I love mysteries. I also love wild landscapes. Rocks? Not so much.
I may have just offended a small part of my audience; sorry to all the geologists out there. Let me cut to the chase: out in the barren expanse of California’s Death Valley, there exists a decades-old mystery. It’s the mystery of the sailing stones. How can rocks move across a featureless, flat landscape, leaving distinct furrows in their wake? What, or who, is pushing them along?
The rocks range in size from small to several kilos. The tracks are sometimes dead straight, other times curved or zig-zagged. A trail may weave past other nearby rocks which have apparently remained stationary.
The unique conditions of Death Valley are incredible in themselves. It almost never rains, so the barren flatland (called Racetrack Playa, after the ‘racing’ stones) is only disturbed by wind and occasional snowmelt from the surrounding mountains.
When the stones were discovered, they were a baffling phenomenon. Efforts were made to work out how the trails were made. No-one ever witnessed a rock moving, but a tagging program confirmed that the rocks were indeed shifting about the place. Some rocks shifted regularly; others with trails never budged again.
A number of possible explanations have been advanced. The Playa is so flat that the strong winds are felt even very close to ground level. It’s possible that a massive gust of wind could get a stone moving. Once the stone is moving, it could be dragged along by the wind for a distance before it comes to rest again.
Another explanation is that of ice sheets on the Playa. When snow melts in the surrounding mountains, water flows over the flat Playa in a thin sheet. The temperature drops sharply overnight, forming a sheet of ice. When the ice starts to melt the next day, it breaks up into thin, flat ice floes, which may be driven across the surface by the howling wind.
That makes sense, but how does this move the rocks? The idea is that they become embedded in the ice sheets. It’s quite easy for a whole ice sheet, with just the base of a rock sticking down into the dirt, to move around in a high wind or if the water underneath it is moving. The rock, trapped in the sheet, digs a furrow which remains after the water evaporates.
One of the most perplexing things about the sailing stones is their ability to move in different directions. The ice-sheet hypothesis can explain that quite easily; if there is an ice sheet breaking up into bits, and the bits move around, it’s simple to picture them bumping into each other or otherwise changing direction before they melt. This also helps to explain why some stones will move and others nearby may not.
Despite reasonable evidence that this takes place, it still hasn’t been directly observed. (EDIT August 2014: It has now! Hooray. Thanks for the tipoff Henk!) There’s something romantic about the Sailing Stones, that in one of the most barren and inhospitable places on the Earth’s surface, natural conditions bring inanimate rocks to life and give them stories, history and trails of their own.