Science

An Icy Answer to the Mystery of the Moving Death Valley Stones

International New York Times | 1 September 2014

Researchers embedded special global positioning system devices in rocks of various sizes brought from outside the park. Credit Mike Hartmann

Researchers have solved a longstanding mystery that has befuddled scientists and tourists alike: why rocks on a dry lake bed in Death Valley National Park in California occasionally move.

The explanation for the phenomenon at Racetrack Playa — named for the long trails that the rocks, some of which weigh hundreds of pounds, leave in the mud — is that the stones are pushed by wind-driven ice that forms and then breaks up under certain conditions.

“It’s a very rare phenomenon,” said Richard D. Norris, a paleobiologist at the Scripps Institution of Oceanography and lead author of a paper describing it in PLOS One. “A brief moment in time.”

Dr. Norris and James M. Norris, a cousin and co-author, actually saw the rocks moving in December when they went to check on the project. “It was complete dumb luck,” Dr. Norris said. “Almost every rock on the playa moved.”

Scientists have offered theories about these “sailing stones” since at least the 1940s. Many thought that wind or ice played a role, but not in the ways that the Norrises and colleagues uncovered. For their study, they embedded special global positioning system devices, designed by James Norris, in rocks of various sizes brought from outside the park, because the park service did not allow them to disturb the existing playa stones.

Richard Norris in 2012 and one of the moving stones that had left a trail, probably more than a decade earlier, at Racetrack Playa in Death Valley. Credit Richard Norris

The playa, about three square miles and pancake flat, is usually dry, though rainwater runoff from the surrounding mountains occasionally fills part of it to a depth of a few inches. It’s when this shallow pond forms, and the nights are below freezing and the days are sunny and warm, that the rocks may move, the researchers said.

On those occasions, the cold night air leaves a sheet of ice barely an eighth of an inch thick. Then the warmth of the sun causes the sheet to break up.

“One moment, it’s quiet and still,” Dr. Norris said. “The next, it’s crackle, crackle, pop, pop, pop.”

As more ice melts, some of these lesser sheets have room to move. Driven by light breezes, the sheets push up against rocks and nudge them. The wind drives the water, too, which also helps push the ice against the rocks.

The movement is slow, no more than about 15 feet a minute. “The rocks are creeping along,” Dr. Norris said, “like a baby on its stomach.”

The playa is normally dry, though occasionally rainwater runoff from the surrounding mountains fills part of it to a depth of a few inches. It’s when this shallow pond forms, and the nights are below freezing and the days are sunny and warm, that the rocks may move, the researchers said. Credit Jim Norris

The researchers have not calculated the forces involved — that is grist for a future study, they said — but clearly, they are enough to move the rocks, aided by the reduced friction that comes from their sitting in the shallow water.

The movements are episodic — the conditions may be just right for a few minutes, and then the sun causes more ice to break and the movement stops. And once the water evaporates, it may be years before the rocks move again, even if the playa floods again, because the precise temperature and wind conditions may not occur.

“What you see 99 percent of the time you’re out there are these fossilized trails,” Dr. Norris said. “Everything has just stopped and gone into hibernation.”

The discovery also explains why many of the trails are parallel to one another. The researchers found that the moving ice sheets were sometimes so big that they would nudge rocks hundreds of feet apart, moving them in the same direction.

Based on existing theories, the cousins had thought that winds as high as 80 or 90 miles per hour might have moved the stones. So early in the two-year study, they discussed buying a sturdy all-season tent for their site visits.

They were surprised to see the rocks moving on a sunny day in a light breeze.

“We didn’t need the tent,” Dr. Norris said.