There are large stones in the Racetrack Playa, a flat dry lake (playa) above the northwestern side of Death Valley in California. Some weigh up to 700 pounds. Many of them have long trails behind them indicating that they’ve moved somehow. There are no footprints, tire tracks or other hints as to how they slid across the flat ground and no witnesses to the event. Aliens? Fraternity prank? Body-building ghosts?
In 2011, a team led by Richard Norris from Scripps Institution of Oceanography put motion-activated GPS units on 15 of the rocks and monitored them with time-lapse cameras and a high-resolution weather station. They planned to wait and watch up to ten years (nice work if you can get it) but needed only two before the alarm went off.
According to their report published in the current edition of the journal PLOS ONE, a rare sequence of events leads up to the movement of the stones. First, precipitation must fall in a sufficient quantity to cover the playa with water but leave the rocks exposed. The temperature must then drop to a point where the water freezes into “windowpane ice” – thin sheets that are light enough to move but thick enough to not break easily.
As the temperature warms again - sometimes just a sunny day is sufficient – the ice melts slightly and cracks, forming large panels. The light winds (10 miles per hour) that blow across the plaza are strong enough to move the ice panels, which push the rocks and leave trails in the mud under the ice. The rocks may move only a few inches per second but can stay in motion for up to 16 minutes before stopping – in one case the team saw a rock travel over 200 feet.
The study isn’t over yet, says Norris.
We documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks. So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion. But we have not seen the really big boys move out there….Does that work the same way?
Sounds like the team is heading back for more rock-and-roll.