For nearly a century, scientists have struggled with the mystery of the sailing stones of Death Valley. These massive rocks, weighing up to 320 kilograms, scraped out tracks as long as 224 meters in parallel formation, giving the valley its second name, “Racetrack Playa.” What scientists did not understand was how these immense stones managed to move, or “sail.” No forces powerful enough seemed to exist in the environment.
Image courtesy of Richard Norris et al, PLoS One
Finally, at the end of August, a team of researchers from the Scripps Institute of Oceanography published their surprising findings from three years of observations in Death Valley, during which they quite literally saw the process in motion. This discovery finally emerged after years of debate within the scientific community over conflicting theories about the sailing stones, and the answer is of interest to scientists and tourists alike.
It is not pure force, but rather the right combination of conditions that thrusts these massive stones along the lakebed. The Scripps team found that the rocks only move when a thin layer of ice forms overnight from rainwater runoff from the surrounding mountains. The next day, when the sun shines down on Death Valley, the thin ice sheet breaks up into panels, which flow steadily in the direction of even a light wind. The panels push the massive rocks along with them at about two to five meters per minute.
The rainwater runoff must be seven millimeters to form “windowpane” ice three to six millimeters thick – thin enough to be broken into panels, but still strong enough push the sailing stones forward. The exact movement of the sailing stones depends on the magnitude and direction of the wind, as well as the water under the ice. Light, steady breezes of about four to five meters per second help the rocks move along their path.
Image courtesy of ScienceNews
Some previous theories had predicted ice and wind to play a role in the sailing stones, but not in the same way that the researchers discovered. Powerful wind, thick ice sheets, and algal films that reduce the friction between the rocks and the lakebed were all previous conjectures. However, the researchers found that the wind that sweeps through the dry lake does very little to move the rocks. The ice sheets that form are not thick enough to move the rocks directly. And only winds of up to 80 meters per second – about as fast as a NASCAR race car – could move the stones even with the help of an algal film. Only thin, floating ice panels pushed with a gentle breeze are able to move the stones.
Led by paleobiologist Richard Norris, the Scripps team started their work on Racetrack Playa in 2011. To measure the movement of the rocks, they monitored the stones and environmental conditions with time-lapse cameras, GPS systems, and a weather station that measured the velocity of gusts every second. Because the National Park Service did not allow the researchers to use the native rocks in the playa for their experiment, the team attached the GPS systems to 15 rocks similar to those in Racetrack Playa and placed them in the dry lake. Dr. Norris and the other researchers were not expecting to actually see any motion because the rocks seldom move – at most once every decade. It was by pure chance that they were present when the phenomenon occurred on December 21 last year. The researchers heard the ice begin to crack around noon and saw the spectacle firsthand.
Image courtesy of Richard Norris
The discovery has explained other phenomena surrounding the sailing stones of Death Valley as well. In some areas, the ice panels themselves scrape through the sand and leave tracks in their wake, which explains why there are some trails with no stone marking the end. Some pairs of rocks also lose synchronization with each other along their trails, which is likely a result of splitting ice sheets that maneuver around one stone but not the other.
The sailing stones were a mystery to the public as well as to scientists. Visitors to Death Valley now have an explanation for the tourist attraction, and scientists now have a case study of a surprising force of nature: thin panels of ice floating on water that together force massive rocks hundreds of meters forward. The rocks look the same as before – sitting motionlessly at the end of the tracks streaking the playa – but now we understand the story that these sailing stones tell.