This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
"I love to sail forbidden seas, ..."
Herman Melville in "Moby Dick" (1851)
Strange tracks cover a muddy plain, located in a remote part of Death Valley National Park, named appropriately Racetrack-Playa. Most of these tracks end behind large boulders of dolostone or syenite, some however start and end without an apparent object nearby. In the first case, it seems reasonable to assume that the rocks moving along the ground formed the furrows - however nobody ever observed the actual process of formation.
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The sailing rocks of Death Valley were studied since 1948, when geologists Jim McAllister and Allen Agnew mapped the area and noted the tracks. In March 1952 geologist Thomas Clement tried to observe the rocks moving, but a heavy thunderstorm forced him into his tent. Only the next morning he noted fresh formed tracks on the ground and a thin layer of water covering the ground. As most of the tracks coincide with the overall wind direction (southwest to northeast), it was assumed that the wind pushed the rocks over the wet and slippery mud. However this hypothesis could explain only a part of the moving rocks, especially the smaller ones. Curiously there is no correlation between the size of the rock and length of the track, even if it seems that larger boulders seem to travel less than smaller ones.
In the following years the strangest ideas tried to explain the mystery of Racetrack-Playa: extraterrestrials, geologists or animals pushing the stones, a hoax to fool tourists, earthquakes, magnetic or gravitational anomalies and unknown wind and water currents.
Geographer George Stanley Druhot (1914-1983) assumed a dominant role of ice, not only as slippery surface, but also as a sort of sail, when ice forms plates around the boulders and increases the surface on which the wind can act.
In 2010 a research team from various institutions (NASA, Slippery Rock University (!)- Pennsylvania, University of Wyoming) reanalyzed the geological and meteorological conditions at Racetrack-Playa, finding evidence to support Stanley´s idea. Geologist Paula Messina showed that the ground is covered by argillaceous sediments and bacterial mats, forming under wet conditions a very slippery surface. The climatic data showed also that ice can in fact form during wintertime in Death Valley, when also most tracks on Racetrack-Playa are formed. The ice hypothesis (and similar models) explains also the tracks without apparent object nearby, as the chunk of ice melted after the formation of the track, and the deepening of some furrows behind the respective boulder, as the plate of ice surrounding the boulder melts, the rock tends to sink deeper into the mud.
Most researchers agree that a simplistic - one factor assuming - model fails to explain all the sailing rocks. It´s probably the odd combination of mud and bacteria, forming a slippery ground, the topography, forcing the wind into one prevalent direction, the sizes of the boulders, the particular temperature changes experienced in Death Valley and the occasional formation of ice, that like a ghostly hand moves the rocks around the desert.
Bibliography:
MILLER, M.B. (2005): Geological landscapes of the Death Valley region. Earth-Science Review 73:17-30