The video above – from the Slithering Stones Research Initiative – shows a famous sailing or sliding or slithering stone of Death Valley’s Racetrack Playa in motion. See it? It’s the big rock in the foreground.
Although their tracks across Racetrack Playa – a dry lake bed in Death Valley – have been observed and studied since the early 1900s, no one had ever seen the stones in motion, until recently.
In August 2014, a group of (very patient) researchers aided by the Scripps Institution of Oceanography, NASA and others announced they had solved the mystery. Richard D. Norris and his cousin James M. Norris said the motion comes from very thin windowpane ice that sometimes covers the dry lake bed. When the ice begins to melt in late morning sun, it may break up under light winds. Floating ice panels may then push the rocks, causing them to move and leave tracks in the desert floor. The journal PLOS ONE published their study.
The two cousins launched their investigation of sailing stones in 2011. That’s when they founded what they called the Slithering Stones Research Initiative. They established a weather station near Racetrack Playa and added 15 of their own stones to the playa. The added stones had GPS tracking units attached.
Then, they watched. On December 4 and December 20, 2013, their set-up – which used time-lapse photography – caught on camera rocks that were sliding across the playa at up to 15 feet (3-5 meters) per minute. They saw many other instances of sailing stones as well, becoming the first people in the world to see the stones in motion. They wrote:
The largest observed rock movement involved >60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 meters between December 2013 and January 2014 in multiple move events.
They said watching the stones move enabled them to see the cause:
In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3- to 6-millimeter “windowpane” ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of ~4–5 meters/second.
Floating ice panels tens of meters in size push multiple rocks at low speeds of 2–5 meters/minute along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice.
We first heard of Death Valley’s sliding stones from a Facebook friend, Chris Tinker. He captured the image above and wrote of the stones on our website. He spoke of the idea that the rocks were blown by the strong winter winds, which can reach 90 mph on the Death Valley desert floor. In fact, that’s not what happens, as James Norris and Richard Norris saw.
According to Wikipedia, these sailing stones are slabs of dolomite and syenite ranging from a few hundred grams to hundreds of kilograms.
Racetrack Playa stones only move once every two or three years and most tracks last for three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander.
Nice to know what causes them to move! If you have time, check out the video below, which features the Norris brothers telling their story.
Bottom line: Watch the video in this post, and become one of the people on Earth who have seen the famous sliding stones of Death Valley’s Racetrack Playa in motion. The rock movement occurs when very thin windowpane ice covering the dry lake bed begins to melt in late morning sun. The ice breaks up under light winds, and floating ice panels push the rocks, causing them to move and leave tracks in the desert floor.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.