The movement of the rocks occurs when large, thin sheets of ice floating on an ephemeral winter pond break up in the sun.
So a small puddle forms in the shade, under the rock, which freezes and causes the rock to slide forward a little bit. Over time this scoots the rock across the ground and leaves a trail that looks like it was pushed but with no footprints nearby.
Very cool!
Not quite, to my understanding. Imagine a shallow pond that covers a rock maybe halfway up. The top of the pond freezes, forming a sheet of ice that's attached to the equator of the rock and also floating on top of the liquid water underneath. Eventually this ice sheet breaks into pieces, and wind blows across the top of the ice. With enough surface area on the ice, the wind shear force on the ice is great enough to push the rock along the ground.
From the article:
These observations contradicted earlier hypotheses of strong winds or thick ice floating rocks off the surface. Instead, rocks move when large ice sheets a few millimeters thick floating in an ephemeral winter pond start to break up during sunny mornings.
A few points in your comment just didn't make sense to me, so I read through the article for clarification.
My first bit of confusion was that I'd hardly expect an ephemeral pond to rise to the level of halfway up these rocks on a wide flat expanse like this in a desert. That's a lot of water. Death Valley gets much less than 3 inches a year. Maybe it's possible, but the article makes no such claims.
Also, the idea that these rocks float seems at best dubious. Moreover if they did float, I find it hard to believe they would leave such prominent tracks in the clay. But they don't need to float. All you need is wet clay to reduce friction and a large ice sail; now you have movement exactly as the article described.
Thanks for clarifying!
How is this creepy?
It was creepy when I was a kid, before they figured out what makes them move.
That's fair. Didn't think about that.
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