Sailing Stones

One of the most interesting mysteries of Death Valley National Park is the sliding stones at Racetrack Playa (a playa is a dry lake bed).

These stones can be found on the floor of the playa with long trails behind them. Somehow the stones slide across the playa, cutting a furrow in the sediment as they move.

Remarkably, multiple stones commonly show parallel tracks, including apparently synchronous high angle turns and sometimes reversals in travel direction.

Some of the stones weigh more than 300 kg. That makes the question: “what powerful force could be moving them?”

Scientists have investigated this question since the first report in 1948, but no one has seen the phenomenon in action – until now.

Because the stones can sit for a decade or more without moving, Dr Jackson and his colleagues decided to monitor them remotely by installing a weather station capable of measuring gusts to one-second intervals and fitting 15 stones with custom-built, motion-activated GPS units.

Their experiments showed that moving the stones 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 stones. 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.

These observations upended previous theories that had proposed hurricane-force winds, slick algal films, or thick sheets of ice as likely contributors to stone motion.

Instead, the stones moved under light winds of about 3-5 m per second and were driven by ice less than 3-5 mm thick, a measure too thin to grip large stones and lift them off the playa, which several papers had proposed as a mechanism to reduce friction. Further, the stones moved only 2-6 m per minute, a speed that is almost imperceptible at a distance and without stationary reference points.

Individual stones remained in motion for anywhere from a few seconds to 16 minutes.

In one event, the scientists observed stones three football fields apart began moving simultaneously and traveled over 60 m before stopping.

“We documented five movement events in the two and a half months the pond existed and some involved hundreds of stones. So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in stone motion,” said Dr Richard Norris from Scripps Institution of Oceanography, who is the first author of a paper published in the journal PLoS ONE.