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

In summary, a new paper claims that the rocks at the Racetrack Playa show evidence of sliding due to the force of wind.
  • #71
Seems windish to me:

The prevailing winds that blow across Racetrack Playa travel from southwest to northeast. Most of the rock trails are parallel to this direction.

Nasa took some interns there this year:

They confirmed earlier observations that some of the big rocks have moved farther than the small ones.


And look!

Rilee later fed this information into a model that can be used to determine where on the playa a photo was taken even if no GPS coordinates were documented. Soon, any visitor to Racetrack Playa will be able to upload photos for analysis at www.racetrackplaya.org.

http://geology.com/articles/racetrack-playa-sliding-rocks.shtml
http://geology.com/nasa/racetrack-playa/
 
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  • #72
Pythagorean said:
So... how do the tile domains form as the mud dries?

Contraction perhaps?
I once owned some property that had a high clay content, and when it rained and then dried in hot weather, it developed numerous fissure cracks.
Not sure if that type of scenario is going on there though.
 
  • #73
Sorry, I´m not a native English speaker: ?what does tile domain mean? The more level or bowl shaped type of soil surface features?

1) What most Racetrack pictures show varies from a pronounced to a slightly domed surface of soil peds. This is a columnar structure mostly associated with sodic, very fine textured clay soils in arid to semiarid regions. It is not prismatic in soil science terms (as prismatic peds show a flat top) and usually they are subsurface features (but in the Racetrack case obviously there is no humus accumulation nor biological activity to form a topsoil). Yes, drying causes soil contraction and thereby cracking - but it needs many cycles of swelling and drying for a pronounced columnar structure to form. With each wetting, the soil doesn´t swell completely, so aggregates formed from former drying don´t get destroyed by wetting. On some tracks the surface seems quite homogenized by the sliding, and on some others the columnar structure seems to have reformed. May be these features could help in a rough relative dating of tracks?

I found a his English page that explains the formation of a columnar structure.
http://soils.missouri.edu/tutorial/page9.asp

2) I`ve also found another hint on a possible cause of direction changes. I´ll send the link later, but there´s a photo that shows well that the floor is not flat on a microrelief scale, because old tracks may act as barriers that force the moving rocks into another direction (if not forcing them to stop moving).

3) Large rocks may travel for larger distances than small ones because they offer more surface for the wind.
 
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  • #74
This is the link to the microrelief image I referred to: see Photo 54 http://www.coleskingdom.com/photos/a...name=racetrack
 
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  • #75
Sorry, have to correct or specify my words:
when I said "Might there not be salts in the topsoil working as an agent for providing a sliding ground when hydrated..." I did not mean something like a pure salt crust or concentration at the very surface, but salt in the clayely surface layer, so hidration might explain the very smearyness of the surface.
 
  • #76
Sorry if this has been mentioned, but I thought this phenomenon was pretty well addressed by John Reid and a group of Hampshire College students in the mid 90s. They peformed on-site studies and experiments and concluded that the rocks were moved by wind-driven sheets of ice, confirming previous conjectures by Stanley. I read through their original paper, but can't locate it now, nor can I find the full text on the internet. Here is the reference:

Sliding rocks at the Racetrack, Death Valley: What makes them move?
Geology; September 1995; v. 23; no. 9; p. 819-822;
http://geology.geoscienceworld.org/cgi/content/abstract/23/9/819

A portion of the abstract:

As proposed by Stanley (1955), it seems that the rocks, resting on mud, were locked into a single floating ice sheet, in this case at least 850 X 500 m. Final resting places of these rocks are much more widely scattered than their starting points, suggesting that the sheet broke into smaller plates. Large ice sheets can move rocks even with light winds and may explain the gentle curvature of tracks hundreds of metres long, a pattern very unlikely with gusty high winds and no ice."

Again, sorry if this was previously brought up.
 
  • #77
Hi,
the ice sheet hypothesis (yes, as far as I remember, it has been mentioned on this forum several times and I think also a link to the authors) as I understand has been cuestioned as a mechanism, that´s also mentioned in the new article at
http://geology.com/articles/racetrac...ng-rocks.shtml
http://geology.com/nasa/racetrack-playa/

I didn´t understand, looking at the tracks, how this ice sheet explanation should work in practice, in relation to what can be seen on the surface of the playa. Anyway, ice is still part of other hypothesis as you may read in the article mentioned above, but I wonder : ? why is ice formation at all necessary to explain the whole thing, especially if in the same hypothesis mud also acts as a sliding ground? What function has the ice then? Is it necessary to get the rock started and then it slides on soft, may be very water saturated mud? Why wouldn´t the whole thing work, from the beginning to the end, through a watersaturated ground as a surface for sliding?
 
