Editing Internal structure of Earth (section)

=== 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">
{{cite journal |first1=Marc |last1=Monnereau |first2=Marie |last2=Calvet |first3=Ludovic |last3=Margerin
 |first4=Annie |last4=Souriau|author-link4=Annie Souriau|date=21 May 2010|title=Lopsided growth of Earth's inner core|journal=Science|volume=328 
 |issue=5981 |pages=1014–1017|doi=10.1126/science.1186212 |bibcode=2010Sci...328.1014M|pmid=20395477 |s2cid=10557604}}
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{{cite journal
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 |last2=Flinn |first2=E.A.
 |last3=Massé |first3=R.P.
 |year=1974
 |title=Differential PKiKP travel times and the radius of the inner core
 |journal=Geophysical Journal International
 |volume=39 |issue=3 |pages=457–463
 |doi=10.1111/j.1365-246x.1974.tb05467.x
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</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.{{cn|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}}