There's a place in Death Valley National Park where a mystery that has puzzled scientists and park visitors for decades finally has been solved.
Across a dry lake in the park known as the Racetrack Playa, hundreds of rocks scattered along the ground -- many weighing up to several hundred pounds -- seem to move all on their own. They leave behind long, winding trails in the lakebed, evidence that something has been pushing or sliding them across.
Researchers have guessed at the forces moving them since the rocks were discovered nearly a century ago, but no one had seen them move until last December, when a team of scientists led by Richard Norris, a paleobiologist with the Scripps Institution of Oceanography, caught them on camera.
In a study published Aug. 27 in the online scientific journal PLOS ONE, Norris reports on his team's two-year-long effort to photograph and track the movement of the stones, and the amazing luck they had in actually seeing them move.
"Science sometimes has an element of luck," Norris said in a news release. "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."
Norris and his team began their experiment in the winter of 2011, when they devised a plan to monitor the rocks' movement with a high-resolution weather station that could measure wind gusts in very small time frames. They also fitted 15 rocks -- which they brought from outside their park, as National Park Service officials wouldn't allow them to move any of the rocks already there -- with GPS devices that tracked their motion.
Next, they waited for something to happen, what study co-author Ralph Lorenz of the Applied Physics Laboratory at Johns Hopkins University said he thought would be "the most boring experiment ever."
When the team visited Racetrack Playa in December 2013, they found it covered in a pond of water about three inches deep. And then they noticed something interesting: the rocks began moving.
Here's how the Scripps Institution of Oceanography described it in their news release:
"Their observations show that moving the rocks requires a rare combination of events. First, the playa fills with water, which must be deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks.
"As nighttime temperatures plummet, the pond freezes to form thin sheets of "windowpane" ice, which must be thin enough to move freely but thick enough to maintain strength. On sunny days, the ice begins to melt and break up into large floating panels, which light winds drive across the playa, pushing rocks in front of them and leaving trails in the soft mud below the surface.
On Dec. 21, Norris and his study co-author Jim Norris said they heard "popping and cracking sounds coming from all over the frozen pond surface," according to Scripps, adding: "I said to Jim, 'This is it!'
Thanks to being able to witness the rocks' movement in real time, it became clear why no one had reported seeing them move in the past. The rocks moved only a few inches every second, a speed that would be virtually impossible to notice at a distance.
"It's possible that tourists have actually seen this happening without realizing it," Jim Norris said in an interview with Scripps.
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