{{For|the cores of other planetary bodies|Planetary core}}
{{short description|The innermost part of the Earth, a solid ball of iron-nickel alloy}}
{{broader|Structure of the Earth#Core}}
[[File:Earth poster.svg|thumb|upright=1.6|The internal structure of Earth]]
The [[Earth|Earth's]] '''inner core''' is the Earth's innermost part. It is primarily a [[solid]] [[ball (mathematics)|ball]] with a [[radius]] of about {{convert|1220|km|mi|abbr=off}}, which is about 70% of the [[Moon]]'s radius.[{{Cite journal | first1=Marc | last1=Monnereau | first2=Marie | last2=Calvet | first3=Ludovic | last3=Margerin | first4=Annie | last4=Souriau | title=Lopsided Growth of Earth's Inner Core | journal=[[Science (journal)|Science]] | date=May 21, 2010 | volume=328 | issue=5981 | pages=1014–1017 | doi=10.1126/science.1186212 | bibcode=2010Sci...328.1014M | pmid=20395477 | postscript= {{inconsistent citations}}}}][{{cite journal | author = E. R. Engdahl | author2 = E. A. Flynn | author3 = R. P. Massé | lastauthoramp=yes | title = Differential PkiKP travel times and the radius of the core | journal = Geophys. J. R. Astron. Soc.| volume = 40 | issue = 3 | pages = 457–463 | date = 1974 | doi = 10.1111/j.1365-246X.1974.tb05467.x|bibcode = 1974GeoJ...39..457E }}]
There are no samples of the Earth’s core are available for direct measurement, as there are for the [[Earth mantle|Earth's mantle]]. The only direct information that we have about comes from analysis of [[seismic waves]] and the [[Earth's magnetic field|magnetic field]].[{{Cite journal|last=Allègre|first=Claude J.|last2=Manhès|first2=Gérard|last3=Göpel|first3=Christa|date=April 1995|title=The age of the Earth|url=http://dx.doi.org/10.1016/0016-7037(95)00054-4|journal=Geochimica et Cosmochimica Acta|volume=59|issue=8|pages=1445–1456|doi=10.1016/0016-7037(95)00054-4|issn=0016-7037}}]
The inner core is believed to be composed of an [[iron–nickel alloy]] with some other elements. The temperature at the inner core's surface is estimated to be approximately {{convert|5700|K|C}} or 9806 °F, which is about the temperature at the surface of the [[Sun|Sun]].[{{cite journal | author = D. Alfè | author2 = M. Gillan | author3 = G. D. Price | lastauthoramp=yes | title = Composition and temperature of the Earth's core constrained by combining ab initio calculations and seismic data | journal = Earth and Planetary Science Letters | volume = 195 | issue = 1–2 | pages = 91–98 | date = January 30, 2002 | url = http://www.homepages.ucl.ac.uk/~ucfbdxa/pubblicazioni/epsl2002.pdf | doi = 10.1016/S0012-821X(01)00568-4 | bibcode=2002E&PSL.195...91A}}]
==Discovery==
The Earth was discovered to have a solid inner core distinct from its molten [[outer core]] in 1936, by the Danish seismologist [[Inge Lehmann]],[{{cite book|editor=Edmond A. Mathez |title=EARTH: INSIDE AND OUT |date=2000 |publisher=American Museum of Natural History |url=http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html |deadurl=yes |archiveurl=https://web.archive.org/web/20080430105106/http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html |archivedate=2008-04-30 }}] who deduced its presence by studying seismograms from earthquakes in New Zealand. She observed that the [[seismic wave]]s reflect off the boundary of the inner core and can be detected by sensitive [[seismographs]] on the Earth's surface. This boundary is known as the [[Keith Edward Bullen|Bullen]] discontinuity,[{{cite web |url=http://www.uh.edu/~jbutler/physical/chapter19notes.html |title=Class Notes – The Earth's Interior |author=John C. Butler |date=1995 |work=Physical Geology Grade Book |publisher=[[University of Houston]] |accessdate=30 August 2011}}] or sometimes as the Lehmann discontinuity.