跑马场海滩，移动石头之谜

The Racetrack Playa is located above the northwestern side of Death Valley in Death Valley National Park, Inyo County, California, United States Of America. It is a lake bed that is flat and is always dry.

The length of the Playa is about 4 kilometers, north to south and about 2 kilometers, east to west. On this cracked earth, playa rocks move magically from place to place, leaving smooth tracks behind as the evidence of their movement on the lake bed, which is otherwise uniform.

Racetrack rocks only move once every two or three years and most tracks last for three or four years. Some of these rocks weigh several hundred pounds. That makes the question: “How do they move?” a very challenging one.

The research had been going on since 1990s. No human or animal intervention was found in the movement. Originally it was thought that strong winter winds could possibly be the reason after it had rained, by making the rocks slippery.

Another theory suggested that ice sheets moved the rocks. After it rained, a thin layer was formed on the playa by the strong winds. At night when the temperature falls below freezing, thin sheets of ice were formed even around the rock thereby enabling it to move with the wind.

The mystery was finally solved in November 2013, after it rained. The water up to three inches deep covered the playa and then froze during cold winter nights, forming sheets of ice.

They observed that when the sun comes out, the ice begins melting and breaking up into large floating panels. These ice panels, driven by light winds, push the rocks ahead of them, leaving trails in the soft mud below the surface.

Researchers observed many ice-embedded rocks moving slowly across the playa on several dates in December 2013 and January 2014. This evidence of the rocks in motion has been shared in a video by the Scripps Institution of Oceanography.

By February 2014 when the lake had dried up and new trails left by the recently-moved rocks could be seen in the playa sediment surface.

Some of the rocks had been equipped with a small GPS recorder and their records indicate that some rocks had moved over seven hundred feet during at least four episodes of movement.

The conclusion was reached by a team led by paleobiologist Richard Norris of the Scripps Institution of Oceanography, UC San Diego.

This work demonstrated the movement of the rocks and attributed it to wind moving the rocks while they were embedded in a large ice sheet floating on a thin layer of water.

Asked if the mystery of sliding rocks has finally been solved, Richard Norris replied, “We documented five movement events in the 2 1/2 months the pond existed and some involved hundreds of rocks.

So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion. But we have not seen the really big boys move out there.Does that work the same way?”