Weekend Argus (Saturday Edition)
New data on Vredefort, the biggest impact crater on the planet
JOHANNESBURG would have gone first, then Pretoria followed by the Limpopo town of Bela Bela, all smashed by a mega explosion like never seen before.
Fortunately this mega explosion happened long before humans and their cities arose and even before complex life began. It happened two billion years ago when an asteroid slammed into what is now Vredefort in the Free State.
What it left is the biggest known crater on the planet.
Scientists have known for a long time that the crater was big, stretching over a 100km, but now it seems it is even bigger.
And from new research, scientists are hoping that the Vredefort impactor might one day help in spotting and preventing the next big asteroid strike.
It had been thought the object that struck the Vredefort Dome area was probably about 15km in diameter and hit the earth travelling at the speed of 15km/s. What it left was a crater 175km in diameter.
Now simulations have led researchers from the University of Rochester in the US to believe the impactor was 20km in diameter and the crater 250 to 280km in size.
This is far larger than the asteroid that killed off the dinosaurs 66 million years ago and formed the Chicxulub crater, off the coast of Mexico.
“Unlike the Chicxulub impact, the Vredefort impact did not leave a record of mass extinction or forest fires given that there were only single-cell lifeforms and no trees existed two billion years ago,” Miki Nakajima, an assistant professor of Earth and environmental sciences at the University of Rochester said in a statement.
“However, the impact would have affected the global climate potentially more extensively than the Chicxulub impact did.”
The research was recently published in the Journal of Geophysical Research and the main author was then-undergraduate student Natalie Allen. She quickly discovered that she had her work cut out for her. The problem was the amount of erosion that had happened over the last two billion years, which made defining the size of the crater difficult.
“We actually used a lot of evidence from inside the crater itself. Teams for over the last 100 years or so have collected rock samples from inside the crater and these are very interesting.
“We call them metamorphic features and they are evidence of really high pressures and temperatures. Things like shatter cones are a very famous example of them and they are large radial structures that go out from the source of a large pressure event like what happens with nuclear explosions or impacts like this.”
With this new geological evidence and measurements, the researchers could, through simulations, work out the likely size of the asteroid that made the crater.
Increasingly, efforts have been made to understand killer asteroids and find them before they strike Earth.
“Understanding the largest impact structure that we have on Earth is critical,” said Allen.
“Having access to the information provided by a structure like the Vredefort crater is a great opportunity to test our model and our understanding of the geologic evidence so we can better understand impacts on Earth and beyond.”