View source for Internal structure of Earth

{{Short description|Interior of the earth}}
{{Distinguish|Earth structure}}
{{Geophysics|all}}
[[File:Earth Internal Structure.svg|thumb|307x307px|Geological cross section of Earth, showing the different layers of the interior.]]
The '''internal structure of Earth''' are the layers of the [[Earth]], excluding its [[Earth's atmosphere|atmosphere]] and [[hydrosphere]]. The [[structure]] consists of an outer [[silicate mineral|silicate]] solid [[crust (geology)|crust]], a highly [[viscosity|viscous]] [[asthenosphere]], and solid [[Mantle (geology)|mantle]], a liquid [[Earth's outer core|outer core]] whose flow generates the [[Earth's magnetic field]], and a solid [[Earth's inner core|inner core]].

Scientific understanding of the internal structure of [[Earth]] is based on observations of [[topography]] and [[bathymetry]], [[observation]]s of [[rock (geology)|rock]] in [[outcrop]], samples brought to the surface from greater depths by [[volcano]]es or volcanic activity, analysis of the [[seismic wave]]s that pass through Earth, measurements of the [[Gravity of Earth|gravitational]] and [[Earth's magnetic field|magnetic fields]] of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior.

==Global properties==
{{Further|Earth#Physical characteristics}}{{Expand section|date=August 2022}}
{| class="wikitable"
|+Chemical composition of the upper internal structure of [[Earth]]<ref name=":1">{{Cite book |url=http://assets.press.princeton.edu/chapters/s7559.pdf |title=The Structure of Earth and Its Constituents |publisher=Princeton University Press |pages=4}}</ref>
!Chemical element
![[Continental crust]] (%)
![[Upper mantle]] (%)
![[Pyrolite|Pyrolite model]] (%)
!Chondrite model (1) (%)
!Chondrite model (2) (%)
|-
|[[Magnesium oxide|MgO]]
|4.4
|36.6
|38.1
|26.3
|38.1
|-
|[[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]]
|15.8
|4.6
|4.6
|2.7
|3.9
|-
|[[Silicon dioxide|SiO<sub>2</sub>]]
|59.1
|45.4
|45.1
|29.8
|43.2
|-
|[[Calcium oxide|CaO]]
|6.4
|3.7
|3.1
|2.6
|3.9
|-
|[[Iron(II) oxide|FeO]]
|6.6
|8.1
|7.9
|6.4
|9.3
|-
|Other oxides
|7.7
|1.4
|1.2
|N/A
|5.5
|-
|[[Iron|Fe]]
|N/A 
|N/A
|N/A
|25.8
|N/A
|-
|[[Nickel|Ni]]
|N/A 
|N/A
|N/A
|1.7
|N/A
|-
|[[Silicon|Si]]
|N/A
|N/A
|N/A
|3.5
|N/A
|}
Note: In chondrite model (1), the light element in the core is assumed to be Si. Chondrite model (2) is a model of chemical composition of the mantle corresponding to the model of core shown in chondrite model (1).<ref name=":1" />[[File:The Blue Marble (remastered).jpg|alt=see caption|thumb|A [[The Blue Marble|photograph of Earth]] taken by the crew of [[Apollo 17]] in 1972. A processed version became widely known as ''[[The Blue Marble]]''.<ref name="Petsko">{{cite journal |last=Petsko |first=Gregory A. |author-link=Gregory Petsko |date=28 April 2011 |title=The blue marble |journal=[[Genome Biology]] |volume=12 |issue=4 |page=112 |doi=10.1186/gb-2011-12-4-112 |pmc=3218853 |pmid=21554751 |doi-access=free }}</ref><ref name="NASAmarble">{{cite web |date=1 November 2012 |title=Apollo Imagery – AS17-148-22727 |url=https://spaceflight.nasa.gov/gallery/images/apollo/apollo17/html/as17-148-22727.html |url-status=dead |archive-url=https://web.archive.org/web/20190420043021/https://spaceflight.nasa.gov/gallery/images/apollo/apollo17/html/as17-148-22727.html |archive-date=20 April 2019 |access-date=22 October 2020 |publisher=NASA}}</ref>]]Measurements of the force exerted by [[Earth's gravity]] can be used to calculate its [[mass]]. Astronomers can also calculate [[Earth mass|Earth's mass]] by observing the motion of orbiting [[satellite]]s. Earth's average [[density]] can be determined through gravimetric experiments, which have historically involved [[pendulum]]s. The mass of Earth is about {{val|6|e=24|u=kg}}.<ref name="AA">''M''<sub>E</sub> = 5·9722×10<sup>24</sup> kg ± 6×10<sup>20</sup> kg. "[http://asa.usno.navy.mil/static/files/2016/Astronomical_Constants_2016.pdf 2016 Selected Astronomical Constants] {{Webarchive|url=https://web.archive.org/web/20160215143441/http://asa.usno.navy.mil/static/files/2016/Astronomical_Constants_2016.pdf |date=2016-02-15 }}" in {{citation |title=The Astronomical Almanac Online |url=http://asa.usno.navy.mil/ |publisher=[[United States Naval Observatory|USNO]]–[[United Kingdom Hydrographic Office|UKHO]] |access-date=2016-02-18 |archive-date=2016-12-24 |archive-url=https://web.archive.org/web/20161224174302/http://asa.usno.navy.mil/static/files/2016/Astronomical_Constants_2016.pdf |url-status=dead }}</ref> The average density of Earth is {{val|5.515|u=[[Gram per cubic centimetre|g/cm<sup>3</sup>]]}}.<ref name="NASA">{{cite web |title=Planetary Fact Sheet |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |access-date=2 January 2009 |work=Lunar and Planetary Science |publisher=NASA |archive-date=24 March 2016 |archive-url=https://web.archive.org/web/20160324033129/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |url-status=live }}</ref>

