Dune fields are large features of eolian or arid environments. They are associated with hot climate deserts such as the Sahara. Dune fields are not, however, exclusively restricted to these types of environments. Many dune fields are found in temperate climates where the processes of aridity in an arid climate combine to form dunes , but at a much slower rate than hot, arid climates.
The basic processes that contribute to dune formation are straightforward. It is the range of minute geological processes that generates controversy in dune research. When a geographical region experiences prolonged drought accompanied by high evaporation rates, the soils lose vegetation. Plant roots secure loose particles that make up soils. When wind sweeps across the barren soil , the first stages of dune and desert formation occur.
Wind is an excellent agent for separating grains sizes and weights in soils. Although not as effective as water for transporting sediments, it is responsible for a tremendous amount of silt and sand relocation around the planet. Wind picks up lighter and smaller particles, such as the types of clay minerals , and moves them far away from their source. Atmospheric dust can circulate the globe and may stay in the air for days, weeks, and even years. It later settles in all types of places, even the polar areas.
Unlike these lightweight minerals, heavier minerals such as quartz remain in their original spot and begin to accumulate. They are rolled around by wind but not removed. The rolling makes the grains smooth and of the type more commonly associated with dunes than with water deposits. The grains are typically light in color and are what make dune fields light beige to white in hue.
Dune fields are active regions of moving sands that form characteristic shapes including the well-known crescent dune. The macro and microscopic movement of sand particles is an area still being intensively studied. One method of particle movement is called saltation . This is a process in which the wind is not strong enough to pick up the grains, but, instead, moves the grains along the ground in a hopping and rolling fashion. As the grains climb up the faces of dunes facing the wind, they reach the crest where they bounce off the top. Movies of this action have been carefully studied. This airborne motion of grain movement can be seen as wisps of sand curling from the tops of dunes much like the snows blowing from mountain tops.
The sediments that accumulate on the windward slope are called topset deposits. When they reach the crest, they form an unstable and temporary surface called the brink. When enough sediments are captured on the brink they eventually tumble over the edge onto the slipface. This motion provides the advancement of the dune as it migrates in the direction of the wind. A temporary halt in dune movement can make a thin layer of sediments that become slightly bonded to one another. This layer becomes visible in side view and is even more recognizable in ancient deposits.
The sequence of dune formation has been widely studied. Increased or decreased wind strength is the force that makes the wide variety of dune shapes. This has led to a wide variety of terms used to describe dunes, dune fields, and other structures. Barchan dunes are the traditional crescent shaped dunes where the tips lie pointing away from the direction of the wind. Parabolic dunes are also crescent-shaped, but in this case, the tips lie facing the wind. Star dunes represent a dune formed from wind that blows from a variety of directions. Draas is an antiquated term used to describe huge dune fields that are often only observable from space . An erg is a region in the Sahara that is occupied by deep and complex sand dunes. Size is one of the factors that distinguish ergs from draas.
Dune fields themselves are complex environments. Within the field, there are many microenvironments that lie between the dunes and at the bottom of dune valleys. Moisture may even accumulate and form small ponds. Scientists continue to study dunes and dune fields. They are one of the least understood structures in geology because of the difficulty in studying them. However, dune fields occur over a significant portion of Earth's surface and certainly command more attention.
See also Beach and shoreline dynamics; Desert and desertification; Seawalls and beach erosion