A dune is a wind-blown pile of sand. Over time, dunes become well-sorted deposits of materials by wind or water that take on a characteristic shape and that retain that general shape as material is further transported by wind or water.
Desert dunes classifications are based upon shape include barchan dunes, relic dunes, transverse dunes, lineal dunes, and parabolic dunes. Dunes formed by wind are common in desert area and dunes formed by water are common in coastal areas. Dunes can also form on the bottom of flowing water (e.g., stream and river beds).
When water is the depositing and shaping agent, dunes are a bedform that are created by saltation and deposition of particles unable to be carried in suspension. Similar in shape to ripples—but much larger in size—dunes erode on the upstream side and extend via deposition the downstream or downslope side. Regardless of whether deposited by wind or water, dunes themselves move or migrate much more slowly than any individual deposition particle.
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 slip-face. 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 sand forming dunes is usually composed of the mineral quartz eroded from rocks, deposited along streams or oceans or lakes, picked up by the wind, and redeposited as dunes. Sand collects and dunes begin to form in places where the wind speed drops suddenly, behind an obstacle such as a rock or bush, for example, and can no longer transport its load of sand.
Dunes move as wind bounces sand up the dune's gently-sloping windward side (facing the wind) to the peak of the slope where the wind's speed drops and sends sand cascading down the steeper lee side (downwind). As this process continues, the dune migrates in the direction the wind blows. The steeper lee side of the dune, called the slip face, maintains a 34° angle (called the angle of repose), much greater than the flatter (10°-12°) windward side. The sand may temporarily build up to an angle greater than 34°, but eventually it avalanches back to the angle of repose. Given enough sand and time, dunes override dunes to thicknesses of thousands of feet, as in the Sahara Desert, or as in the fossilized dunes preserved in the sandstone of Zion and Arches National Parks in Utah. In the famous Navajo Sandstone in Zion National Park, crossbeds (sloping bedding planes in the rock) represent the preserved slip faces of 180 million year old former dunes.
Three basic dune shapes—crescent, linear, and star—range in size up to 330 ft (100 m) high and up to 1,000 ft (300 m) long and wide. Barchan and parabolic dunes are crescent-shaped like the letter C. Barchans form where the sand supply is minimal. The two ends of the barchan's crescent point downwind toward the direction the dune moves. In contrast, the pointed ends of a parabolic dune stab into the wind, a mirror image of a barchan. Bushes or some other obstruction anchor the tips of a parabolic dune.
Transverse and longitudinal dunes form as long, straight, or snake-like ridges. Transverse ridges run perpendicular to a constant wind direction, form with an abundance of sand available, and are asymmetric in cross section (the windward side gently-sloped, the slip face steep). The ridges of longitudinal dunes, however, run parallel to a slightly varying wind direction and are symmetrical in cross section—they have slip faces on either side of the ridge. Longitudinal dunes are also known as linear or seif (Arabic for sword) dunes.
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.
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 about 30% of Earth's surface and certainly command more attention.
The formation and movements of dune fields are also of great interest to extraterrestrial or planetary geologists. Analysis of satellite images of Mars, for example, allow calculation of the strength and direction of the Martian winds and provide insight into Martian atmospheric dynamics. Dunes fields are a significant Martian landform and many have been observed to have high rates of migration.
See also Desertification; Erosion; Sediment and sedimentation; Sedimentary environment; Sedimentary rock.
Goudie, Andrew S., et. al. Aeolian Environments, Sediments, and Landforms. Hoboken, NJ: John Wiley & Sons, 2000.
Tack, Francis, and Paul Robin. Dunes. Paris: Vilo International, 2002.
United States Geological Survey. "Types of Dunes." (cited February 24, 2003) <http://pubs.usgs.gov/gip/deserts/dunes/>.
K. Lee Lerner
Science EncyclopediaScience & Philosophy: Direct Variation to Dysplasia