What causes Death Valley’s sailing stones to move?

People used to think that strong winds – which can reach 90 mph in the Death Valley desert – caused the rocks to move. In fact, that’s not what happens.

Although their tracks across Racetrack Playa – a dry lake bed in Death Valley – have been observed and studied since the early 1900s, no one had ever seen the stones in motion, until recently.

In August 2014, a group of (very patient) researchers aided by the Scripps Institution of Oceanography, NASA and others announced they had solved the mystery. Richard D. Norris and his cousin James M. Norris said the motion comes from very thin windowpane ice that sometimes covers the dry lake bed. When the ice begins to melt in late morning sun, it may break up under light winds. Floating ice panels may then push the rocks, causing them to move and leave tracks in the desert floor. The journal PLOS ONE published their study.

The two cousins launched their investigation of sailing stones in 2011. That’s when they founded what they called the Slithering Stones Research Initiative. They established a weather station near Racetrack Playa and added 15 of their own stones to the playa. The added stones had GPS tracking units attached.

One of the GPS-instrumented rocks and its track across Racetrack Playa. The GPS unit, with its battery pack, was placed in a cavity bored into the top of the rock. Photo via PLOS ONE.

A sliding stone in Death Valley, via EarthSky Facebook friend Chris Tinker

Sailing stone tracks, via PLOS ONE.

Then, they watched. On December 4 and December 20, 2013, their set-up – which used time-lapse photography – caught on camera rocks that were sliding across the playa at up to 15 feet (3-5 meters) per minute. They saw many other instances of sailing stones as well, becoming the first people in the world to see the stones in motion. They wrote:

The largest observed rock movement involved >60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 meters between December 2013 and January 2014 in multiple move events.

They said watching the stones move enabled them to see the cause:

In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3- to 6-millimeter “windowpane” ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of ~4–5 meters/second.

Floating ice panels tens of meters in size push multiple rocks at low speeds of 2–5 meters/minute along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice