Earth's oldest impact crater found in Greenland

You'd have to go to the moon or Mars to find a bigger impact crater than the one discovered in Greenland.

An asteroid 30 kilometres across smashed into Greenland three billion years ago, creating a crater that was once 25 kilometres deep and 600 kilometres wide.

That dwarfs the oldest known impact crater on Earth, the Vredefort crater in South Africa, both in age and size. The Vredefort is about two billion years old and about 300 kilometres across.

The new find is centred to the east of what is now the town of Maniitsoq on Greenland's western coastline. What remains of the original crater is only about 100 kilometres wide.

This, according to the geologists who discovered the crater, explains why it remained undiscovered until now. Only the deepest parts of the crater survive. All the near-surface and easily recognisable features of the crater have been worn away. "The rocks we see today were about 25 kilometres down when the impact occurred," says Adam Garde of the Geological Survey of Denmark and Greenland in Copenhagen, Denmark, who led the team.

After three years of intensive work, Garde and his colleagues now say they have enough evidence to support their claim.

The most compelling evidence is the presence of granite-like rocks that are crushed, melted and pulverised in a way that can only be explained by a sudden, massive impact. The deformed granite is spread throughout an area measuring 35 by 50 kilometres, centred on the supposed impact site.

Such large-scale deformation of granite could not have happened over such a large area through any known terrestrial geologic process. "You might see something similar in a geologic fault zone, but not in a circle 100 kilometres across," says team member Iain McDonald of Cardiff University.

Also, the quartz deposits in the impact zone have micro-cracks and flaws that are fairly consistent with those found in quartz from known impact sites. When the researchers measured the angles between the cracks and the orientation of each quartz crystal, the patterns broadly matched those of an impact shockwave, not the random patterns formed by known terrestrial processes of tectonic pushing and pulling.

"The patterns conformed to what you would expect from impacts, not from those that happen through terrestrial geology," says McDonald.

Not everyone is completely convinced that the geologists have found an impact crater. "It points towards it being a crater, but frustratingly, I don't know if it will ever be proven," says John Spray, a specialist in impact science at the University of New Brunswick in Fredericton, Canada, who is familiar with the work. "On the other hand, I don't think anyone can definitively say it isn't a crater."

Nonetheless, Spray is pleased. "I think they've done a great job, and no-one could probably have done more," he says. "Also, it stimulates impact specialists like myself to look for other extremely old impacts ? there are probably multiple impact points on earth, but we don't recognise them."

The claimed impact occurred 2.4 billion years before the Cambrian "explosion" of life 580 million years ago. The asteroid impact that wiped out the dinosaurs was even more recent, occurring 65 million years ago.

When the alleged asteroid struck Greenland, the Earth's only inhabitants were algae and cyanobacteria, so its effects on evolution are unknowable. If it struck today, it would probably wipe out much of life on Earth.

Journal reference: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2012.04.026

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