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August 29, 2014

Scientists Solve the Case of the Sliding Stones

Image showing rock trails
Rock trails: View is looking north on December 20, 2013. A steady, light wind (4–5 meters per second) has blown water to the northeast, exposing newly formed rock trails. The lower image shows an overlay of lines to emphasize the congruent shape of adjacent rock trails as well as the proximity of rock trails to rocks that did not move. The image has not been enhanced. (Image credit: PLOS One)

Looks like the mystery of the sliding stones has been solved.

The improbable movement of massive rocks across the flat, usually dry mud of Racetrack Playa in Death Valley National Park, California, has been the subject of speculation for almost a century. Theories behind the movements ranged from hurricane-force winds pushing stones over slick mud, to thick ice sheets carrying the rocks across the desert—but the rare phenomenon had never been directly observed.

That is, until a team that includes APL scientist Ralph Lorenz got word in late 2013 that the rocks were once again on the move. The team rushed to the scene, where several instruments (including cameras, a weather station, and fake rocks outfitted with GPS transmitters) were already in place. As they watched water and thin ice from a recent, melting snowfall move across the area, the researchers found their answer: dozens of rocks moving at about five centimeters per second, at different times, pushed by a breeze across thin, transparent layers of ice.

Later, when the ice melted and winds dried the water, the rocks were left at the ends of long trails that traced their movement across the mud. Lorenz actually captured footage of the freshly formed trails still underwater, when they were inaccessible by foot and mostly invisible from shore, by flying a camera-equipped kite over the scene.

“We had seen it happen with our own eyes, but more importantly we had before-and-after pictures, measurements of the movement speed, and good local weather data’” Lorenz said. “And just like that, we had solved the mystery.”

The research team, which along with Lorenz includes Richard Norris of the Scripps Institution of Oceanography, James Norris and Jib Ray of Interwoof, and Brian Jackson of Boise State University, published its findings this week in Earth Surface Dynamics Discussions and PLOS One.

Time-lapse images of a moving rock
Time-lapse look at a moving rock: This image was acquired with a handheld digital camera on January 9, 2014. The image on the left shows the wide-angle view; the interior black frame indicates the view in other frames. In close-up frames, the blue arrows show stationary rocks and the red arrow shows a rock in motion (moving from left to right). Total movement lasted about 18 seconds. Dark, flat areas on the pond are panels of approximately 3-millimeter-thick ice surrounded by rippled water several centimeters deep. Ice thickness was estimated from inshore ice panels. Broken ice panels accumulated on the upstream side of the moving rock in the last two images. Images have been cropped but not otherwise edited. (Image credit: PLOS One)

Media contact: Michael Buckley, 240-228-7536, michael.buckley@jhuapl.edu