Thermo-chemical evolution of the Earth

The thermal history of Earth’s outer layers (above 660 km) is independent of processes in the interior, and vice versa, due to large cooling time-scales. Internal radioactive heating warms the lower mantle, which heats and melts the core. Thermal isolation permits estimating a lower mantle chemical composition. Evolution of the outer layers also involves external and internal gravitational processes. Briefly, Earth began with nucleation of an iron core, followed by cold accretion of the lower mantle from an iron-depleted sub-nebulae. The outer layers were assembled from chondrites and ices, which oxidized as accretion progressed. Spin dissipation, high radioactivity, coupled with melts and volatiles being buoyant, caused warming and differentiation above 660 km and provided high temperatures and thin crusts in the Archean. Impact heating seems unimportant. Cooling proceeded slowly until plate tectonics developed due to solar force imbalance, which add heat while promoting cooling and mass recycling above 660 km.