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In almost perfectly flat dry lakebed at the upper end of Death Valley there  some strange rocks known as 'sailing stones'  are dotted about the otherwise almost smooth lake bed, some of them weigh up to 36kg, but behind them are tracks sometimes tens of metres long as if they were moving. The tracks can be straight or zig zagged and some of the stones have been marked and their positions seem to change relative to stakes put in the ground, but noone has ever seen a stone move.

The movements seem to occur on the rare occasions whern the lake bed is covered with a shallow layer of water, there is a strong wind and the temperature is very low. Both ice bergs and winds which can reach 90mph only a couple of inches above the lake bed have been suggested as ways of moving the stones but neither has been decisive.

Ralph Lorenz, a scientist at Johns Hopkins University has come up with a new explantion, a neat combination of both. The lake floods a couple of inches deep, and as it does so a small ice berg forms around the stone, the water level increases slightly and beacuse ice floats this gives the stone lift, reducing the friction with the ground. The high winds are then able to push the stones along forming the long tracks. They have done some exeriments which support their hypothesis, though it will be hard to know what is happening for sure until someone actually sees one moving.

This is a lovely mystery, but studying areas like the racetrack playa may be useful in understanting places like the shallow lakes on Saturn's moon Titan, where there are some hydrocarbon lakes which seem to occasionally dry out.

References

Part of the show Checking the Atmosphere and Changing the Climate from the 20th Feb 2011

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Comments

If they are moved by the wind how can neighbouring stones travel in different directions ?

Alternative no-wind hypotheses here RD, Tue, 22nd Feb 2011

Having been to racetrack lake (playa) and seen the tracks, they seem to defy explanation.  Some of the tracks are hundreds of meters long, others only a few meters.  All different sizes and shapes of rocks "sail" across the playa.  And RD is right, the tracks do cross each other.  one other thing, the wind does blow there, ferociously at times.  The wind has always blown when I've been in Death Valley (mostly in winter months), sometimes so forcibly that it's hard to stand.  As my partner once said, as a gust almost sent our truck into the ditch, "The wind doesn't blow here, it sucks"

I believe wind is the motive force- but I also agree that you need sufficient water and freeze-thaw cycles to reduce friction to the point that the wind can move these boulders.  Who knows- maybe these are really wind depression valleys instead of fault blocks?? Bass, Wed, 23rd Feb 2011

Like a vortex street behind the stone ?


http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street

However the stones are reported to have moved, not just developed trails.

Say a stone was raised up when the wet ground froze (ice expands) and when it thawed the stone sat back down again and a little mud under the stone was expelled in this process. If mud under the stone was not expelled symmetrically during the thaw, (say due to ridges on the underside of the stone), that asymmetry could propel the stone horizontally, (very slowly).
RD, Wed, 23rd Feb 2011

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