Jan 312016
 
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As we all learned in our childhood, solid rocks belong to the abiotic environment and cannot move by their own selves. They have no will of their own and besides, no locomotor system.

The rocks in Racetrack Playa, located in the Death Valley National Park in south-west USA – a hostile place of annual heat records (the hottest temperature on earth since recording was measured in the Death Valley in July 1913 and came to 56.7°C)[1] – however seem to overrule this fundamental law of biology.

The name Racetrack Playa is no accident: over decades, tens to hundreds of rocks have been found with tracks behind them as if they were slowly sliding leaving grooves in the dusty soil (left picture). The tracks are often parallel and run in the same direction looking as if the rocks were participating in a slow-motion race (right picture).

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Rock with a distinct track (left)[2] and aerial image of rocks moving in the same direction (right)[3].

This phenomenon was first discovered in 1948 and started versatile speculations about its origin. Some of the rocks weigh more than a hundred kilos, so help by humans is only possible with heavy equipment but no such traces can be found around them. Mud and even slime-producing algae as well as the weather were considered.[4]

Wind in conjunction with ice floes, as the most possible critical factors for rock movement, were supposed for years but no direct observation was made since studying in person is not recommended due to the temperatures and the restricted access in the Death Valley. But during the winter of 2013/2014, the group of Richard D. Norris and James M. Norris was able to monitor the motion using GPS in combination with information from weather stations.[5] Several rocks were provided with GPS transmitters and the area was observed by time lapse photography. Between November and February most of the Playa was covered by a shallow rainwater pool which froze at night-time. During sunny and windy days the ice melted partly and the rocks were driven on their ice sheets by the wind and running water. On this occasion, they pushed the mud beneath them aside forming long flat furrows. Some rocks only glided a few meters, some travelled up to 66 m and some shared an ice sheet which produced parallel lines. Under some rocks, the ice was already crushed so they showed no movement at all. At the end of February, the temperatures rose, the water evaporated and the spurs were exposed. Norris’ results prove that freezing temperatures for the formation of ice sheets and wind forces of 3-5 m/s are necessary for a rock movement of 2-5 m/min whereas the velocity is also dependent on the individual texture of the stone’s surface and weight.[5]

This is an excellent example of a long unexplained phenomenon that finally found elucidation by rigorous research. Do such allegedly mysterious occurrences lose their charm by an objective, scientific clarification like this? No! On the contrary, they show how complex and versatile the interactions of nature’s mechanisms are even by such a peculiar phenomenon as the wind-driven “wandering” rocks in the desert.

– Tatjana Daenzer

Read more:
[1] http://wmo.asu.edu/world-highest-temperature.
[2] “Runningrock2” by Tahoenathan – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Runningrock2.jpg#/media/File:Runningrock2.jpg
[3] “Racetrack playa 2013-12-20” by Richard D. Norris, James M. Norris, Ralph D. Lorenz, Jib Ray, Brian Jackson. Licensed under CC BY 1.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Racetrack_playa_2013-12-20.jpg#/media/File:Racetrack_playa_2013-12-20.jpg
[4] R. P. Sharp, D. L. Carey, J. B. Reid, P. J. Polissar, M. L. Williams, Geol. Soc. Am. 1996, 765–767.
[5] R. D. Norris, J. M. Norris, R. D. Lorenz, J. Ray, B. Jackson, PLoS One 2014, 9, 1–11.

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