==Layers==
[[File:Slice earth.svg|thumb|Schematic view of Earth's interior structure. {{olist |item_style=margin-bottom:0 |{{legend|#dd8714|[[continental crust]]}} |{{legend|#b8d2fe|[[oceanic crust]]}} |{{legend|#81ff7a|upper [[Earth's mantle|mantle]]}} |{{legend|#2dcf20|lower mantle}} |{{legend|#f4a828|[[Earth's outer core|outer core]]}} |{{legend|#f72b2e|[[inner core]]}}}}{{olist |item_style=margin-bottom:0| list-style-type=upper-alpha |[[Mohorovičić discontinuity]] |[[core–mantle boundary]] |outer core–inner core boundary}}]]

The structure of Earth can be defined in two ways: by mechanical properties such as [[rheology]], or chemically. Mechanically, it can be divided into [[lithosphere]], [[asthenosphere]], [[Mesosphere (mantle)|mesospheric mantle]], [[Earth's outer core|outer core]], and the [[Earth's inner core|inner core]]. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.<ref name=":0" /> The geologic component layers of Earth are at increasing depths below the surface.<ref name=":0">{{cite encyclopedia |title=Earth's structure, global |encyclopedia=Encyclopedia of solid earth geophysics |publisher=Springer Science & Business Media |last=Montagner |first=Jean-Paul |date=2011 |editor-last1=Gupta |editor-first1=Harsh |isbn=9789048187010}}</ref>{{Rp|page=146}}

