European scientists have discovered a 100 km-wide crater, the result of a massive asteroid or comet impact a billion years before any other known collision on Earth.
An artistic expression of how a large meteorite impact into the sea might have looked in the first second of the impacting (Carsten Egestal Thuesen / GEUS)
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.
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.
Now, a team of scientists from Cardiff University, the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen, Lund University in Sweden and the Institute of Planetary Science in Moscow has upset these odds. The team has discovered the remains of a giant 3 billion year old impact near the Maniitsoq region of West Greenland.
Circle on map shows the location of the meteorite impact structure near the town Maniitsoq in Greenland. Image below shows Maniitsoq in 1890 (John Møller)
“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,” said Dr Iain McDonald of the Cardiff University’s School of Earth and Ocean Sciences, co-author of a paper 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,” Dr McDonald explained. “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% of them contain important natural resources of minerals or oil and gas. The largest and oldest known crater prior to this study, the 300 km wide Vredefort crater in South Africa, is 2 billion years in age and heavily eroded.
“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,” Dr McDonald said.
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Bibliographic information: Garde AA et al. 2012. Searching for giant, ancient impact structures on Earth: The Mesoarchaean Maniitsoq structure, West Greenland. Earth and Planetary Science Letters 337–338, 1 July 2012, pp. 197–210; doi: 10.1016/j.epsl.2012.04.026
Source link: https://www.sci.news/geology/article00439.html
