold, platinum and other siderophileelements.[13]

The matter that comprises Earth is connected in funda-mental ways to matter of certain chondrite meteorites,and to matter of outer portion of the Sun.[14][15] Thereis good reason to believe that Earth is, in the main, like achondrite meteorite. Beginning as early as 1940, scien-tists, including Francis Birch, built geophysics upon thepremise that Earth is like ordinary chondrites, the mostcommon type of meteorite observed impacting Earth,while totally ignoring another, albeit less abundant type,called enstatite chondrites. The principal difference be-tween the two meteorite types is that enstatite chondritesformed under circumstances of extremely limited avail-able oxygen, leading to certain normally oxyphile ele-ments existing either partially or wholly in the alloy por-tion that corresponds to the core of Earth.

2.3 Crust 3

Dynamo theory suggests that convection in the outer core,combined with the Coriolis effect, gives rise to Earth’smagnetic field. The solid inner core is too hot to holda permanent magnetic field (see Curie temperature) butprobably acts to stabilize the magnetic field generated bythe liquid outer core. The average magnetic field strengthin Earth’s outer core is estimated to be 25 Gauss (2.5mT), 50 times stronger than the magnetic field at thesurface.[16][17]

Recent evidence has suggested that the inner core ofEarth may rotate slightly faster than the rest of theplanet;[18] however, more recent studies in 2011 foundthis hypothesis to be inconclusive. Options remain forthe core which may be oscillatory in nature or a chaoticsystem. In August 2005 a team of geophysicists an-nounced in the journal Science that, according to their es-timates, Earth’s inner core rotates approximately 0.3 to0.5 degrees per year faster relative to the rotation of thesurface.[19][20]

The current scientific explanation for Earth’s tempera-ture gradient is a combination of heat left over from theplanet’s initial formation, decay of radioactive elements,and freezing of the inner core.

2.2 Mantle

World map showing the position of the Moho.

Main article: Mantle (geology)

Earth’s mantle extends to a depth of 2,890 km, mak-ing it the thickest layer of Earth. The upper mantleis divided into the lithospheric mantle and the astheno-sphere. The upper and lower mantle are separated by thetransition zone. The lowest part of the mantle next tothe core-mantle boundary is known as the D″ (D primeprime) layer. The pressure at the bottom of the mantleis ~140 GPa (1.4 Matm). The mantle is composed ofsilicate rocks that are rich in iron and magnesium rel-ative to the overlying crust. Although solid, the hightemperatures within the mantle cause the silicate mate-rial to be sufficiently ductile that it can flow on very longtimescales. Convection of the mantle is expressed at thesurface through the motions of tectonic plates. As there isintense and increasing pressure as one travels deeper into

the mantle, the lower part of the mantle flows less eas-ily than does the upper mantle (chemical changes withinthe mantle may also be important). The viscosity of themantle ranges between 1021 and 1024 Pa·s, depending ondepth.[21] In comparison, the viscosity of water is approx-imately 10−3 Pa·s and that of pitch is 107 Pa·s.

2.3 Crust

Main article: Crust (geology)

The crust ranges from 5–70 km (~3–44 miles) in depthand is the outermost layer. The thin parts are the oceaniccrust, which underlie the ocean basins (5–10 km) andare composed of dense (mafic) iron magnesium silicateigneous rocks, like basalt. The thicker crust is continentalcrust, which is less dense and composed of (felsic) sodiumpotassium aluminium silicate rocks, like granite. Therocks of the crust fall into two major categories – sialand sima (Suess,1831–1914). It is estimated that simastarts about 11 km below the Conrad discontinuity (asecond order discontinuity). The uppermost mantle to-gether with the crust constitutes the lithosphere. Thecrust-mantle boundary occurs as two physically differentevents. First, there is a discontinuity in the seismic veloc-ity, which is known as the Mohorovičić discontinuity orMoho. The cause of the Moho is thought to be a changein rock composition from rocks containing plagioclasefeldspar (above) to rocks that contain no feldspars (be-low). Second, in oceanic crust, there is a chemical dis-continuity between ultramafic cumulates and tectonizedharzburgites, which has been observed from deep parts ofthe oceanic crust that have been obducted onto the conti-nental crust and preserved as ophiolite sequences.Many rocks nowmaking up Earth’s crust formed less than100million (1×108) years ago; however, the oldest knownmineral grains are 4.4 billion (4.4×109) years old, indicat-ing that Earth has had a solid crust for at least that long.[22]

3 Historical development of alter-native conceptions

Main article: Hollow EarthIn 1692 EdmundHalley (in a paper printed in Philosophi-cal Transactions of Royal Society of London) put forth theidea of Earth consisting of a hollow shell about 500 milesthick, with two inner concentric shells around an inner-most core, corresponding to the diameters of the planetsVenus, Mars, and Mercury respectively.[23] Halley’s con-struct was a method of accounting for the (flawed) val-ues of the relative density of Earth and the Moon thathad been given by Sir Isaac Newton, in Principia (1687).“Sir Isaac Newton has demonstrated theMoon to bemoresolid than our Earth, as 9 to 5,” Halley remarked; “why

4 5 REFERENCES

Edmond Halley's hypothesis.

may we not then suppose four ninths of our globe to becavity?"[23]

4 See also

• Core-mantle boundary

• Geological history of Earth

• Lehmann discontinuity

• Mohorovičić discontinuity

• Rain-out model

• Receiver function

5 References

[1] A. M. Dziewonski, D. L. Anderson (1981). “Preliminaryreference Earth model” (PDF). Physics of the Earth andPlanetary Interiors 25 (4): 297–356. doi:10.1016/0031-9201(81)90046-7. ISSN 0031-9201.