=== Crust and lithosphere ===
{{Main|Earth's crust|Lithosphere}}
[[File:Tectonic plates (empty).svg|alt=Map of Earth's tectonic plates|thumb|upright=1.4|[[List of tectonic plates|Earth's major plates]], which are:{{Hlist|{{Legend inline|#fee6aa|[[Pacific Plate]]}}|{{Legend inline|#fb9a7a|[[African Plate]]}}|{{Legend inline|#ac8d7f|[[North American Plate]]}}|{{Legend inline|#7fa172|[[Eurasian Plate]]}}|{{Legend inline|#8a9dbe|[[Antarctic Plate]]}}|{{Legend inline|#fcb482|[[Indo-Australian Plate]]}}|{{Legend inline|#ad82b0|[[South American Plate]]}}}}]]
Earth's crust ranges from {{convert|5|to(-)|70|km}}<ref>{{cite news |url=https://www.zmescience.com/other/science-abc/layers-earth-structure/ |title=What are the layers of the Earth? |date=21 August 2018 |access-date=28 June 2019 |first=Mihai |last=Andrei |work=ZME Science |archive-date=12 May 2020 |archive-url=https://web.archive.org/web/20200512013554/https://www.zmescience.com/other/science-abc/layers-earth-structure/ |url-status=live }}</ref> in depth and is the outermost layer.<ref>{{cite news |url=https://sciencing.com/earths-structure-crust-inner-core-16911.html |title=Earth's Structure From the Crust to the Inner Core |date=25 April 2017 |access-date=28 June 2019 |first=Lisa |last=Chinn |work=Sciencing |publisher=[[Leaf Group]] Media |archive-date=30 July 2020 |archive-url=https://web.archive.org/web/20200730091442/https://sciencing.com/earths-structure-crust-inner-core-16911.html |url-status=live }}</ref> The thin parts are the [[oceanic crust]], which underlies the ocean basins (5–10&nbsp;km) and is [[mafic]]-rich<ref name="Rogers">{{cite book |url=https://books.google.com/books?id=WA9ST5S_2v0C&pg=PA44 |title=An Introduction to Our Dynamic Planet |date=2008 |publisher=[[Cambridge University Press]] and [[The Open University]] |isbn=978-0-521-49424-3 |editor-last=Rogers |editor-first=N. |page=19 |access-date=2022-08-08 |archive-date=2016-05-02 |archive-url=https://web.archive.org/web/20160502212957/https://books.google.com/books?id=WA9ST5S_2v0C&pg=PA44 |url-status=live }}</ref> (dense iron-magnesium [[silicate mineral]] or [[igneous rock]]).<ref name="Jackson1997">{{cite book |title=Glossary of Geology |date=1997 |publisher=American Geological Institute |isbn=0922152349 |editor1-last=Jackson |editor1-first=Julia A. |edition=4th |location=Alexandria, Virginia |chapter=mafic}}</ref> The thicker crust is the [[continental crust]], which is less dense<ref>{{cite encyclopedia |author=<!--editors. Regardless of what the website gives you as a citation, using "Britannica" as a surname and "The Editors of Encyclopædia" as a forename is really dumb--> |title=Continental crust |encyclopedia=Encyclopædia Britannica |date=5 September 2023 |url=https://www.britannica.com/science/continental-crust |access-date=6 October 2024 }}</ref> and is [[felsic]]-rich (igneous rocks rich in elements that form [[feldspar]] and [[quartz]]).<ref>{{cite book |last1=Schmidt |first1=Victor A. |url=http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/other/contents.html |title=Planet Earth and the New Geosciences |last2=Harbert |first2=William |year=1998 |isbn=978-0-7872-4296-1 |edition=3rd |page=442 |chapter=The Living Machine: Plate Tectonics |publisher=Kendall/Hunt |access-date=2008-01-28 |archive-url=https://web.archive.org/web/20100124060304/http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/other/contents.html |archive-date=2010-01-24 |url-status=dead}} {{cite web |title=Unit 3: The Living Machine: Plate Tectonics |website=Planet Earth and the New Geosciences |first1=Victor A. |last1=Schmidt |first2=William |last2=Harbert |publisher=Adam Mickiewicz University |location=Poznańb|url=http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html |url-status=dead |archive-url=https://web.archive.org/web/20100328212854/http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/chap03.html |archive-date=2010-03-28}}</ref> The rocks of the crust fall into two major categories – [[sial]] (aluminium silicate) and [[Sima (geology)|sima]] (magnesium silicate).<ref>{{Cite journal |last=Hess |first=H. |date=1955-01-01 |title=The oceanic crust |url=https://elischolar.library.yale.edu/journal_of_marine_research/854 |journal=Journal of Marine Research |volume=14 |issue=4 |pages=424 |quote=It has been common practice to subdivide the crust into sial and sima. These terms refer to generalized compositions, sial being those rocks rich in Si and Al and sima those rich in Si and Mg.}}</ref> It is estimated that sima starts about 11&nbsp;km below the [[Conrad discontinuity]],<ref name="Kearey">{{cite book |last1=Kearey |first1=P. |url=https://books.google.com/books?id=HYqZntfg25UC&dq=%22conrad+discontinuity%22&pg=PA19 |title=Global Tectonics |last2=Klepeis |first2=K. A. |last3=Vine |first3=F. J. |publisher=[[John Wiley & Sons]] |year=2009 |isbn=9781405107778 |edition=3 |pages=19–21 |accessdate=30 June 2012}}</ref> though the discontinuity is not distinct and can be absent in some continental regions.<ref name="Lowrie">{{cite book |last=Lowrie |first=W. |url=https://books.google.com/books?id=7vR2RJSIGVoC&dq=%22conrad+discontinuity%22&pg=PA149 |title=Fundamentals of Geophysics |publisher=[[Cambridge University Press]] |year=1997 |isbn=9780521467285 |pages=149 |accessdate=30 June 2012}}</ref>

