Editing Core–mantle boundary

{{Short description|Discontinuity where the bottom of the planet's mantle meets the outer layer of the core}}
[[File:Slice earth.svg|thumb|Schematic view of the interior of Earth. {{olist |{{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|[[Earth's inner core|inner core]]}}}}{{olist |list-style-type=upper-alpha |[[Mohorovicic discontinuity]] |'''core–mantle boundary''' |outer core–inner core boundary}}]]

The '''core–mantle boundary''' ('''CMB''') of Earth lies between the planet's [[silicate]] [[Earth's mantle|mantle]] and its liquid [[iron–nickel alloy|iron–nickel]] [[Earth's outer core|outer core]], at a depth of {{cvt|2,891|km|mi}} below Earth's surface. The boundary is observed via the discontinuity in [[seismic wave]] velocities at that depth due to the differences between the [[acoustic impedance]]s of the solid mantle and the molten outer core. [[P-wave]] velocities are much slower in the outer core than in the deep mantle while [[S-wave]]s do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic [[silicate perovskite|perovskite]] mineralogy of the deep mantle named [[post-perovskite]]. [[Seismic tomography]] studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific [[Large low-shear-velocity provinces]] (LLSVP).<ref name="LLSVP">{{cite journal|author1=Lekic, V.|author2=Cottaar, S.|author3=Dziewonski, A.|author4=Romanowicz, B.|name-list-style=amp|year=2012|title=Cluster analysis of global lower mantle|journal=[[Earth and Planetary Science Letters]]|volume=357-358|issue=1–3|pages=68–77|doi=10.1016/j.epsl.2012.09.014|bibcode=2012E&PSL.357...68L}}</ref>

The uppermost section of the outer core is thought to be about 500–1,800 K hotter than the overlying mantle, creating a thermal boundary layer.<ref>{{Cite journal|last1=Lay|first1=Thorne|last2=Hernlund|first2=John|last3=Buffett|first3=Bruce A.|date=2008|title=Core–mantle boundary heat flow|journal=[[Nature Geoscience]]|volume=1|issue=1|pages=25–32|doi=10.1038/ngeo.2007.44|issn=1752-0894|bibcode=2008NatGe...1...25L}}</ref> The boundary is thought to harbor topography, much like Earth's surface, that is supported by solid-state convection within the overlying mantle.{{Citation needed|date=August 2018}} Variations in the thermal properties of the CMB may affect how the outer core's iron-rich fluids flow, which are ultimately responsible for [[Earth's magnetic field]].{{Citation needed|date=August 2018}}

