Deep drilling reveals how impact crater's hidden ring formed

by Mary Caperton Morton
Tuesday, January 15, 2019

When the 15-kilometer-wide Chicxulub meteorite slammed into what is now Mexico’s Yucatán Peninsula 66 million years ago, it was moving at more than 20 kilometers per second. The impact blasted a hole 200 kilometers wide and more than 30 kilometers deep in Earth’s surface. The forces involved in such impacts are colossal — many orders of magnitude greater than the largest human-made explosions — and scientists have traditionally relied on models to explain what happens in the moments after impact. But in a new study looking at shocked rocks retrieved from the depths of the buried Chicxulub Crater, scientists have determined how the crater’s “peak ring” formed in mere minutes.

When a large meteorite hits Earth, the cratering process occurs in a series of stages. A transient crater first forms, lasting just seconds before the steep walls collapse inward. Then, as the middle of the crater collapses outward, the crater margins move inward; where these two masses of rock meet, “there is a space problem,” so the rocks pile up, making what’s called a peak ring, says Ulrich Riller, a structural geologist at the University of Hamburg in Germany and lead author of the new study in Nature.

Peak rings can tower hundreds of meters above the crater floor. Such rings are seen in lunar craters on the moon, as well as on other planetary bodies, sometimes as multiple concentric rings in a single crater. Their formation is dependent on the size of the crater, Riller says. Chicxulub is the third-largest confirmed meteorite crater on Earth and the only crater with an intact peak ring. The only other craters on Earth large enough to host peak rings — Vredefort in South Africa and Sudbury in Canada — are too eroded to preserve the structures.

For peak rings to form, “the rock needs to be very fluid to move so quickly, but it must get very rigid again to sustain the shape of the elevated rings. And all of that takes place within less than 10 minutes,” Riller says. “It’s almost incomprehensible to think about such huge volumes of rock moving so quickly.”

Chicxulub’s peak ring is not visible at the surface because the crater is buried under 600 meters of sediment. Unfortunately, “we can’t see or touch the crater,” Riller says, but “the sediments have kept the internal structure pristine.” In 2016, the International Ocean Discovery Program and the International Continental Scientific Drilling Program drilled into the crater to a depth of 1,335 meters, extracting core samples of rock affected by the impact.

Riller and his colleagues looked for evidence of weakening and strengthening in the drill cores, finding numerous zones of deformation called cataclasites, where rock was smashed into sand-sized particles, bordering domains of less-deformed rock. “The idea is that these rock domains are bouncing against each other, thereby crushing the rocks at the borders,” Riller says. And the force of the impact generates high-frequency vibrations that cause the rock to temporarily behave like a fluid, he says, in a process called acoustic rock fluidization.

The team also found numerous fault zones cutting across the cataclasites. “That tells us that immediately after this bouncing of blocks is over, the rocks regain strength enough to undergo thrust faulting,” Riller says. “So we see a clear progression of weakening and then restrengthening that has never been documented before.”

The observations offer new insight into the formation of peak rings in impact craters, says Christian Koeberl, a geochemist and impact specialist at the University of Vienna in Austria, who was not involved in the new study. “Several models have been proposed to explain the formation of peak rings, but I’d say that acoustic fluidization makes the most sense in many respects,” he says. Other mechanisms invoke extreme heat or large fault movements after impacts to explain rapid rock deformation. “Each model has something going for it. The models tell us what is physically possible, but not what really happens,” Koeberl says. “This study is one of the first opportunities we’ve had to get the data we need to sort through the models.”

The next step will be to use the physical data to improve the models. “The geological observations can help to refine the physical properties that are [input into] the models,” Riller says. “Then we can ask next-order questions like, ‘What was the timing of these events? What happens in what order?'”

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