Two new studies published in the journal Geology have cast light on the mysteries of Earth’s largest and oldest known crater.
The Vredefort Impact Crater, which is located in South Africa, is thought to have formed when a 10 km wide meteor collided with earth about 2 billion years ago, creating a crater 300 km across. The impact imparted so much energy to the earth that the rocks within the crater melted, forming a sea of lava.
At least that’s the theory – but geologists have been unable to produce any actual evidence of this enormous lava sea.
The difficulties are due to the fact that about 10 kilometers of rock have been eroded in the two billion years since the impact, leaving only the deepest “roots” of the crater intact. (take a look at the remnants on Google maps here)
Despite these challenges, one of the new studies published in Geology may finally provide the elusive evidence the scientists have long sought.
Desmond Mosor and his colleagues were doing some geochronology work when they stumbled upon an exciting find – rocks from a magma dike containing zircons dating from the time of the impact. Since zircons can only form at high temperatures – and would have been damaged by an impact event – the undamaged crystals indicate that the magma was associated with the impact. They are, the team argues, part of the original lava sea.
Additional evidence of the magnitude of the impact was found and reported by a separate team working 10,000 kilometres away in Karelia, Russia, which, given the arrangement of the continents at the time of the impact, was only about 2,500 km away from the crater centre.
Matthew Huber, one of the lead authors of this second paper, happened to notice that some rock samples described as limestone ooids by geologists looked suspiciously like spherules – streamlined pieces of volcanic glass representing molten material hurled into space by the impact. After examining the specimens in person, Huber’s team became convinced that the spherules were related to the impact. The range of ages suggested by attempts to date the spherules includes the suspected time of the impact. As Huber noted, correlation doesn’t always imply causation.
However, spherules are thought to form only as the result of impacts, and geologists have yet to find evidence of another crater from that era of geologic time.
The fact that Vredefort crater, which has been studied for almost a century, is still revealing its secrets just shows how much we still don’t know about the earth.
For another take on the discoveries, check out this article)