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August 27, 2014 5:33 PM   Subscribe

The sliding rocks of Racetrack Playa [previously, previously] have finally been observed in motion!
posted by moonmilk (27 comments total) 23 users marked this as a favorite
Yes! I just saw this and I am unreasonably excited by it. Racetrack Playa is one of my favorite odd places.
posted by LobsterMitten at 5:36 PM on August 27, 2014

I love that they actually put a GPS tracker on one of the stones. Like tagging wildlife.
posted by moonmilk at 5:37 PM on August 27, 2014 [2 favorites]

Finally! Maybe now I'll get some sleep!
posted by nevercalm at 5:50 PM on August 27, 2014 [4 favorites]

Science rocks.
posted by rtha at 6:09 PM on August 27, 2014 [4 favorites]

I think Morning Edition will have a segment on this study tomorrow.

No, I'm not psychic. My station ran a teaser.
posted by workerant at 6:34 PM on August 27, 2014 [1 favorite]

Wow. Good science. The explanation makes a lot of sense; I'm surprised nobody came up with it before. I only have one quibble: they describe 5-6 meters a minute as "slow".


Thank you.
posted by Joe in Australia at 6:36 PM on August 27, 2014 [16 favorites]

It would really suck to live downstairs from those rocks. What, are they moving furniture up there at 2am!?
posted by moonmilk at 6:39 PM on August 27, 2014

This is fun.

I love this but I kinda wanted to see a time lapse video rather than a GPS track. I'm a bit suprised that someone hasn't staked a camera. I'm sure that will change.

"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."

I thought a playa was deadly flat but I guess a seasonal wind can scoop out a shallow hollow depending on the density of the material. It is easier to intuit wind pushing ice panels pushing rocks than wind pushing large rocks. The surface area of the rocks just isn't enough. I suppose Nature does some Curling but it seems dislodging the rocks from the mud would require more energy than is supplied by anything short of a tornado or a hurricane, the former would leave a spiral mark and the latter doesn't occur in Death Valley.

Big rocks tend to come to the surface with frost heave - as melting occurs the smaller rocks drop beneath the larger rocks so it's not a suprise to see the bigger rocks on the surface. Death Valley has a temperature range from 134°F [somewhat disputed but you get the idea] to 15°F at Furnace Creek.

Mini glacial erractics [though they are driven growing ice sheet as opposed to wind action on a floating ice sheet] makes sense.

Thanks for the post moonmilk.
posted by vapidave at 6:43 PM on August 27, 2014

Sometimes I think these scientists don't even want to find aliens.
posted by Pyry at 6:47 PM on August 27, 2014 [3 favorites]

That's marvellous. Also, if I ever become a rapper dedicated to equestrianism, I'll be calling myself Racetrack Playa.
posted by misterbee at 6:53 PM on August 27, 2014 [3 favorites]

posted by holyrood at 7:10 PM on August 27, 2014

Great explanation, the problem in the past has been that all the schemes involved decreasing friction so the wind can easily slide the rocks. But the rocks leave deep trenches - obviously not a low friction scenario. Any wind strong enough to make the rock move, would make it tumble instead. However, if its frozen into a sheet of ice, with little sticking down, it won't tumble, and there's extra surface area for the wind to act on.
posted by 445supermag at 7:31 PM on August 27, 2014

The funny thing is, this was almost a version of the classic 'corpse lying on the floor of a room with a broken window and a puddle beside the body' mystery, and we didn't get it.
posted by jamjam at 7:33 PM on August 27, 2014 [1 favorite]

The prevailing theory for a while has been that they were frozen into sheets of ice and then moved by wind action. (Had to be sheets because groups of rocks move in tandem, making parallel tracks that have jogs at the same points etc.) As I understand it, the surprise was that it's thin ice sheets just pushing, not thick ice sheets carrying.
posted by LobsterMitten at 7:51 PM on August 27, 2014

This is fantastic! I was out at racetrack playa last November in an attempt to do some star photography but despite the drought here in CA the racetrack was entirely flooded under about a foot of water. I awoke just before sunrise and have photos of the racetrack frozen over - a freeze that only lasted about an hour as the sun melted it back to standing water. It was windy that night, I wonder if any rocks moved :)
posted by jnnla at 9:10 PM on August 27, 2014 [1 favorite]

Thanks for that, Long Way To Go!

I like this quote from that article: “A baby can get going a lot faster than your average rock.”

Especially a baby on ice on a windy day.
posted by moonmilk at 9:41 PM on August 27, 2014 [1 favorite]

I only have one quibble: they describe 5-6 meters a minute as "slow".

That's pretty much my top speed first thing in the morning.
posted by Faint of Butt at 4:30 AM on August 28, 2014 [1 favorite]

I love that they actually put a GPS tracker on one of the stones. Like tagging wildlife.

I'm not thrilled they bored a hole in it and filled it with cement and electronics. I get that we'd like to understand the mystery behind one of our national wonders, but maybe we can avoid vandalizing it in the process?
posted by Slap*Happy at 5:08 AM on August 28, 2014

It's a rock. Are you saying there's something special about it, beyond its location?
posted by Kirth Gerson at 6:40 AM on August 28, 2014

Didn't they actually bring their own rocks (versus modifying the natural rocks)?

From the article "Limestone blocks were obtained from the Panamint Springs Member of the Permian-aged Darwin Canyon Formation in Darwin Canyon, California (N36.28936, W117.53727), and were modified using a concrete boring tool to create a cavity for the GPS logger"
posted by jindc at 7:34 AM on August 28, 2014 [2 favorites]

So, when are rock-mounted GoPro videos going to start showing up on YouTube?
posted by TedW at 8:28 AM on August 28, 2014

If there were snails there, they'd be surfing these rocks for a thrill.
posted by moonmilk at 8:34 AM on August 28, 2014

Science rocks.

I observed what you did there.
posted by humboldt32 at 12:05 PM on August 28, 2014 [1 favorite]

"Mystery of how rocks move across Death Valley lake bed solved," from the LA Times, has nice photos and a nifty graphic. Via Elementary Penguin's Twitter.

I was going to post this but this is a fine and dandy post. Was going to use the tag 'episodicmovement' and was quite excited about that.
posted by joseph conrad is fully awesome at 6:15 PM on August 28, 2014 [1 favorite]

'episodicmovement' is fully awesome, as is that LA Times article.
posted by moonmilk at 7:20 PM on August 28, 2014 [1 favorite]

NY Times article

Which points to this Scripps/UCSD press release which has a lot of nice details:

They set up the experiment in winter 2011, placing rocks with motion-activated GPS devices and a weather station to measure winds. Two of the co-authors (the Norrises) were visiting the Racetrack to check on the equipment in December 2013. They "arrived in Death Valley to discover that the playa was covered with a pond of water seven centimeters (three inches) deep." The rocks moved on this visit, under the following conditions:
- water on the playa, "deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks"
- nighttime temperatures make thin but sturdy "windowpane" ice
- daytime sun breaks up the ice
- light wind moves the ice sheets and they push the rocks around.
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.”[...]

“We documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks”, says Richard 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?”
posted by LobsterMitten at 11:24 PM on September 1, 2014 [2 favorites]

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