Leave No Stone Unturned: What Propels the Racetrack Playa Moving Rocks?
Racetrack Playa is a dried-up lake located in Death Valley, California, USA. This dry lake is notable for its sailing stones, which are known also as sliding stones, rolling stones, or moving rocks. The sailing stones are a geological phenomenon that has been observed since the early 1900s. Although they have been studied for a long time, for decades no one managed to provide an adequate explanation for this curious phenomenon. Recently, however, a team of scientists have succeeded in unravelling the mystery of the Racetrack Playa Sailing Stones.
Examining the Sailing Stones
Sailing stones are recorded to exist in other parts of the world as well, though those of Racetrack Playa are the most famous. The sailing stones are named as such due to their apparent movement over the dried bed lake of Racetrack Playa. As a result of the stones’ movements, trails, which may stretch for hundreds of meters, are left behind them in the cracked mud. The trails were the only indication that the rocks move, as no one is known to have actually seen the stones move in person. This only changed in the age of GPS tracking technology.
Sailing Rocks or Sailing Stones at the Racetrack Playa. (Mike Baird/CC BY 2.0)
This geological phenomenon has been known by humans since the early part of the 20th century. In addition, research has been conducted over the decades in an attempt to explain how these stones could move on their own.
Various hypotheses have been put forward, though some more plausible that others. Some people, for instance, attributed the movement of the sailing stones to aliens, whilst others claimed that the stones have magical properties. Yet others speculated that the stones move due to magnetism, or as a result of mysterious energy fields.
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Star Trails over the Racetrack Playa moving stones, Death Valley National Park, California. (Diana Robinson/CC BY NC ND 2.0)
Testing the Hypotheses on How the Stones Move
Other researchers have carried out experiments to test the validity of their hypotheses. For example, in 1948, two geologists, Jim McAllister and Allen Agnew, proposed that the stones were able to move as a result of dust devils, in combination with the dried lake’s intermittent flooding. This hypothesis was tested four years later by another geologist. For the experiment, a stretch of the lakebed was soaked, and a plane’s propeller was used to create strong winds, which simulated the dust devils. The results were inconclusive.
Sailing Stones at the Racetrack Playa. (John Fowler/CC BY 2.0)
Apart from wind, another popular hypothesis was that ice was the cause of the stones’ movement. One experiment to determine if ice was the culprit was conducted during the early 1970s. Two geologists, Robert Sharp and Dwight Carey visited the site twice a year, and meticulously recorded the movement of 30 stones. They planted wooden stakes around the stones, speculating that if ice sheets were responsible for the stones’ movement, the ice would freeze to the stakes, stopping the stones from moving. In spite of this, some stones still managed to move.
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Roaming rocks at Racetrack Playa. (John Fowler/CC BY 2.0)
In 2006, a planetary scientist by the name of Ralph Lorenz was setting up a series of miniaturized weather stations in the Death Valley as part of a project with NASA. It was during this project that Lorenz came to know about the sailing stones and decided to have a go at solving this mystery. Lorenz reviewed the scientific literature for other instances of rocks moving on their own, though in environments different from that of the Racetrack Playa. He learned that ice, being buoyant, helped float rocks onto Arctic tidal beaches. This idea was then applied to the sailing stones of Racetrack Playa.
A Slippery Situation?
Lorenz and his team observed that some of these trails looked as though one stone had hit another and was somehow repelled or bounced off. The scientists then hypothesized that this may be possible if the stone had a ‘collar of ice’ around it.
To test this, Lorenz decided to conduct an experiment using a small stone, a Tupperware with water, and a tray of water with sand at the bottom. The stone was placed in the water, with a bit of the stone sticking out. This was then placed in the freezer, so that the water could form a collar of ice around the stone. The stone was then flipped, so that its un-iced part was on the bottom and placed in the tray. When the ice was gently blown, the stone moved quite easily, leaving a trail in the sand. The team also calculated that under certain winter conditions in the area, this hypothesis might actually work.
Nevertheless, this seemed to work only with the small stones, and the movement of the big ones (some weighing more than 300 kg (661.39 lbs.)), had not been fully explained yet. But Lorenz was certainly getting closer to the solution.
Moving rocks at Racetrack Playa. (Arno Gourdol/CC BY NC SA 2.0)
The Final Solution of the Racetrack Playa Mystery
The final solution seems to have been arrived at through a study set up in 2011 by Richard and James Norris, in conjunction with Ralph Lorenz. The team fitted 15 rocks with motion activated GPS units, and also set up a high-resolution weather station. They expected a long wait, as some of the rocks would not move at all over 10 years. But in 2013 the team got extremely lucky. The conditions were right, and upon visiting the site they found the playa covered with a pool of water just 3 inches (7cm) thick. And then the magic happened. The rocks began to move.
"Science sometimes has an element of luck," Richard Norris said, according to NPS. "We expected to wait five or ten years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person."
It turns out, the rocks are moved due to a rare combination of circumstances involving just the right ice. The amount of water on the playa must be right, only enough to allow a thin layer of floating ice. In the sun of the day, the ice breaks up to form large floating segments. These are moved around the playa with very slight winds, and the force of these heavy sheets is enough to push the rocks in front of them through the soft mud in which they sit. The problem of the sliding stones was solved!
Top image: A Racetrack Playa Sailing Stone. Source: Trey Ratcliff/ CC BY NC SA 2.0
By Wu Mingren
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