Studying the Earth’s earliest history is a tough job. Information about it is buried under layers of younger sediments that have accumulated over billions of years. The article, published in the Journal of Geophysical Research: Planets, shows that scientists have to deal with yet another thief of the geological past – time.
Scientists from South Africa, the UK and the US set out to find out why we haven’t found craters older than about two billion years on Earth, when there are other traces of older asteroid impacts. Erosion processes and geological activity are responsible for “flashing them to the ground”, but scientists were interested in how quickly they can engulf a gigantic impact structure.
To find the answer, they looked at the Vredefort impact crater in South Africa. It is about 300 kilometers in diameter and was formed about two billion years ago, making it the second oldest on Earth. The impact was so strong that just before the space rock hit the ground, the mantle and crust of the Earth lifted up to form a gigantic dome. Today, however, it is merely a low hill. Matthew Huber of the University of the Western Cape in South Africa, the lead author of the study, explained that Vredefort is one of the last structures from that period that can be studied today, and only the deepest fragments remain.
“It’s a stroke of luck that these old structures have survived at all,” he said. – There are many questions that we could answer if we could look at the older craters. But that’s how geology is, we have to make history out of what’s available.
Ten kilometers of erosion
The results of the analysis of the samples and the modeling of the impact turned out not to be kind to the seekers of ancient craters. Although the Vredefort region retained remnants of rocks and minerals transformed by the asteroid impact, most of the structure was made of rocks indistinguishable from those surrounding the former crater. Scientists have estimated that over the past two billion years, erosion has “eaten” a layer of ground as much as 10 kilometers thick.
“It took us a while to really understand the data,” Huber explained. – Ten kilometers of erosion and all geophysical evidence of the impact simply disappears, even for the largest craters.
Scientists explained that this was the best time to observe Vredefort – if the erosion processes intensify, the impact structure will completely disappear. That’s why the odds of finding buried craters more than two billion years old are low.
– To an Archean impact crater [najstarszy eon w dziejach Ziemi, trwający od 4 do 2,5 mld lat temu – przyp. red.] has survived to this day, it would have to have conditions for it. However, the earth is full of unique places, so maybe something unexpected is hiding somewhere, Huber added.
American Geophysical Union
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