For years, Death Valley’s “sailing stones” attracted visitors and puzzled scientists. In a remote area of the national park, large stones move across an arid playa leaving tell-tale tracks behind, some as long as a football field — and no one could explain what force propelled them. Some blamed “magnetism”; others, aliens. Previous experiments ruled out dust devils and powerful winds as the cause. For decades, the rocks’ movement seemed like one of those natural mysteries that would never be solved. But now, a Johns Hopkins physicist has finally figured it out.
Ralph Lorenz came to Death Valley in 2006 to work on a project for NASA. (The park serves as an approximation of Mars.) During the project’s off-season, Lorenz used the NASA equipment to study the rocks. Working from earlier theories about underground ice shelfs, Lorenz researched arctic tidal beaches and other extreme environments. But the real aha! moment came when he brought the experiment home — to his kitchen table, in fact:
“I took a small rock, and put it in a piece of Tupperware, and filled it with water so there was an inch of water with a bit of the rock sticking out,” he told Smithsonian Magazine. “I put it in the freezer, and that then gave me a slab of ice with a rock sticking out of it.” When he turned that rock/ice upside down and floated it in a tray of water with sand on the bottom — a tabletop approximation of Death Valley’s deep geography — he could move the ice-embedded rock by blowing on it gently.
In scientific terms, Lorenz says, “Basically, a slab of ice forms around a rock, and the liquid level changes so that the rock gets floated out of the mud. It’s a small floating ice sheet which happens to have a keel facing down that can dig a trail in the soft mud.” It’s an explanation that doesn’t require huge ice sheets or powerful gusts of wind, and many consider it the most plausible explanation to date.
Nonetheless, some Death Valley visitors refuse to accept Lorenz’s theory. “People always ask, ‘what do you think causes them to move?’ But if you try to explain, they don’t always want to hear the answers,” park ranger Alan Van Valkenburg told Smithsonian. “People like a mystery—they like an unanswered question.”
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