Northern America

The rocks that move

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On a dry lakebed in California’s Death Valley, stones weighing up to 300kg zip across the surface without human intervention. Despite 60 years of study, geologists are still unsure why.

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Ever since the first scientific paper on it was published in 1948, the Racetrack Playa, a smooth desert floor in Death Valley National Park, has had geologists scratching their heads. With irregular frequency, sometimes every third or fourth year, giant rocks travel inexplicably across the surface; some in straight lines, others in zig-zag patterns; others, again, across a certain distance, before turning 180 degrees and continuing in another direction. The evidence is there to see; each rock leaves a deep trail along the desert floor. But no one has ever seen them move.

The surrounding environment – Death Valley – is a land of extremes; harsh natural forces that geologists believe impact the stones’ movement. There are towering mountain peaks, naked valleys, lakes and deserts; gusting winds, sandstorms, thunderstorms and flash floods. In winter, temperatures are reasonably pleasant, with cool nights that occasionally reach the freezing point. In summer, however, it is one of the hottest and driest places on earth. Temperatures above 50˚C are not uncommon (the record, from July 1913, stands at 57˚C), and the annual rainfall is less than two inches per summer. In 1929 and 1953, no rain was recorded at all. So severe are the conditions that the park’s management staff have created a survival guide for hikers.

In the north-western part of this uncompromising area lies the Racetrack Playa, a 4.5-kilometre-long, 2-kilometre-wide lakebed, created 10,000 years ago when a lake evaporated and left behind a 300-metre layer of mud. Here, on the southern end, rocks tumble down two steep hills and onto the playa, before racing off towards the north. “The fact that they move at all is fascinating,” says Allen Glazner, professor in geology at the University of North Carolina, “and that the Racetrack is the only place where this has happened”.

Theories have been formed about the stones’ driving force. Animal intervention and earthquakes have been suggested, as well as mere gravity. But all earthquakes are recorded, and none have coincided with the rocks’ movements. Animals? Footprints would have left visible tracks in the mud. As for gravity, the lakebed has a mere 5-centimetre incline towards the north. The stones are actually travelling uphill.

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The most likely theory is based on the extreme weather conditions. In wintertime, after rain, a thin layer of water covers the surface. “Two or three days after a storm, it creates a kind of Teflon surface on the playa,” says Paula Messina, geology professor at San José State University, who has published several leading studies on the rocks. “Once this is slippery enough, it doesn’t take much wind to make the rocks move.”

It is a plausible argument, and a widely accepted one. But questions remain. Studies show that the stones’ movement are unrelated to their sizes and shapes. Large rocks can move further than small ones. Some travel in groups, others alone. Patterns can go in straight lines, curves and circles. “The complexity of the tracks, and the way that one stone could hit another and then tumble, was a surprise,” says Glazner. According to one theory, thin ice sheets form under the rocks, then carry several of them at a time. But this was partly discredited by another study, in which a corral of stakes was erected around a group of rocks. Eventually, one stone moved outside it. The others remained in place.

Messina has another theory. Her studies suggest the strong winds that supposedly move the rocks arrive through two lowered mountain passes in the south. These work as natural wind tunnels. “It’s like if you take a garden hose,” she explains. “If it’s running and you stick your finger over it, the water should run a lot faster.” Interestingly, she says, “the rocks have the most erratic tracks where the confluence of these two wind tunnels emerge”. That would appear to clarify why some of the patterns are so irregular.

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But will we ever see them move? “I have talked to people about putting sensors on rocks, and putting out cameras,” says Glazner. “The problem is that the observer would have to be there when the wind was blowing at 90 or 100 miles per hour [145 or 160 kilometres per hour], which is unlikely.” And as Messina points out, if the wind can move rocks that weigh 300kg, what will it do to a normal person?

A further obstacle is that the national park is 95 per cent designated wilderness, which means research deemed ‘invasive’, such as radio transmitters, is forbidden. Some years ago, however, one study did get permission to erect equipment. For two winters, monitors were set up in November. When they collected the data in May, the results revealed little. Not a single stone had moved.

“Barring someone observing them, it is impossible to know what is really happening,” says Messina. Not that she minds. “I’m kind of happy no one has ever seen them move. It maintains this level of mystery. It’s so mysterious, and yet we know that it’s something natural.”

Photos: Bill Perry, Bryan Brazil, Nagel Photography [all via Shutterstock.com].

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