Mysterious Sliding Boulders

Photograph courtesy John B. Reid, Jr.

One of the strangest occurrences that Dr. Bob has ever come across is the mysterious sliding boulders of Death Valley National Park, California. There are trails showing that boulders up to 705 pounds (320 kg) have moved large distances over flat terrain. No one has actually seen these boulders move.

Death Valley National Park is a protected desert region with an area of 2,067,628 acres. It receives less than 2 inches (5 cm) of rain a year. Some of the world's highest air temperatures (134F/57C) and ground temperatures (165F/74C) have been measured there. Within the park is an area called the Racetrack Playa. A playa is a nearly level area at the bottom of an undrained desert basin that is sometimes temporarily covered with water. The Racetrack Playa is where the mysterious sliding boulders are found.

What is causing these boulders to move? Gravity sliding is clearly out of the question. The Racetrack Playa is incredibly flat, so much so, that on a calm day only 2 inches ( 5 cm) of water will entirely cover the playa.

Some scientists have suggested that the boulders are blown by strong winds after a rain. They theorize that the rain forms a slippery layer of mud that the boulders can slide on.

The theory that I think is the most convincing, however, is one that was initially proposed in 1955 in which sheets of ice were thought to be involved. Recently Dr. John Reid, Jr. (a geologist) and his colleagues at Hampshire College in Massachusetts, USA, have provided additional data to support that this is the case.

It appears that the boulders do not slide very often because an unusual set of weather conditions must occur. There must first be enough rain to form a shallow lake on the playa, followed by freezing temperatures to form a sheet of ice. When this happens, the boulders become trapped in a sheet of ice that floats on the shallow lake. When there is wind, all the trapped boulders and the ice sheet are pushed around the shallow lake. The wind blowing over the ice causes "frictional drag." In other words, there is friction between the moving air and the ice sheet. As the air blowing over the ice is slowed down by this friction, it exerts a pushing force on the ice which helps to move it. This is how the wind pushes the ice sheet . The bottoms of the boulders leave the tracks that we see in the pictures. When the ice melts, the boulders assume their new resting places.

One of the unusual things about the sliding boulders is that the tracks they make are very similar. The following diagram shows what the boulder trails would look like. Note that the three wiggling boulder tracks (boulders 1, 2, and 3) look very similar. These three trails were probably made when these boulders were caught in the same ice sheet. There are also two other trails (boulders 4 and 5) that look alike but that are much different than the trails formed by boulders 1, 2, and 3. These trails were probably made at a different time when these two boulders were caught in a different ice sheet.

Patterns like these would not be expected if the wind pushed boulders of different sizes that were not held in ice. In that case, boulders of different size and shape would be pushed by the wind at different speeds and the trails that they would leave would not be so similar. For example, we would expect that a large flat boulder would be hard to push by the wind while a tall boulder might be easier to push. If this were the case, we would expect the tall boulder to move much further than the large flat boulder. On the other hand, if both of these boulders were locked in an ice sheet, we would expect them to move in the same way and have very similar trails as is observed and shown in the diagram.

Dr. Reid has also done experiments that show that very powerful winds would be required to move large boulders in the mud without the ice sheets. For example, he calculated that moving a cubic boulder weighing 705 pounds (320 kg) would require winds of 580 miles per hour (260 meters/second) to move it, something that would not happen on Earth. If the rocks are locked in an ice sheet , however, much lower wind speeds are required to move them (10 - 60 miles per hour, or 4 - 27meters/second).

The mysterious sliding boulders of Racetrack Playa and the theories to explain why they move are certainly Interesting Science Stuff at its best. I am sure that in the future, scientists will figure out this very strange occurrence.

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