The Sahara Desert, the world’s largest hot desert, is often pictured as an endless expanse of towering sand dunes. The specific geological term for these immense, undulating sandscapes is an “erg.” These vast seas of sand are complex, dynamic environments sculpted by wind and time. Understanding the erg involves recognizing its definition and the physical processes that create it.
Defining the Erg
An erg is a broad, flat area of desert covered by wind-blown sand, defined by specific size and composition requirements. To qualify as an erg, it must cover a minimum area of 125 square kilometers (about 48 square miles). Furthermore, wind-transported sand (aeolian sand) must cover more than 20% of that surface area. Smaller accumulations of dunes are referred to as simple dune fields.
The Sahara is home to several of the world’s largest ergs, including the Grand Erg Oriental and the Grand Erg Occidental, which stretch across Algeria and neighboring countries. Within these massive sand bodies, the dunes take on various shapes, reflecting complex wind patterns. Common dune forms include linear dunes, crescent-shaped barchans, and massive, pyramidal star dunes, which can reach heights of over 100 meters.
The Mechanics of Formation
The creation of an erg requires a large, continuous source of sand, persistent wind energy, and a topographical feature to trap the material. Sand grains are weathered from rock formations and often transported into the desert basin by ancient river systems. Once exposed, the wind moves the grains through three distinct aeolian transport mechanisms.
The most common method is saltation, which accounts for the majority of material displacement. This process involves medium-sized sand grains (0.1 to 0.5 millimeters) being lifted and bounced along the surface by the wind. When a saltating grain lands, its impact dislodges other stationary grains, initiating a chain reaction of movement.
Larger, heavier sand grains (0.5 to 2 millimeters) move primarily through surface creep. These grains are too heavy for direct wind transport, so they are nudged and rolled along the ground by the force of impacting saltating particles. In contrast, the finest dust particles (smaller than 0.1 millimeters) are carried by suspension. These tiny particles can be lifted high into the atmosphere and transported great distances.
Ergs form and are sustained in large sedimentary basins where the wind’s power wanes or where topographical barriers create zones of wind convergence. These low-lying areas act as natural traps, allowing massive quantities of sand to accumulate over long geological timescales.
Ergs in Context: Contrasting Desert Landscapes
Ergs constitute only a fraction of the Sahara Desert’s vast expanse, covering approximately 20% of its total surface area. The majority of the Sahara is composed of entirely different types of landscapes. The most widespread terrain is the reg, a flat, stony plain or gravel desert.
Regs are defined by a surface layer of tightly packed pebbles and gravel, often called desert pavement. This layer is left behind after the wind removes finer sand and dust particles. These plains cover about 70% of the Sahara and are remnants of ancient riverbeds and seas.
The third major type of Saharan landscape is the hamada, which consists of elevated, barren, and hard rocky plateaus. Hamadas are distinguished by the complete absence of sand, as aeolian processes have removed nearly all loose material, exposing the bedrock. These high, rocky regions stand in stark contrast to the low-lying, soft sand of the ergs.