[Although [[Lehmann discontinuity|another discontinuity]] is named after Lehmann, this usage still can be found: see for example: {{cite book |title=The basics of earth science |author=Robert E Krebs |date=2003 |publisher=Greenwood Publishing Company |url=https://books.google.com/books?id=-4ndyH7u6T0C&pg=PA142 |isbn=978-0-313-31930-3}},and [http://geology.about.com/library/weekly/aa031598.htm ''From here to "hevean," or the D layer''], About.com] A few years later, in 1940, it was hypothesized that this inner core was made of solid iron; its rigidity was confirmed in 1971.[{{cite book |title=International Handbook of Earthquake and Engineering Seismology; part A |editor1-last=Lee |editor1-first=William H. K. |editor2-last=Kanamori |editor2-first=Hiroo |editor3-last=Jennings |editor3-first=Paul C. |editor4-last=Kisslinger |editor4-first=Carl |page=926 |url=https://books.google.com/books?id=aFNKqnC2E-sC&pg=PA926 |date=2002 |publisher=Academic Press |isbn=978-0-12-440652-0}}]
The [[outer core]] was determined to be molten from observations showing that [[compressional wave]]s pass through it, but elastic [[simple shear|shear]] waves do not – or do so only very weakly.[{{cite news | url = http://news.harvard.edu/gazette/1996/08.15/PuttingaNewSpin.html | title = Putting a New Spin on Earth's Core | author = William J. Cromie | date = 1996-08-15 | publisher = Harvard Gazette | accessdate = 2007-05-22}}] The solidity of the inner core had been difficult to establish because the elastic [[shear wave]]s that are expected to pass through a solid mass are very weak and difficult for seismographs on the Earth's surface to detect, since they become so attenuated on their way from the inner core to the surface by their passage through the liquid outer core. Dziewonski and Gilbert established that measurements of [[normal modes|normal modes of vibration]] of Earth caused by large earthquakes were consistent with a liquid outer core.[{{cite journal | title = Solidity of the Inner Core of the Earth inferred from Normal Mode Observations |author1=A. M. Dziewonski |author2=F. Gilbert | journal = Nature | volume = 234 | issue = 5330 | pages = 465–466 | date = 1971-12-24 | doi = 10.1038/234465a0 |bibcode = 1971Natur.234..465D }}] In 2005, shear waves were detected passing through the inner core; these claims were initially controversial, but are now gaining acceptance.[{{cite web | url = http://www.livescience.com/environment/050414_earth_core.html| title = Finally, a Solid Look at Earth's Core | author = Robert Roy Britt | accessdate = 2007-05-22 | date = 2005-04-14}}]
==Size==
The radius of the current inner core is determined by the travel time of the [[seismic wave]]s reflected at the inner core-outer core boundary,[{{Cite journal|last=Engdahl|first=E. R.|last2=Flinn|first2=E. A.|last3=Masse|first3=R. P.|date=1974-12-01|title=Differential PKiKP Travel Times and the Radius of the Inner Core|url=http://dx.doi.org/10.1111/j.1365-246x.1974.tb05467.x|journal=Geophysical Journal International|volume=39|issue=3|pages=457–463|doi=10.1111/j.1365-246x.1974.tb05467.x|issn=0956-540X}}]
==Temperature and pressure==
The temperature of the inner core can be estimated by considering both the theoretical and the experimentally demonstrated constraints on the melting temperature of impure iron at the pressure which iron is under at the boundary of the inner core (about 330 [[GPa]]). These considerations suggest that its temperature is about {{convert|5700|K|sigfig=2}}.[{{cite book | editor = David. R. Lide | title = CRC Handbook of Chemistry and Physics | pages = j14–13 | edition = 87th | date = 2006–2007 | url = http://hbcpnetbase.com/ }}] Iron can be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (see the [[Clausius–Clapeyron relation]]).[{{cite journal | author = Anneli Aitta | title = Iron melting curve with a tricritical point | journal = Journal of Statistical Mechanics: Theory and Experiment | volume = 2006 | issue = 12 | date = 2006-12-01 | pages = 12015–12030 | url = http://stacks.iop.org/JSTAT/2006/P12015 | doi = 10.1088/1742-5468/2006/12/P12015 |arxiv = cond-mat/0701283 |bibcode = 2006JSMTE..12..015A }} or see preprints https://arxiv.org/pdf/cond-mat/0701283 , https://arxiv.org/pdf/0807.0187 .]
