<P> The core--mantle boundary (CMB in the parlance of solid earth geophysicists) of the Earth lies between the planet's silicate mantle and its liquid iron - nickel outer core . This boundary is located at approximately 2891 km (1796 mi) 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 impedances 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 above the CMB possibly made of a novel phase of the basic 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 (LLSVPs). 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". </P> <P> The bottom of D" has been observed in some regions to be marked by a seismic velocity discontinuity (sometimes known as the' Gutenberg discontinuity,' after German geophysicist Beno Gutenberg). </P>

What is the name of the boundary between the mantle and the outer core