Moving rocks of the Racetrack Playa

We all can comprehend how rocks break apart due to heat and frost action and fall due to gravity. But, for a long while, there was a popular mystery surrounding moderately-sized rocks moving across a dead flat surface in Death Valley leaving a trail behind them.

The famous location where this happens is Racetrack Playa (The Racetrack), a remote valley within Death Valley National Park, California. But it also occurs at the Little Bonnie Claire Playa in Nevada, both of which have similar geographical settings. The Racetrack playa collects water runoff but because there is no drainage outlet the water infiltrates and evaporates.

The heavy rocks along the playa have eroded from the surrounding mountains. But once on the very level plain, they seemingly slid across the mud-cracked surface of the dry lake bed without human intervention. The stones were thought to slide, sail, or wander by some mysterious natural means or through paranormal or mystical “energy”. I can’t remember when I first heard of them but it was when I was a kid in the late 70s, I think. The moving rocks were featured in many books on American mysteries. It was generally considered more likely to be an unobserved natural phenomenon rather than aliens or earth energies. However, Death Valley has many weird locations and associated stories, so the moving stones added to its reputation as a spooky place.

In 1948, USGS scientists who found the tracks speculated that wind was moving the rocks during wet periods. Yet, that couldn’t be the whole story. A fuller explanation for this movement began to solidify in the early-2000s.

The location is not that easy to access, requiring an off-road vehicle. No one had yet seen the rocks move, just the trails left behind. Technology like GPS and remote camera equipment was the key to discovering what was going on. Brian Dunning, a science writer, first noticed that sheets of thin ice were blown across the playa on a cold morning. He filmed a video showing the thin sheet of water moving rapidly across the surface with respect to a strong wind.

A team of academic researchers was later able to attach GPS units to some of the playa rocks. They also used time-lapse photography and installed weather instruments at the location. These measurements led to the conclusion that even a moderate breeze was able to move thin ice sheets over meltwater that buoyed the large rocks. They recorded movements of hundreds of meters in a few events that took place from December 2013 and January 2014.

Their observations show that moving the rocks requires a rare combination of events. First, the playa fills with water, which must be deep enough to allow the formation of floating ice during cold winter nights but shallow enough to expose the rocks. As nighttime temperatures plummet, the pond freezes to form sheets of “windowpane” ice, which must be thin enough to move freely but thick enough to maintain strength. On sunny days, the ice begins to melt and break up into large floating panels, which light winds drive across the playa pool. The ice sheets shove rocks in front of them and the moving stones leave trails in the soft mud bed below the pool surface.

National Park Service website

Why had no one seen the rocks move? Water on the playa is rare. And, visitors are prohibited from being there when the ground is wet because foot traffic would tear up the fragile surface. Additionally, the rocks often moved together so it was hard to tell they were moving at all even when you were right there watching. The ice masked the tracks that were revealed only when the surface dried out. The unique combination of a 0.5 in of water, ice formation, and stiff wind is also rare. For these reasons, other playas don’t typically exhibit moving rocks.

You can read the paper they published here: Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion

Some people haven’t gotten the message that the question was solved. Or, they badly want to maintain the mystery because the answer is too mundane to them. That’s a terrible reason to pass on misinformation about the earth. It’s much more satisfying to accept that humans solved a curious puzzle through thinking and technology.