Earth's lithosphere consists of the crust and the uppermost [[Earth's mantle|mantle]].<ref>{{Cite book |url= |title=Human Geoscience |publisher=[[Springer Science+Business Media]] |year=2020 |isbn=978-981-329-224-6 |editor-last=Himiyama |editor-first=Yukio |location=Singapore |page=27 |oclc=1121043185 |editor-last2=Satake |editor-first2=Kenji |editor-last3=Oki |editor-first3=Taikan}}</ref> The crust-mantle boundary occurs as two physically different phenomena. The [[Mohorovičić discontinuity]] is a distinct change of [[seismic wave]] velocity. This is caused by a change in the rock's density<ref>{{Citation |last1=Rudnick |first1=R. L. |title=3.01 – Composition of the Continental Crust |date=2003-01-01 |url=http://www.sciencedirect.com/science/article/pii/B0080437516030164 |journal=Treatise on Geochemistry |volume=3 |page=659 |editor-last=Holland |editor-first=Heinrich D. |publisher=Pergamon |bibcode=2003TrGeo...3....1R |doi=10.1016/b0-08-043751-6/03016-4 |isbn=978-0-08-043751-4 |access-date=2019-11-21 |last2=Gao |first2=S. |editor2-last=Turekian |editor2-first=Karl K.|url-access=subscription }}</ref> – immediately above the Moho, the velocities of primary seismic waves ([[P wave]]) are consistent with those through [[basalt]] (6.7–7.2&nbsp;km/s), and below they are similar to those through [[peridotite]] or [[dunite]] (7.6–8.6&nbsp;km/s).<ref name="Badescu">{{cite book |first1=R. B. |last1=Cathcart |url=https://books.google.com/books?id=5bZBEM31K1MC&pg=PA169 |title=Macro-engineering: a challenge for the future |first2=M. M. |last2=Ćirković |date=2006 |publisher=Springer |isbn=978-1-4020-3739-9 |editor1-first=Viorel |editor1-last=Badescu |page=169 |editor2-first=Richard Brook |editor2-last=Cathcart |editor2-link=Richard Cathcart |editor3-first=Roelof D. |editor3-last=Schuiling |name-list-style=amp}}</ref> Second, in oceanic crust, there is a chemical discontinuity between [[ultramafic]] cumulates and tectonized [[Peridotite|harzburgites]], which has been observed from deep parts of the oceanic crust that have been [[obduction|obducted]] onto the continental crust and preserved as [[ophiolites|ophiolite sequences]].{{Clarify|reason=Too technical, a simpler explanation is warranted|date=August 2022}}

Many rocks making up Earth's crust formed less than 100 [[million years]] ago; however, the oldest known mineral grains are about 4.4 [[billion years]] old, indicating that Earth has had a solid crust for at least 4.4 billion years.<ref>{{cite web |url=http://spaceflightnow.com/news/n0101/14earthwater/ |title=Oldest rock shows Earth was a hospitable young planet |agency=National Science Foundation |archive-url=https://web.archive.org/web/20090628164134/http://spaceflightnow.com/news/n0101/14earthwater/ |archive-date=2009-06-28 |website=Spaceflight Now |date=2001-01-14 |access-date=2012-01-27}}</ref>

