The Shifting Sands Of The Sahara Are A Lesson In Dune Dynamics
The many ergs of the Sahara Desert are a beautiful lesson in the shifting history of sand in the desert, and the shapes and scales of dunes
Top image: The Issaouane Erg in Algeria on January 16, 2005. Credit: NASA
Most of the Sahara Desert is not sand and dunes, but other, equally harsh landscapes. Image credit:NASA
The Sahara Desert is impressively large, but for the most part fails to live up to the deserts of our imagination, containing gravel-strewn plains and painfully scrubby vegetation. But in pockets of the desert where tall plateaus and remorseless mountains break the wind, sediment gathers into vast sand seas called ergs. The sand in these ergs is teased by the wind into epic dunes, shifting sand that can take thousands of years to grow into monstrous structures.
The Sahara Desert contains ancient lava flows and volcanoes (dark brown), escarpments and plateaus (grey-green), and sand (orange-red) amongst less picturesquely desolate landscapes. Image credit: NASA
Transverse dunes grow perpendicular to the prevailing wind direction in this tiny patch of sand between mesas and plateaus in southern Mauritania on January 8, 2014. Image credit: NASA
The erg contains a variety of dunes, from wee linear dunes form parallel to the prevailing wind, to crescent-shaped barchan dunes with their rounded backs to the wind and steep slip-faces in the lee, to picturesque star dunes in areas of complex, multi-directional winds that change with seasons and are disrupted by storms.
Longitudinal dunes, barchan dunes, and star dunes in the Issaouane Erg, a 38,000 square kilometer (nearly 15,000 square mile) erg in eastern Algeria between the northern Tinrhet Plateau and the southern Fadnoun Plateau. Image credit:NASA
Star dunes in the Grand Erg Oriental on October 27, 2012 reflect the season east-west shift in wind direction, and the even more chaotic changes from local conditions and occasional storms. Image credit: NASA
Rare rainfall gathers in the basins between dunes, the water first dissolving salts then leaving them behind as it evaporates under the unrelenting sun. The evaporite deposits form interdune salt deposits called sabkah, blue-white areas nested in the sea of iron oxide rich red sands.
Yellow-orange sand of the Tifernine Dune Field abruptly ends at the dark brown consolidated rocks of the Tinrhert Plateau in the southernmost tip of the Grand Erg Oriental. Image credit: NASA
Longitudinal dunes stretch between lakes in the Ounianga Basin on November 14, 2009. The lakes evaporate at a rate of over 6 meters per year, yet only one of the ten is saline due to a freshwater aquifer feeding the lakes. Image credit: NASA
With this much sand, complexity is inevitable. In this case, it means the superposition of dunes, with the smallest dunes climb up the slopes of the larger dunes, migrating in a never-ending dance of sand and wind.
Linear dunes aligned with the direction of prevailing winds in Erg Iabès on March 11, 2006. Image credit: NASA
When barchan dunes and star dunes crawling across each other, they’re reflectingthe transition between a single dominant wind direction and a more complex variable-direction wind regime.
Star dunes crawling over longitudinal dunes, each ridge separated by salt flats in the Erg Oriental. Image credit:NASA
In other places, the long strips of longitudinal dunes separated by salt flats are broken by superimposed star dunes. The contained chaos of a multi-armed mess set in tidy rows tells a two-stage history of strong southern winds creating the initial longitudinal dunes, weakening over time with weak gusts blowing out arms from central points.
Longitudinal (linear) dunes parallel to the wind and transverse dunes perpendicular to it dominate this stretch of the Fachi-Bilma erg. The transverse dunes are a particular subset called zibar dunes, where the finest sediments are winnowed by the wind, leaving the coarsest grains to form dune crests. Image credit: NASA
Each scale of dune reflects a different time domain, with the largest dunes taking the longest to accumulate.
Superposition of dune types as smaller dunes migrate over the slopes of larger dunes in the Grand Erg Oriental. The zig-zags of the mega-dunes reflect a shift in dominant wind direction of the millenia-long history of the dunes. Image credit: NASA
The largest mega-dunes grew over thousands of years, potentially dating back to the first arid days of the Sahara 2.5 million years ago, while the intermediate-scale meso-dunes grow, crest, and migrate on the scale of decades. The very smallest dunes are the fastest, taking just years to crawl along the surface of their larger relatives.
The draa of the Marzuq Sand Sea are superimposed with longitudinal, transverse, and star dunes in this image from December 20, 2008. Image credit: NASA
Draa, the largest of the dunes, can reflect all the shapes of the smaller dunes depending on the wind history over their formation. From zig-zags of a variable past to long arms of a relatively steady wind regime, they are the result of the same mechanics just written large over far longer time scales.
For more information on any of the individual images, follow the image credit to read about it on NASA’s Earth Observatory.