A large area full of sand dunes in a sandy desert is called an erg. As they are formed by the wind, it is likely that their shape, or their number, may change over a period of time. A large erg can spread over many kilometres.

An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The word is derived from the Arabic word arq (???), meaning “dune field”. Strictly speaking, an erg is defined as a desert area that contains more than 125 km2 (48 sq mi) of aeolian or wind-blown sand and where sand covers more than 20% of the surface. Smaller areas are known as “dune fields”. The largest hot desert in the world, the Sahara, covers 9 million square kilometres (3.5×106 sq mi) and contains several ergs, such as the Chech Erg and the Issaouane Erg in Algeria. Approximately 85% of all the Earth’s mobile sand is found in ergs that are greater than 32,000 km2 (12,355 sq mi). Ergs are also found on other celestial bodies, such as Venus, Mars, and Saturn’s moon Titan.

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