What Causes the Rocks to Slide on Death Valley's Racetrack Playa?

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The Racetrack Playa in Death Valley National Park is known for its unique geological phenomenon where boulder-sized rocks appear to slide across the dry lakebed, leaving distinct trails. Despite extensive observation, no one has witnessed the actual movement of these rocks. Theories about the mechanism behind this phenomenon include the influence of strong winter winds, particularly when the playa is wet, creating slippery conditions that facilitate movement. Some suggest that trapped air or bubbles beneath the rocks during rain could reduce friction, allowing them to slide. Others propose that thermal expansion and contraction, as well as variations in surface topography, may contribute to the rocks' movement. The complexity of the environment, including wind patterns and moisture levels, likely plays a significant role in determining how and when the rocks move, leading to varied paths and occasional changes in direction. The phenomenon remains a subject of fascination and speculation, with calls for further scientific investigation, including the potential use of cameras to capture the elusive movement.
  • #31
I wonder if wet mud would erode in the wind. Clays are very cohesive and because of this they can take nearly as much flow velocity to move as boulders. Check out a Hjulstrom curve
 
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  • #33
matthyaouw said:
I wonder if wet mud would erode in the wind. Clays are very cohesive and because of this they can take nearly as much flow velocity to move as boulders. Check out a Hjulstrom curve

I couldn't understand your point at first. Dried, the clay would bond to a stone quite tightly, right? But if enough water desolved the bonds between clay and rock, it could break loose under a gust of wind, then continue under lighter winds, which is pretty much consistant with the extract that Ivan posted.

On top of that, the place should be an excellent site for mud sailing in the right weather.
 
  • #34
http://sfgirl-thealiennextdoor.blogspot.com/2008/08/sailing-racetrack-playa-of-death-valley.html"
During periods of heavy rain, water washes down from the nearby mountain slopes onto the playa and form a shallow short-lived lake. Soon, the thin veneer of water evaporates and leaves behind a layer of soft gooey slippery mud. As it dries the mud shrinks and cracks into a mosaic of interlocking polygons.


Forget the rocks. What did the rocks do? I'd like a close up view of the ridges left on either side of a rock. Does they look like plowed furrows or dried muddy ridges, or something inbetween?
 
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  • #35
Two random photos selected

racetrack_playa_pirate_scott.jpg


sliding-rock.jpg
 
  • #36
Ivan Seeking said:
Two random photos selected

I'll bet we're not going to get better close-ups than those. #1 is the best of the two. I'd like to get in real close and look evidence of water softening on the edges of the cracked clay.

Either way it could be inconclusive, unless one could tell the difference between water softening and wind errosion without experimentation. Who wants to grab and test some samples?
 
  • #37
Phrak said:
I'll bet we're not going to get better close-ups than those. #1 is the best of the two. I'd like to get in real close and look evidence of water softening on the edges of the cracked clay.

Either way it could be inconclusive, unless one could tell the difference between water softening and wind errosion without experimentation. Who wants to grab and test some samples?
It's 1,600 × 1,067 pixels at Wikipedia.
http://upload.wikimedia.org/wikipedia/commons/c/c7/Racetrack_Playa_(Pirate_Scott).jpg
 
  • #39
Here is a paper from the folks at Cal Tech
http://www.jstor.org/pss/30068068
 
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  • #40
This is just a thought, but I think I might know what's happening here.

First, let's examine this statement from http://geology.com/articles/racetrack-playa-sliding-rocks.shtml
----------------------------

The climate in this area is arid. It rains just a couple of inches per year. However, when it rains, the steep mountains which surround Racetrack Playa produce a large amount of runoff that converts the playa floor into a broad shallow lake. When wet, the surface of the playa is transformed into a very soft and very slippery mud.

----------------------------

Now, and here is what I believe might be the key: The surface prior to water saturation is cracked. Relatively deep cracks and many of them.

When water falls on top, air is trapped in the cracks. During contraction of those cracks by virtue of the water, this scenario would likely cause the formation of massive amounts of tiny, practically imperceivable bubbles rising to the surface and underneath any large rock.

Then, with this bubbling "cushion", other external forces moves the rock.

Anyway, that's my hypothesis.
 
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  • #41
But what is the coefficient of friction of a rock on tiled clay surface, lubricated by a layer of mud?
 
  • #42
Phrak, I understand what you are saying. Which is why it's so puzzling.
However, let's consider the following(these are my presumptions, not saying that they are correct):

1) There is a presumption that this phenomenon is not a "prank"

2) There is a further presumption that this phenomenon is specific to that location, specific to the geology and topology of that location, and specific to the unique, sometimes transient extreme environmental conditions at that location.

