Last update: November 14th, 2019 at 11:00 am
Looking almost straight down onto the Sahara Desert, a crew member aboard the International Space Station took this late afternoon photograph of the Grand Erg Oriental. Astronauts have a unique vantage point from which to view the large areas of Algeria and Libya covered by seas of sand.
Winds have organized vast quantities of sand into straight lines in what geologists call “compound linear chains.” The chains are about 1.5 kilometers (0.9 miles) wide in this view and rise 150 meters (500 feet) above the smooth, intervening flats. The compound chains are made up of numerous smaller linear dunes with sharply defined crests (also known as seifdunes, after the Arabic word for sword). Linear dunes sometimes converge to a point with long tentacle-like arms called star dunes. An astronaut snapped a photo of individual, well-developed star dunes just 20 seconds after this photo was taken. (Click here to view it.)
Geologists now know that different wind patterns are responsible for different dune shapes. Winds that blow from one direction build linear dunes and, ultimately, chains like those in this image. The slight variation in wind direction pushes sand from one side of the dune and then from the other, making the sharp crests of the small linear dunes. These winds also stretch out the dune in the average direction of the winds (southward in this part of Algeria).
By contrast, winds that blow with roughly equal strength from several directions make the star dunes. This suggests that the wind regime changed with time, first building the chains over a long period of time, and then becoming more multidirectional, so that the star dunes formed on top of the chains.
Transverse dunes forms at right angles to the wind direction. Relatively small transverse dunes can be seen at many points in this image. They were made by north winds channeled by the chains, especially along the outer flanks and in hollows within the chains.