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| Credits as above. Picture grabbed from ScienceDaily.com. |
How can a rock weight several hundred pounds move hundreds of meters across a "dry lake"? And, why do they move in tandem? One of the early pioneers in solving this mystery was Robert Sharp* of Caltech, and many others have speculated about this problem. Sharp monitored stones over a period of seven years stretching from 1968-1974, and concluded that movement was related to wet stormy weather. Sharp and Carey documented a greatest cumulative movement of 262 m, and "greatest single-episode movement, of 201 m. These were of a small 250 g stone, but other moved stones weighed as much as 25 kg. They concluded that movement "most likely occurs within one to several days after playa wetting, and velocities on the order of 0.5 to 1 m/sec are inferred from track characteristics." Sharp and Carey reported eyewitness accounts of ice sheets containing frozen stones being propelled by wind on other southern California playas, and inferred that the stone tracks at Racetrack were made in this way. However, there were observations that led them to conclude that the stones could not move within extensive ice sheets: they moved out of an encirclement of iron stakes that Sharp and Carey had placed and their spacing changed during movement.
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| Stationary rocks (blue arrows) and rock moving from left to right (red arrow) |
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The team used quarried rocks,
one shown here with its
GPS unit |
Now, the process has actually been observed by a team led by Richard Norris of Scripps Institute of Oceanography.** In what one of the co-authors, Ralph Lorenz, described as potentially the most boring experiment ever, the scientists outfitted 15 rocks with motion-activated GPS sensors and placed them on the playa to await movement. Only two years into the project, not only did the rocks move, but Norris and Norris were there at the time. When they arrived in December 2013, there seven centimeters (3") of water on the playa, and they concluded that not only must water be present, but it must be deep enough to form "floating ice" during cold winter nights, but shallow enough to leave at least part of the rocks exposed. Panes of ice form during the night, and on sunny days the ice can begin to melt and break up into panels that float across the water if there is wind. These panels actually push the rocks in front of them. The wind speeds were about 3-5 meters/second (10 mph), and the ice that moved was about 1/4" thick. These speeds are much lower than inferred from track characteristics by Sharp and Carey. The rocks moved at 2-6 meters a minute, and moved for a few seconds to 16 minutes. The rock trails formed under the ice, and became visible only when the water underlying the ice is blown away by winds.
Floating ice moves around the playa under the influence of winds. When it encounters rocks, it may pile up on the upstream side, increasing the effective cross-sectional area of the rocks to both upstream ice and water and thus facilitating movement. On the other hand, sometimes the ice fragments upon encountering a rock. Norris and Norris suspect that this phenomenon might explain the Sharp and Carey observation of the corral behavior: the rock that didn't move out of the corral was just downstream of a stake that may have shattered the ice. Stones with low profiles might be submerged beneath the ice, some rocks may be too big for the available forces under some wind conditions, and others may not totally or partially encounter ice.
But, the researchers concluded, the mystery may not be completely solved: they didn't get to see the really big ones move.
The authors also point out that the sliding rocks are not unique to Racetrack Playa or even the U.S. Ice-driven rock trails are observed on the bottom of Great Slave Lake in northern Canada and on the shores of the Baltic Sea. The mechanism may apply to rock trails on dry lake surfaces in Spain and South Africa where the lakes are at high elevation and exposed to cold winters.
*Robert P. Sharp and Dwight L. Carey, Sliding stones, Racetrack Playa, California, GSA Bulletin, 87(12), 1704-1717.
**Richard D. Norris, James M. Norris, Ralph D. Lorenz, Jib Ray, Brian Jackson, Sliding rocks on Racetrack Playa, Death Valley National Park: First observation of rocks in motion. PloS ONE, 2014; 9(8) e105948 DOI:10.1371/journal.pone.0105948
link to article is here
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