Mobile rocks explained

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Mike Hartmann

Rocks that mysteriously slide across a dry lake bed are thought to be nudged along by large ice sheets. But previously, no one could fully explain how the rocks — some weighing more than 300 kilograms — scoot across Racetrack Playa in California.

Now a team led by Richard Norris at the Scripps Institution of Oceanography in La Jolla, California, has caught the rocks in action after tagging them with Global Positioning Satellite markers. On 20 December 2013, pools of ice in the lake started cracking in the morning sun. A gentle breeze then bumped 'windowpane' ice sheets against the rocks, moving them at a rate of 2–5 metres per minute. When the ice melted, more than 60 rocks had budged, leaving freshly formed trails (pictured) behind them. By the end of the winter, the farthest-moving rock had travelled 224 metres.

PLoS ONE 9, e105948 (2014)


  1. Report this comment #64009

    Larry N. Smith said:

    This is so cool. I've been going out there since 1976, and was there in March 2014 with a small group of students and saw the fresh tracks. Norris et al. was one lucky group. The rocks had not moved for many years, and they got lucky to capture the events. It will be great to go again and see the distribution of fresh and older tracks, and think about the subtle differences in playa elevations. But, in a way, it's too bad we don't have that mystery to ponder.

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