Rocks and How they Roll- Or in this Case, How they Slide
By Matthew Tyler, Marine Biology & Environmental Science, 2017
The ‘sailing stones,’ a phenomenon exclusive to Death Valley, has defied definitive explanation since the 1940s. These stones are a number of rocks on a dry lake bed that occasionally move, carving long trails into the hardened mud behind them. The mystery of these migrating rocks, which can weigh over forty-five kilograms, has finally been solved by cousins Dr. Richard Norris and James Norris and their team of researchers from Scripps Institute of Oceanography.
When one thinks of Death Valley National Park, they think of inhospitable desert and heat. After all, Death Valley is where the highest air temperatures on Earth have been recorded. Death Valley is indeed a place of extremes, but not necessarily in the ways people think. The elevated Racetrack Playa, the site of the moving rocks, is a dried lakebed that can rarely be flooded with a few inches of runoff from the nearby mountains. When the conditions are just right, the researchers discovered, and the flooding of the playa intersects with below-freezing nights, 3 to 6 millimeter ice sheets form across the mosaic of mud underneath. As morning approaches, some of the ice begins to melt, and if the winds are strong enough, ice sheets tens of meters in size may slide across the ground and push rocks as they go. This motion is not fast –only between 2 to 5 meters a minute– but one might argue that any movement rocks make is fast.
The Scripps team brought over 60 of their own rocks, as the park service requested, and planted GPS devices in them to track movement. Amazingly, on one visit to the park, Dr. Richard Norris claims the team saw nearly every rock they placed moved. Some rocks moved as far as 224 meters in just one winter month of the experiment. He described their observations as “complete dumb luck,” given that years can pass in between bouts of the stones moving, but it certainly does not take away from their discovery. What’s next for the team? Calculating the forces involved in the movement of the rocks, perhaps.
