Thursday, September 25, 2014

Sliding Stones in Death Valley...and Ultrarunning

Have you ever seen this phenomenon written up?  Seems there are stone that somehow slide around Death Valley, thus far eluding explanation.

[image credit Bad Astronomy]

Until now.  From the always-great read Bad Astronomy, Phil Plait explains:

When I was a kid, one of the coolest mysteries going was the moving stones of Racetrack Playa. This is a dry lake bed in Death Valley, California, where large rocks are embedded in the dried mud. However, many of the rocks have clearly been moving; there are long tracks behind them in the caked, baked mud pushed up like rails along the tracks’ sides.
What could be moving these stones? No one knew. They would sit for years, then suddenly be found to have moved many meters. Could wind push them? Maybe ice formed after rain, forming rafts that floated the rocks up. Speculation abounded, and I remember watching TV shows about the rocks, and reading about them in sketchy “Mysteries of the Paranormal” type books when I was a wee lad.
Now, however, this enduring mystery has been solved. And I mean,solved. Like, we know what’s causing this. A team of scientists and engineers were able to capture the motion on camera, finally revealing the mechanism behind this bizarre behavior.

In a nutshell, the playa is very dry, getting only a few centimeters of rain per year. In the winter, when it does rain, the slightly tilted playa gets accumulations of water a few centimeters thick at one end. It gets cold enough for the water to freeze on top. When the Sun comes out, the ice begins to melt, forming large chunks called rafts. The wind blows these rafts (which are typically a few millimeters thick), which then hit the rocks and push on them. The ground is softened by the water, so the rocks can move more easily ... and then they do. 

Gosh, I love science!

The link to Ultrarunning, of course, is that Death Valley is where the Badwater race takes place, a race that I have absolutely NO desire to ever run.  If you do, my hat's off to you, but this clearly is a case of different strokes for different folks.

And so on.

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