=== Mantle ===
{{Main|Earth's mantle}}
[[File:Subduction-en.svg|left|thumb|300x300px|Earth's crust and mantle, [[Mohorovičić discontinuity]] between bottom of crust and solid uppermost mantle]]
Earth's mantle extends to a depth of {{convert|2,890|km|abbr=on}}, making it the planet's thickest layer.<ref>{{cite news |url=https://www.forbes.com/sites/trevornace/2016/01/16/layers-of-the-earth-lies-beneath-earths-crust/ |title=Layers Of The Earth: What Lies Beneath Earth's Crust |first=Trevor |last=Nace |date=16 January 2016 |access-date=28 June 2019 |magazine=[[Forbes]] |archive-date=5 March 2020 |archive-url=https://web.archive.org/web/20200305220618/https://www.forbes.com/sites/trevornace/2016/01/16/layers-of-the-earth-lies-beneath-earths-crust/ |url-status=live }}</ref>
[This is 45% of the 
{{convert|6,371|km|abbr=on}} radius, and 83.7% of the volume -  0.6% of the volume is the crust].
The mantle is divided into [[upper mantle|upper]] and [[lower mantle]]<ref>{{cite journal |url=https://education.nationalgeographic.org/resource/mantle/ |title=Mantle |first=Jeannie |last=Evers |date=11 August 2015 |access-date=28 June 2019 |journal=[[National Geographic]] |publisher=[[National Geographic Society]] |archive-date=12 May 2016 |archive-url=https://web.archive.org/web/20160512171916/https://www.nationalgeographic.org/encyclopedia/mantle/ |url-status=live }}</ref> separated by a [[Transition zone (Earth)|transition zone]].<ref>{{cite journal |journal=[[Nat Commun]] |volume=9 |date=28 March 2018 |issue=9 |page=1266 |doi=10.1038/s41467-018-03654-6 |pmc=5872023 |pmid=29593266 |title=Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii |first1=Chunquan |last1=Yu |first2=Elizabeth A. |last2=Day |first3=Maarten V. |last3=de Hoop |first4=Michel |last4=Campillo |first5=Saskia |last5=Goes |first6=Rachel A. |last6=Blythe |first7=Robert D. |last7=van der Hilst |bibcode=2018NatCo...9.1266Y }}</ref> The lowest part of the mantle next to the [[core-mantle boundary]] is known as the D″ (D-double-prime) layer.<ref name="DDoublePron">{{cite news |url=https://www.science.org/content/article/d-layer-demystified |title=''D'' Layer Demystified |publisher=[[American Association for the Advancement of Science]] |magazine=[[Science News]] |date=24 March 2004 |access-date=5 November 2016 |last=Krieger |first=Kim |archive-date=10 July 2022 |archive-url=https://web.archive.org/web/20220710170436/https://www.science.org/content/article/d-layer-demystified |url-status=live }}</ref> The [[pressure]] at the bottom of the mantle is ≈140 G[[pascal (unit)|Pa]] (1.4 M[[Atmosphere (unit)|atm]]).<ref>{{cite journal |url=https://www.unr.edu/Documents/science/mackay/coring%20the%20earth.pdf |title=Coring the Earth |first=Rachel |last=Dolbier |journal=[[W. M. Keck Earth Science and Mineral Engineering Museum]] |publisher=[[University of Nevada, Reno]] |pages=5 |access-date=28 June 2019 |archive-date=7 September 2015 |archive-url=https://web.archive.org/web/20150907215534/http://www.unr.edu/Documents/science/mackay/coring%20the%20earth.pdf |url-status=dead }}</ref> The mantle is composed of [[silicate]] rocks richer in iron and magnesium than the overlying crust.<ref>{{cite news |url=https://www.universetoday.com/40229/what-is-the-earths-mantle-made-of/ |title=What is the Earth's Mantle Made Of? |date=26 March 2016 |access-date=28 June 2019 |first=Fraser |last=Cain |work=[[Universe Today]] |archive-date=6 November 2010 |archive-url=https://web.archive.org/web/20101106115153/https://www.universetoday.com/40229/what-is-the-earths-mantle-made-of/ |url-status=live }}</ref> Although solid, the mantle's extremely hot silicate material can [[Ductility|flow]] over very long timescales.<ref>{{cite news |url=https://sciencing.com/different-properties-asthenosphere-lithosphere-8447830.html |title=The Different Properties of the Asthenosphere & the Lithosphere |date=22 October 2018 |access-date=28 June 2019 |first=Ethan |last=Shaw |work=Sciencing |publisher=[[Leaf Group]] Media |archive-date=30 July 2020 |archive-url=https://web.archive.org/web/20200730055603/https://sciencing.com/different-properties-asthenosphere-lithosphere-8447830.html |url-status=live }}</ref> [[Convection]] of the mantle propels the [[plate tectonics|motion of the tectonic plates]] in the crust. The [[Earth's internal heat budget|source of heat]] that drives this motion is the decay of [[radioactive isotopes]] in Earth's crust and mantle combined with the initial heat from the planet's formation<ref>{{cite journal|last=Preuss|first=Paul|date=July 17, 2011|title=What Keeps the Earth Cooking?|url=http://newscenter.lbl.gov/2011/07/17/kamland-geoneutrinos/|journal=[[Lawrence Berkeley National Laboratory]]|publisher=[[University of California, Berkeley]]|access-date=28 June 2019|archive-date=21 January 2022|archive-url=https://web.archive.org/web/20220121083440/https://newscenter.lbl.gov/2011/07/17/kamland-geoneutrinos/|url-status=live }}</ref> (from the [[potential energy]] released by collapsing a large amount of matter into a [[gravity well]], and the [[kinetic energy]] of accreted matter).

