Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: RD on 20/10/2009 17:14:22

Title: Is this the linear equivalent of a crop circle ?
Post by: RD on 20/10/2009 17:14:22

i.e. is this phenomenon a prank ? ...

 [ Invalid Attachment ]

Quote

The sailing stones (sliding rocks, moving rocks) are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention. They have been recorded and studied in a number of places around Racetrack Playa, Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is subject to research.

http://en.wikipedia.org/wiki/Sailing_stones


I suppose it may be possible that tremors could cause a ridged stone to move in a straight line like a bristlebot ...

 

Title: Is this the linear equivalent of a crop circle ?
Post by: Bass on 20/10/2009 22:04:13

I've been to Racetrack Lake (Playa) and seen the tracks- they are real.  My guess is that the floor of the playa becomes slick with just the right amount of moisture and that winds (which can blow you over at times there) then push the rocks across the slick surface.

Title: Is this the linear equivalent of a crop circle ?
Post by: JimBob on 21/10/2009 02:41:26

Again, the Mountain Man comes through with the right answer.

I just wish he would take a bath more than once a year. I can smell him in Texas!

Title: Is this the linear equivalent of a crop circle ?
Post by: Bass on 21/10/2009 06:06:26

Quote from: JimBob on 21/10/2009 02:41:26

Again, the Mountain Man comes through with the right answer.

I just wish he would take a bath more than once a year. I can smell him in Texas!

AHHH! The smell of money!

don't want to wash all that gold off

Title: Is this the linear equivalent of a crop circle ?
Post by: LeeE on 21/10/2009 21:35:51

These stones are fascinating and I first read about them about twenty years ago.

FWIW, I don't think that they are moved by the force of the wind at all.  This is because they are effectively sitting in a shallow pit/trench below the surrounding ground level, and the sides of the tracks they leave have raised edges.  This seems to mean that two factors need to be addressed: the stone needs to be pushed uphill, against gravity, and it also needs to force aside at least some of the clay in front of it to leave the raised edges.

It is interesting to note that in the full-size version of the photo from the wiki article, shown above (link below):

http://upload.wikimedia.org/wikipedia/commons/c/cb/Death_8_bg_082303.jpg (http://upload.wikimedia.org/wikipedia/commons/c/cb/Death_8_bg_082303.jpg)

...the size of the 'tessellated' polygon divisions (there's probably a proper term for them but I don't know what it is) in the track are much larger than on the surrounding surface.

Also, in another picture in the wiki article:

http://upload.wikimedia.org/wikipedia/commons/c/c7/Racetrack_Playa_(Pirate_Scott).jpg (http://upload.wikimedia.org/wikipedia/commons/c/c7/Racetrack_Playa_(Pirate_Scott).jpg)

...it looks as though there has been a flow of clay over the surrounding surface (look at the base of the corner nearest the camera, where a layer of clay appears to have flowed over part of one of the cracks between the surrounding polygons.  It's also apparent in this photo that there is a clear difference in colour between the clay in the track and the flows, and that of the surrounding clay; that colour difference is almost certainly significant because it has occurred for a reason.

Both of these photos indicate to me that at some point in the process there is a partial liquefaction of the clay around and beneath the stones.

My best guess is that ice forms in the clay, pretty uniformly, both beneath and around the stones (and probably across large areas of the playa), but that it thaws at different rates depending upon the wind direction and sun angle.  As the ice forms in the clay it expands (because water expands when it freezes), and then when it thaws the clay contracts again.  However, the clay beneath the stones will thaw more slowly because the stone will not only insulate the clay beneath it from sunlight and wind but may also act as a heat sink.  The wind too, may act differentially as it will result in a turbulent region behind the stones, on the downwind side, which may contribute to differential thawing.  In addition, if wind-chill is a factor, this should be greater upwind of the stone than downwind in the turbulent region.

As the ice thaws around the stone, but not beneath it, the surrounding clay contracts leaving the stone on a slightly raised platform from where it then falls forward into the area of surrounding clay that has thawed the most i.e. downwind due to the turbulence there having increased the rate of thawing, aided by the force of the wind on the upwind side.

Just a guess, like I said, but none of the other explanations seem to fit all of the phenomenon, whereas the irregular shape of the stones will allow for differing degrees of downwind turbulence between different stones, explaining how some stones will move whilst others don't, even when they are close together and seemingly subject to the same environmental factors.

Title: Is this the linear equivalent of a crop circle ?
Post by: RD on 22/10/2009 06:00:05

I think you are on the right track* with the mud expanding when frozen LeeE.

As seen in the bristlebot video I linked to, repeated vertical displacement can cause horizontal motion (@ 0:40 - 0:50 (http://www.youtube.com/watch?v=rUSTXUis_ys))

These repeated vertical displacements could be provided to the stones by the mud under them expanding when frozen then thawing, (no wind necessary for movement). This mechanism would permit neighbouring stones to move in different directions, (e.g. tracks cross).


[* pun intended]