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  • #78
As to the "regelation hypothesis" - another ice oriented one - mentioned in the article on
http://geology.com/articles/racetrac...ng-rocks.shtml
http://geology.com/nasa/racetrack-playa/

I´m not a physician to tell if this is possible, but I think of very high pressures (like happens under a glacier) to get ice molten by pressure. And then also, what gets and holds the movement of the rocks according to this hypothesis?
The suggested "bubble indicators" taken for this hypothesis may have an explanation that seems more straightforward to me : on sodic soils (like the external Solonchaks, that are sodic at the surface) bubbles may form at the surface through degassing of the mud, when a crust forms through drying of the peptised surface layer after a wetting event. The bowlshaped prints of these gas bubbles remain visible when this fine crust is detached through wind erosion. See also "vesicular crusts" or "vesicular horizons" (another mechanism of bubble formation in desert soils) on google.
 
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  • #79
susa said:
why is ice formation at all necessary to explain the whole thing, especially if in the same hypothesis mud also acts as a sliding ground? What function has the ice then? Is it necessary to get the rock started and then it slides on soft, may be very water saturated mud? Why wouldn´t the whole thing work, from the beginning to the end, through a watersaturated ground as a surface for sliding?

Thanks for those more recent links. I had pretty much forgotten about this phenomenon after reading Reid's work but I see that it's still of interest, and is useful in teaching geophysical processes.

As far as the need for the ice, as the NASA study states, they performed water-absorption experiments and found that the clay got slippery, but concluded that most rocks needed other help. The problem with wet clay by itself is that, even though the coefficient of friction would be reduced, a heavy rock with compress it and the rock would likely settle into the mud, offsetting the reduction in friction. An ice layer would make a clay/water/ice mixture that would reduce friction while maintaining sufficient solidity. As you probably know, the slipperiest surface is not ice, but wet ice.

The hypothesis of semi-frozen wet clay and high winds seems to me to be testable. Place such a mixture in a wind tunnel, put a heavy Dolomite boulders on top, and crank up the wind speed until something moves. Try different mixtures and boulder size/shape/weight and compare the results with the Playa. Would be a good term project for a geophysics class.
 
  • #80
"As far as the need for the ice, as the NASA study states, they performed water-absorption experiments and found that the clay got slippery, but concluded that most rocks needed other help. The problem with wet clay by itself is that, even though the coefficient of friction would be reduced, a heavy rock with compress it and the rock would likely settle into the mud, offsetting the reduction in friction. An ice layer would make a clay/water/ice mixture that would reduce friction while maintaining sufficient solidity. As you probably know, the slipperiest surface is not ice, but wet ice.

The hypothesis of semi-frozen wet clay and high winds seems to me to be testable."

I don´t know what was the "water-absorption experiment" that tested if the soil "gets slippery": checking the soil material? Putting 100 l of water on a spot and looking what happens? It´s quite a common sense result that the soil material gets slippery. But you can´t test in the field (or at least not in that "field") what happens in a flashflood. `With " watersaturated ground" I don`t just mean that the soil material is somehow saturated, but that there is also nearly no infiltration of floodwater in the soil: a water film forms and gives the rock buoyant pressure from below (it does not sink a lot because it can´t), while at the same time the soil particles on top of the aggregates peptize and get slippery. Just because there are cracks seen from the top, it does not mean that these are profound. On some spots infillings of the cracks are even recognizable.

As far as I understand you suggest the possibility of a situation where a) the soil is very wet b) so that the top cm of the soil profile can become a semifrozen ice-soil mixture when it`s cold enough to freeze (semifrozen because then soil is still soft enough to get the furrows formed) and c) winds are present. ?What prevents the rock from sinking in this scenario? a) Compressed ice that gets fluid at the bottom of the rock (regelation-hypothesis?) ?What prevents the water then from infiltration? b) ?Permafrost? (yes, this is just a joke).