== D″ region ==
An approximately 200&nbsp;km thick layer of the lower mantle directly above the CMB is referred to as the ''D″ region'' ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core–mantle boundary zone.<ref name="Peltier">{{cite book <!--|chapter=Mantle dynamics and the D-doubleprime layer implications of the post-perovskite phase -->|contribution=Mantle Dynamics and the D" Layer: Impacts of the Post Perovskite Phase |author=WR Peltier |url=http://www.atmosp.physics.utoronto.ca/~peltier/pubs_books/W.R.%20Peltier,%20Mantle%20Dynamics%20and%20the%20D%20doubleprime%20Layer.Implications%20of%20the%20Post-perovskite%20Phase,%20AGU%20Geodynamics%20Series%20Monograph,%20AGU%20Books,%20pages%20217-227,%202007.pdf |year=2007  |pages=217–227 |title=Post-Perovskite: The Last Mantle Phase Transition|work=Volume 174 in AGU Geophysical Monographs|editor1=Kei Hirose |editor2=John Brodholt |editor3=Thome Lay |editor4=David Yuen |isbn=978-0-87590-439-9 |publisher=[[American Geophysical Union]]}}</ref> The D″ name originates from geophysicist [[Keith Edward Bullen|Keith Bullen]]'s designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the [[Earth's crust|crust]] as 'A' and the inner [[Internal structure of Earth#Core|core]] as 'G'. In his 1942 publication of his model, the entire lower mantle was the D&nbsp;layer. In 1949, Bullen found his 'D'&nbsp;layer to actually be two different layers. The upper part of the D layer, about 1,800&nbsp;km thick, was renamed D′ (D prime) and the lower part (the bottom 200&nbsp;km) was named D″.<ref>Bullen K., Compressibility-pressure hypothesis and the Earth’s interior. Monthly Notices of the Royal Astronomical Society, Geophysical Supplements, 5, 355–368., 1949 </ref>  Later it was found that D" is non-spherical.<ref> Creager, K.C. and Jordan, T.H. (1986). Asperical structure of the core-mantle boundary. Geophys. Res. Lett. 13, 1497-1500</ref> In 1993, Czechowski found that inhomogeneities in D" form structures analogous to continents (i.e. core-continents). They move in time and determine some properties of [[Hotspot (geology)|hotspot]]s and [[mantle convection]].<ref>Czechowski L. (1993) Geodesy and Physics of the Earth pp 392-395, The Origin of Hotspots and The D” Layer</ref> Later research supported this hypothesis.<ref>Torsvik, Trond H.; Smethurst, Mark A.; Burke, Kevin; Steinberger, Bernhard (2006). "Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle". Geophysical Journal International. 167 (3): 1447–1460. Bibcode:2006GeoJI.167.1447T. doi:10.1111/j.1365-</ref>
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(Commenting this out until someone can make it make sense.) Slow small-scaled structures have been discovered on the core-mantle boundary; these are dubbed [[Ultra low velocity zone|ultra-low velocity zone]]s (ULVZs). It is suggested that large scale inhomogeneities of D" are formed by high density matter flowing on the core. Correlation between these inhomogeneities (c(ore)-continents)  and position of [[Hotspot (geology)|hotspots]] was found by 
<ref name="Czechowski">{{cite |title= The Origin of Hotspots and The D” Layer.|author1=Czechowski L.|journal=In: Montag H., Reigber C. (eds) Geodesy and Physics of the Earth. International Association of Geodesy Symposia|volume=112|pages=392–395}}</ref>
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== Seismic discontinuity ==
A seismic discontinuity occurs within Earth's interior at a depth of about 2,900 km (1,800 mi) below the surface, where there is an abrupt change in the speed of seismic waves (generated by earthquakes or explosions) that travel through Earth.<ref>{{Cite journal|date=1981-06-01|title=Preliminary reference Earth model|journal=[[Physics of the Earth and Planetary Interiors]]|language=en|volume=25|issue=4|pages=297–356|doi=10.1016/0031-9201(81)90046-7|issn=0031-9201|bibcode=1981PEPI...25..297D|last1=Dziewonski|first1=Adam M.|last2=Anderson|first2=Don L.}}</ref> At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material and cannot transmit through liquids, so it is thought that the unit above the discontinuity is solid, while the unit below is in a liquid or molten form.

The discontinuity was discovered by [[Beno Gutenberg]], a seismologist who made several important contributions to the study and understanding of the Earth's interior. The CMB has also been referred to as the [[Gutenberg discontinuity]], the Oldham-Gutenberg discontinuity, or the Wiechert-Gutenberg discontinuity.{{Citation needed|date=August 2018}}  In modern times, however, the term [[Lithosphere–asthenosphere boundary|Gutenberg discontinuity or the "G"]] is most commonly used in reference to a decrease in seismic velocity with depth that is sometimes observed at about 100 km below the Earth's oceans.<ref>{{Cite journal|last=Schmerr|first=N.|date=2012-03-22|title=The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary|journal=Science|volume=335|issue=6075|pages=1480–1483|doi=10.1126/science.1215433|pmid=22442480|issn=0036-8075|bibcode=2012Sci...335.1480S|s2cid=206538202}}</ref>
== See also ==
* [[Core–mantle differentiation]]
* [[Ultra low velocity zone]]

==References==
<references/>

==External links==
*[http://unisci.com/stories/20014/1130014.htm Earth's Core–Mantle Boundary Has Core-Rigidity Zone]
*{{citation |title=Earth's interior: Redefining the Core–Mantle Boundary |author=Audrey Slesinger |url=http://www.geotimes.org/jan01/earthsinterior.html |access-date = 2011-03-24 |date=January 2001 |magazine=Geotimes |publisher=[[The American Geological Institute]]}}
*[https://web.archive.org/web/20050705074959/http://www.aip.org/pnu/2004/split/679-1.html Mineral phase change at the boundary]
*[http://seismo.berkeley.edu/annual_report/ar01_02/node37.html Superplumes at the boundary] {{Webarchive|url=https://web.archive.org/web/20060213053142/http://seismo.berkeley.edu/annual_report/ar01_02/node37.html |date=2006-02-13 }}
*[http://geology.about.com/library/weekly/aa021300a.htm About.com article on the name of D″] {{Webarchive|url=https://web.archive.org/web/20081006223111/http://geology.about.com/library/weekly/aa021300a.htm |date=2008-10-06 }}

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[[Category:Geophysics]]
[[Category:Structure of the Earth]]