|Sahara Desert: Grand Erg Oriental between Algeria and Libya, 2016|
Looking almost straight down onto the Earth’s 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 seif dunes, after the Arabic word for sword). Linear dunes sometimes converge to a point with long tentacle-like arms called star dunes
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.
SOURCE: Astronaut photograph ISS046-E-48626 was acquired on February 26, 2016, with a Nikon D4 digital camera using an 1150 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and NASA’S Earth Science and Remote Sensing Unit, Johnson Space Center
|Star dunes comprise about 8.5 percent of the world’s sand dunes. Other areas they can be found include the Badain Jaran in China, the Gran Desierto de Altar in Mexico, and the eastern Rub’ al Khali in the Arabian peninsula.|
HOW SAND DUNES ARE FORMED
In some areas of Earth, winds tend to blow in roughly the same general direction all year. The Grand Erg Oriental, a sprawling sea of sand dunes in the Saharan Desert, is not one of them.
The winds in northeastern Algeria tend to be complex and changing. Easterly summer winds shift in the winter, becoming westerly. Meanwhile, passing storms and local geographical features further muddle the picture.
If winds came consistently from one direction, crescent-shaped barchan dunes would reign. But the dominant dune type along the southern edge of Grand Erg Oriental (shown above) are large, pyramid-shaped star dunes, which only form in areas where winds blow from multiple directions.
The image was acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite on October 27, 2012. It was made from a combination of near-infrared and visible light. In this type of false-color image, sand is tan and shadows are black or gray. The blue-tinted areas are likely mineral-rich evaporites. The image is centered at 29.8°north latitude, 7.9°east longitude, near the town of Gadamis. As is common with star dunes, some of the dunes have long interlacing arms connecting to nearby dunes
OTHER STAR DUNES ON EARTH
|STAR DUNES: BADAIN JARAN DESERT (GOBI REGION BETWEEN CHINA AND MONGOLIA)|
|STAR DUNES IN GRAN DESIERTO DE ALTAR, SONORAN DESERT, MEXICO|
|RUB AL-KHAIL DESERT "STAR DUNES" IN THE UNITED ARAB EMIRATES|