A report published in the journal ''Science'' [{{cite journal | author = S. Anzellini | author2 = A. Dewaele | author3 = M. Mezouar | author4 = P. Loubeyre | author5 = G. Morard | lastauthoramp=yes | title = Melting of Iron at Earth's Inner Core Boundary Based on Fast X-ray Diffraction | journal = Science | volume = 340 | issue = 6136 | date = 2013 | pages = 464–466 | url = http://www.sciencemag.org/content/340/6131/464 | doi = 10.1126/science.1233514| bibcode = 2013Sci...340..464A }}] concludes that the melting temperature of iron at the inner core boundary is 6230 ± 500 K.
==Composition==
There is still no direct evidence about the composition of the inner core. However, based on the relative prevalence of various chemical elements in the [[Solar System]], the theory of [[Formation and evolution of the Solar System#Terrestrial planets|planetary formation]], and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of an [[iron–nickel alloy]].
At the known pressures and estimated temperatures of the core, it is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That result implies the presence of lighter elements in the core, such as [[silicon]], [[oxygen]], or [[sulfur]], in addition to the probable presence of nickel.[{{Cite journal|last=Stixrude|first=Lars|last2=Wasserman|first2=Evgeny|last3=Cohen|first3=Ronald E.|date=1997-11-10|title=Composition and temperature of Earth's inner core|journal=Journal of Geophysical Research: Solid Earth|volume=102|issue=B11|pages=24729–24739|doi=10.1029/97JB02125|issn=2156-2202|bibcode=1997JGR...10224729S}}]
Also, if the inner core grows by precipitation of frozen particles falling onto its surface, then some liquid can also be trapped in the pore spaces. In that case, some of this residual fluid may still persist to some small degree in much of its interior.{{cn|date=March 2019}}
==Growth and structure==
The Earth's inner core is thought to be slowly growing as the liquid [[outer core]] at the boundary with the inner core cools and solidifies due to the gradual cooling of the Earth's interior (about 100 degrees Celsius per billion years).[
Many scientists had initially expected that the inner core would be found to be [[homogeneous mixture|homogeneous]], because that same process should have proceeded uniformly during its entire formation. It was even suggested that Earth's inner core might be a [[single crystal]] of iron.][
However, seismic waves were found to pass more rapidly through some parts of the inner core than through others, indicating that in fact there is some large-scale variation in its properties.][
In addition, the properties of the inner core's surface vary from place to place across distances as small as 1 km. This variation is surprising, since lateral temperature variations along the inner-core boundary are known to be extremely small (this conclusion is confidently constrained by [[magnetic field]] observations).{{cn|date=March 2019}}
Discoveries in 1994 suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick.][
Speculation also continues that the inner core might have exhibited a variety of internal [[Deformation (engineering)|deformation]] patterns. This may be necessary to explain why seismic waves pass more rapidly in some directions than in others.][{{cite journal |author1=G Poupinet |author2=R Pillet |author3=A Souriau | title = Possible heterogeneity of the Earth's core deduced from PKIKP travel times | journal = Nature | volume = 305|issue=5931 | pages = 204–206 | date = 1983 | doi=10.1038/305204a0|bibcode=1983Natur.305..204P }}] Because [[thermal convection]] alone appears to be improbable,[{{cite journal | author = T. Yukutake | title = Implausibility of thermal convection in the Earth's solid inner core | journal = Phys. Earth Planet. Inter. | volume = 108 | issue = 1 | pages = 1–13 | date = 1998 | doi = 10.1016/S0031-9201(98)00097-1 | bibcode=1998PEPI..108....1Y}}] any buoyant convection motions will have to be driven by variations in composition or abundance of liquid in its interior. S. Yoshida and colleagues proposed a novel mechanism whereby deformation of the inner core can be caused by a higher rate of freezing at the equator than at polar latitudes,[{{cite journal | author = S.I. Yoshida | author2 = I. Sumita | author3 = M. Kumazawa | lastauthoramp=yes | title = Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy | journal = Journal of Geophysical Research: Solid Earth | volume = 101 | issue = B12 | pages = 28085–28103 | date = 1996 | doi = 10.1029/96JB02700 | bibcode=1996JGR...10128085Y}}] and S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time.[{{cite journal | author = S. I. Karato | title = Seismic anisotropy of the Earth's inner core resulting from flow induced by Maxwell stresses | journal = Nature | volume = 402 | issue = 6764 | pages = 871–873 | date = 1999 | doi = 10.1038/47235|bibcode = 1999Natur.402..871K }}]
