Core–mantle boundary

<h2 id="d-region">D″ region</h2>
<p>An approximately 200 km thick layer of the lower mantle directly above the CMB is referred to as the <i>D″ region</i> ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core–mantle boundary zone.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> The D″ name originates from geophysicist <a href="/facts/Keith_Edward_Bullen/jH09oyrz">Keith Bullen</a>'s designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the <a href="/facts/Earth%27s_crust/VSy2u31r">crust</a> as 'A' and the inner <a href="/facts/Internal_structure_of_Earth/678dYXU7">core</a> as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1949, Bullen found his 'D' layer to actually be two different layers. The upper part of the D layer, about 1,800 km thick, was renamed D′ (D prime) and the lower part (the bottom 200 km) was named D″.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>  Later it was found that D" is non-spherical.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> 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 <a href="/facts/Hotspot_(geology)/drIyXSLs">hotspots</a> and <a href="/facts/Mantle_convection/YV2IE8KQ">mantle convection</a>.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Later research supported this hypothesis.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a>
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<h2 id="seismic-discontinuity">Seismic discontinuity</h2>
<p>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.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> 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.
</p><p>The discontinuity was discovered by <a href="/facts/Beno_Gutenberg/jTJqBeNs">Beno Gutenberg</a>, 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 <a href="/facts/Gutenberg_discontinuity/aMs9vwMc">Gutenberg discontinuity</a>, the Oldham-Gutenberg discontinuity, or the Wiechert-Gutenberg discontinuity.  In modern times, however, the term <a href="/facts/Lithosphere%E2%80%93asthenosphere_boundary/z6bTNqIJ">Gutenberg discontinuity or the "G"</a> 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.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a>
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<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Core%E2%80%93mantle_differentiation/VgB0vxsz">Core–mantle differentiation</a></li>
<li><a href="/facts/Ultra_low_velocity_zone/vFCez0CI">Ultra low velocity zone</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://unisci.com/stories/20014/1130014.htm">Earth's Core–Mantle Boundary Has Core-Rigidity Zone</a></li>
<li>Audrey Slesinger (January 2001), <a href="http://www.geotimes.org/jan01/earthsinterior.html">"Earth's interior: Redefining the Core–Mantle Boundary"</a>, <i>Geotimes</i>, <a href="/facts/The_American_Geological_Institute/yTVsKPQY">The American Geological Institute</a>, retrieved 2011-03-24</li>
<li><a href="https://web.archive.org/web/20050705074959/http://www.aip.org/pnu/2004/split/679-1.html">Mineral phase change at the boundary</a></li>
<li><a href="http://seismo.berkeley.edu/annual_report/ar01_02/node37.html">Superplumes at the boundary</a> <a href="https://web.archive.org/web/20060213053142/http://seismo.berkeley.edu/annual_report/ar01_02/node37.html">Archived</a> 2006-02-13 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li>
<li><a href="http://geology.about.com/library/weekly/aa021300a.htm">About.com article on the name of D″</a> <a href="https://web.archive.org/web/20081006223111/http://geology.about.com/library/weekly/aa021300a.htm">Archived</a> 2008-10-06 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Lekic, V.; Cottaar, S.; Dziewonski, A. & Romanowicz, B. (2012). "Cluster analysis of global lower mantle". Earth and Planetary Science Letters. 357–358 (1–3): 68–77. Bibcode:2012E&PSL.357...68L. doi:10.1016/j.epsl.2012.09.014. <a href="/wiki/Earth_and_Planetary_Science_Letters" target="_blank">/wiki/Earth_and_Planetary_Science_Letters</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Lay, Thorne; Hernlund, John; Buffett, Bruce A. (2008). "Core–mantle boundary heat flow". Nature Geoscience. 1 (1): 25–32. Bibcode:2008NatGe...1...25L. doi:10.1038/ngeo.2007.44. ISSN 1752-0894. <a href="/wiki/Nature_Geoscience" target="_blank">/wiki/Nature_Geoscience</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>WR Peltier (2007). "Mantle Dynamics and the D" Layer: Impacts of the Post Perovskite Phase". In Kei Hirose; John Brodholt; Thome Lay; David Yuen (eds.). Post-Perovskite: The Last Mantle Phase Transition (PDF). American Geophysical Union. pp. 217–227. ISBN 978-0-87590-439-9. {{cite book}}: |work= ignored (help) <a href="978-0-87590-439-9" target="_blank">978-0-87590-439-9</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Bullen K., Compressibility-pressure hypothesis and the Earth’s interior. Monthly Notices of the Royal Astronomical Society, Geophysical Supplements, 5, 355–368., 1949
 <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Creager, K.C. and Jordan, T.H. (1986). Asperical structure of the core-mantle boundary. Geophys. Res. Lett. 13, 1497-1500 <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Czechowski L. (1993) Geodesy and Physics of the Earth pp 392-395, The Origin of Hotspots and The D” Layer <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>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- <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>Dziewonski, Adam M.; Anderson, Don L. (1981-06-01). "Preliminary reference Earth model". Physics of the Earth and Planetary Interiors. 25 (4): 297–356. Bibcode:1981PEPI...25..297D. doi:10.1016/0031-9201(81)90046-7. ISSN 0031-9201. <a href="/wiki/Physics_of_the_Earth_and_Planetary_Interiors" target="_blank">/wiki/Physics_of_the_Earth_and_Planetary_Interiors</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
<li id="fn:9"><p>Schmerr, N. (2012-03-22). "The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary". Science. 335 (6075): 1480–1483. Bibcode:2012Sci...335.1480S. doi:10.1126/science.1215433. ISSN 0036-8075. PMID 22442480. S2CID 206538202. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li>
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Core–mantle boundary open-in-new

Core–mantle boundary