Racetrack Playa researcher Richard Norris standing by a trail likely formed more than a decade before this December 16, 2012 photo. Trails can last for years or decades between events. Photo from Richard Norris courtesy of Scripps Oceanography

The phenomenon of the “sailing stones” on Racetrack Playa in Death Valley National Park has baffled scientists for decades.

By some mysterious force of nature, rocks move along the flat-as-a-pancake playa and leave long trails behind. What causes the stones to move?

Parallel trails carved into the wet, mud-cracked surface of Racetrack Playa in Death Valley. Photo by Jim Norris courtesy of Scripps Oceanography

One popular theory was that strong winter winds upward to 90 mph combined with just enough rain to make the clay slippery caused the stones to “sail.”

Another is that ice sheets pick up the rocks, or ice forms around the rock enabling it to move with the wind, leaving a series of rock trails.

But now, the mystery is solved.

Scientists can say conclusively that these synchronized trails left by rocks, some up to 700 pounds, are caused by thin sheets of ice pushing the rocks across the desert floor under certain conditions, a theory that had been previously dismissed in 1976 after a test.

The conclusion was reached by a team led by paleobiologist Richard Norris of the Scripps Institution of Oceanography, UC San Diego, with the results published Wednesday in the journal PLOS ONE.

Scripps Oceanography details the phenomenon in this six-minute video (it also illustrated the event on a whiteboard):

As part of the Slithering Stones Research Initiative, researchers custom built motion-activated GPS units and fitted them into 15 rocks and placed them on the playa in the winter of 2011, with permission from the National Park Service. They expected it would take five to 10 years before something happened.

A GPS tracking unit was fitted into 15 rocks that were placed on the Racetrack Playa. Photo by Richard Norris courtesy of Scripps Oceanography

Ralph Lorenz, one of the paper’s authors from Applied Physics Laboratory at John Hopkins University, called it “the most boring experiment ever.”

But in December 2013, something happened.

“Science sometimes has an element of luck,” Norris said. “Only two years into the project, we just happened to be there at the right time to see it happen in person.”

Three inches of water covered the playa and shortly after their arrival, rocks began moving. The study showed that sailing rocks require a rare combination of these events:

1. The playa fills with water deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks. 2. As overnight temperatures drop, the pond freezes to form thin sheets of “windowpane” ice. 3. When the sun comes out, the ice begins melting and breaking up into large floating panels. These ice panels, driven by light winds, push the rocks ahead of them, leaving trails in the soft mud below the surface. When the playa dries out months later, the trails become clear.

Example of “windowpane” ice collected on the Racetrack Playa. It was much thinner than expected. Photo by Richard Norris courtesy of Scripps Oceanography

“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 [Norris, a cousin], ‘This is it!’”

Indeed it was.

Forget hurricane-force winds, the rocks were moved by quarter-inch thick ice panels by light winds of 10 mph. The rocks moved only a few inches per second or a speed deemed imperceptible at a distance without a stationary reference point.

“It’s possible that tourists have actually seen this happening without realizing it,” said Jim Norris of the engineering firm Interwoof in Santa Barbara. “It is really tough to gauge that a rock is in motion if all the rocks around it are also moving.”

Lorenz said the last suspected movement previously was in 2006, so rocks may move only about 1 millionth of the time, and there is evidence to suggest that the frequency of rock movement has declined since the 1970s because of climate change.

Racetrack Playa is partly flooded shortly after the December 21, 2013 move event in which hundreds of rocks scribbled trails in the mud under the floating ice. Photo by Richard Norris courtesy of Scripps Oceanography

Asked if the mystery of sliding rocks has finally been solved, Richard Norris replied, “We documented five movement events in the 2 1/2 months the pond existed and some involved hundreds of rocks. So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion. But we have not seen the really big boys move out there….Does that work the same way?”