[2] T. H. Jordan (1979). “Structural Geol-ogy of the Earth’s Interior”. Proceedingsof the National Academy of Sciences 76 (9):4192–4200. Bibcode:1979PNAS...76.4192J.doi:10.1073/pnas.76.9.4192. PMC 411539. PMID16592703.

[3] Monnereau, Marc; Calvet, Marie; Margerin, Lu-dovic; Souriau, Annie (May 21, 2010). “Lop-sided Growth of Earth’s Inner Core”. Science 328(5981): 1014–1017. Bibcode:2010Sci...328.1014M.doi:10.1126/science.1186212. PMID 20395477.

[4] Hazlett, James S. Monroe; Reed Wicander; Richard(2006). Physical geology : exploring the earth; [the wrathof Hurricane Katrina ; Could you survive a Tsunami?;catastrophic earthquakes; global warming] (6. ed.). Bel-mont: Thomson. p. 346. ISBN 9780495011484.

[5] Stixrude, Lars; Cohen, R.E. (January 15, 1995).“Constraints on the crystalline structure of the in-ner core: Mechanical instability of BCC iron athigh pressure”. Geophysical Research Letters 22(2): 125–128. Bibcode:1995GeoRL..22..125S.doi:10.1029/94GL02742.

[6] Benuzzi-Mounaix, A.; Koenig, M.; Ravasio, A.; Vinci,T. (2006). “Laser-driven shock waves for the studyof extreme matter states”. Plasma Physics and Con-trolled Fusion 48 (12B). Bibcode:2006PPCF...48B.347B.doi:10.1088/0741-3335/48/12B/S32.

[7] Remington, Bruce A.; Drake, R. Paul; Ryutov,Dmitri D. (2006). “Experimental astrophysics withhigh power lasers and Z pinches”. Reviews of Mod-ern Physics 78. Bibcode:2006RvMP...78..755R.doi:10.1103/RevModPhys.78.755.

[8] Benuzzi-Mounaix, A.; Koenig, M.; Husar, G.; Faral, B.(June 2002). “Absolute equation of statemeasurements ofiron using laser driven shocks”. Physics of Plasmas 9 (6).Bibcode:2002PhPl....9.2466B. doi:10.1063/1.1478557.

[9] Cohen, Ronald; Stixrude, Lars. “Crystal at the Center ofthe Earth”. Retrieved 2007-02-05.

[10] Stixrude, L.; Cohen, R. E. (1995). “High-Pressure Elas-ticity of Iron and Anisotropy of Earth’s Inner Core”. Sci-ence 267 (5206): 1972–5. Bibcode:1995Sci...267.1972S.doi:10.1126/science.267.5206.1972. PMID 17770110.

[11] BBC News, “What is at the centre of the Earth?.Bbc.co.uk (2011-08-31). Retrieved on 2012-01-27.

[12] Ozawa, H.; al., et (2011). “Phase Transition of FeOand Stratification in Earth’s Outer Core”. Science334 (6057): 792–794. Bibcode:2011Sci...334..792O.doi:10.1126/science.1208265.

[13] Wootton, Anne (2006). “Earth’s Inner Fort Knox”. Dis-cover 27 (9): 18.

[14] Herndon, J. M. (1980). “The chemical composition ofthe interior shells of the Earth”. Proc. R. Soc. Lond A372(1748): 149–154. JSTOR 2398362.

[15] Herndon, J. M. (2005). “Scientific basis of knowledgeon Earth’s composition” (PDF). Current Science 88 (7):1034–1037.

[16] FirstMeasurement OfMagnetic Field Inside Earth’s Core.Science20.com. Retrieved on 2012-01-27.

[17] Buffett, Bruce A. (2010). “Tidal dissipation and thestrength of the Earth’s internal magnetic field”. Na-ture 468 (7326): 952–4. Bibcode:2010Natur.468..952B.doi:10.1038/nature09643. PMID 21164483.

[18] Chang, Kenneth (2005-08-25). “Earth’s Core SpinsFaster Than the Rest of the Planet”. The New York Times.Retrieved 2010-05-24.

5

[19] Kerr, R. A. (2005). “Earth’s Inner Core Is Running aTad Faster Than the Rest of the Planet”. Science 309(5739): 1313a. doi:10.1126/science.309.5739.1313a.PMID 16123276.

[20] Chang, Kenneth (26 August 2005) “Scientists Say Earth’sCenter Rotates Faster Than Surface” The New York TimesSec. A, Col. 1, p. 13

[21] Uwe Walzer, Roland Hendel, John Baumgardner MantleViscosity and the Thickness of the Convective Down-wellings.

[22] Breaking News | Oldest rock shows Earth was a hospitableyoung planet. Spaceflight Now (2001-01-14). Retrievedon 2012-01-27.

[23] N. Kollerstrom (1992). “The hollow world of EdmondHalley”. Journal for History of Astronomy 23: 185–192.archive

6 Further reading• Kruglinski, Susan. “Journey to the Center of theEarth.” Discover.

• Lehmann, I. (1936) Inner Earth, Bur. Cent. Seis-mol. Int. 14, 3–31

• Schneider, David (October 1996)A Spinning CrystalBall, Scientific American

• Wegener, Alfred (1915) “The Origin of Continentsand Oceans”

6 7 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

7 Text and image sources, contributors, and licenses

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