The Rub' al Khali, the "Empty Quarter", is the sand desert (erg) encompassing most of the southern third of the Arabian Peninsula. The desert covers some 650,000 km2 (250,000 sq mi) including parts of Saudi Arabia, Oman, the United Arab Emirates, and Yemen. It is part of the larger Arabian Desert.
Sand dunes to the east of Liwa Oasis in the Emirate of Abu Dhabi, near the UAE's border with Saudi Arabia
Typical pale gravel plains surrounded by huge sand dunes, as seen in the Emirate of Abu Dhabi
Water found below ground level in the Rub' al-Khali in Shaybah, Saudi Arabia
Satellite image of sand dunes in the Empty Quarter
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 ʿarq (عرق), 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 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), the largest being the Rub' al Khali, the Empty Quarter of the Arabian Peninsula. Ergs are also found on other celestial bodies, such as Venus, Mars, and Saturn's moon Titan.
Issaouane Erg, Algeria
Linear Dunes, Namib Sand Sea
Erg Chebbi, Morocco
Satellite image of Rub' al Khali (Arabia's Empty Quarter), the world's largest erg with an area of more than 600,000 km2 (230,000 sq mi)