The Roving Rocks of Racetrack Playa
From Earth magazine, Feb. 1996
By Larry O'Hanlon
Among the most sought-after and never-witnessed phenomena in Death Valley National Park, California, are the roving rocks on Racetrack Playa. By some freak of wind, ice and mud, large chunks of dolomite have been able to roam about, gouging long trails on the surface of this remote dry lake bed. However, these rocks – some as heavy as 700 pounds – have never been witnessed in motion, making them the target of scientific speculation and investigation as well as old-fashioned desert tall tales.
Aside from the obviously silly explanations – like vague, fourth-hand accounts of the rocks levitating – the seeker of Racetrack truth must be wary of even the honorable National Park Service. Because the NPS can't always be up-to-date with the latest discoveries, it employs some of the world's best tale spinners to fill in the gaps and leave visitors sated and amazed by nature's miraculous handiwork. An unsuspecting traveler might leave Death Valley with fantastic tales of squirrels with super-rodent strength that once pushed the rocks together for shelter in difficult winters.
Less fanciful are the hypotheses of geologists who have ventured out to this empty patch of mud. Over the past 40 years several studies have been published about Racetrack, each trying to explain what makes the rocks move. Geologists have blasted the rocks with wind from airplane propellers to make them budge, and they've even dubbed each rock with a proper name, like Jo and Carol, in order to keep better track of its wanderings. To date, there are two main scientific explanations for how the rocks move.
One scenario holds that sheets of ice help the rocks along on their journeys. During especially wet winters several inches of water can collect on the playa surface, turning it briefly into a lake. On cold nights it's surmised that thin sheets of ice form, locking in rocks. When the strong winds blow through the valley and hit the ice sheets at an angle, the wind pushes with enough force to move the ice and the rocks embedded in it along the slippery surface.
"It's the ice sheets that make it possible," insists geologist John Reid of Hampshire College in Amherst, Massachusetts. "There is no way on earth the wind is going to blow with enough force. You'd need incredibly high winds to get rocks to move by themselves [without ice]."
To prove his point, Reid visited Racetrack with undergraduate students several times between 1987 and 1994, armed with everything from surveying instruments to block and tackle. He wanted to settle, once and for all, just how much force it takes to get the largest rocks moving.
Reid and his students first boxed in the rocks they wished to test and then flooded the dry mud surface inside the box to simulate a rain soaked playa. Next they hooked up block and tackle and measuring devices and pulled until the boulders budged. After crunching some numbers, Reid's team figured out that it would take a 175-mile-per-hour wind to move a moderate 44-pound rock and a 280 mph wind to move the largest 700-pound rock without the help of ice. Their conclusion, which they recently published in the journal Geology, is that wind alone just can't do the trick.
What's more, Reid says, there is direct evidence that ice sheets play an important role. By mapping out the rock locations and the routes of the tracks, Reid and his students found that the rocks purportedly carried in ice sheets leave a peculiar signature on the playa. He likens the pattern to that made by sliding your fingertips rigidly down a steamy mirror and gently twisting your wrist at the same time. The tracks are not quite parallel, but they move in concert, as if the rocks that made them were riding in the same mobile, rotating ice sheet.
At first glance it might seem that Reid and his crowd have found a tidy explanation for the moving rocks. Not quite, according to geologist John Shelton, one of the pioneers in tackling the Racetrack mystery. Shelton published one of the first scientific papers on Racetrack Playa in 1953, and at 82 years old he is still pondering the enigmatic rocks.
"The Hampshire boys [and girls] measured the friction, but under nothing like real conditions," Shelton says. Those "real" conditions, he says, certainly involve freezing temperatures. (Reid's team worked on warm days.) Freezing conditions could create much less friction between the playa surface and the rocks, Shelton says. The ingredients would be a vigorous wind and a firm and slippery enough surface so the rocks don't plow deeply and get bogged down, as they did for Reid's team.
The iceless scenario goes like this: The playa gets soggy with rain and freezes overnight to several inches down in the mud. The next morning the first thing to thaw would be the paper-thin surface layer of clay and gelatinous algae. Voila! One very slippery surface over hard, frozen mud. If the wind blew hard enough under these conditions, Shelton says, the rocks could theoretically slide along with much less friction than the Hampshire team measured.
"The wind alone can do it," Shelton says. But he also thinks that ice may occasionally play a role. Both scenarios could be correct at different times of the year, he notes.
Perhaps the only way to figure out if one, both, or neither explanation is correct is for someone to witness the rocks in action – if that's even possible. It could be that the rare, once- or twice-in-a-decade events happen under such horrible weather conditions that no one could reach Racetrack in time to catch the rocks in motion.
In fact some folks, including Reid himself, hope scientists never quite figure it out the problem and thus ruin Racetrack's mystique. "I'd hate it if the average person thought this was the end of the story," he says.
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