Due to increasing pressure deeper in the mantle, the lower part flows less easily, though chemical changes within the mantle may also be important. The viscosity of the mantle ranges between 10<sup>21</sup> and 10<sup>24</sup> [[pascal second|pascal-second]], increasing with depth.<ref>{{cite journal |first1=Uwe |last1=Walzer |first2=Roland |last2=Hendel |first3=John |last3=Baumgardner |author3-link=John Baumgardner |archive-url=https://web.archive.org/web/20060826020002/http://www.chemie.uni-jena.de/geowiss/geodyn/poster2.html |title=Mantle Viscosity and the Thickness of the Convective Downwellings |url=http://www.chemie.uni-jena.de/geowiss/geodyn/poster2.html |archive-date=26 August 2006 |access-date=28 June 2019 |journal=[[Los Alamos National Laboratory]] |publisher=[[Universität Heidelberg]]}}</ref> In comparison, the viscosity of water at {{convert|300|K}} is 0.89 millipascal-second <ref>{{Cite book |url= |title=[[CRC Handbook of Chemistry and Physics]] |publisher=[[CRC Press]] |year=2017 |isbn=978-1-4987-5429-3 |editor-last=Haynes |editor-first=William M. |edition=97th |location=Boca Raton, Florida |at=Section 6 page 247 |oclc=957751024 |editor-last2=David R. |editor-first2=Lide |editor-last3=Bruno |editor-first3=Thomas J.}}</ref> and [[Pitch (resin)|pitch]] is (2.3 ± 0.5) × 10<sup>8</sup> pascal-second.<ref name="pitchdrop">{{cite web |last1=Edgeworth |first1=R. |last2=Dalton |first2=B.J. |last3=Parnell |first3=T. |title=The Pitch Drop Experiment |url=http://www.physics.uq.edu.au/physics_museum/pitchdrop.shtml |url-status=dead |archive-url=https://web.archive.org/web/20130328064508/http://www.physics.uq.edu.au/physics_museum/pitchdrop.shtml |archive-date=28 March 2013 |access-date=15 October 2007 |publisher=The University of Queensland Australia}}</ref>

=== Core ===
{{Main|Earth's inner core|Earth's outer core}}
[[File:Dynamo Theory - Outer core convection and magnetic field generation.svg|alt=A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization of magnesium oxide, silicon dioxide, and iron(II) oxide. Convection of Earth's outer core is displayed alongside magnetic field lines.|thumb|A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization of [[magnesium oxide]], [[silicon dioxide]], and [[iron(II) oxide]]]]

Earth's outer core is a fluid layer about {{convert|2260|km|mi|abbr=on}} in height (i.e. distance from the highest point to the lowest point at the edge of the inner core) [36% of the Earth's radius, 15.6% of the volume] and composed of mostly [[iron]] and [[nickel]] that lies above Earth's solid [[Earth's inner core|inner core]] and below its [[mantle (geology)|mantle]].<ref>{{cite web |title=Earth's Interior |url=https://www.nationalgeographic.com/science/earth/surface-of-the-earth/earths-interior/ |website=Science & Innovation |publisher=National Geographic |access-date=14 November 2018 |date=18 January 2017 |archive-date=18 January 2019 |archive-url=https://web.archive.org/web/20190118060622/https://www.nationalgeographic.com/science/earth/surface-of-the-earth/earths-interior/ |url-status=dead }}</ref> Its outer boundary lies {{convert|2890|km|mi|abbr=on}} beneath Earth's surface. The transition between the inner core and outer core is located approximately {{convert|5150|km|mi|abbr=on}} beneath Earth's surface. Earth's inner core is the innermost [[structure of the Earth|geologic layer]] of the planet [[Earth]]. It is primarily a solid ball with a radius of about {{cvt|1220|km|mi}}, which is about 19% of [[Earth radius|Earth's radius]] [0.7% of volume] or 70% of the [[Moon]]'s radius.<ref name="monner2010">
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 |doi-access=free}}
</ref>

