The density field and temperature are believed to vary adiabaticaly with depth, but they are averaged horizontally. The only exception being the core mantle boundary and the thin boundary layer's adjacent to the surface of the lithosphere. The oceanic plates are thought to be very closely connected to the motion of the mantle, and to be coincident with the cold boundary layers of convective circulation. The continents on the other hand, are believed to form both a chemical and thermal boundaries thicker than oceanic lithosphere, with increasing thickness underneath the cratons. The layered mantle dynamics differs in many aspects from the above Click here for illustration .
Geochemical data supports that the upper mantle has much less radioactivity than the lower.
This inference is supported by the fact that heat transfer at depths of 670-Km is possible only by diffusion. The convective motions of the upper level are thought to be of different scale, and also unrelated to lithospheric plate motions. This would make them responsible for most of the heat flowing across the upper mantle. But the depths and temperatures are now much higher, causing the layered model to have weaker thermal boundaries at the core mantle separation. Not enduring a large heat flow coming from the core, the overall cooling of the Earth will be much slower.