J.Geophys. Res. http://doi.org/r87 (2014)

Questions surround the timing of the onset of Earth's mantle convection and its subsequent evolution. Numerical modelling suggests that the mantle developed in two distinct stages.

Masaki Ogawa at the University of Tokyo, Japan, simulated mantle convection, together with magmatic activity and movements of the surface lithosphere, over Earth's lifetime. During the first one to two billion years of the simulation, the deep mantle is strongly heated by the core and radioactive decay of elements. Hot material ascends from the lower mantle in large bursts that generate surface magmatism and cause the lithosphere to move chaotically. The vigorous bursts and chaotic movements help to mix the mantle. With time, the rate of heat production wanes, reducing the number of mantle bursts. During this second stage of mantle evolution, rigid tectonic plates form at Earth's surface. The plates migrate in steady motions and slabs of lithosphere subduct into the mantle, taking surface water with them, accumulating in stable piles at the core–mantle boundary. Plumes generated from the top of the piles help return material to the upper mantle.

These second-stage processes occur over about one billion years, however, resulting in a mantle that becomes increasingly poorly mixed.