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International Space Sta...
2006-09-20 0:0:0
 
Description ISS013-E-65526 (8 Aug. 2006) --- Issaouane Dune Sea, Eastern Algeria is featured in this image photographed by an Expedition 13 crewmember on the International Space Station. This view from one of the smaller dune seas in the central Sahara shows the complex but regular patterns produced by winds in deserts where abundant sand is available. Geologists now know that dune seas (also called ergs) comprise at least three orders of dune size. In this image the largest and oldest appear here as chains oriented about 60 degrees apart, that is, one oriented almost north-south, the other southwest-northeast. The "streets" between the dune chains (also called mega-dunes) are swept clean of sand in places, revealing the original surface, with light colored muds and salt derived from very occasional rains. The chains have probably taken hundreds of thousands of years to accumulate, starting when the Sahara began to become significantly dry roughly 2.5 million years ago. Rivers became smaller, failed to reach the sea and deposited their sand load in the desert. Wind did the rest, blowing the sand into aerodynamic dune forms. According to scientists, chain trends coincide with two of the four major trends identified in the Great Eastern Sand Sea immediately to the north. Each trend likely implies a different formative wind direction--attesting to the climate shifts that have occurred since sand began to accumulate in the central Sahara. Smaller dunes are superimposed on the mega-dunes. Sinuous crest lines are the mesoscale (intermediate in size) forms, forming octopus-like crests, especially evident as the arms of star dunes. Whereas the mega-dunes are apparently stationary, studies based on aerial photographs in other parts of the world show that these dune crests move in the course of decades. The smallest dunes appear in patches on the eastern sides of the mega-dunes as a tracery of closely spaced crests. Small dunes move fast and reform quickly as stronger winds shift with the seasons. Sand grains are blown continuously from upwind dunes, across the dune-free flats. Small dunes form when the grains slow down and accumulate at the next large dune. The small dunes ride up and over the backs of the mega- and meso-dunes. Interestingly the crest orientation of the small dunes is different from that of the mesoscale dunes throughout the image. This is a common effect of wind direction shifting locally depending on dune height: the increased friction caused by larger dunes causes formative winds to blow to the left of the (weaker) winds that form the small dunes. The friction effect of larger dunes is to the right in the southern hemisphere, well illustrated on the coast of the Namib Desert.
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