Over the past few decades, many geologists have puzzled over why rocks at the bottom of a completely dry lake were leaving behind traces. Since their discovery, these "floating rocks" remained a mystery, as observing their movements was virtually impossible. Another problem was that they took years to move. Thanks to advances in time-lapse photography and GPS tracking, researchers have finally been able to figure out what's going on.
Located in the Racetrack Playa area of Death Valley National Park, California, the "floating rocks" have long remained a mystery. The rocks are known to move along the valley floor., apparently without any intervention, leaving long traces.
«A playa is a dry lake bed that once held water but has now evaporated, leaving a dry bottom. The rate of evaporation is greater than the rate of refill, meaning the lake almost always remains dry. Playa Racetrack is one such dry lake bed, located at an elevation of 1,132 meters above sea level in Death Valley in Inyo County, California. It is 4.5 kilometers long and 2.1 kilometers wide. The lake fills only during heavy rainfall. After the rains and under the scorching sun, a thin layer of water evaporates, leaving a cracked, hexagonal, mosaic-like dry bed.
The salt marsh is known for its "floating stones"—accumulations of dolomite and syenite rocks weighing from a few hundred grams to several hundred kilograms. These rocks move without any human or animal intervention, leaving visible tracks, or "race tracks," on the lake bottom. The tracks often reach 100 meters in length and are typically less than 2.5 centimeters deep. The stones vary in size, ranging from 15 to 46 centimeters in diameter.
This phenomenon was first noticed in 1915 during exploration of the area for natural resources. It soon attracted the attention of geologists and was published in the Bulletin of the Geological Society of America, sparking numerous hypotheses about its causes.
In 1915, a gold prospector named James Crook visited the excavation site, and then, in 1948, two other geologists, Jim McAllister and Allen Agnew, mapped the area. Soon, the phenomenon began to be studied and observed at other salt flats, such as Little Bonnie Claire Playa in Nye County, Nevada. In 1952, a National Park Service ranger conducted various measurements, and several geologists put forward various hypotheses.
In May 1972, a program to track the movement of stones at the racetrack was initiated, marking 30 stones. Markers were installed at the locations of these stones, and their positions were recorded for seven years.
The movements of falling rocks are difficult to record, as they only occur once every three years or so. Researchers attempted to construct a 1.7-meter-diameter corral around several selected rocks to determine whether ice floe movements were the cause. However, the rocks moved unimpeded by the corral's reinforcing bars. This meant that the thickness of the ice around the rocks must have been less than the distance between the reinforcing bars. Additional research was conducted in the 1990s, and by then, the most likely hypotheses were that both wind and ice were driving the movements.
Using GPS trackers and digital time-lapse cameras, the researchers finally discovered that ice forms around these rocks when a layer of water forms on them during cold winter nights. During the day, the ice sheets crack and, being buoyant, begin to drift with the wind over shallow water, dragging the rocks along with them.
In 2009, thanks to the development of time-lapse cameras, researchers were able to capture the flooding of the salt marsh. Further advances allowed the researchers to obtain images from a wind sensor, significantly reducing the over 2,700 hours of downtime. On December 20, 2013, the researchers again began tracking the movements of more than 60 rocks using GPS and time-lapse photography, and continued to do so until January 2014. These observations contradicted previous theories that the rocks were lifted from the surface by wind or thick ice.
When the lake fills with shallow water, a thin layer of ice, just a few millimeters thick, forms on the surface and around the rocks. During the day, when the sun shines brightly, this layer of ice breaks up into sheets, which, along with the rocks, are moved by the wind at speeds of up to five meters per minute (0.3 kilometers per hour). Some rocks have moved up to 224 meters during the observation period. Thus, moving rocks requires a special combination of ice, wind, and sun.
[Sources: Wikipedia, DiscoverMagazine]