The inner core was discovered in 1936 by [[Inge Lehmann]] and is composed primarily of iron and some nickel. Since this layer is able to transmit shear waves (transverse seismic waves), it must be solid. Experimental evidence has at times been inconsistent with current crystal models of the core.<ref>{{Cite journal | first1=Lars | last1=Stixrude | first2=R.E. | last2=Cohen | title=Constraints on the crystalline structure of the inner core: Mechanical instability of BCC iron at high pressure | journal=Geophysical Research Letters | date=January 15, 1995 | volume=22 | issue=2 | pages=125–28 | doi=10.1029/94GL02742 | bibcode=1995GeoRL..22..125S | url=http://discovery.ucl.ac.uk/135995/ | access-date=January 2, 2019 | archive-date=August 8, 2022 | archive-url=https://web.archive.org/web/20220808131935/https://discovery.ucl.ac.uk/id/eprint/135995/ | url-status=live }}</ref> Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000&nbsp;K below those from shock laser (dynamic) studies.<ref>{{Cite journal | first1=A. | last1=Benuzzi-Mounaix | first2=M. | last2=Koenig | first3=A. | last3=Ravasio | first4=T. | last4=Vinci | title=Laser-driven shock waves for the study of extreme matter states | journal=Plasma Physics and Controlled Fusion | year=2006 | volume=48 | issue=12B | pages=B347 | doi=10.1088/0741-3335/48/12B/S32 |bibcode = 2006PPCF...48B.347B | s2cid=121164044 }}</ref><ref>{{Cite journal | first1=Bruce A. | last1=Remington | first2=R. Paul | last2=Drake | first3=Dmitri D. | last3=Ryutov | title=Experimental astrophysics with high power lasers and Z pinches | journal=Reviews of Modern Physics | year=2006 | volume=78 | issue=3 | pages=755 | doi=10.1103/RevModPhys.78.755 | bibcode=2006RvMP...78..755R | url=https://zenodo.org/record/1233971 | access-date=2019-06-26 | archive-date=2020-05-23 | archive-url=https://web.archive.org/web/20200523055212/https://zenodo.org/record/1233971 | url-status=live }}</ref> The laser studies create plasma,<ref>{{Cite journal | first1=A. | last1=Benuzzi-Mounaix | first2=M. | last2=Koenig | first3=G. | last3=Husar | first4=B. | last4=Faral | title=Absolute equation of state measurements of iron using laser driven shocks | journal=Physics of Plasmas | date=June 2002 | volume=9 | issue=6 | pages=2466 | doi=10.1063/1.1478557 |bibcode = 2002PhPl....9.2466B }}</ref> and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid. This is an area of active research.

In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called [[planetary differentiation]] (see also the [[iron catastrophe]]), while less-dense materials would have migrated to the [[crust (geology)|crust]]. The core is thus believed to largely be composed of iron (80%), along with [[nickel]] and one or more light elements, whereas other dense elements, such as [[lead]] and [[uranium]], either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see [[felsic|felsic materials]]). Some have argued that the inner core may be in the form of a single iron [[crystal]].<ref>{{cite book |last=Schneider |first=Michael |chapter-url=http://www.psc.edu/science/Cohen_Stix/cohen_stix.html |chapter=Crystal at the Center of the Earth |title=Projects in Scientific Computing, 1996 |publisher=Pittsburgh Supercomputing Center |date=1996 |access-date=8 March 2019 |archive-date=5 February 2007 |archive-url=https://web.archive.org/web/20070205041442/http://www.psc.edu/science/Cohen_Stix/cohen_stix.html |url-status=live }}</ref><ref>{{cite journal|doi=10.1126/science.267.5206.1972|title=High-Pressure Elasticity of Iron and Anisotropy of Earth's Inner Core|year=1995|last1=Stixrude|first1=L.|last2=Cohen|first2=R.E.|journal=Science|volume=267|issue=5206|pages=1972–75|pmid=17770110|bibcode = 1995Sci...267.1972S |s2cid=39711239}}</ref>

Under laboratory conditions a sample of iron–nickel alloy was subjected to the core-like pressure by gripping it in a vise between 2 diamond tips ([[diamond anvil cell]]), and then heating to approximately 4000 K. The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south.<ref>[https://www.bbc.co.uk/news/uk-14678004 BBC News, "What is at the centre of the Earth?] {{Webarchive|url=https://web.archive.org/web/20200523084941/https://www.bbc.co.uk/news/uk-14678004 |date=2020-05-23 }}. BBC.co.uk (2011-08-31). Retrieved on 2012-01-27.</ref><ref>{{cite journal|doi=10.1126/science.1208265|pmid=22076374|title=Phase Transition of FeO and Stratification in Earth's Outer Core|year=2011|last1=Ozawa|first1=H.|last2=al.|first2=et|journal=Science|volume=334|issue=6057|pages=792–94|bibcode = 2011Sci...334..792O |s2cid=1785237}}</ref>

