DID LARGE IMPACTS SEED THE FIRST CONTINENTS?

Studies of the Moon have long suggested that its record of impacts between about 4.5 and 3.7 Ga implied that a similar history occurred on Earth (Wilhelms, 1987). The larger of lunar impacts produced continental scale basins such as South Pole Aitken (2500 km diameter), probably Procellarum (3200 km diameter), and Imbrium, Crisium and several cryptic basins (Schmitt, 2003) greater than 1000 km in diameter. The question follows as to the effects of such impacts on Earth, modified by higher impact velocities and the presence of water on and in the early terrestrial crust. Evidence now points to the Procellarum Basin forming-event having occurred at about 4.35 Ga (summarized by Schmitt, 2016) with the South Pole Aikten event between that date and about 4.1 Ga.

Kilometers-thick melt-sheets in terrestrial impact basins of continental scale were the likely result of large terrestrial impacts. Other than the fact that such impacts occurred in the inner solar system, based on the lunar record, the primary evidence of the existence of differentiated impact melt-sheets lies in reports of zircons with crystallization ages of about 4.4 Ga and isotopic evidence that they formed in the presence of water (Valley, et al., 2014). Modeling studies of the interaction of a solid melt-sheet with early terrestrial mantle convection would be great interest in evaluating this hypothesis.

Of additional interest is the coincidence of the formation of large terrestrial impact basins with the organization of complex organic molecules and their eventual transition to replicating life forms. The broad existence of smectitic phyllosilicates formed from impact-generated glasses may have provided the substrates necessary for organizing this evolutionary process (Schmitt, 2015).