In Death Valley, there's a flat, remote area called Racetrack Playa with a truly bizarre natural phenomenon: hundreds of stones that appear to gradually move themselves over time.
Each of the stones drifts a few inches per year, leaving a trail etched into the dusty ground behind it. Scientists have studied these "sailing stones" since the 1940s, proposing a range of mechanisms — including dust devils, flooding, and large ice sheets — that might be responsible.
Most recently, in 2011, Ralph Lorenz, a physicist at Johns Hopkins, suggested that small rafts of ice occasionally formed around the stones, lifting them up off the ground and allowing them to be blown by gentle breezes.
But now, Lorenz and a few other researchers have used time-lapse cameras and GPS sensors attached to the rocks to directly observe the stones moving for the first time — and in a paper published Wednesday in PLOS ONE, they explain that the real answer involves vast sheets of ice.
How the sailing stones move
It turns out that a number of weather conditions have to align perfectly to lead to the stones moving. First, the playa needs to get drenched by a few inches of rain — something that happens only a few times per year in the arid region.
Next, nighttime temperatures need to dip below freezing, so the whole playa is coated in a thin sheet of ice. The following day, warmer temperatures and a steady wind need to break the sheet into large panes, floating atop a layer of water.
When these ice sheets are blown into the rocks, the researchers found, they can slowly push the stones across the watery mud, at speeds somewhere between between 5 and 15 feet a minute, for durations of 15 minutes or less.
"It's possible that tourists have actually seen this happening without realizing it," said Jim Norris, one of the paper's authors, told Phys.org. "It is really tough to gauge that a rock is in motion if all the rocks around it are also moving."
It's also extremely rare for all these conditions to align: the researchers estimate it only happens a few time per year. But when it does happen, large sheets of ice can push many rocks at a time, explaining why in some cases, different trails are strikingly parallel.
In a single movement event on December 20, 2013, more than 60 different rocks moved on the same day. And what's remarkable is that even though the researchers were only on hand for a few weeks of the multi-year experiment, they were lucky enough to arrive just a few days before the rocks started moving.
Here's a sped-up time lapse of one rock moving on January 9th, made from a video released by the researchers. It can be a bit tough to make out, but there's a stone that starts at the center of the frame and is pushed from left to right by a large sheet of ice (which looks darker than the rippling water), then gets left behind as the ice pane keeps moving: