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The spectacular craters on the Moon formed from impacts with asteroids and comets between 3 and 4 billion years ago. The early Earth, with its far greater gravitational mass, must have experienced even more collisions at this time—but the evidence has been eroded away or covered by younger rocks.

Looking southwest from Fossilik Hill, the center of the impact structure is visible (Credit: Cardiff U.)

The previously oldest known crater on Earth formed 2 billion years ago and the chances of finding an even older impact were thought to be, literally, astronomically low.

The discovery of the giant 3-billion-year-old impact near the Maniitsoq region of West Greenland upsets those odds.

“This single discovery means that we can study the effects of cratering on the Earth nearly a billion years further back in time than was possible before,” says Iain McDonald of Cardiff University’s School of Earth and Ocean Sciences, coauthor of the study published in Earth and Planetary Science Letters.

Finding the evidence was made all the harder because there is no obvious bowl-shaped crater left to find. Over the 3 billion years since the impact, the land has been eroded down to expose deeper crust 25 km below the original surface. All external parts of the impact structure have been removed, but the effects of the intense impact shock wave penetrated deep into the crust—far deeper than at any other known crater—and these remain visible.

However, because the effects of impact at these depths have never been observed before it has taken nearly three years of painstaking work to assemble all the key evidence. “The process was rather like a Sherlock Holmes story,” says McDonald.

“We eliminated the impossible in terms of any conventional terrestrial processes, and were left with a giant impact as the only explanation for all of the facts.”

Only around 180 impact craters have ever been discovered on Earth and around 30 percent of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 kilometer-wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded.

McDonald adds that “It has taken us nearly three years to convince our peers in the scientific community of this but the mining industry was far more receptive. A Canadian exploration company has been using the impact model to explore for deposits of nickel and platinum metals at Maniitsoq since the autumn of 2011.”

Adam A. Garde, senior research scientist at the Geological Survey of Denmark and Greenland (GEUS), led the international team of researchers from Cardiff University, Lund University, and the Institute of Planetary Science in Moscow, whose work was funded by GEUS and the Danish Carlsberg Foundation.

More news from Cardiff University: www.cardiff.ac.uk/news