Core–mantle boundary

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 a discontinuity in seismic wave velocities, which is interpreted as due to a distinct change in density at this depth. This change is due to the change from the solid silicate mantle to liquid iron-nickel. Recent evidence suggests a boundary layer represented by a structural change in the 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 and relation to 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’’.

Another name for the D’’ is the "Gutenberg Discontinuity".

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

External links

• 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’’