There is an East–West asymmetry in the inner core seismological data. There is a model which explains this by differences at the surface of the inner core – melting in one hemisphere and crystallization in the other.[{{Cite journal | last1 = Alboussière | first1 = T. | last2 = Deguen | first2 = R. | last3 = Melzani | first3 = M.| title = Melting-induced stratification above the Earth's inner core due to convective translation | journal = Nature | volume = 466 | issue = 7307 | pages = 744–747 | year = 2010 | pmid = 20686572 | doi = 10.1038/nature09257|bibcode = 2010Natur.466..744A |arxiv = 1201.1201 }}] The western hemisphere of the inner core may be crystallizing, whereas the eastern hemisphere may be melting. This may lead to enhanced magnetic field generation in the crystallizing hemisphere, creating the asymmetry in the Earth's magnetic field.[[http://www.nature.com/ngeo/journal/v5/n8/fig_tab/ngeo1516_F1.html "Figure 1: East–west asymmetry in inner-core growth and magnetic field generation."] from {{cite journal | last1 = Finlay | first1 = Christopher C. | year = 2012 | title = Core processes: Earth's eccentric magnetic field | url = | journal = Nature Geoscience | volume = 5 | issue = 8| pages = 523–524 | doi = 10.1038/ngeo1516 | bibcode = 2012NatGe...5..523F }}]
==Dynamics==
Because the inner core is not rigidly connected to the Earth's solid [[mantle (geology)|mantle]], the possibility that it [[rotation|rotates]] slightly more quickly or slowly than the rest of Earth has long been entertained.[{{Cite journal
| title = Mechanics of inner core super-rotation
| date = 1996
| journal = Geophysical Research Letters
| pages = 3401–3404
| volume = 23
| issue = 23
| last1 = Aaurno | first1 = J. M.
| last2 = Brito | first2 = D.
| last3 = Olson | first3 = P. L. |bibcode = 1996GeoRL..23.3401A |doi = 10.1029/96GL03258
| postscript = {{inconsistent citations}} }}][{{Cite journal
| title = Evidence for inner core super-rotation from time-dependent differential PKP traveltimes observed at Beijing Seismic Network
| date = 2003
| journal = [[Geophysical Journal International]]
| pages = 509–514
| volume = 152
| issue = 3
| doi = 10.1046/j.1365-246X.2003.01852.x
| last1 = Xu | first1 = Xiaoxia
| last2 = Song | first2 = Xiaodong |bibcode = 2003GeoJI.152..509X
| postscript = {{inconsistent citations}} | citeseerx = 10.1.1.210.8362}}] In the 1990s, seismologists made various claims about detecting this kind of super-rotation by observing changes in the characteristics of [[seismic waves]] passing through the inner core over several decades, using the aforementioned property that it transmits waves more quickly in some directions. Estimates of this super-rotation are around one degree of extra rotation per year.
Growth of the inner core is thought to play an important role in the generation of [[Earth's magnetic field]] by [[Dynamo theory|dynamo]] action in the liquid outer core. This occurs mostly because it cannot dissolve the same amount of light elements as the outer core and therefore freezing at the inner core boundary produces a residual liquid that contains more light elements than the overlying liquid. This causes it to become [[buoyant]] and helps drive convection of the outer core.{{Citation needed|date=May 2007}} The existence of the inner core also changes the dynamic motions of liquid in the outer core as it grows and may help fix the magnetic field since it is expected to be a great deal more resistant to flow than the outer core liquid (which is expected to be turbulent).{{Citation needed|date=May 2007}}
== Age ==
Theories about the age of the core are necessarily part of theories of the [[history of Earth]] as a whole. This has been a long debated topic and is still under discussion at the present time. It is widely believed that the Earth’s solid inner core grows from an initially completely liquid core as the Earth cooled down. However, the onset time of this process is still unresolved.[
Two main approaches have been been used to infer the age of the inner core: [[thermodynamics|thermodynamic]] modeling of the cooling of the Earth, and analysis of [[paleomagnetism|paleomagnetic]] evidence. The estimates yields by both methods still vary over a large range: from 0.5 to 2 billion years old.
