Racetrack Playa Mystery In Death Valley Solved Assignment

Racetrack Playa researcher Richard Norris standing by a trail likely formed more than a decade before this December 16, 2012 photo. Trails can last for years or decades between events. Photo from Richard Norris courtesy of Scripps Oceanography

The phenomenon of the "sailing stones" on Racetrack Playa in Death Valley National Park has baffled scientists for decades.

By some mysterious force of nature, rocks move along the flat-as-a-pancake playa and leave long trails behind. What causes the stones to move?

Parallel trails carved into the wet, mud-cracked surface of Racetrack Playa in Death Valley. Photo by Jim Norris courtesy of Scripps Oceanography

One popular theory was that strong winter winds upward to 90 mph combined with just enough rain to make the clay slippery caused the stones to "sail."

Another is that ice sheets pick up the rocks, or ice forms around the rock enabling it to move with the wind, leaving a series of rock trails.

But now, the mystery is solved.

Scientists can say conclusively that these synchronized trails left by rocks, some up to 700 pounds, are caused by thin sheets of ice pushing the rocks across the desert floor under certain conditions, a theory that had been previously dismissed in 1976 after a test.

The conclusion was reached by a team led by paleobiologist Richard Norris of the Scripps Institution of Oceanography, UC San Diego, with the results published Wednesday in the journal PLOS ONE.

Scripps Oceanography details the phenomenon in this six-minute video (it also illustrated the event on a whiteboard):

As part of the Slithering Stones Research Initiative, researchers custom built motion-activated GPS units and fitted them into 15 rocks and placed them on the playa in the winter of 2011, with permission from the National Park Service. They expected it would take five to 10 years before something happened.

A GPS tracking unit was fitted into 15 rocks that were placed on the Racetrack Playa. Photo by Richard Norris courtesy of Scripps Oceanography

Ralph Lorenz, one of the paper's authors from Applied Physics Laboratory at John Hopkins University, called it "the most boring experiment ever."

But in December 2013, something happened.

"Science sometimes has an element of luck," Norris said. "Only two years into the project, we just happened to be there at the right time to see it happen in person."

Three inches of water covered the playa and shortly after their arrival, rocks began moving. The study showed that sailing rocks require a rare combination of these events:

1. The playa fills with water deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks. 2. As overnight temperatures drop, the pond freezes to form thin sheets of "windowpane" ice. 3. When the sun comes out, the ice begins melting and breaking up into large floating panels. These ice panels, driven by light winds, push the rocks ahead of them, leaving trails in the soft mud below the surface. When the playa dries out months later, the trails become clear.

Example of “windowpane” ice collected on the Racetrack Playa. It was much thinner than expected. Photo by Richard Norris courtesy of Scripps Oceanography

"On Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface," said Richard Norris. "I said to Jim [Norris, a cousin], 'This is it!'"

Indeed it was.

Forget hurricane-force winds, the rocks were moved by quarter-inch thick ice panels by light winds of 10 mph. The rocks moved only a few inches per second or a speed deemed imperceptible at a distance without a stationary reference point.

"It's possible that tourists have actually seen this happening without realizing 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."

Lorenz said the last suspected movement previously was in 2006, so rocks may move only about 1 millionth of the time, and there is evidence to suggest that the frequency of rock movement has declined since the 1970s because of climate change.

Racetrack Playa is partly flooded shortly after the December 21, 2013 move event in which hundreds of rocks scribbled trails in the mud under the floating ice. Photo by Richard Norris courtesy of Scripps Oceanography

Asked if the mystery of sliding rocks has finally been solved, Richard Norris replied, "We documented five movement events in the 2 1/2 months the pond existed and some involved hundreds of rocks. 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?"

No word whether the Slithering Boulder Research Initiative is now forming.

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Photo by Scott Beckner via Flickr CC license

The discovery of the mechanism that causes “sailing stones” in Death Valley is actually a great example of observational science, it just too bad these scientists ruined the announcement by blaming “climate change” instead of the things that ACTUALLY drive the flooding of the playa, like regional weather patterns, the PDO, and ENSO. Watch the video.

Here are frames of that video, with annotations, click for a very large image:

Here is how they describe it, from the Scripps Oceanograpgy website:

Racetrack Playa is home to an enduring Death Valley mystery. Littered across the surface of this dry lake, also called a “playa,” are hundreds of rocks – some weighing as much as 320 kilograms (700 pounds) – that seem to have been dragged across the ground, leaving synchronized trails that can stretch for hundreds of meters.

What powerful force could be moving them? Researchers have investigated this question since the 1940s, but no one has seen the process in action – until now.

In a paper published in the journal PLOS ONE on Aug. 27, a team led by Scripps Institution of Oceanography, UC San Diego, paleobiologist Richard Norris reports on first-hand observations of the phenomenon.

Because the stones can sit for a decade or more without moving, the researchers 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 National Park Service would not let them use native rocks, so they brought in similar rocks from an outside source.) The experiment was set up in winter 2011 with permission of the Park Service. Then – in what Ralph Lorenz of the Applied Physics Laboratory at the Johns Hopkins University, one of the paper’s authors, suspected would be  “the most boring experiment ever” – they waited for something to happen.

But in December 2013, Norris and co-author and cousin Jim Norris arrived in Death Valley to discover that the playa was covered with a pond of water seven centimeters (three inches) deep. Shortly after, the rocks began moving.

“Science sometimes has an element of luck,” Richard 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 nighttime 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 Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface,” said Richard 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 meters per second (10 miles per hour) and were driven by ice less than 3-5 millimeters (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. Further, the rocks moved only a few inches per second (2-6 meters 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 realizing 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 traveled over 60 meters (200 feet) 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 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.”

Sigh. Surely they know better.

Here is the paper from PLOS One: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0105948

Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion

Citation: Norris RD, Norris JM, Lorenz RD, Ray J, Jackson B (2014) Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion. PLoS ONE 9(8): e105948. doi:10.1371/journal.pone.0105948

Abstract

The engraved trails of rocks on the nearly flat, dry mud surface of Racetrack Playa, Death Valley National Park, have excited speculation about the movement mechanism since the 1940s. Rock movement has been variously attributed to high winds, liquid water, ice, or ice flotation, but has not been previously observed in action. We recorded the first direct scientific observation of rock movements using GPS-instrumented rocks and photography, in conjunction with a weather station and time-lapse cameras. The largest observed rock movement involved >60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 m between December 2013 and January 2014 in multiple move events. In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3 to 6 mm, “windowpane” ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of ~4–5 m/s. Floating ice panels 10 s of meters in size push multiple rocks at low speeds of 2–5 m/min. along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice.

h/t to WUWT reader Joel O’Brien

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August 29, 2014 in Curious things.
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