The Geology of the Sahara

Today, the vast, hostile Sahara remains the mother of all deserts. High points like 11,204-foot (3,415 m) Mount Koussi in Chad rise like stone fortresses in a great battlefield of sand. A series of ridges and mountain ranges are separated by great, stretched, low-lying depressions, like the Quyattara Depression of Egypt that lies 436 feet (143 m) below sea level.

Named for the Arabic word for desert, sahra, this vast region sits on top of the warped, deformed, and folded African Shield, composed of some of the oldest rocks on the planet, forged when life was limited to single-celled creatures living in the oceans. These older rocks underlying the present-day Sahara are mostly granite, schist, or gneiss, all mixtures of igneous and metamorphic rocks forged deep beneath the surface. The rocks that form the African Shield were laid down near the surface, buried, remelted, re-fused, and returned finally to the surface. The African Shield comprises the hard bedrock of Africa and has held the continent together during the hundreds of millions of years that it has been split, shifted, twisted, and rammed into Europe as a result of the movement of the crustal plates.

This stable mass of rock has been covered over with younger sediments, laid down in horizontal, largely unaltered layers. Much of the Sahara is covered with limestone, made mostly of the skeletons of microscopic sea creatures raining down onto the bottom of a vanished sea.

Most of the limestone and sandstone covering the surface of the Sahara were deposited in the Mesozoic era (245–65 million years ago). This was the heyday of the dinosaurs, encompassing their rise in the Triassic period and their mysterious extinction at the end of the Cretaceous period. In the Mesozoic era, both mammals and birds emerged to spread over the surface of the Earth when the dinosaurs faltered. Moreover, at the same time, the evolution of plants transformed life on land. Strange ferns, cycads, ginkgophytes, and other types dominated the surface at the beginning of the period. But by the end, flowering plants and conifers had largely displaced them.

The limestone and sandstone laid down when the dinosaurs were gobbling one another were established in vast lakes and seas when the hard underlying rock of the African Shield got heated and deeply buried in the restless jostling of continents. The basins caused by the stretching of the crust over the down-warping rocks of the shield filled with huge bodies of water like the ancient Mega-Chad. Many of these rocks now have that distinctive feature of the desert, a rich, reddish-brown coating of iron and manganese compounds weathered into desert varnish. The plateaus of the Sahara are covered with such dark, varnished rock.

Closer to the center of the great desert, several massive volcanic eruptions caused by the stresses and strains of these continental collisions left great, isolated mountain ranges or massifs. These include the highest points of the Sahara, the Tibesti and Ahaggar mountains, plus Mount Uwaynat.