{| class="wikitable"
|+Age estimates from different studies
!Studies
!Age estimates(billion years)
|-
|Labrosse et al.(2001) without radioactive element in the core][
|1±0.5
|-
|Labrosse et al.(2001) with radioactive element in the core][
|<3
|-
|Labrosse (2003) with radioactive element in the core][
|~2
|-
|Labrosse (2015)][
|<0.7
|-
|Ohta et al.(2016)][
|<0.7
|-
|Konôpková et al.(2016)][
|<4.2
|-
|Biggin et al.(2015)][ and R. Arevalo.][]]
[[File:Outer_core_convection_rolls.jpg|thumb|upright=1.25|Schematic of the Earth's inner core and outer core motion and the magnetic field it generates.]]
One of the ways to estimate the age of the inner core is by modeling the cooling of the Earth, constrained by a minimum value for the [[heat flux]] at the [[core–mantle boundary]] (CMB). That estimate is based on the prevailing theory that the Earth's magnetic field is primarily triggered by [[convention]] currents in the liquid part of the core, and the fact that a minimum heat flux is required to sustain those currents. The heat flux at the CMB at present time can be reliably estimated because it is related to the measured heat flux at Earth’s surface and to the measured rate of [[mantle convection]].][
In 2001, S. Labrosse and others, assuming that there were no [[radionuclide|radioactive element]]s in the core, gave an estimate of 1±0.5 billion years for the age of the inner core — considerably less than the estimated age of the Earth and of its liquid core (about 4.5 billion years)][ In 2003, the same group concluded that, if the core contained a reasonable amount of radioactive elements, the inner core's age could be a few hundred million years older.][
[[File:Earth_formation.jpg|thumb|The Earth at its early stage.]]
In 2012, theoretical computations by M. Pozzo and others indicated that the [[electrical resistivity and conductivity|electrical conductivity]] of iron and other hypothetical core materials, at the high pressures and temperatures expected there, were two or three times higher than assumed in previous research. These predictions were confirmed in 2013 by measurements by Gomi and others.][ The higher values for electrical conductivity led to increased estimates of the [[thermal conductivity]], to 90 W/m/K; which, in turn, lowered estimates of its age to less than 700 million years old.][
However, in 2016 Konôpková and others directly measured the thermal conductivity of soldi iron at inner core conditions, and obtained a much lower value, 18-44 W/m/K. With those values, they obtained an upper bound of 4.2 billion years for the age of the inner core.][
=== Paleomagnetic evidence ===
Paleomagnetic evidence is also helpful to constrain the Earth’s core evolution. The presence or absence of the solid inner core could result in very different dynamic processes in the core which could lead to a different [[Magnetic field of earth|magnetic field]] during Earth’s history.][{{Citation|last=Aubert|first=Julien|title=Observations and Models of the Long-Term Evolution of Earth’s Magnetic Field|date=2010|url=http://dx.doi.org/10.1007/978-1-4419-7955-1_12|work=Terrestrial Magnetism|pages=337–370|publisher=Springer New York|isbn=9781441979544|access-date=2019-02-10|last2=Tarduno|first2=John A.|last3=Johnson|first3=Catherine L.}}] Previously, this change in the paleomagnetic field was not recognized due to the lack of sufficient robust measurements. Until 2015, Biggin observed long-term intensity variations of the paleomagnetic field by examining an extended set of [[Precambrian]] samples. They observed a prominent increase in the paleomagnetic field strength and variance around 1 to 1.5 billion years ago and related the change to the birth of Earth’s solid inner core.[{{Cite journal|last=Dye|first=S. T.|date=September 2012|title=Geoneutrinos and the radioactive power of the Earth|url=http://dx.doi.org/10.1029/2012rg000400|journal=Reviews of Geophysics|volume=50|issue=3|doi=10.1029/2012rg000400|issn=8755-1209}}]
