Meteorite impacts may have triggered an ancient subduction
by Sarah Derouin Thursday February 15th, 2018
The Earth in the Hadean Aeon between 4.56 billion and 4 billion years ago was far too hot to support active plate tectonics as we know it today, where cold, established plates walk. slowly around the Earth. Yet some evidence, including tiny zircon crystals dating back to the Hadean, suggests that a form of plate tectonics was active about 4.1 billion years ago – about a billion years before many researchers believe that modern plate tectonics began. The mechanisms that could have initiated and maintained early tectonics are unclear, but according to a new study, a constant bombardment of the primitive Earth by meteorites could have triggered temporary bursts of early tectonism.
Early Earth was characterized by long stretches of what is known as stagnant eyelid tectonics, with a thin, floating lithospheric shell resting motionless on a warm mantle. In the new study, the researchers simulated what would have happened when the ancient, thin, hot-skinned Earth was struck by meteorites of varying sizes.
“No one had considered the effects that impactors of different sizes could have had on the early Earth, mainly because it is extremely difficult to simulate the Earth under these conditions,” says Craig o’neill, planetary geologist at Macquarie University in Australia and co-author of the study, published in Nature Geoscience. âThe Earth was so hot that the movements [in Earthâs interior from impacts] happened very quickly and on a very small scale, which made it difficult to simulate, âsays O’Neill.
The team used information from collections of other planetary craters and populations of asteroid belts to help limit the sizes and other details of impactors that likely struck early Earth, ultimately simulating meteorites with rays ranging from less than 70 kilometers to more than 500 kilometers.
The researchers found that the pressure of an impact could thin the lithosphere and cause a large-scale rise in mantle magma, which, when it reached near the surface, spread laterally. This horizontal spreading stressed the lithosphere, creating a brief explosion of tectonic activity lasting about 5-10 million years in which parts of the lithosphere subdued into the mantle. After these episodes of subduction, a stagnant cover reformed on the surface, remaining intact until the next impact.
It didn’t take a particularly large impact to get the process started, says O’Neill. “Sometimes the impactors were only about 70 kilometers wide, but the moment they struck the lithosphere, the subduction started.” Previously, impacts like these were thought to be too small to matter in triggering tectonic activity. âWe have shown that in a system initiated for tectonic activity, [small impactors] can tip it overboard, âhe says.
In a search highlight in the journal Solid Earth Sciences on early plate tectonics, Weidong Sun of the Chinese Academy of Sciences wrote that the stiffness of the plates on the hot Hadean Earth is the key factor in whether the impacts could have triggered tectonics. The impacts, as presented in O’Neill’s study, “provide a mechanism to establish weak spots in the crust conducive to plaque formation,” Sun wrote. But “under sufficiently high flows, [they] could destroy the plates. In this regard, impact is not the ideal trigger for plaque subduction, âhe wrote. Nevertheless, âthe effects of a moderate impact crater could [have been] significantly different in the Hadean âwhen the conditions in the mantle were very different from today, he noted.
At the start of the project, O’Neill said the team was unsure whether their impact simulations would trigger tectonic activity on early Earth. âThere are physical reasons why impacts can help or hinder subduction; but in the end they seemed to be able to [subduction] quite effectively âin Hadean. It’s unclear if, or for how long, the process might have lasted in the Archean, he says, adding that the team is focusing on this for their next research question.