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

[Phrak]

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?

 

[OAQfirst]

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

 

[Phrak]

It's 1,600 × 1,067 pixels at Wikipedia.
http://upload.wikimedia.org/wikipedia/commons/c/c7/Racetrack_Playa_(Pirate_Scott).jpg

Thanks, OAQ. But still not enough to reveal the shapes of the pieces of the ridges of Earth left behind --though the visible ridge is certainly not a monolithic trail of solidified mud.

 

[Ivan Seeking]

Here is a paper from the folks at Cal Tech
http://www.jstor.org/pss/30068068

 

[pallidin]

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.

 

[Phrak]

But what is the coefficient of friction of a rock on tiled clay surface, lubricated by a layer of mud?

 

[pallidin]

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.

 

[Ivan Seeking]

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.

 

[pallidin]

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??

 

[Ivan Seeking]

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?

 

[pallidin]

Ever played air hockey?

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


Edit: Sorry for all the edits... late night.

 

[runner_one]

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.

 

[Ivan Seeking]

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?

 

[runner_one]

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.

 

[Ivan Seeking]

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?

 

[runner_one]

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

 

[Ivan Seeking]

Did you feel compelled to check and be sure that they did not move overnight?

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?

 

[PRDan4th]

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.

 

[runner_one]

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"

 

[Mk]

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

 

[dhundsness]

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.

 

[DaveC426913]

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.

 

[zoobyshoe]

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.

 

[dhundsness]

What's the measured speed of these things? Surely someone has bothered to do some daily/weekly surveys.

 

[Ivan Seeking]

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.

 

[Ivan Seeking]

I suspect that the reason the process has never been witnessed, may be that it only happens at times when conditions are so unfavorable that no one would be there.

 

[pallidin]

No one has ever witnessed the underlying process in action.

Exactly. Why is it that no-one has used our current technology to see this in action?
I find it unimaginable that a university grad has not tackled this.

 

[Ivan Seeking]

Exactly. Why is it that no-one has used our current technology to see this in action?
I find it unimaginable that a university grad has not tackled this.

It could easily be an undergrad project, for that matter.

Arguably, a UT Dallas chapter of the SPS solved the "Marfa Lights Mystery" in one evening.
http://www.spsnational.org/wormhole/utd_sps_report.pdf

 

[dhundsness]

Wind would cause objects to rotate or even tumble. These tracks are so remarkably linear, where the rock has clearly held a constant orientation. Some rocks appear to move sideways rather than lengh-wise, without rotating, which is not how wind would push something. Small lightweight rocks with high profiles don't appear to tumble. And rocks with low, flat profiles appear to move just as much as the bulkier rocks.

I cannot tell if these trails have a *starting* point, at least not from the photos. An event like a wind storm would create a trail from a starting point to an ending point. But if there are no starting points, that suggest a slow gradual process happening continuously, right under everyone's noses.

A few hundred bucks for a solar-powered gps tracker and weather station tacked to one of these things would get some real data.

 

[DaveC426913]

A few hundred bucks for a solar-powered gps tracker and weather station tacked to one of these things would get some real data.

Yeah, it's not quite as easy as you might think. Like other unexplained phenom, the targets don't tend to cooperate so much. When, where, etc. How many rocks can you afford to tag? Which on'es next most likely to go?" How many days, weeks, months do you trudge out to them to replace their batteries on the off-chance that one will cooperate?

 

[Ivan Seeking]

Yeah, it's not quite as easy as you might think. Like other unexplained phenom, the targets don't tend to cooperate so much. When, where, etc. How many rocks can you afford to tag? Which on'es next most likely to go?" How many days, weeks, months do you trudge out to them to replace their batteries on the off-chance that one will cooperate?

An interesting argument in defense of many unsupported claims, here we have a bunch of rocks sitting on a lake bed, and we know that they move, but after a century, no one has been able to catch them moving! Too funny!

A+, Dave, jolly good. I had never quite connected those dots before.

 

[DaveC426913]

A+, Dave, jolly good. I had never quite connected those dots before.

Oh dear, I just defended UFO sightings, didn't I? :blush:

 

[Ivan Seeking]

Oh dear, I just defended UFO sightings, didn't I? :blush:

Not pointing to anything in particular, but not knowing when something might happen, is sufficient to make verification difficult, even if we know where it will happen. I find that to be a highly salient point.

 

[susa]

Might there not be salts in the topsoil working as an agent for providing a sliding ground when hydrated through seasonal humidity fluctuations or even on a daily basis through night and early morning cooling? Such an environmental setting suggests at least some salt accumulation in the toplayer of the basin soils. Also many images show a clear prismatic structure with rounded tops that remind me to a tipical swelling and shrinking feature of Solonetz soils – although these latter are alkaline soils… May be spatially varying salt contents then even could explain (partly) track length and direction changes: where friction gets too strong, rocks might stop and eventually slightly rotate, and under different exposure to the winds move in a different direction.

I can’t imagine how the ice sheet mechanism should work. ?According to this idea where would this sheet form? Right on top of the surface? Wouldn’t the surface then still be too rough to provide for sliding? Also many images show that heavy rocks apparently sunk a little into a soft enough surface layer, leveling the surface in the track by beheading the rounded tops of prisms and creating small dikes on each track side.

 

[Pythagorean]

So... how do the tile domains form as the mud dries?