Field of Science

The Geo- Files:The unearthly cases in Geology: Sailing stones

Strange tracks, hundred of meters long, cover a muddy plain, 4,5 kilometres long and 2,2 wide, in the middle of the desert of Death Valley, and give the name to this enigmatic place: Racetrack-Playa.

Most of these tracks end behind dolostone and syenite rocks or large boulders, some of them however start and end without an apparent object nearby. In the first case its seems reasonable to assume that the rocks scratching on the ground formed the parallel furrows, however nobody ever observed them in motion, and some boulders weigh more than 100 kilograms, it's unknown which force could move them, sometimes even uphill (anyway considering that the plain is almost horizontal it´s not such an accomplishment).

The wandering rocks of Death Valley are studied since 1948, when geologists Jim McAllister and Allen Agnew mapped the bedrock of the area and made note of the tracks. Actually a research team of 17 scientists and students from various institutions (NASA, Slippery Rock University (!)- Pennsylvania, University of Wyoming.) are studying them.

In March 1952 the geologist Thomas Clement was one of the first to try to observe the rocks move, but a heavy thunderstorm forced him into his tent. Only the next morning he noted fresh tracks on the ground, and a thin layer of water covering the soil. Most of the tracks also coincide with the wind pattern, channelized by the adjacent mountain ranges in direction from southwest to northeast.
This observations could explain some observed movements, especially of smaller rocks, however to move large boulders the wind alone is insufficient, also there are still tracks not moving into the direction of the wind.
Some boulders, weighing up to 320kg, travelled 18m in a month, smaller ones travelled in the same period 880m, however there is no correlation between form and weight to the distance and the speed that the rocks travel. Some travel straight, some display a curved pattern, some travel pair wise, other chaotic or even describing a circle. From the accumulation of mud and mud splatter on the rocks it was possible to calculate an average speed of 7 kilometres per hour.

The strangest theories arouse to explain this mystery: extraterrestrial influence, animals pushing the stones, a hoax to fool tourists, earthquakes, magnetic or gravitational anomalies and unknown wind and water currents.

In the fifties the naturalist George Stanley postulated a dominant role of ice, not only as slippery surface, but also acting as a kind of sail, when ice plates remain attached to the boulders.
The rocks itself seems nothing extraordinary, aside their movements, however the research by geologist Paula Messina showed that the former lake bottom of Racetrack Playa is covered by argillaceous material and bacterial mats, that in wet conditions can produce a slippery surface.

The ongoing studies by NASA showed that ice can form under the climatic conditions of Death Valley. The ice hypothesis could also explain the apparent maximum of movement in the winter time, the furrows with no apparent object at their end (left by the melted ice), and the apparent deepening of some of the tracks, the loss of ice supporting the rocks causes them to sink deeper in the ground.

However, most modern research agrees that a simplistic, one factor assuming model fails to explain all the strange moving rocks. The presence of different factors and their interaction, slippery underground, algae or mud, wind, rain and ice all together play a role in the movements of the sailing stones.

Online resources:

BOJANOWSKI, A. (13.09.2010): Tal des Todes - Das Geheimnis der streunenden Felsen. SpiegelOnline Wissenschaft (Accessed 16.09.2010)
Image from Wikipedia: Racetrack Playa

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