An erg is a desert area covered by wind-blown sand dunes.
The word comes from the Arabic ʿirq, meaning a vein or sand ridge. In geomorphology, an erg is more than an isolated dune: it is a broad sand sea or dune field with little vegetation. A commonly used technical threshold is at least 125 square kilometres of aeolian sand, with sand covering more than 20% of the surface.
Ergs form where dry conditions leave loose sediment exposed to persistent winds. The sand may originate in ancient riverbeds, deltas, dry lake basins, floodplains or beaches, then travel downwind before accumulating into dunes. Major ergs occur in the Sahara, Arabian Peninsula, Central Asia and Australia; the Rubʿ al Khali is the largest.
A common mix-up is treating every sandy desert as an erg. Deserts can also be dominated by rocky plateaus, gravel plains or salt flats. Smaller accumulations are often called dune fields, while an erg may contain several dune shapes, including crescentic, linear and star dunes. Ergs also occur beyond Earth, including on Mars, Venus and Saturn’s moon Titan.