Sand Dunes At Sunset Are Rainbow Lines Of Loveliness

These long, slender dunes are part of Erg Chech, a massive sand sea in Africa. The slightly sinuous ridges are catching the last rays of a setting sun, popping up above the shadowy sand valleys in an eye-twisting perspective puzzle.

Erg Chech at sunset in southwestern Algeria. Image credit: USGS/NASA/Landsat 7

This is the Erg Chech at sunset, a massive sand sea that embodies the endless dunes and lack of vegetation that characterizes our most stereotypical ideas of a desert environment. Ergs like this one form when prevailing winds hit an obstacle like a mountain range, dumping sand until it grows into a vast ocean of dunes. Erg Chech is part of the Sahara, a desolate desert-scape in southwestern Algeria.


The long, linear dunes lit by the last rays of sunset are longitudinal dunes. They form near-parallel to the prevailing winds in areas, and can stretch kilometers long.

Longitudinal dunes could form at the intersection of helical roll vortices. Image credit: Steven R. Hanna

Like many things in geomorphology, the exact origin of longitudinal dunes is not entirely understood. The long, skinny dunes may form because of helical roll vortices: prevailing spiralling twists of wind. These adjacent semi-stable rotating winds deposit sand where they intersect, creating the longitudinal dunes at the boundaries between vortices. Alternately, they could form in areas with two near-parallel prevailing wind directions, again with the wind coming together at just the right angle to create a dead-zone of deposition. What is more clear is the mechanism of what happens when wind from two directions hits the crest of a dune at an angle. If bidirectional winds hit the dune crest obliquely so the angle of attack is between 30◦ to 40◦, the wind will deflect, merge and flow along the crest. This carries sand along the crest of the dune, slowly stretching it longer and longer over the years. Other regions of the erg are populated by star dunes, their star shape reflecting being buffeted by winds coming from all directions.


A 5.7 gram fragment of the Chergach meteorite collected from Erg Chech. Image credit: Jon Taylor

In 2007, the Chergach meteorite exploded over the desert. The stony chondrite meteorite rained down rock fragments into the sand below over an elliptical strewn field roughly 20 kilometers long. More than 100 kilograms of samples were recovered during the fall and winter.


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