A solution to the longstanding mystery of why rocks move erratically across an isolated patch of California's Death Valley has finally emerged as researchers published a study showing the driving force was sheets of wind-driven ice.
Trails from the movement of the rocks, which show them changing direction suddenly in their movement across the so-called Racetrack Playa, have long befuddled scientists and the general public. People wondered: How were the rocks moved?
Paleobiologist Richard Norris of the Scripps Institution of Oceanography, who led the study, saw the rare phenomenon first-hand last December while standing with his cousin, engineer James Norris, at the spot.
They recently published their findings in the journal PLOS One, showing that even though the stones can sit for a decade or more without moving, on certain occasions they go on a slow trip that results from an unusual combination of ice and wind in an area normally known for scorching hot temperatures.
That happens when the dry lake bed they are in freezes over with a thin layer of ice which then breaks apart in a light wind, sending large sheets of ice against the rocks with enough force to move them a few yards per minute, Norris said.
Because of the ability of the large ice sheets to catch the wind, and aided by the underlying flow of water, the rocks, which weigh as much as 700 pounds (318 kg), are pushed along in a way that could not occur from the force of the wind alone, he said.
A scientific theory dating back to the 1950s had suggested that thick ice and heavy winds could be behind the movement of the rocks, but the study found the ice is far thinner and the wind much lighter than first thought.
Popular theories for what drives the rocks have ranged from a sudden tilting of the Earth to the action of giant magnets under the surface of the ground.
"I think it has massive popular appeal because it is one of these things that's very widely known about but kind of marveled at,” Norris said.
Norris said other people might have seen the rare phenomenon of wind-driven ice pushing the Death Valley rocks, but they probably did not understand what they were seeing because it happens so slowly and the trail the rocks leave is obscured until the water dries out.
His team used motion-activated GPS units built into rocks and also cameras to document the phenomenon in preparation for publishing their findings.