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By Kurt Loft
of The Tampa Tribune
A 700-pound rock rests at the center of a dry lake bed. Night lays a blanket of darkness over the playa. Then, as a hot sun rises the next morning the rock has moved, leaving a mysterious trail across the desert floor.
Known as dancing rocks, rolling stones and boogie boulders, these oddities of nature for decades have kept geologists scratching their heads. A handful of explanations have been offered, but no one is certain precisely how the rocks of Death Valley's so-called "Racetrack" lake bed move.
Geologist Paula Messina of San Jose State University in California hopes to break new ground, using Global Positioning System satellites to track dozens of rocks of all shapes and sizes.
"There's a global interest in this, even though it's such an isolated phenomenon," Messina says by telephone. "I wanted to see a pattern of what all of the rocks do, and I wanted to look at it from above. It clicked as an ideal project to pursue."
Messina, who taught geology in the New York City school system for 20 years before taking a job in San Jose, was fascinated by the serene, eerie atmosphere of Death Valley, which she calls a geologist's dream.
"I was always interested in pictures of the West when I was a kid, and for a city kid I wanted to venture there, to be outside," she says. "When I first went to Death Valley, I had heard of these sliding rocks, and when I went back in 1983 I was so blown away by what I was seeing I had to come back to study what the patterns were."
The mystery of the sliding rocks has been the subject of scientific curiosity since at least 1948, when James F. McAlister, a researcher with the U.S. Geological Survey, published the first official paper on the subject. In 1955, another scientist, George M. Stanley, suggested the rocks slid atop ice sheets that formed at higher altitudes.
In 1976, two scientists disputed the ice hypothesis, saying the rocks traveled along different paths, while an ice sheet would create embedded, parallel patterns. They placed a circle of stakes around the rocks to test the ice-sheet theory. Some rocks moved outside the circle and others didn't, suggesting ice wasn't a factor.
Although Death Valley is the lowest and hottest point in the Western Hemisphere, the rocks originate at about 4,000 feet, where winter brings freezing temperatures and a variety of weather conditions. At least four theories have been explored (excluding alien intervention):
- Vibrations from earthquakes
- Swelling clay that pushes up on the rocks
- Ice sheets that slide downward, carrying objects with them
- The combination of high winds and slick clay
The most popular theory is wind. After rains, a fine, slippery layer of clay forms across the playa, reducing friction. When two columns of strong wind whip across the site, they may push the rocks across the lake bed. When crosscurrents of wind meet, a pocket of low pressure forms, creating mini-tornadoes that may steer some rocks in all directions. This weather pattern could create straight lines and zigzags.
Some scientists believe the shape of the rocks determines their path. Those with angular bottoms have "keels" to steer them in a line, while rocks with flat or rounded bottoms can move in any direction. Messina says the wind-swept terrain also determines a rock's path.
"I say rocks moving in concert with how the topography was shaped around them," she says. "So it was clearly wind blowing over the landscape - a wind tunnel effect."
Dancing rocks in the California desert might not mean much to the average person, but they represent a gold mine for ambitious geologists.
"This started out as a curiosity, but there's a lot of research in the area," she says. "It's s microcosm to study extreme conditions of how things are dispersed in the desert."
The playa - a desert basin that temporarily becomes a shallow lake after heavy rains - doesn't welcome rock watchers. Few people venture to this desolate area, and for good reason.
"If the wind is at hurricane strength, some of these rocks can move rapidly, up to 4 mph," she says. "But no one has seen them in motion. When you have the weather conditions needed to move a 700-pound boulder, you wouldn't want to be there."