One of the hottest places on the surface of the Earth is home to one of its most puzzling geological mysteries.
The sliding rocks, from pebbles to 500 pound boulders, of Death Valley National Park's Racetrack Playa have long puzzled scientists and park visitors. The rocks seem to defy nature, moving hundreds of yards at a time, sometimes uphill. Some slide in straight lines or circles, others zig and zag. They leave behind long, though erratic, grooves in the flat basin that raise more questions than they answer. The most obvious: What makes these amazing rocks move?
Although they were first reported about 90 years ago and studied extensively for decades, the results are still subject to debate. Part of the issue is that scientists must overcome several challenges to collect results. For instance, trails left by the sliding rocks are usually short-lived; a single rainstorm can wash away smaller trails, and even the deepest grooves last no more than a few years. The most challenging aspect of the mystery, though, is that no one has ever actually seen the rocks move.
"I try to explain that to people by pointing out that it is probably a pretty extreme set of weather conditions that causes the rocks to slide," says Alan Van Valkenburg, park interpreter at Death Valley. "We're talking about winds strong enough to move rocks that are several hundred pounds. No one would want to be out there-they'd move, too!"
The playa's remoteness-it is in the northern half of the park at the end of a 30-mile trip on a treacherous dirt road-also explains why no one has as yet seen the rocks slide, and the conditions under which the rocks move are sporadic. "We'll see the rocks not move for years and then they suddenly move halfway across the lake bed," says Van Valkenburg. "When they move, they don't creep along."
The most widely accepted theory for how the rocks slide in the playa centers on mud and high winds. The theory suggests that water travels from nearby mountain slopes onto the playa and creates mud. When that mud becomes slippery-but not so soft that the rocks sink-it creates a highly slick surface. Strong, high winds capable of moving boulders of several hundred pounds come through the nearby mountains.
An in-depth study of the playa in the 1990s confirmed much of the theory, finding that the movement of most of the rocks cor-related to the strongest winds, according to the U.S. Geological Survey. But other theories persist, such as the belief that sheets of ice at the surface of the lakebed create a raft upon which the rocks slide. The haphazard nature of the rocks' movement casts doubt on that theory, because a sheet of ice would likely send all nearby rocks in a similar direction.
Among the theories that have been discredited is that gravity is the real culprit-that the rocks slide downhill on a sharply narrow slope. Scientists later noted that the rocks tend to slide toward the northern end of the playa, which is slightly higher than its southern end-and the rocks were, in fact, moving uphill. Other theories were much more easily discredited, says Van Valkenburg: "People have sometimes accused the Park Service of moving them, or attributed it to flying saucers."
The fascination that visitors have with the playa makes it one of the Park Service's most talked-about spots, officials say-but it also endangers the area. Overly excited visitors sometimes try to drive on the lakebed or walk on it when it is muddy, which obscures tracks left behind by the rocks. Others break the law by removing the rocks.
"I guess that people think they are taking a magic rock away with them," says Van Valkenburg. "But once the rock leaves the playa, it loses its magic."
