So far, Earth is the only planet we know of that has continents.
Exactly how they formed and evolved is not clear, but we do know, because the edges of continents thousands of kilometers apart coincide, that, long ago, Earth’s land mass was concentrated into a large supercontinent .
Since this is not what the planet looks like today, something must have caused this supercontinent to break up. Now, we have new evidence suggesting that giant meteorite impacts played a role.
The smoking gun consists of crystals of the mineral zircon, excavated from a craton in Western Australia, a piece of the Earth’s crust that has remained stable for over a billion years.
Known as the Pilbara Craton, it’s the best-preserved chunk of crust on the planet…and the zircon crystals in it contain evidence of ancient meteorite impacts before the continents broke apart.
“Study of the oxygen isotope composition in these zircon crystals revealed a ‘top-down’ process that began with the melting of the rocks near the surface and progressed deeper, consistent with the geological effect from giant meteorite impacts,” explained geologist Tim Johnson of Curtin University. in Australia
“Our research provides the first strong evidence that the processes that eventually formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, but occurring billions of years earlier.”
The work was carried out on 26 rock samples containing zircon fragments, dating between 3.6 billion and 2.9 billion years.
The research team carefully analyzed the oxygen isotopes; specifically, the proportions of oxygen-18 and oxygen-16, which have 10 and 8 neutrons, respectively. These ratios are used in paleogeology to determine the temperature of formation of the rock in which the isotopes are found.
Based on these ratios, the team was able to distinguish three different and fundamental stages in the formation and evolution of the Pilbara craton.
The first stage is the formation of a large proportion of zircons consistent with partial melting of the crust. The researchers show that this partial melting was likely the result of bombardment by meteorites, which heated the planetary crust on impact.
The oldest cluster of these zircons, the team interprets, was the result of a single giant impact that led to the formation of the craton.
The second stage was a period of reworking and stabilization of the crustal core, followed by the third stage: a period of melting and granite formation. This stabilized core then, much later, would evolve to become the present continents, just like the cratons found in other continents of the world.
However, many meteorites have struck the Earth in the past, in numbers far greater than the number of continents. Only the largest impacts could generate enough heat to create the cratons, which appear to be twice as thick as their surrounding lithosphere.
These findings are consistent with previously proposed models for the formation of cratons worldwide, but constitute, according to the researchers, the strongest evidence yet for the theory.
However, it is only one craton, of about 35 known. To make the evidence even stronger, the team will need to compare their results with more samples from other cratons, to see if their model is globally consistent.
“Data related to other areas of ancient continental crust on Earth appear to show similar patterns to those recognized in Western Australia,” Johnson said. “We would like to test our findings in these ancient rocks to see if, as we suspect, our model is more widely applicable.”
The research has been published in Nature.