Meteorites may have created Earth’s first continents
by Timothy Oleson Wednesday December 23rd, 2015
Massive meteorite impacts on Earth are destructive events, digging huge craters into the crust and raining debris on the planet’s surface. But such huge impacts may also have created some of its earliest continental cores, called cratons, during Archean times.
Archean cratons, such as the Pilbara Craton in Western Australia and the Kaapval Craton in Southern Africa, solidified over 2.5 billion years ago and are made up of distinctive crustal rocks called granite and granite terranes. green rocks resting on deep roots and floating in the lithosphere. Many ideas about how these ancient provinces formed – often involving plate tectonic or mantle plume processes – have been suggested in the past, although the actual mechanism remains unclear.
Contrary to existing notions, a newly hypothetical mechanism, described in a recent study in the journal Lithosphere by Vicki Hansen from the University of Minnesota at Duluth, summons an alien trigger. Hansen suggests that large meteorites hitting early Earth in the Archean could have penetrated through what was likely thinner lithospheric skin at the time and into the mantle. The enormous energy of the impacts would have melted the mantle locally, with the more floating parts of the melt then pouring into the void left on the surface and forming a new crust and lithosphere with different compositions from the surrounding rock.
The mechanism is essentially the same as a Hansen previously proposed to explain how features on Venus called crustal plateaus, perhaps analogous to Archean cratons on Earth, could have formed. Although neighboring planets differ considerably now – Earth has active plate tectonics while Venus does not, for example – they were much more similar in their youth, Hansen noted in the study. For starters, each probably had a thin lithosphere compared to today and was bombarded by impactors – two components needed in their model. The bolide-craton hypothesis, she wrote, offers a plausible alternative to other current hypotheses for the formation of the Archean craton, although it is “also not incompatible with the other mechanisms.”