The Moving Rocks of Death Valley

The moving rocks of Death Valley puzzled visitors and scientists until the mystery was finally resolved in 2014. Previous theories included hurricane-like winds, dust devils, and thick sheets of ice. It turns out, the actual way they move is much simpler. The stones can remain still for years until certain specifications are met. First, the dry lake bed must fill with rainwater, deep enough to allow ice to form on the surface while exposing the rocks. At night, a thin sheet of ice forms on the surface. The cold air freezes the top layer of water, forming an ice sheet which floats on the surface because ice is less dense than water. The reason water freezes starting on the surface is because when water is below 4 degrees Celsius, the density decreases with a decrease in temperature due to the hydrogen bonds of water pushing the molecules apart. The lower density causes the cold water to move to the top, which freezes first, creating a layer of ice that can drift on top of the water. Then the Sun melts the ice during the day into several pieces. Light winds of 3 to 5 meters per second and ice only 3 to 5 mm thick push the rocks around. The rocks crawl at a pace of 2 to 5 meters per minute. As the rocks move, a track forms in the muddy playa. There are also some rockless tracks. These form from ice panels that drag on the ground.