For more than 50 years, scientists have puzzled over rocks that seemed to move across the dry landscape of Racetrack Playa in Death Valley National Park. They could see their trails but only speculated how they might have moved from point A to point B, not seeing one actually in the act of transit.
The "wandering stones" were known to move because they left behind trails. But researchers finally were able to record their actual movement and find the mechanism behind it. (Image source: Arno Gourdol/Flickr)
Researchers recently recorded the first observation of the rocks moving using GPS and time-lapsed cameras.
Initially, Ralph Lorenz with Johns Hopkins University Applied Physics Laboratory told Nature he thought watching rocks with GPS would be "the most boring experiment in the history of science." He was pleasantly surprised.
While scientists previously speculated that these "wandering stones" were aided by high wind, water or ice, they now have a more specific answer.
"In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3 to 6 mm, 'windowpane' ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of [about] 4–5 [meters per second]," the study authors wrote in the abstract published in the journal PLOS One. "Floating ice panels 10 s of meters in size push multiple rocks at low speeds of 2–5 m/min. along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice."
Scientists were actually on hand to witness the rocks moving in December 2013, when they trekked out to the park to check the equipment. Nature reported that when they arrived, they found a pond covered with ice. After a few days, the ice began to melt.
“At 11:37 a.m., very abruptly, there was a pop-pop-crackle all over the place in front of us — and I said to my cousin, ‘This is it,'" Richard Norris with Scripps Institution of Oceanography told Nature.
Slowly but surely, the moving ice pushed the rocks. The pace, however, was too slow to see with the naked eye, according to Nature.
“A baby can get going a lot faster than your average rock,” Norris said.
After some time though, the iconic trails left behind the rocks were visible — and their movement was captured on cameras.
Watch some of the researchers' footage of the phenomenon:
Lorenz told Nature these findings are "transformative."
“It’s not just an anecdotal report, but we have before and after pictures, and meteorological information simultaneous with the event," he said.