Mantle convection begins.
Early mantle differentiation by mantle fractionation rather than plate tectonic which would not come about until the next period. Based on the moon's geological record, the time before 4.0 BY was a time of massive meteorite impacts. No rocks preserved from this period. Oldest rocks are gneiss is 3.96 and 3.927 BY Detrital zircon in archean sandstone and quartzite dates between 4.1 and 4.27 BY Early Archean 4.0-3.2 BY ago Development of the Lithosphere begins. Heat lost from earth's surface differentiates into an upper lithosphere, somewhat cooler than the underlying asthenosphere, therefore more solid and more able to resists underlying mantle convection.