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Death Valley Moving Stones Solved Assignment

Mystery Behind the Sailing Stones of Death Valley

Rocks at Racetrack Playa seem to move on their own, leaving mysterious trails behind.


These are the “sliding rocks” or “sailing stones” of Death Valley. First documented by miners back in the 1900s, these rocks range from pebbles to 600-pound (272-kilogram) boulders and seem to move of their own accord. The only evidence of their activity is a series of long, perplexing trails left in the valley’s dried mud.

Scientists have been trying to solve the mystery of the sliding rocks since 1948, and have since proposed all manner of explanation: dust devils, flooding, ice sheets, hurricane-force winds, and algal films.

Geologists previously speculated that some combination of wind, rain and ice would have a role. But few expected that the answer would involve ice as thin as windowpanes, pushed by light breezes rather than strong gales.
But thanks to new photographic and meteorological evidence presented by Richard Norris of the Scripps Institution of Oceanography and James Norris of Interwoof, we may finally have a conclusive answer as to what makes the stones stir. The researchers’ findings were published in the journal PLOS ONE.

A Goldilocks Phenomenon

According to Norris, a geological oceanographer and paleontologist, Death Valley’s rocks move under a delicate mix of water, ice, sun, and wind. Norris and his cousin were able to document the rock movements by inserting GPS tags into chunks of limestone and syncing their movements with readings taken from a custom-built weather station.

Unlike one of the previous explanations, which had thick ice capturing the rocks and carrying them along like a miniature glacier, Norris said their evidence shows that thin ice floes break up and pile against the rocks. This creates enough friction to cause the rocks to skim across the muddy surface of a temporary pond. If you were there to see it, the rocks would look like ice-breaker ships plowing through sea ice—though in this instance, it’s the ice that’s moving the ships.
But the conditions have to be just right, what Norris called a sort of Goldilocks phenomenon. If the ice is too thick, or the day is too sunny, or the wind isn’t steady enough, then nothing happens.

Oh, and you have to have standing water—a rare phenomenon in itself for an area that receives less than 2 inches of rainfall annually.

“The process of ice breaking up and shoving rocks around happens every year if you go up into Saskatchewan or Ontario, but you don’t normally associate it with a hot, dry place like Death Valley,” said Norris. “And yet here’s the same kind of process unfolding occasionally—very occasionally—in this place that we associate with a very different kind of climate.”

Scientists have long known that whatever it is that causes the stones to move, it doesn’t happen very often. In fact, the mudflat where you’ll find the rocks and their trails, called Racetrack Playa, can go a decade or more without showing any new signs of movement.

A balance of very specific conditions is thought to be needed for stones to move:
  • A flooded surface
  • A thin layer of clay
  • Wind
  • Ice floes
  • Warming temperatures causing ice breakup

The researchers’ findings were published in the journal PLOS ONE.

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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