Death Valley National Park is the site of a mysteryâthe "sailing stones" of Racetrack Playaâthat has intrigued visitors and baffled scientists for decades. The playa is a dry lake, and its surface is marked by a web of "trails" left in the soil by the unexplained movement of hundreds of rocks. Now a team of researchers has solved the puzzle...and the answer involves a very unlikely process in the hottest place in America.
The rocks, some of them weighing as much as 700 pounds, appear to have been dragged across the surface of the dry lake, leaving trails that can stretch for hundreds of yards. Multiple rocks commonly show parallel tracks including apparently synchronous high angle turns and sometimes reversals in travel direction. Until recently, there has been no plausible explanation for the force that was moving the rocks, and no one was known to have actually seen the stones in motion.
It's a puzzle that lures hardy visitors to the remote site, and scientists have been investigating this phenomenon since the 1940s. Now, for just a touch of irony, the answer to this mystery deep in the desert has been discovered by a team from the Scripps Institution of Oceanography, led by UC San Diego paleobiologist Richard Norris.
Because the stones can sit for a decade or more without moving, the researchers did not originally expect to see any motion in person. Their solution was to monitor the rocks remotely by installing a weather station adjacent to Racetrack Playa, several time-lapse camera systems overlooking the southeast corner of the playa, and 15 GPS-instrumented rocks on the playa surface. They then visited the playa 5â8 times a year to exchange battery packs and download weather data.
Would This Be "The Most Boring Experiment Ever?"
The National Park Service did not want the scientists to disturb native rocks for the study, so the team brought in similar rocks from an outside source. The experiment was set up in the winter of 2011... and then the researchers waited for something to happen. One of the study participants, Ralph Lorenz of the Applied Physics Laboratory at the Johns Hopkins University, said later he suspected it would be âthe most boring experiment ever."
âThe last suspected movement was in 2006, and so rocks may move only about one millionth of the time,â said Lorenz. âThere 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.â
In December 2013, team leader Richard Norris and co-author and cousin Jim Norris arrived in Death Valley to discover that the playa was covered with a pond of water three inches deep.
Shortly thereafter, the rocks began moving.
âScience sometimes has an element of luck,â Richard Norris said. âWe 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.â
Based on their observations, moving the rocks requires a rare combination of events, especially considering the harsh desert environment found in Death Valley.
Ice in the Desert?
First, say the scientists, 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 âwindowpaneâ ice, which must be thin enough to move freely but thick enough to maintain strength.
Then, 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.
"This is it!"
âOn December 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface,â said Richard Norris. âI said to Jim, 'This is it!ââ
The team's observations upended previous theories that had proposed everything from hurricane-force winds to dust devils, slick algal films and thick sheets of ice as likely contributors to rock motion. Instead, it was determined that rocks moved under light winds of about 10 miles per hour and were driven by ice less than 0.25 inches thick.
That's a thickness of ice too thin to grip large rocks and lift them off the playa, which several previous papers had proposed as a mechanism to reduce friction. Furthermore, the team discovered that the rocks moved only a few inches per second, a speed that is almost imperceptible from a distance and without stationary reference points.
âItâs possible that tourists have actually seen this happening without realizing it,â said Jim Norris, of the engineering firm Interwoof in Santa Barbara, California. âIt is really tough to gauge that a rock is in motion if all the rocks around it are also moving.â
Individual rocks remained in motion for anywhere from a few seconds to as long as 16 minutes. In one event, the researchers observed rocks three football fields apart that began moving simultaneously and traveled over 200 feet before stopping. Rocks often moved multiple times before reaching their final resting place.
The short video below by Jim Norris shows a time-lapse sequence of some of the rocks in motion.
The researchers also solved a related mystery when they observed trails left in the playa's surface without rocks, features that the Park Service had previously suspected were the result of tourists stealing rocks. This study determined that these trails were formed by "grounding ice panels."
Richard and Jim Norris, with study co-author Jib Ray of Interwoof, started studying the Racetrackâs moving rocks to solve the âpublic mystery,â and set up the âSlithering Stones Research Initiativeâ to engage a wide circle of friends in the effort. They needed the help of volunteers who repeatedly visited the remote dry lake, quarried the rocks that were fitted with GPS sensors, and maintained the custom-made instruments.
âWhat is striking about prior research on the Racetrack is that almost everybody was doing the work not to gain fame or fortune, but because it is such a neat problem,â said Jim Norris.
Is the mystery of the sliding rocks finally solved? The answer seems to be ... "partially."
âWe documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks," says Richard Norris, âSo 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 thereâ¦.Does that work the same way?â
Given the remote location of the playa and the potentially long period of time until conditions are just right, perhaps the answer to that part of the mystery will continue to tantalize researchers for some years to come.
If you'd like to visit the site yourself, be sure you're properly prepared. Based on their names, you might think a trip down the Racetrack Road to a viewpoint called the Grandstand would be quick and easy...but you'd be wrong on both counts.
The drive to Racetrack Playa covers 26 miles of often bone-jarring travel over an unpaved road into a very remote area. The park website says, "Normally, [the road] is recommended for high-clearance vehicles with heavy-duty tires, as it can be rough and washboard." You'll find important details about visiting the playa here.
The findings of the team led by Richard Norris are detailed in a paper published on August 27 in the journal PLOS ON. You'll find a copy of their paper at this link.