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The Sliding Rock Phenomenon

by Ivan Seeking
Tags: phenomenon, rock, sliding
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Ivan Seeking
#1
Jan22-05, 03:34 AM
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The Racetrack Playa, at an elevation of 1131 m, is a dry lakebed nestled in the Panamint Range in Death Valley National Park, California. Though almost perfectly flat, it shows evidence of dynamic traction (sliding) of boulder-sized and smaller rock fragments that tumble onto it from two abutting cliffs and surrounding alluvial fans (Figure 1). Scars of sliding rock activity in the form of recessed furrows have been noted since the beginning of the twentieth century...yet to date no one has witnessed the actual surface process that causes the rocks to slide. [continued]
http://geomaps.wr.usgs.gov/parks/deva/racetrack.pdf

[link updated] 6/20/09
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Andrew Mason
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Jan22-05, 01:48 PM
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This is a very interesting phenomenon.

FWIW, I will give you my take on it:

1. at some time during the year there obviously has to be difference between the forces acting on the rock and the forces acting on the ground beneath it AND this difference is greater than the static friction force between the ground and the rock at that time.

2. wind is insufficient to provide that force directly to the rock

3. wind might be sufficient to provide that force indirectly by pushing the surrounding ice sheet which pushes the rock with it.

4. ice movement caused by the expansion of the ice sheet against the nearby 'shore' of the dry lake bed could provide sufficient force as well.

5. perhaps it is possible that there is ground movement in winter (outward from the center of the lake bed) caused by expansion of the lake bed surface due to moisture, with the rocks being prevented from being carried along with it because they are trapped in the ice sheet. In summer the bed shrinks but carries the rocks with it then.

In any event, the ice is the only explanation that I can see.

To explore further, I would suggest that they do pressure tests in the ice on either side of the rocks during the winter and also measure movement of the ice sheet and movement of the underlying lake bed. That should give the answer.

AM
matthyaouw
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Jan23-05, 07:54 AM
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I'm tempted to mention some kind of heave movement, caused by thermal expansion and contraction, but I'm being purely speculative here.

tonyjames
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Jan4-07, 07:45 AM
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The Sliding Rock Phenomenon

Some interesting comments , i'd like to add to the idea that heat plays a roll and maybe gravity even though the rocks can appear to move slightly up an incline.
If the rocks are storing up heat during the day time and the ground is cooling rapidly at night (due to surface area) maybe it is possible for minor expansion and contraction of elements in the uneven ground surface to cause deviations in topography around the rock which cause it to move down a very slight gradient.
As the rock moves (with its stored heat) so the whole process moves along the ground which is heated slightly by the rock. The uneven surface of the ground may account for the apparent random direction in which the rocks end up moving. As for wind blowing the rocks around,, i would like to see the measurement of forces needed to to get a vaiety of rock sizes moving in the first place, as i dont believe this is likely.
It would be good to see some measurements of temperature deviation between the rock surfaces and ground surfaces as this mechanism should not be underestimated.
Mk
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Jan4-07, 09:13 AM
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Quote Quote by tonyjames View Post
Some interesting comments , i'd like to add to the idea that heat plays a roll and maybe gravity even though the rocks can appear to move slightly up an incline.
If the rocks are storing up heat during the day time and the ground is cooling rapidly at night (due to surface area) maybe it is possible for minor expansion and contraction of elements in the uneven ground surface to cause deviations in topography around the rock which cause it to move down a very slight gradient.
As the rock moves (with its stored heat) so the whole process moves along the ground which is heated slightly by the rock. The uneven surface of the ground may account for the apparent random direction in which the rocks end up moving. As for wind blowing the rocks around,, i would like to see the measurement of forces needed to to get a vaiety of rock sizes moving in the first place, as i dont believe this is likely.
It would be good to see some measurements of temperature deviation between the rock surfaces and ground surfaces as this mechanism should not be underestimated.
Wow. That sounds extremely good.

How could wind blow the rocks? They wouldn't have a trail then!
Andre
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Jan16-07, 07:21 AM
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And thus the Glacial world is full of surprises. Like this closely related problem:

http://instaar.colorado.edu/cosmolab...et_al_2003.pdf

Look especially at figure 3. How does a fresh 11,000 year old rock end up a strongly weathered old bedrock, given that a sliding ice sheet / glacier wears out the bedrock in a completely different way?

Then there is the cold based hypothesis, which assumes that the bottom ice is frozen solid to the bedrock while the sliding happes at higher levels in the ice. I can see some problems with that, one for instance that this does not seem to happen anywhere today and even frozen solid, ice always remains elastic under atmospheric conditions and will distort under pressures.

So, how about a shallow ice sheet, unable to exert strong enough forces on the bedrock to wipe out the old weathering and more sliding rocks?
O2Polluter
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Feb3-09, 03:03 PM
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I've been out to the Playa --- long 30 mile 4X4 drive in down washoboard roads. Stop at the old western sign and leave a tea kettle at Tea Kettle Junction --- it's a tradition!

