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16 December 2011

Sand Sea in Southwestern Libya


(Click on image for larger view.)

In southwestern Libya, near the borders of Algeria and Niger, lies a sand sea named Idhan Murzuq, also Sahra Marzuq or the Murzuq Desert. This extreme desert environment sees little rain, and the sand sea is home to complex dunes shaped by dry winds. Extending from the northeast corner of the roughly round sand sea, like a whipping tail, is a corridor of sand. Dry as it is today, this corridor lines what used to be a river channel: Wadi Barjuj.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image on December 7, 2011. Sand dunes are especially visible in the northwestern part of Idhan Murzuq, but the entire sand sea contrasts with its rockier surroundings.

A study published in 2008, which relied on data from multiple NASA sensors, reconstructed Libya’s geology and hydrology over the past 7 million years. Over that time span, Libya intermittently experienced humid, rainy conditions. One finding of the study was that Wadi Barjuj was probably once a tributary in a river system that drained into the Mediterranean Sea though eastern Libya.

Rivers naturally migrate and change over time, and Wadi Barjuj was no exception. Volcanoes began to rise northeast of Wadi Barjuj, and sometime between 5 million and 2 million years ago, they blocked the river’s path. As this happened, intermittent periods of extreme humidity continued, and with its route blocked, rather than feeding a river system draining into the sea, Wadi Barjuj began feeding a lake. The researchers studying this region’s geologic history named the water body Lake Megafazzan, and estimated its maximum size at about 135,000 square kilometers (52,000 square miles). During humid periods, they explained, Libya was “a veritable lake district.”

Massive as it was, the lake did not cover all of southwestern Libya, and today’s Idhan Murzuq was likely above water during most of the last several million years. It was not, however, a sand sea throughout that entire period. The researchers found evidence of sand seas starting to form in the region after volcanoes blocked the path of Wadi Barjuj, and after the massive lake began backing up toward the southwest.

References
CIA World Factbook. (2011, November 29). Libya. Accessed December 10, 2011.
Drake, N.A., El-Hawat, A.S., Turner, P., Armitage, S.J., Salem, M.J., White, K.H., McLaren, S. (2008). Palaeohydrology of the Fazzan Basin and surrounding regions: The last 7 million years.Palaeogeography, Palaeoclimatology, Palaeoecology, 263(3–4), 131–145.
NASA image courtesy LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott.

Instrument:
Terra - MODIS - NASA