We hope that geologists will be inspired to go back and to look at sites where perhaps, in the past, their likelihood of being an impact crater has been rejected. This approach could be used to identify another ancient impact structure that we haven’t yet recognised.
Dr Iain McDonald
An international team of scientists has discovered Earth’s oldest known impact crater near Maniitsoq in Greenland. The crater, which is approximately three billion years old, predates South Africa’s Vredefort crater – the previous holder of the ‘oldest known’ title – by almost a billion years.
Scientists from the Geological Survey of Denmark and Greenland (GEUS), Cardiff University, Lund University and Moscow’s Institute of Planetary Science, made the discovery as part of research funded by GEUS and Denmark’s Carlsberg Foundation. Due to the tremendous age of the impact site, its land has been eroded and no bowl-shaped crater remains. However, the researchers, whose work has been published in the journal Earth and Planetary Science Letters
, were able to identify the crater from the effects of an intense impact shockwave that penetrated deep into the Earth’s crust.
Dr Iain McDonald, Lecturer at Cardiff University’s School of Earth & Ocean Sciences, spoke to ScienceOmega.com
about the hard-fought nature of this record-breaking discovery.
"To a degree, it was an accidental discovery," he explained. "It was GEUS’s Adam Garde, my colleague and the leader of this project, who made the discovery. Adam had worked in that area for many years as a young geologist. He came away having mapped the area but feeling that he had never fully understood it. There were many things that puzzled him. In 2009, he and I were preparing for a meeting in Nuuk. When we sat down, Adam put together all of these details that had been puzzling him for so long, and he formulated the idea that it could be an impact crater.
"We first presented this hypothesis to the staff of the mining company with whom we were meeting. Adam expected his idea to be treated with great scepticism, but I sat there thinking that this was actually possible. He was describing nothing that was totally crazy.
"Over the following days, the mining company flew us in helicopters to various sites that they wanted to visit. We had the chance to visit locations on the edge of the crater and sit down with some of the company’s geologists to discuss our field observations. Gradually we began to realise that there was something in this. Everything that we started to see in the field began to make sense when placed within the context of what Adam was suggesting. This really set the ball rolling.
An artistic impression of how a large meteorite impact might have looked
"Adam then returned to his rock collection and began looking at samples that he’d never examined with the idea of an impact crater in mind. With a fresh eye, he began to recognise features such as intense crushing, intense melting and fragmentation, but on a scale far larger than could be explained by any terrestrial process. This was one of the strongest lines of evidence. The only explanation that could explain the presence of these features on such a scale was a giant impact. However, when we looked at other lines of evidence such as microstructures in the quartz crystals, we had a difficult time convincing the geological community that this was an impact crater."
"At most other craters, these microstructures are straight. At Maniitsoq, they were not. There was always an element of curvature, so many of our peers were reluctant to accept them as evidence for impact. If we had been looking at effects caused by a normal terrestrial process, the cracks in this quartz would have had a random distribution from 0° to 90°. When we measured the angular relationships between the lines at Maniitsoq, relative to one another and to the crystal axis in which they were sitting, we did not find them to be random. Whenever we measured these angles against the universal stage, we began to find patterns of cracks similar to those found at other impact craters."
Many had predicted that a crater predating the two billion year old Vredefort crater would not be found. Due to the immense age of such craters, original rocks tend to be eroded over millennia. Dr McDonald went on to explain more about the process involved in identifying the age of an impact crater.
"We use radioactive decay within minerals," he said. "Essentially, when a rock is very, very hot – when it is completely molten – all of its isotopes get mixed up. This means that whenever something crystallises as a mineral, a certain amount of a particular element, such as uranium or rubidium, will be locked in. These elements decay to other elements with known half-lives.
"At Maniitsoq, we are mostly using a mineral called zircon, which is linked to the element uranium. Over a certain amount of time, uranium will produce lead. The relative amounts of uranium and lead that we find in a rock allow us to calculate its age."
Dr McDonald went on to explain why he and his colleagues believe that this crater’s advanced age is responsible for the apparent ‘anomalies’ found within the quartz crystals.
Looking south-west from Fossilik Hill into the centre of the impact structure
"The rocks in the area around Maniitsoq formed, or were forming, about three billion years ago," he said. "This links to the difficulty that we had in understanding why this impact was different to those that led to the creation of other known craters. We think that when the impact occurred, the land in that part of Greenland was part of a mountain belt that was forming. The rocks in that area would have already been very, very hot, and of course, the mountain-building process would have continued after the formation of the crater. The microstructures would be squeezed by the mountain-building, and the quartz would have lost its perfect straightness. At other craters, there has been no subsequent deformation or mountain-building, so beautiful, perfectly straight planes are present."
Despite the challenges that the team encountered in establishing the Maniitsoq site as an impact crater, Dr McDonald believes that their findings could lead to the discovery of other, previously disregarded craters.
"The straightness of cracks in quartz crystals has been used as a gatekeeper mechanism," he explained. "It remains one of the best criteria that we have for distinguishing impact craters from terrestrial processes. In the past, researchers have looked at potential candidates for old craters and have ended their investigations after finding that their quartz cracks are not perfectly straight. I don’t think that we would have been successful in convincing our peers of the Maniitsoq crater’s validity, if it wasn’t for the additional evidence that we had collected.
"We hope that geologists will be inspired to go back and to look at sites where perhaps, in the past, their likelihood of being an impact crater has been rejected. This approach could be used to identify another ancient impact structure that we haven’t yet recognised."
I concluded our conversation by asking Dr McDonald about the further research that he hopes to conduct around this subject.
"Whenever craters form, they eject molten rock that falls back onto the surface in the form of impact spherules," he explained. "Both in Australia and at other sites in South Africa, scientists have found layers of sediment containing impact spherules that are older than those at Vredefort. These might be areas of interest for other researchers. At Maniitsoq, we’re confident that we have identified a big crater, so it would be great if we were able to locate some of its impact spherules."