My guess is that the surface which is crackled becomes slippery when wet. The trails themselves are hardened crackled trenches on the playa. Note: there were wet spots when I was out there. And, the wind tunnel effect of the winds coming up off China Lake military bombing range coupled with the winds from Death Valley and the Nellis bombing range make for tornadic activity on occasion when the ground is wet.

I say some rocks that appeared to have dropped out of the sky. Leaving just a crater and not a trail. Yet, they appeared the same as the other rocks on the Playa.

Some of the trails defy explanation for the crossing marks of where the rock crossed a previos path it had made were quite simply impossible physically. Yet, it was very apparent that it was not man made. However, there are other spots on the playa where it is evident that faux trails were made (probably by college students)!
O2Polluter
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Feb3-09, 03:16 PM
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Excuse my typos above. But, in addition, I noticed acoustical phenomena there as well. I could hear aircraft that were not over the playa or anywhere over the valley at all. They either had to be invisible or there was some reflection of sound off the atmosphere or a funnel of sound from aircraft out over China Lake or Nellis. As well, I could occasionally hear mountain climbers from an incredible distance given the distance alone it would have been phenomenal. Given that there were strong episodic winds and I could still hear them on occasion made it even more incredible.
wolram
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Feb3-09, 04:25 PM
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A Wiki article.

The sailing stones are a geological phenomenon found in the Racetrack. The stones slowly move across the surface of the playa, leaving a track as they go, without human or animal intervention. They have never been seen or filmed in motion. Racetrack stones only move once every two or three years and most tracks last for just three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different-sized track in the stone's wake.

The sailing stones are most likely moved by strong winter winds (up to 90 mph), once it has rained enough to fill the playa with just enough water to make the clay slippery. The prevailing winds across Racetrack Playa travel blow from southwest to northeast. Most of the rock trails are parallel to this direction, lending support to this hypothesis.
Ivan Seeking
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Feb3-09, 10:45 PM
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If this only happens when winds approach 90 mph, it is no wonder that no one has ever witnessed the process directly. But one does have to wonder why some rocks change direction while apparently others don't. If the tracks only exist for a few years and they only move every few years, it seems that we wouldn't expect to see a complex history. But I guess this could result from surface features of the lakebed.

I see the original link is dead. Just google for "racetrack playa" for plenty of photos and additional information.
matthyaouw
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Feb4-09, 01:58 AM
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[QUOTE=Ivan Seeking;2061720]But one does have to wonder why some rocks change direction while apparently others don't.QUOTE]

Maybe it's the shape of the rock. If it's laying on a surface that's not smooth, it might move more easily in one direction than another.
Or Maybe it depends how full of water the playa gets. Some rocks might be left high and dry while others slip around. I know it will be small, but I wonder what the height variation across its surface would be...
Or variation in the wind speed. It might only get up to the right level in a few areas in each storm.
nottheone
#12
Mar1-09, 09:40 PM
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Quote Quote by O2Polluter View Post
Some of the trails defy explanation for the crossing marks of where the rock crossed a previos path it had made were quite simply impossible physically.
Can you explain this in more detail? Describe the crossing and what about it makes you think it was impossible?

I had a game when I was a kid of racing horses. The horses had some kind of plastic brush on the bottom and the track vibrated to make the horses move around the track. His mentioning the nearby bases made me think of that. Maybe there is a combination of things going on here. Like wind, ice and vibrations caused by base activity. Could be sonic booms or is there a bombing range nearby? Maybe it hasn't been observed because it requires several things to coincide, not all natural. Wind, surface conditions and man-made vibrations.
HiDesert
#13
Jun16-09, 10:24 AM
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The Wikipedia article says that the tracks were visible in 1948, so if it is a confluence of natural and anthropogenic conditions, it probably doesn't involve sonic booms.

I keep reading about ice, but I have to wonder how likely it is for the smaller stones to leave tracks in a frozen surface. For the big ones, maybe, but a small rock being dragged along a frozen surface doesn't seem likely to produce a track.
Equate
#14
Jun20-09, 12:45 PM
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One has to wonder why no one to this date has setup a web cam to study this phenomenon.
Borek
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Jun20-09, 12:56 PM
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Quote Quote by Equate View Post
One has to wonder why no one to this date has setup a web cam to study this phenomenon.
Are you sure there no cameras mounted there?
Equate
#16
Jun20-09, 06:23 PM
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Quote Quote by Borek View Post
Are you sure there no cameras mounted there?
I know, it's hard to believe, but I am assuming it from the statement in the OP:

...yet to date no one has witnessed the actual surface process that causes the rocks to slide.
Shouldn't be too hard to set up a camera there and solve this "mistery" once and for all. Geez, we are digging up rock samples on Mars for Christ's sake...
Borek
#17
Jun20-09, 06:41 PM
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First - original post is over four years old.

Second - it happens once per several years.

Three - area in question is several square kilometers.

Four - one camera is not enough.
Equate
#18
Jun20-09, 07:47 PM
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I am aware of that.

Still, the task doesn't seem insurmountable to me. Bigger problems have been conquered and I, for one, am interested in those "sliding" rocks and would appreciate an explanation sooner or later.

Can't be that hard to find out, can it?


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