Editing Earth's outer core (section)

==Properties==
{{Expand section|speed of convection|small=no|date=July 2019}}

The outer core of Earth is liquid, unlike its [[Earth's inner core|inner core]], which is solid.<ref>{{cite book |last1=Gutenberg |first1=Beno |title=Physics of the Earth's interior |date=2016 |publisher=Academic Press |isbn=978-1-4832-8212-1 |pages=101–118}}</ref> Evidence for a fluid outer core includes [[seismology]] which shows that [[seismic wave|seismic]] [[S wave|shear-waves]] are not transmitted through the outer core.<ref>{{Cite journal |last=Jeffreys |first=Harold |date=1 June 1926 |title=The Rigidity of the Earth's Central Core |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=1 |pages=371–383 |bibcode=1926GeoJ....1..371J |doi=10.1111/j.1365-246X.1926.tb05385.x |issn=1365-246X |doi-access=free}}</ref> Although having a composition similar to Earth's solid inner core, the outer core remains liquid as there is not enough pressure to keep it in a solid state.

Seismic inversions of [[Seismic wave#Body waves|body waves]] and [[Seismic wave#Normal modes|normal modes]] constrain the radius of the outer core to be 3483 km with an uncertainty of 5 km, while that of the inner core is 1220±10 km.<ref>{{cite book |title=Global earth physics a handbook of physical constants |date=1995 |publisher=[[American Geophysical Union]] |isbn=9780875908519 |editor-last1=Ahrens |editor-first1=Thomas J. |edition=3rd |location=Washington, DC}}</ref>{{rp|94}}

Estimates for the [[temperature]] of the outer core are about {{convert|3000|-|4500|K|abbr=on|sigfig=2}} in its outer region and {{convert|4000|–|8000|K|abbr=on|sigfig=2}} near the inner core.<ref name="DeWijs982">{{cite journal |last1=De Wijs |first1=Gilles A. |last2=Kresse |first2=Georg |last3=Vočadlo |first3=Lidunka |last4=Dobson |first4=David |last5=Alfè |first5=Dario |last6=Gillan |first6=Michael J. |last7=Price |first7=Geoffrey D. |year=1998 |title=The viscosity of liquid iron at the physical conditions of the Earth's core |url=http://www.homepages.ucl.ac.uk/~ucfbdxa/pubblicazioni/nat.pdf |journal=Nature |volume=392 |issue=6678 |pages=805 |bibcode=1998Natur.392..805D |doi=10.1038/33905 |s2cid=205003051}}</ref>  Modeling has shown that the outer core, because of its high temperature, is a low-[[viscosity]] fluid that convects [[turbulence|turbulently]].<ref name="DeWijs982" /> The [[dynamo theory]] sees [[Eddy current|eddy currents]] in the nickel-iron fluid of the outer core as the principal source of [[Earth's magnetic field]]. The average [[magnetic field]] strength in Earth's outer core is estimated to be 2.5 [[Tesla (unit)|millitesla]], 50 times stronger than the magnetic field at the surface.<ref>{{cite news |last1=Staff writer |date=17 December 2010 |title=First Measurement Of Magnetic Field Inside Earth's Core |language=en |work=Science 2.0 |url=http://www.science20.com/news_articles/first_measurement_magnetic_field_inside_earths_core |access-date=14 November 2018}}</ref><ref>{{cite journal |last1=Buffett |first1=Bruce A. |year=2010 |title=Tidal dissipation and the strength of the Earth's internal magnetic field |journal=Nature |volume=468 |issue=7326 |pages=952–4 |bibcode=2010Natur.468..952B |doi=10.1038/nature09643 |pmid=21164483 |s2cid=4431270}}</ref>

As Earth's core cools, the liquid at the inner core boundary freezes, causing the solid inner core to grow at the expense of the outer core, at an estimated rate of 1 mm per year. This is approximately 80,000 tonnes of iron per second.<ref name="ng4_2642">{{cite journal |last1=Wassel |first1=Lauren |last2=Irving |first2=Jessica |last3=Dues |first3=Arwen |year=2011 |title=Reconciling the hemispherical structure of Earth's inner core with its super-rotation |journal=Nature Geoscience |volume=4 |issue=4 |pages=264–267 |bibcode=2011NatGe...4..264W |doi=10.1038/ngeo1083}}</ref>