Conventional plate-tectonic hypotheses differ in detail, but since the 1980s most have assumed that Earth has operated in a plate-tectonic mode, with plume-based whole-mantle convection driven by an eternally hot core, for at least 3 b.y., and that this convection has kept the mantle stirred and mostly unfractionated.
Hamilton's model integrates plate behavior with multidisciplinary geophysics, and has the 3-D circulation of plate tectonics entirely confined to the upper mantle, above the profound seismic discontinuity at a depth of about 660 km.