In Racetrack Playa in Death Valley, California, hundreds of rocks — some weighing as much as 700 pounds — seem to have been dragged across the ground, leaving synchronized trails that can stretch for hundreds of meters. Though many phenomena were speculated (hurricane-force winds, dust devils, slick algal films, thick sheets of ice), no one knew what caused the movement.
To solve the mystery, in 2011 a team of researchers led by paleobiologist Richard Norris, Scripps Institution of Oceanography, UC San Diego, began monitoring the rocks remotely. The research team fit 15 similar rocks with custom-built, motion-activated GPS units (the National Park Service disallowed use of native rocks) and installed a high-resolution weather station capable of measuring gusts to one-second intervals. Then — in what Ralph Lorenz of the Applied Physics Laboratory at the Johns Hopkins University suspected would be “the most boring experiment ever” — the researchers waited for something to happen.
In December 2013, Richard Norris and co-author and cousin Jim Norris discovered that the playa was covered with a pond of water three inches deep. Shortly after, the rocks began moving.
“Science sometimes has an element of luck,” Richard Norris said. “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.”
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 December 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!’”
The rocks moved under light winds of about 3-5 meters per second (10 miles per hour) and were driven by ice less than 3-5 millimeters (0.25 inches) thick, a measure too thin to grip large rocks and lift them off the playa, which several papers had proposed as a mechanism to reduce friction. Further, the rocks moved only a few inches per second (2-6 meters per minute), a speed that is almost imperceptible at a distance and without stationary reference points.
“It’s possible that tourists have actually seen this happening without realizing it,” said Jim Norris. “It is really tough to gauge that a rock is in motion if all the rocks around it are also moving.”
Individual rocks remained in motion for anywhere from a few seconds to 16 minutes. In one event, the researchers observed rocks three football fields apart began moving simultaneously and traveled over 60 meters (200 feet) before stopping. Rocks often moved multiple times before reaching their final resting place. The researchers also observed rock-less trails formed by grounding ice panels — features that the Park Service had previously suspected were the result of tourists stealing rocks.
“The last suspected movement was in 2006, and so rocks may move only about one millionth of the time,” Lorenz said. “There is also evidence that the frequency of rock movement, which seems to require cold nights to form ice, may have declined since the 1970s due to climate change.”
The team’s findings were published in the journal PLOS ONE on August 27.