DEATH VALLEY — The moving stones and accompanying trails of the Racetrack Playa have puzzled onlookers for decades, but researchers say they solved the mystery using GPS devices and weather equipment.
Numerous theories have circulated about how the sliding stones in Death Valley move, ranging from hurricane-force winds to supernatural causes. Some of the stones weigh as much as 700 pounds and while movement has been documented, no one had observed the phenomenon in action to pinpoint its cause until recently.
Researchers determined the stone’s movement is driven by the rare formation of thin ice on a shallow pond that breaks up during the day, according to a study released Wednesday by the Scripps Institution of Oceanography at UC San Diego. Ponds in Death Valley are uncommon, so rocks may go a decade or more without moving, researchers said.
“This happens in the middle of the day when the sun is shining and it is just simply beautiful out here,” researcher Richard Norris said in a video. “We had thought before we began to study the racetrack phenomenon that this must happen during howling winds at times of really miserable weather when you wouldn’t want to be out here, and that was why the phenomenon had never been observed before — because the conditions were simply too nasty for anyone to be willingly out here.”
Researchers brought their own stones with GPS units embedded into them to the playa and installed a weather station to test their theories. Then they waited for two years to see what happened.
One of the researchers said he thought waiting for rocks to do something could be “the most boring experiment ever,” but eventually the stones started moving when a shallow pond was created by snowfall on the playa.
“Science sometimes has an element of luck,” Norris said in a statement. “We expected to wait five or 10 years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person."
Thin sheets of broken “windowpane” ice, propelled by light winds, pushed the rocks up to 15 feet on sunny days. Researchers said it is possible tourists and others have observed the movement before, but didn’t realize it because multiple rocks can move at the same time and there are few stationary reference points.
The study was published in the journal PLOS ONE.