Think of the Sahara Desert. What do you see?

Most of you are probably envisioning a sea of ​​dunes filled with wave after wave of windswept sand scorched by an unrelenting sun. The spectacle seems almost endless – desolation in all directions.

In truth, this description rings true for a minor proportion of Earth’s largest hot desert. Of the approximately 9 million square kilometers of the Sahara Desert (almost the same size as China), only about a quarter is actually covered in sand. The rest is a hard, rocky and rugged surface, its banality sometimes broken by salt flats, mountains or valleys.

The picturesque dunes of the Sahara are relegated to areas called ergs – wide flat expanses of desert covered in sand. These form downwind of dry riverbeds, deltas, floodplains and lakes, where previously drowned sediments dry out and, over the years, are blown away for miles and miles.

You can see how the ergs of the Sahara ostensibly align with the locations of former bodies of water.

The Great Sea of ​​Sand, straddling Egypt and Libya, is one of the most remarkable ergs in the Sahara. It alone covers 72,000 square kilometers, an area only slightly smaller than the state of South Carolina in the United States.

Sahara sand dunes stand out aurally as well as visually. Large curved dunes composed mainly of silica-containing sand grains between 0.1 mm and 0.5 mm in diameter with some moisture can generate enormous resonance noise when the wind passes over them. This roaring humprobably created by the friction between the grains of sand or the compression of the air, can reach 105 decibels, that is to say approximately the equivalent of a helicopter in close proximity!

Although it is technically a myth that the Sahara Desert is mostly covered in sand, in a few thousand years our understanding of the Sahara could completely change (provided we are still here). It is because the Sahara can alternate from dry and inhospitable to wet and verdant every 20,000 years or so. Examining dust deposits dating back 240,000 years, a team of researchers reported in 2019 that the sediments seemed to change from dry to wet in synchronization with slight changes in the tilt of the Earth’s axis. Indeed, the Sahara seems to be a region of the Earth in geological flux.