I hope somebody soon will take students for several two-week investigation camps out there when the combination of the specific conditions of all the hypothesis are potentially expected (not to forget snow melt peaks). Somewhere I read, that the same fenomenon is observable at some other playas near (?outside?) the Death Valley Park (hope to find that link...).
 
  • #81
susa said:
As far as I understand you suggest the possibility of a situation where a) the soil is very wet b) so that the top cm of the soil profile can become a semifrozen ice-soil mixture when it`s cold enough to freeze (semifrozen because then soil is still soft enough to get the furrows formed) and c) winds are present. ?What prevents the rock from sinking in this scenario? a) Compressed ice that gets fluid at the bottom of the rock (regelation-hypothesis?) ?What prevents the water then from infiltration? b) ?Permafrost? (yes, this is just a joke).

I'm not suggesting anything, just interpreting what the NASA students reported. According to their study, "...it's thought that collars of ice can form around the lower parts of the stones, probably because the mass of a rock retains the cold. When more water moves in, the collar helps the rock partially float, so even a heavy rock might slide when the wind blows. The presence of ice collars could explain why some trails start narrow and get wider: the rock gradually sinks into the wet clay as its icy lifejacket melts away."

However, I went back to some of Reid's observations, and he believes there are at least two mechanisms involved, and ice is only one of the possibilities. He also believed that thin mud on a firm substrate allowed some of the smoother rocks to skim with a much lower coefficient of friction.

I hope somebody soon will take students for several two-week investigation camps out there when the combination of the specific conditions of all the hypothesis are potentially expected (not to forget snow melt peaks). Somewhere I read, that the same fenomenon is observable at some other playas near (?outside?) the Death Valley Park (hope to find that link...).

Well, there's been at least two student expeditions out there, Reid's and NASA's. Not that other expeditions wouldn't be worthwile but the problem, as I understand it, is that this is a pretty rare phenomenon and no one has actually observed the rocks moving. That's why I suggested a test in a wind tunnel, doesn't have to be a full scale, expensive one. Could be a scaled down version, as is found in many aero labs. Even if that doesn't find the answer, it could put boundaries on what is or is not likely with wet clays, partially frozen wet clays, etc.

As far as other playas with similar phenomena, Reid refers to this paper in discussing rock trails in South Africa
Eriksson, P. G., Fortsch, E. B., Snyman, C. P., Lingenfelder, J. H., Beukes, B. E., and
Cloete, W., 1996, Wind-blown rocks and trails on a dry lake bed, an alternative
hypothesis: Journal of Sedimentary Research, v. 66, p. 36–38.
 
  • #82
http://www.ldeo.columbia.edu/~polissar/publications/Sharp_et_al_2006_Geology_sliding_rocks_of_death_valley_discussion_and_reply.pdf
 
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  • #83
susa said:
http://www.ldeo.columbia.edu/~polissar/publications/Sharp_et_al_2006_Geology_sliding_rocks_of_death_valley_discussion_and_reply.pdf

Interesting reading. This was one of my favorite mysteries for a long time. Glad to know it's been solved.
 
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  • #84
..when I stumbled over this phenomen 2 days ago I wondered that obviously the impact of "clay swelling" is totally neglected.

It can be assumed that the clay/salt layers in the areas where the rock travel occurs have no consistent thickness; it looks plan on the surface, but can reach different meters into the ground. When it rains the water will generate a slippery surface, but most of the water will run into the crevices and cracks and will accumulate in deeper areas/layers creating inconsistent moisture pattern / gradients. Likely that the clay/salt texture at the surface stays firm enough to carry the weight of stones without letting them sink into the mud. Now, when the water is absorbed by this clay/salt mixture in deeper layers, these layers will expand lifting anything what is on the surface. A small slope caused by such a local lift of only a few centimetres will let the rocks slide on the slippery, but still firm surface. Due to dynamic processes (rain patterns, crevices / cracks close and open, different thickness of layers, impacts of the lifts .… etc.) the “local lifts” can happen anywhere at any time facilitating any directions the rock can travel.

“Swelling clay” can cause severe issues for tunnels (there are cases where tunnels collapsed due to the pressure local areas with the right clay blend generated) and tailings / tailing dams (for tailing dams the impacts could be positive, e.g. dewatering effects, and negative, e.g. wasting of storage volume).