[{{Cite journal|last=Arevalo|first=Ricardo|last2=McDonough|first2=William F.|last3=Luong|first3=Mario|date=February 2009|title=The K/U ratio of the silicate Earth: Insights into mantle composition, structure and thermal evolution|url=http://dx.doi.org/10.1016/j.epsl.2008.12.023|journal=Earth and Planetary Science Letters|volume=278|issue=3-4|pages=361–369|doi=10.1016/j.epsl.2008.12.023|issn=0012-821X}}]
[{{Cite journal|last=Labrosse|first=Stéphane|last2=Poirier|first2=Jean-Paul|last3=Le Mouël|first3=Jean-Louis|year=2001|title=The age of the inner core |journal=Earth and Planetary Science Letters|volume=190|issue=3–4|pages=111–123|doi=10.1016/S0012-821X(01)00387-9|bibcode=2001E&PSL.190..111L|issn=0012-821X}}]
[{{Cite journal|last=Labrosse|first=Stéphane|date=November 2003|title=Thermal and magnetic evolution of the Earth’s core|url=http://dx.doi.org/10.1016/j.pepi.2003.07.006|journal=Physics of the Earth and Planetary Interiors|volume=140|issue=1-3|pages=127–143|doi=10.1016/j.pepi.2003.07.006|issn=0031-9201}}]
[{{Cite journal|last=Labrosse|first=Stéphane|date=October 2015|title=Thermal evolution of the core with a high thermal conductivity|url=http://dx.doi.org/10.1016/j.pepi.2015.02.002|journal=Physics of the Earth and Planetary Interiors|volume=247|pages=36–55|doi=10.1016/j.pepi.2015.02.002|issn=0031-9201}}]
[{{Cite journal|last=Ohta|first=Kenji|last2=Kuwayama|first2=Yasuhiro|last3=Hirose|first3=Kei|last4=Shimizu|first4=Katsuya|last5=Ohishi|first5=Yasuo|date=June 2016|title=Experimental determination of the electrical resistivity of iron at Earth’s core conditions|url=http://dx.doi.org/10.1038/nature17957|journal=Nature|volume=534|issue=7605|pages=95–98|doi=10.1038/nature17957|issn=0028-0836}}]
[{{Cite journal|last=Gomi|first=Hitoshi|last2=Ohta|first2=Kenji|last3=Hirose|first3=Kei|last4=Labrosse|first4=Stéphane|last5=Caracas|first5=Razvan|last6=Verstraete|first6=Matthieu J.|last7=Hernlund|first7=John W.|date=2013-11-01|title=The high conductivity of iron and thermal evolution of the Earth’s core|url=http://www.sciencedirect.com/science/article/pii/S0031920113001052|journal=Physics of the Earth and Planetary Interiors|volume=224|pages=88–103|doi=10.1016/j.pepi.2013.07.010|issn=0031-9201}}]
[{{Cite journal|last=Pozzo|first=Monica|last2=Davies|first2=Chris|last3=Gubbins|first3=David|last4=Alfè|first4=Dario|date=2012-04-11|title=Thermal and electrical conductivity of iron at Earth’s core conditions|url=http://www.nature.com/doifinder/10.1038/nature11031|journal=Nature|volume=485|issue=7398|pages=355–358|doi=10.1038/nature11031|issn=0028-0836}}]
[{{Cite journal|last=Konôpková|first=Zuzana|last2=McWilliams|first2=R. Stewart|last3=Gómez-Pérez|first3=Natalia|last4=Goncharov|first4=Alexander F.|date=June 2016|title=Direct measurement of thermal conductivity in solid iron at planetary core conditions|url=http://dx.doi.org/10.1038/nature18009|journal=Nature|volume=534|issue=7605|pages=99–101|doi=10.1038/nature18009|issn=0028-0836}}]
[{{Cite journal|last=Biggin|first=A. J.|last2=Piispa|first2=E. J.|last3=Pesonen|first3=L. J.|last4=Holme|first4=R.|last5=Paterson|first5=G. A.|last6=Veikkolainen|first6=T.|last7=Tauxe|first7=L.|date=October 2015|title=Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation|url=http://dx.doi.org/10.1038/nature15523|journal=Nature|volume=526|issue=7572|pages=245–248|doi=10.1038/nature15523|issn=0028-0836}}]
[{{Cite journal|last=Smirnov|first=Aleksey V.|last2=Tarduno|first2=John A.|last3=Evans|first3=David A.D.|date=August 2011|title=Evolving core conditions ca. 2 billion years ago detected by paleosecular variation|url=http://dx.doi.org/10.1016/j.pepi.2011.05.003|journal=Physics of the Earth and Planetary Interiors|volume=187|issue=3-4|pages=225–231|doi=10.1016/j.pepi.2011.05.003|issn=0031-9201}}]
[{{Cite journal|last=Bono|first=Richard K.|last2=Tarduno|first2=John A.|last3=Nimmo|first3=Francis|last4=Cottrell|first4=Rory D.|date=2019-01-28|title=Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity|url=http://dx.doi.org/10.1038/s41561-018-0288-0|journal=Nature Geoscience|volume=12|issue=2|pages=143–147|doi=10.1038/s41561-018-0288-0|issn=1752-0894}}]
