– Connie Cheng ’18

How did this rock move by itself?

This question has confounded researchers for decades. Without human or animal intervention, the “sailing stones” of Death Valley appear to move across a flat, dry lakebed known as Racetrack Playa, carving trails behind them into the ground.

Since the 1900s, geologists have pondered the rocks’ mechanism of movement. A variety of conjectures have captured the public’s imagination, ranging from earthquakes, to dust devils, to magnetic fields, to aliens.

The mechanism is so elusive because it is exceptionally difficult to observe the rocks in motion. Their movement is not constant; rather, the stones can sit still for decades. The only evidence of their movement are their tracks, which can stretch for hundreds of meters, sometimes bending sharply, forming loops, or intersecting with one another.

No one has seen the stones sail—that is, until now.

This past August, a team led by UC San Diego paleobiologist Richard Norris announced that they have managed to record the first direct observations of movement by the rocks of Racetrack Playa, using GPS trackers and time-lapse photography.

In December of 2013, Norris and his team received the lucky break of a lifetime. When they arrived at the Racetrack to check on their equipment, they found the lakebed covered in ice, freshly engraved with new trails that were surrounded by shards of broken ice.

The next afternoon, the researchers were taking in the view from a mountainside, when a wind picked up and the stones set sail before their eyes, finally putting a decades-long mystery to rest.

As it turns out, very particular conditions have to be present in order for the rocks to move. Norris and his term realized that during some winters, a shallow layer of rainwater accumulates on the parched lakebed, which freezes overnight. When the sun comes up again, the ice breaks off into thin sheets; in the presence of a wind, the floating ice sheets are pushed against the rocks, which causes them to slide over the lakebed and create their trails in the mud. Once the pool dries, the resulting artwork of curves, loops, and bends drawn by nature is revealed for the world to see.

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Image credits: James Gordon/Flickr, John Buie/Flickr, Sandy Redding/Flickr, James Gordon/Flickr

 

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