The right clay/salt mixture can facilitate a volume variation (dry vs. saturated) by factor 1.3! that can result in enormous high temporarily pressures definitely strong enough to lift big rocks. This can also be linked to osmotic pressures that can build up if the clay/salt blend has the right texture (think about ceramic membranes for RO plants where a relative high osmotic pressure is utilized for desalination). It is also likely that the clay/salt ratio or the clay composition varies locally which even would increase local lifting effects in combination with variations in the moisture gradients and the different thickness of layers.

The surface (with small variations in heights) that can be seen on sites where the rock movements occur is only a snapshot: over a longer period with rain and dry seasons it will likely looks more like “rolling waves” of an ocean due to temporarily local expansions and shrinking (….an expansion factor 1.3 can achieve a distinct local lifting!). The pictures I could study so far in the internet also indicate that the movements only happen when the surface is wet (hence slippery): the tracks (from the “bow wave” ) behind the rocks seemed to be generated when the surface was wet and then dried out. If the rock would have been traveled on a dry surface it can be assumed that the wind would have destroyed a track made from lose dry clay dust. So the major driver behind the movement will be rain water which will accumulate temporarily in particular areas (due to cracks) of layers and will cause local swelling and lifting.

http://en.wikipedia.org/wiki/Sailing_stones : while the wikipedia article also focuses more on wind and icy surface layers as the culprits, in a way it also gives hints (like the other sources) that winter rains and the clay composition (feldspar rich syenite?) will play a major part. Local, temporarily lifting generating slopes due to local clay swelling in winter month in combination with an icy, slippery surface layer and strong winds seems to be a reasonable explanation…..strong winds will be supportive, but I don’t think that the wind could be strong enough to move the stones without the impact of gravity / slopes.
 
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  • #85
I posted the photos here last year of my trip to the racetrack. I am staying in Las Vegas now, and plan to make another trip to the racetrack at some point in the near future.
I was wondering if anyone who has posted here was going to be in this area in the next couple of months and would like to make the trip out to there with me? I have no set date or time for the trip, so I have a open schedule. In lieu of actually going out there with me does anyone have any suggestions or requests of specific data or photos I should concentrate on?
At some point I must return to Tennessee but hopefully I can make it back out to the racetrack before I do.
 
  • #86
  • #87
Anyone looked at dreikanters / ventifacts? These are wind-blown stone which can look similar to the pic in the wiki article. They are four sided, with surfaces etched into them and polished by windblown sand. If the mass of the stone is changed enough by the etching they will turn over and present a new face to the wind. If this was happening with sailing stones that would explain why they can suddenly change direction or stop.

Also, if motion occurs only during sandstorms (inevitable on windy days in desert) then a) most likely observers would be in shelter, and b) large movements would occur as a single event rather than be built up over a period, since it takes more energy to start the slide than to continue it.
 
  • #88
Is there any seismic activity in the area? If you take a hard bound book and place a pebble on it, shake it back and forth, the pebble will migrate across the surface. If the ground was semi frozen at the time, I would imagine that these boulders could move quite easily.
 
  • #89
This has already been tested as I have seen a documentary on it. The conclusion was that the mountains caused a temperature difference between the surface where the rocks are and the surrounding area, this created a strong wind that was channeled into the rocks. Moisture from rainfall makes it possible for the rocks to be pushed by the wind. If you look at the path each rock takes they are all in a similar direction which indicates a force acting on each rock from the same direction which is what a wind would do.
 
  • #91
Very cool! Thanks billiards!
 
  • #92
^^ what Evo said ^^
 
  • #93
The video is wrong about this being "the most boring experiment ever". Two years wait is nothing compared with http://www.bbc.co.uk/news/science-environment-28402709

Prof Mainstone, however, never saw the pitch in motion. In 1979, the sixth drop went on a weekend. In 1988,
with the experiment proudly displayed at Brisbane's World Expo, Prof Mainstone was fetching a drink when the seventh drop fell. By 2000, a video camera had been set up to capture drop number eight, but it malfunctioned at the crucial moment.

When the ninth drop fell in April this year it was watched by three webcams and thousands of online enthusiasts - but not by Prof Mainstone, who died eight months earlier at the age of 78.

:cry:
 
  • #94
billiards said:
A new paper claims to have finally solved this one:

Finally! It only took about 100 years. :) Until now the explanations involved far too much arm waving. But this one seems to be definitive. Thanks for the post. :approve:
 

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