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The core-mantle boundary (or CMB in the parlance of solid earth geophysicists) lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth. This discontinuity is due to the differences between the acoustic impedences 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-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly atop the CMB possibly comprised of a novel phase of the basic perovskite mineralogy of the deep mantle. Seismic tomography studies have shown significant irregularities within the boundary zone and are suggestive of a possible organized structure as well as the presence of deep mantle plumes.

The ~200 km thick layer of the lower mantle directly above the boundary is referred to as the D’’ ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core-mantle boundary zone. The D’’ name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D’’.

The top of D’’ has been observed in some regions to be marked by a seismic velocity discontinuity (sometimes known as the 'Gutenberg discontinuity').

It is estimated that due to the cooling of Earth, the inner solid core increases in size each year (on the order of cm).

• Earth's Core-Mantle Boundary Has Core-Rigidity Zone
• Geotimes - Redefining the Core - Mantle boundary
• Mineral phase change at the boundary
• Superplumes at the boundary
• About.com article on the name of D’’

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