The mystery of rocks in California's Death Valley that seem to move on their own has been solved at last, say scientists who've captured the "sailing stones" in action -- thanks to ice, water and wind.
Researchers have long tried to determine what made the rocks move and leave long trails across the valley floor, with theories ranging from strong winds or the Earth's magnetic field to possible pranksters. Some people have even held that the wandering stones were the work of aliens.
Researchers have puzzled over the phenomenon since 1941, but had never been able to catch the moving stones "in the act" -- until now.
Two years ago a team led by scientists from the Scripps Institution of Oceanography attached GPS units to some of the rocks on Racetrack Playa, home of the sailing stones.
Then they sat back and waited, only to find their patience rewarded much sooner than they had thought possible.
"We expected to wait five or ten 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," Scripps paleobiology professor Richard Norris says.
So how do the rocks travel?
The phenomenon takes a set of precise conditions that occur only rarely, the researchers say.
The playa on which the rocks sit must be flooded to a shallow depth, just enough for a thin "windowpane" of ice to form around the base of rocks.
This requires a rare winter storm, nights of sub-freezing temperatures followed by sunny days with winds.
As the sun melts the ice and it breaks up into "windowpanes," the winds can push the rocks along on their frozen "feet," sliding them through melting mush for hundreds of yards.
Richard Norris and his cousin James Morris, who works for the firm providing the GPS units, happened to be on hand and captured images of the moving rocks, which can traverse the playa at around 10 mph, changing course as the wind direction changes.
"On Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface," said Richard Norris. "I said to Jim, 'This is it!'"
Individual rocks, some weighing as much as 700 pounds, might move for just a few seconds or could travel along the flat, friction-free playa surface for as long as 16 minutes, he said.