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[{{cite journal | first=William J. | last= Broad | title = The Core of the Earth May Be a Gigantic Crystal Made of Iron | journal = NY Times | volume = | pages = | date = 1995-04-04 | url = https://www.nytimes.com/1995/04/04/science/the-core-of-the-earth-may-be-a-gigantic-crystal-made-of-iron.html?pagewanted=all | accessdate= 2010-12-21 | issn=0362-4331 }}]
[{{Cite web|url=http://www.berkeley.edu/news/media/releases/96legacy/iron.html|title=Earth's inner core not a monolithic iron crystal, say UC Berkeley seismologist|author=Robert Sanders|date=1996-11-13|accessdate=2007-05-22}}]
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[{{cite journal | author = J.A. Jacobs | title = The Earth's inner core | journal = Nature | volume = 172 | issue = 4372 | pages = 297–298 | date = 1953 | doi = 10.1038/172297a0|bibcode = 1953Natur.172..297J }}]
[{{Cite journal|author= Kazuro Hirahara|author2=Toshiki Ohtaki |author3=Yasuhiro Yoshida | lastauthoramp=yes |title=Seismic structure near the inner core-outer core boundary|journal= Geophys. Res. Lett.|volume=51|pages=157–160|date=1994|url=http://www.agu.org/pubs/crossref/1994/93GL03289.shtml|doi=10.1029/93GL03289|bibcode=1994GeoRL..21..157K|issue =16}}]
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[[Category:1936 in science]]
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==On Earth==
{{further|Earth ellipsoid|Gravity of Earth}}
The [[Earth]] has a rather slight equatorial bulge: it is about {{cvt|43|km|mi}} wider at the equator than pole-to-pole, a difference which is close to 1/300 of the diameter. If the Earth were scaled down to a globe with diameter of 1 meter at the equator, that difference would be only 3 millimeters. (More precisely, in the [[World Geodetic System|WGS-84]] standard [[Earth ellipsoid]], used for map-making and the [[GPS]] system, the radius of the Earth is {{cvt|6378.137|km|mi}} at the equator and {{cvt|6356.7523142|km|mi}} center-to-pole; meaning a difference of {{cvt|21.3846858|km|mi}} in the radii and {{cvt|42.7693716|km|mi}} in the diameters, and a relative [[flattening]] of 1/ 42.77 km more than that measured between the poles ({{cvt|12713.56|km|mi}}). An observer standing at [[sea level]] on either [[geographical pole|pole]], therefore, is 21.36 km closer to Earth's central point than if standing at sea level on the Equator. The value of Earth's radius may be approximated by the average of these radii.
As a result of Earth's equatorial bulge, the highest point on Earth, measured from the center and outwards, is the peak of Mount [[Chimborazo (volcano)|Chimborazo]] in [[Ecuador]] rather than [[Mount Everest]]. But since the ocean also bulges, like Earth and [[atmosphere of Earth|its atmosphere]], Chimborazo is not as high above sea level as Everest is.
The standard formula for this force is the relationship . However, [[velocity]] at the surface is equal to the product of radius and rotational velocity, and therefore the force is directly proportional to radius. Viewing the globe as a series of rotating discs, the radius ''R'' toward the poles decreases and thus a smaller force is produced for the same rotational velocity (approaching zero at the pole). Moving towards the Equator, ''v^2'' increases much faster than ''R'', thus producing the greatest force at the Equator.
In addition, because Earth's [[inner core|dense core]] is included in the [[Cross section (geometry)|cross-section]]al disc at the Equator, it contributes more to the mass of the disc. Similarly, there is a bulge in the water envelope of the oceans surrounding Earth; this bulge is created by the greater [[centrifugal force]] at the Equator and is independent from [[tide]]s. Sea level at the Equator is 21.36 km higher than that at either pole, in terms of distance from the center of the planet.