Grand Erg Oriental, Algeria
Taken through a window on the International Space Station by the EarthKAM camera, this photograph shows the boundary between a major dune field and dark hills along the border between Algeria and Libya. These landscapes are among the driest parts of the Sahara Desert. For scale, the dune margin shown in this photo is slightly more than 100 kilometers long.
Large dune fields are known to geologists as "ergs," the Arabic term for these extensive regions of sand. This eastern erg (known as the Oriental) of Algeria includes hundreds of dune mounds. From more detailed images we know that these are "star dunes". The erg itself occupies a vast area of approximately 600 kilometers by 200 kilometers.
Many winding water courses are visible on the right half of the photo. These typically dry channels drain occasional rain water towards the edge of the vast erg. Sediment carried by such streams has accumulated over a few million years to make the dunes. The margin in the image marks the line between a zone dominated by wind as the main landforming agent and a zone dominated by water movement.
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Credit: NASA Earth Observatory