Earth's oldest and largest craters are slowly disappearing

Disappearing craters

“It’s almost a fluke that the old structures we do have are preserved at all,” said Matthew S. Huber, a planetary scientist at the University of the Western Cape in South Africa who studies impact structures and the lead author of the study. “There are a lot of questions we’d be able to answer if we had those older craters. But that’s the normal story in geology. We have to make a story out of what’s available.”

Buried or hidden craters can be detected using geophysical tools like seismic imaging or gravity mapping. And to confirm their existence, geologists then search for physical elements like ejecta and impact minerals to study them further.

Huber and his team of scientists wanted to find out how fast these craters were disappearing due to land erosion before their last remnants could be unearthed.

The answer lay in South Africa

So the team went to South Africa to study the world’s oldest and largest known impact structure to have ever formed. According to NASA, about two billion years ago, an asteroid measuring around 10 kilometers across hurtled toward Earth and made a 62-mile-wide (100-kilometer) dent on its surface. The impact occurred southwest of what is now Johannesburg, South Africa.

To find out, the researchers dug into the structure. Over the years, all that remains of the impact structure is a semicircle of low hills and rock formations. 

“That pattern is one of the last geophysical signatures that is still detectable, and that only happens for the largest-scale impact structures,” Huber said. 

“Erosion makes these structures disappear from the top down,” Huber added. “So we went from the bottom up.”

The researchers took sample rock cores and analyzed their physical properties. They found that while some ejecta and impact minerals remained, the rocks in the outer ridges of the Vredefort structure were different from the non-impact rocks around them, said the press release.

“That was not exactly the result we were expecting,” Huber said. “The difference, where there was any, was incredibly muted. It took us a while to really make sense of the data. Ten kilometers of erosion and all the geophysical evidence of the impact just disappears, even with the largest craters.”

If more erosion takes place in the next couple of years, the impact structure will totally disappear.

“In order to have an Archean impact crater preserved until today, it would have to have experienced really unusual conditions of preservation,” Huber said. “But then, Earth is full of unusual conditions. So maybe there’s something unexpected somewhere, and so we keep looking.”

The study was published in the Journal of Geophysical Research Planets.

Study abstract:

The record of terrestrial impact events is incomplete with no Archean impact structures discovered, despite the expected abundance of collisions that must have occurred. Because no Archean impact structures have been identified, the necessary conditions to preserve an impact structure longer than 2 Byr are unknown. One significant effect of shock metamorphism is that the physical properties of the target rocks change, resulting in distinctive geophysical signatures of impact structures. To evaluate the preservation potential of impact structures, we evaluate the deeply eroded Proterozoic Vredefort impact structure to examine the changes in physical properties and the remnant of the geophysical signature and compare the results with the well-preserved Chicxulub impact structure. The major structural features of Vredefort are similar to the expected profile of the Chicxulub structure at a depth of 8–10 km. The Vredefort target rocks, while shocked, do not preserve measurable changes in their physical properties. The gravity signature of the impact structure is minor and is controlled by the remnant of the collapsed transient crater rim and the uplifted Moho surface. We anticipate that erosion of the Vredefort structure by an additional 1 km would remove evidence of impact, and regardless of initial size, erosion by >10 km would result in the removal of most of the evidence for any impact structure from the geological record. This study demonstrates that the identification of geologically old (i.e., Archean) impact structures is limited by a lack of geophysical signatures associated with deeply eroded craters.