Deserts are defined by low precipitation, where the potential for evaporation exceeds the moisture received. This arid environment creates a unique geological setting where the primary forces shaping the landscape are wind and the rare, but powerful, action of water. The absence of continuous plant cover leaves the ground exposed, allowing wind and sporadic flash floods to constantly rework surface materials. These conditions result in distinctive landforms sculpted by the movement of air and sand or built up by the sudden deposition of waterborne sediment.
Eolian Landforms: Structures Shaped by Wind
Wind acts as an effective agent of both erosion and deposition in dry regions, giving rise to landforms collectively known as eolian features. Two primary erosional processes are abrasion and deflation. Abrasion occurs when wind-driven sand grains impact rock surfaces, effectively sandblasting them to create features like ventifacts.
Deflation involves the lifting and removal of loose, fine-grained sediment from the desert surface. This process can lead to the formation of broad, shallow depressions called deflation hollows or blowouts. Where deflation has removed the finer sand and silt, a surface of tightly packed, coarser pebbles remains, known as desert pavement.
The primary depositional landforms created by wind are sand dunes, which form when wind velocity drops and the transported sand accumulates. Sand is moved primarily through a bouncing motion known as saltation, with larger grains moving by surface creep. The shape and size of a dune depend on the consistency of the wind direction and the available sand supply.
Crescent-shaped barchan dunes are common, forming where wind blows consistently from one direction and the sand supply is limited. The horns of these dunes point downwind. When sand is abundant and the wind remains unidirectional, barchan dunes can merge into long, perpendicular ridges known as transverse dunes.
Seif dunes, also called linear dunes, appear as long, narrow ridges that run parallel to the prevailing wind direction. These forms often develop in areas with a somewhat bidirectional wind regime. Star dunes, characterized by a central pyramidal mound with three or more radiating arms, form in regions where wind consistently blows from multiple directions.
Fluvial Landforms: Features of Ephemeral Water Flow
Although deserts receive little rain, the precipitation that occurs is often intense, leading to rapid runoff and powerful flash floods that carve and deposit large amounts of sediment. These temporary water flows create distinct fluvial landforms. Channels that carry this ephemeral water are known as wadis in the Middle East and North Africa, or arroyos, washes, or coulees in the American Southwest.
These channels are dry most of the time but can quickly fill with water and debris during a storm event. When these streams exit a confined mountain canyon onto a flatter valley floor, the water loses velocity rapidly and deposits its sediment load. This deposition forms a fan-shaped feature called an alluvial fan, built from coarse, poorly sorted materials.
Over long periods, multiple adjacent alluvial fans can grow and merge along a mountain front, creating a continuous, gently sloping apron of sediment known as a bajada. The bajada forms a transitional zone between the steep mountain slopes and the flat basin floor.
At the lowest point of an enclosed desert basin, where water collects and cannot drain to the sea, a temporary lake may form following a rain event. This ephemeral body of water is called a playa lake. As the water quickly evaporates, it leaves behind an extremely flat, dry lake bed called a playa. Playas are underlain by fine-grained clay, silt, and often a crust of precipitated salts, which is why they are sometimes called salinas.
Erosional Relief: Mesas, Buttes, and Isolated Peaks
The desert landscape features large, stable erosional remnants that testify to long-term differential weathering of rock layers. This process begins with extensive, elevated regions known as plateaus. Over geologic time, water erosion cuts into the plateau, leaving behind isolated, broad, flat-topped hills called mesas.
A mesa is characterized by its steep, cliff-like sides and a protective caprock layer of hard, erosion-resistant rock that shields the softer layers underneath. The defining characteristic of a mesa is that its width is greater than its height. As erosion continues, the mesa is reduced in size, eventually becoming a butte.
Buttes are smaller, isolated, flat-topped hills that are typically taller than they are wide. They represent a later stage of erosion where the remnants of the original caprock still protect the small column of rock below. This progression from plateau to mesa to butte illustrates the gradual retreat of the ancient rock layers.
Another distinct erosional feature is the inselberg, a German term meaning “island mountain.” These are isolated, steep-sided rock hills that rise abruptly from an otherwise gently sloping plain. Inselbergs are often composed of highly resistant rock, such as granite, formed by the long-term burial and subsequent re-exposure of the resistant rock mass as the surrounding, less-resistant material is stripped away.