The primordial Earth

Four and a half billion years ago, the proto-Earth was completing its formation. During the last accretion phases, its growing gravity had increased the impact velocities, so that their energies had been transformed into more and more heat. Hence the proto-Earth became progressively covered with a thick layer of molten lava, possibly to a very great depth.

The large-scale differentiation that separated the denser iron core from the mantle of lighter silicates was triggered by this intense heat. The last large impact occurred somewhat later, notably the one which, by a tangential grazing collision, caused the appearance of a transient ring around the Earth that rapidly became the Moon. The smaller cometary impacts, however, persisted and ended by establishing, not only the atmosphere and the oceans, but also the minor differentiation that separated the terrestrial crust from the underlying mantle.

The chemical and isotopic evidence that the terrestrial crust formed so early on was an enigma for geologists. It seems to be resolved by the cometary bombardment, when chondritic silicates were plowed deeply into the surface of the Earth after the separation of the core from the mantle.