The composition of Earth bears strong similarities to that of certain [[chondrite]] meteorites, and even to some elements in the outer portion of the Sun.<ref>{{cite journal|author=Herndon, J.M.|title=The chemical composition of the interior shells of the Earth|journal= Proc. R. Soc. Lond.|year=1980|volume=A372|pages=149–54|jstor=2398362|issue=1748|doi=10.1098/rspa.1980.0106|bibcode=1980RSPSA.372..149H|s2cid=97600604}}</ref><ref>{{cite journal|author=Herndon, J.M.|title=Scientific basis of knowledge on Earth's composition|journal=Current Science|year=2005|volume=88|issue=7|pages=1034–37|url=http://nuclearplanet.com/CS50410.pdf|access-date=2012-01-27|archive-date=2020-07-30|archive-url=https://web.archive.org/web/20200730081139/http://nuclearplanet.com/CS50410.pdf|url-status=live}}</ref> Beginning as early as 1940, scientists, including [[Francis Birch (geophysicist)|Francis Birch]], built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth. This ignores the less abundant [[enstatite]] chondrites, which formed under extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.{{citation needed|date=November 2023}}

[[Dynamo theory]] suggests that convection in the outer core, combined with the [[Coriolis effect]], gives rise to [[Earth's magnetic field]]. The solid inner core is too hot to hold a permanent magnetic field (see [[Curie temperature]]) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure {{convert|2.5|mT|abbr=off}}, 50 times stronger than the magnetic field at the surface.<ref>{{cite journal|doi=10.1038/nature09643|title=Tidal dissipation and the strength of the Earth's internal magnetic field|year=2010|last1=Buffett|first1=Bruce A.|journal=Nature|volume=468|issue=7326|pages=952–94|pmid=21164483|bibcode=2010Natur.468..952B|s2cid=4431270}}</ref>

The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the [[solar wind]]. The rate of cooling by conduction and convection is uncertain,<ref>{{cite web |url=https://www.nbcnews.com/science/science-news/earths-core-cooling-faster-previously-thought-researchers-say-rcna12732 |title=Earth's core cooling faster than previously thought, researchers say |date=19 January 2022 |author=David K. Li |publisher=[[NBC News]]}}</ref> but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out.<ref>{{cite web |url=https://education.nationalgeographic.org/resource/core/ |access-date=15 July 2024 |title=Core |publisher=[[National Geographic]]}}</ref>{{better source needed|date=July 2024}}

== Seismology ==
{{Main|Seismology}}
The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel ([[seismic velocity]]) is different in other layers. The changes in seismic velocity between different layers causes refraction owing to [[Snell's law]], like light bending as it passes through a prism. Likewise, reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror.

==See also==
* [[Hollow Earth]]
* [[Geological history of Earth]]
* [[Large low-shear-velocity provinces]]
* [[Lehmann discontinuity]]
* [[Rain-out model]]
* [[Seismic tomography]] – technique for imaging the subsurface of Earth using seismic waves
* [[Travel to the Earth's center]]
* [[Solid earth]]

==References==
{{Reflist}}

==Further reading==
{{refbegin}}
* {{cite journal|last=Drollette|first=Daniel |date=October 1996 |title=A Spinning Crystal Ball |journal=Scientific American |volume=275 |number=4 |pages=28–33|doi=10.1038/scientificamerican1096-28 |bibcode=1996SciAm.275d..28D }}
* {{cite journal |last=Kruglinski |first=Susan |title=Journey to the Center of the Earth |journal=Discover |date=June 2007 |url=http://discovermagazine.com/2007/jun/journey-to-the-center-of-the-earth |access-date=9 July 2016 |archive-date=26 May 2016 |archive-url=https://web.archive.org/web/20160526021214/http://discovermagazine.com/2007/jun/journey-to-the-center-of-the-earth |url-status=live }}
* {{cite journal|last=Lehmann |first=I |year=1936 |title=Inner Earth |journal=Bur. Cent. Seismol. Int. |volume=14 |pages=3–31}}
* {{cite book|last1=Wegener|first1=Alfred|title=The origin of continents and oceans|url=https://archive.org/details/originsofcontine0000unse|url-access=registration|date=1966|publisher=Dover Publications|location=New York|isbn=978-0-486-61708-4}}
{{refend}}

==External links==
* {{commons-inline}}

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