So, my summary presumption is that the phenomenon is real, but that the natural mechanism behind it is based on complex factors which must come together in some unique way to achieve this.

As such, when addressing issues such as the coefficient of friction, I am looking at what complex natural forces might play a role.
Thus my hypothesis that trapped air/rising bubbles from the cracked surface during a rainstorm might be a key contributing factor mitigating friction.

Even still, it's just my thoughts.
 
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  • #43
pallidin said:
1) There is a presumption that this phenomenon is not a "prank"

I think the notion of a prank was ruled out about fifty years ago. IIRC, the phenomenon has been noted for about a century now. Also, we are talking about death valley, which is remote, desolate, and where it was about 120 degrees yesterday, as is usually the case.

Back when this was first discovered, which is cited as being around the turn of the century, Death Valley was about as remote of a place as one could find.
 
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  • #44
Ivan Seeking said:
I think the notion of a prank was ruled out about fifty years ago. IIRC, the phenomenon has been noted for about a century now. Also, we are talking about death valley, which is remote, desolate, and where it was about 120 degrees yesterday, as is usually the case.

Back when this was first discovered, which is cited as being around the turn of the century, Death Valley was about as remote of a place as one could find.

OK, that was my presumption. What about my hypothesis??
 
  • #45
pallidin said:
OK, that was my presumption. What about my hypothesis??

The bubbles? It sounds to me like a reasonable idea, but I'm not a geologist. I can certainly imagine that something like you suggest might be possible at least in principle.

Ever played air hockey?
 
  • #46
Ivan Seeking said:
Ever played air hockey?

Exactly, that's what I'm getting at.


Edit: Sorry for all the edits... late night.
 
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  • #47
Hello all, I'm new here and thought I might jump in here and let you all know I visited the racetrack this week and have some good photos of the stones and trails. If anyone is interested here is a link to them (no ads) http://www.coleskingdom.com/photos/album.php?dir_name=racetrack I will upload more later.
 
  • #48
runner_one said:
Hello all, I'm new here and thought I might jump in here and let you all know I visited the racetrack this week and have some good photos of the stones and trails. If anyone is interested here is a link to them (no ads) http://www.coleskingdom.com/photos/album.php?dir_name=racetrack I will upload more later.

Cool! Was this trip motivated by casual or professional interests?

Were you left with any particular impressions wrt the mechanism?
 
  • #49
Ivan Seeking said:
Cool! Was this trip motivated by casual or professional interests?

Were you left with any particular impressions wrt the mechanism?

More Casual, although I have been fascinated by the racetrack ever since I first learned of it years ago. This was my fist visit there and we spent the night. The road there seems designed to destroy any vehicle short of a ATV, I suspect the park service keeps it that way to deter more visitors who might damage or move the rocks. as for the rocks themselves, I came away more perplexed than ever, although the wind is thought by most to be the prime mover I found some problems with this theory first look at this rock http://runner.coleskingdom.com/pics/racetrack/large/Dsci0123.jpg" All in all it was a fascinating trip.
 
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  • #50
What would you estimate to be the depth of the tracks left behind? Does this tend to vary according to the size [weight] of the rock?
 
  • #51
Ivan Seeking said:
What would you estimate to be the depth of the tracks left behind? Does this tend to vary according to the size [weight] of the rock?
There seems to be no rhyme or reason to the depth of the tracks. for example this rock http://runner.coleskingdom.com/pics/racetrack/large/100_4132.jpg"
which was one of the largest, has the most shallow track while this much smaller rock http://runner.coleskingdom.com/pics/racetrack/large/100_4093.jpg" has a much deeper track. the differences in depth could however be attributed to weathering, (I.E. the larger rock moved much longer ago.) However I did notice that one consistency, the smaller rocks in general seem to always have the deeper tracks. However I did not spend enough time there to verify this observation scientifically
 
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  • #52
Did you feel compelled to check and be sure that they did not move overnight? :biggrin:

Sorry, but as I was thinking about it, I decided that the impulse would have struck me.

What would you say was the deepest track that you saw? They look to be about a quarter of an inch?
 
  • #53
It seems to me that wind must play a role in this phenomenon as the tracks cross in your picture "4132.jpg". The dry lake bed must freeze in the winter with a thin layer of ice. The strong wind moves the rocks slightly downwind compressing the ice layer leaving a slight indentation in the soft sand base of the dry lake. Gravity may also play a role if the surface is not perfectly flat. It is a very curious mystery.
 
