Sometimes a good mystery just won't stay solved. Last year, scientists witnessed Death Valley's famous sliding rocks, also known as “sailing stones”, in action, and thought that they had worked out how they move. Now, however, the same phenomenon has been observed somewhere that the previous explanation doesn't apply, suggesting that the movements could have multiple causes.
Racetrack Playa in Death Valley gets its name from long grooves in the flat surface of the land, almost always with rocks at their ends. The tracks have been formed by rocks sliding across the ground, but geologists have been fascinated by the question of what could drive these heavy objects. Naturally, the rocks have almost never moved when anyone has been watching.
Last year, after attaching motion-activated GPS devices to 15 likely-looking rocks, a team from the Scripps Institution of Oceanography announced success. A thin layer of water had frozen on the surface of the Playa, and the rocks began moving as it started to break up. The circumstances under which movement occurs are rare, requiring sheets of floating ice, driven by winds of just the right strength, pushing the rocks before them.
All great, except for one thing: at Altillo Chica lagoon, Spain, similar tracks have been found regularly since 2012, sometimes stretching beyond 100m. Here, however, ice sheets do not form, as a result of the saltiness of the soil and mildness of the winters.
In Earth Surface Processes and Landforms, three researchers have proposed their own explanation. "Our hypothesis is that they move during the winter, when storms are produced and accompanied by strong winds," said first author María Esther Sanz-Montero of Complutense University, Madrid.
The winter rain in Spain leaves the Altillo plain under roughly 5cm of water, low enough for the sailing stones to stick through. “The wind is capable of creating water currents up to two meters [seven feet] per second, which could be the real cause of the sliding stones,” said Sanz-Montero.
The currents alone would not normally overcome friction between a stone and the ground. However, Sanz-Montero argues there is another factor: microbial communities that fill the soil with gas bubbles, making it exceptionally slippery.
The stones get an initial push from turbulence around them, Sanz-Montero believes, which leaves a small groove downstream of the direction the current is flowing. Once started the slippery mud allows them to keep going, sometimes for great distances.
"On the one hand,” Sanz-Montero said, “You can recognise the sedimentary marks or structures which accompany the stone trails, and you can observe that they follow the direction of the water currents. On the other, the direction of the trails clearly coincides with the prevailing winds in the area during each storm."
The Altillo Chica tracks are less durable than those at Death Valley, fading away over summer, and the causes are different. Nevertheless, potential commonalities, such as microbial communities reducing friction, have attracted the interest of North American researchers.
Sanz-Montero hopes that the findings could be useful in paleoclimatic studies where relics of ancient tracks have been detected.