Boulders Don’t Just Roll. They Bounce.

https://www.nytimes.com/2019/12/18/science/bouncing-boulders-atacama-desert.html

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There’s a place in Chile’s Atacama Desert where trails of depressions punctuate the fine chusca dust. But what might seem like the footsteps left by a giant creature are in fact exquisitely preserved evidence of boulders that tumbled down a nearby cliff face before bouncing to their final resting place.

The site, the Chuculay Boulder Field, is home to thousands of granite goliaths, some as big as houses. And because the desert’s hyper-arid conditions preserve the boulders’ steps, it’s “an ideal place to study rockfall theory and physics,” said Paul Morgan, a geologist at Cornell University.

Mr. Morgan and his collaborators analyzed the trajectories of some of these boulders and presented their research last week at the American Geophysical Union conference in San Francisco. Their findings of how far boulders tumble are useful for designing structures that could protect people and property in rockfall-prone areas.

In July 2018, Mr. Morgan and his collaborators from Cornell and Chile’s Universidad Católica del Norte pitched tents amid the granite giants of Chuculay. A 1,000-foot-high scarp, a geological feature created by a tectonic fault, towered nearby. The site’s rocks probably tumbled down from that scarp during one of the numerous earthquakes experienced in tectonically active Chile, the researchers hypothesize.

While most landscapes on Earth are continually changing, the Atacama Desert is different. It’s over 30 times drier than California’s Death Valley. Without regular rainfall to drive erosion, if something happens, the evidence tends to stick around.

To map the boulders and the scarp in three dimensions, the research team scanned them with quadcopter drones equipped with cameras. They also scrambled up the scarp one day to get their own view.

“It was on the edge of safe,” Mr. Morgan said.

The scientists cataloged the sizes and locations of hundreds of boulders larger than roughly 6 feet in diameter. It made intuitive sense that they found the most boulders beneath the most jagged sections of the scarp.

“The rougher the scarp, the more likely it is to generate multiple rockfalls,” Mr. Morgan said.

They also mapped the trails of impact craters that led back toward the scarp, records of the rocks’ trajectories for 32 boulders. Some of the boulders bounced as many as 25 times before coming to a rest, and certain rocks left behind depressions up to 20 inches deep.

When the researchers analyzed the locations of the impact craters, they were surprised. They had expected that the distances between successive depressions would decrease as the bouncing rocks lost energy. “But sometimes there was a short bounce followed by a long bounce,” said Mr. Morgan.

One explanation is that natural variations in the properties of the desert floor — its slope and composition, for instance — affected how the rocks bounced. Another possibility is that some of the boulders broke apart, and that the craters left behind recorded the bounces of different fragments.

The researchers also found that many of the boulders didn’t simply tumble in a straight line. About a quarter of the rocks bounced sideways and came to rest more than 30 degrees away from where they left the scarp. This could also be explained by differences in the underlying ground, or irregularities in a boulder’s shape — they aren’t perfect spheres.

These results are valuable for designing structures like fences and berms in places where rockfalls are common, said Jeffrey Moore, a geoscientist at the University of Utah, who was not involved in the research.

“We want to know where the boulders are going to wind up,” he said.

Mr. Morgan and his colleagues are continuing to analyze the Atacama’s bouncing boulders. There’s no shortage of research questions, said Richard Allmendinger, a structural geologist at Cornell and Mr. Morgan’s adviser.

“We started studying them simply because we didn’t understand them.”