  • #54
runner_one said:
and I found one rock that appeared to be following the track left by another. ( I will have to find that photo and upload it later.)
I am now back home in Tennessee after the road trip of a lifetime, I have uploaded all 229 images of our Racetrack Playa trip to http://www.coleskingdom.com/photos/album.php?dir_name=racetrack&thumbnails=large"
There are photos there taken by more than one person so forgive the different styles.
They also include photos of the trip out and back.
But as I stated before I found more than one rock that "appeared to" be following the trail left by another. Whether this is a fact, coincidence, or maybe just a trick of the light, or even some practical joke played by a previous visitor I don't know, but as I promised I would link to those when I found them.
http://runner.coleskingdom.com/pics/racetrack/large/DSC02474.JPG"
http://runner.coleskingdom.com/pics/racetrack/large/DSC02485.JPG"
http://runner.coleskingdom.com/pics/racetrack/large/100_4118.JPG"
 
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  • #55
Add self-rolling snowballs to the list...
http://www.telegraph.co.uk/topics/weather/6950788/Snow-stories-rare-self-rolling-snow-balls-found-in-UK.html
 
  • #56
I suspect the daily thermal expansion and contraction of the rocks may cause them to creep a small increment everyday, like an inchworm. This would work only on rocks which happen to have ratchet-like grooves on the bottom in roughly the same direction, so the edges slide in one direction, but grab traction in the other direction. This would explain why different rocks move in different directions, but some don't move at all. And as the bottom surface deteriorates, that would explain why some rocks suddenly change direction.

Winter probably has more effect, with greater temperature extremes, plus ice and rain. Also the ice sheets may help steer the rocks to move more or less in the same directions. The angle of the sun would cause the ice to melt on the sunny side, but remain frozen on the shaded side, causing differential friction. Weather patterns may also influence different rocks to move and change directions in parallel. For example, if there is cloud cover every morning for a month, the sun wouldn't break till it's in the south, while otherwise it would break in the east, changing which side melts first over time.

I imagine you could demonstrate this motion by putting a heavy metal plate with ratcheted grooves on an even wooden(?) surface in a sunny location -- most likely an indoor terrarium to rule out wind and human interference.
 

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  • #57
dhundsness said:
I suspect the daily thermal expansion and contraction of the rocks may cause them to creep a small increment everyday, like an inchworm. This would work only on rocks which happen to have ratchet-like grooves on the bottom in roughly the same direction, so the edges slide in one direction, but grab traction in the other direction. This would explain why different rocks move in different directions, but some don't move at all. And as the bottom surface deteriorates, that would explain why some rocks suddenly change direction.

Winter probably has more effect, with greater temperature extremes, plus ice and rain. Also the ice sheets may help steer the rocks to move more or less in the same directions. The angle of the sun would cause the ice to melt on the sunny side, but remain frozen on the shaded side, causing differential friction. Weather patterns may also influence different rocks to move and change directions in parallel. For example, if there is cloud cover every morning for a month, the sun wouldn't break till it's in the south, while otherwise it would break in the east, changing which side melts first over time.g

I imagine you could demonstrate this motion by putting a heavy metal plate with ratcheted grooves on an even wooden(?) surface in a sunny location -- most likely an indoor terrarium to rule out wind and human interference.
It has to happen muuuuuch faster than that to leave the lengths of trails they leave before the trails are eroded by weather.
 
  • #58
There have recently been two separate programs about this on the History Channel. In one a hapless team of Mythbusters-type characters did a poor debunking of the wind + rain theory. They had a 70mph fan set up but clearly failed to saturate the clay to the point of slipperiness required.

For the rocks to be blown, the clay would have to be dried to "leather hardness" as potters say, which it is out in Death Valley, and then suddenly flooded with a surfeit of water, which it is when it rains. This would create a thickish layer of "slip" (as potters call very watery clay-water mixture), over a very hard bed of dried clay. The debunkers merely sprayed the clay with spray bottles, dampening the surface. No where near enough "slip" was created. That there is plenty of "slip" out in the desert is in evidence from the dried banks of it on either side of the tracks behind the rocks.
 
  • #59
What's the measured speed of these things? Surely someone has bothered to do some daily/weekly surveys.
 
  • #60
dhundsness said:
What's the measured speed of these things? Surely someone has bothered to do some daily/weekly surveys.

No one has ever witnessed the underlying process in action.
 

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