There is something strange happening on the dusty, cracked surface of Death Valley.
Littered across its dry lake, Racetrack Player, are hundreds of rocks that appear to move all by themselves.
The ‘magic’ force behind these huge rocks – some weighing as much as 700lbs (320kg) – has been a mystery to scientists for nearly a century.
But now, scientists in San Diego believe they have solved the puzzle of the ‘sailing stones’ after seeing the phenomenon happen first-hand.
Because the stones can sit for a decade or more without moving, the researchers Scripps Institution of Oceanography, UC San Diego, did not originally expect to see motion in person.
Instead, they decided to monitor the rocks remotely by installing a high-resolution weather station capable of measuring gusts to one-second intervals and fitting 15 rocks with custom-built, motion-activated GPS units.
The experiment was set up in winter 2011 with permission of the Park Service.
Then, in what Ralph Lorenz, a lead researchers described as ‘the most boring experiment ever’, they waited for something to happen.
But in December 2013, Richard Norris and co-author and cousin Jim Norris arrived in Death Valley to discover that the lake, also called playa, was covered with a pond of water 7cm (3 inches) deep. Shortly after, the rocks began moving.
‘Science sometimes has an element of luck,’ Professor Norris said. ‘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.’
Their observations show that moving the rocks requires a rare combination of events.
First, the playa fills with water, which must be deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks.
As night-time temperatures plummet, the pond freezes to form thin sheets of ‘windowpane’ ice, which must be thin enough to move freely but thick enough to maintain strength.
On sunny days, the ice begins to melt and break up into large floating panels, which light winds drive across the playa, pushing rocks in front of them and leaving trails in the soft mud below the surface.
‘On December 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface,’ said Professor Norris. ‘I said to Jim, ‘This is it!’
These observations upended previous theories that had proposed hurricane-force winds, dust devils, slick algal films, or thick sheets of ice as likely contributors to rock motion.
Instead, rocks moved under light winds of about 3-5 metres per second (10mph) and were driven by ice less than 3-5mm (0.25 inches) thick, a measure too thin to grip large rocks and lift them off the playa, which several papers had proposed as a mechanism to reduce friction.
The rocks moved only a few inches per second (2-6 metres per minute), a speed that is almost imperceptible at a distance and without stationary reference points.
‘It’s possible that tourists have actually seen this happening without realising it,’ said Jim Norris of the engineering firm Interwoof in Santa Barbara.
‘It is really tough to gauge that a rock is in motion if all the rocks around it are also moving.’
Individual rocks remained in motion for anywhere from a few seconds to 16 minutes.
In one event, the researchers observed rocks three football fields apart began moving simultaneously and travelled over 200ft (60 metres) before stopping.
Rocks often moved multiple times before reaching their final resting place. The researchers also observed rock-less trails formed by grounding ice panels – features that the Park Service had previously suspected were the result of tourists stealing rocks.
‘The last suspected movement was in 2006, and so rocks may move only about one millionth of the time,’ said Professor Lorenz.
‘There is also evidence that the frequency of rock movement, which seems to require cold nights to form ice, may have declined since the 1970s due to climate change.’
So is the mystery of the sliding rocks finally solved?
‘We documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks’, says Professor Norris.
‘So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion.
‘But we have not seen the really big boys move out there….Does that work the same way?’
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