Dunes
Dunes are 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 blount (parabolic) dunes. Dunes formed by wind are common in desert areas 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.
In desert regions, dune shape is dependent upon a number of factors including the type of sand, the moisture content of the sand, and the direction and strength of the prevailing wind pattern. Barchan dunes are crescent-shaped small dunes with the terminal points of the crescent pointed downwind (on the lee side of the prevailing wind). Transverse dunes are long narrow dunes (a dune line) formed at right angles to the prevailing wind pattern. Transverse dunes may form from the fusion of individual barchan dunes.
Blount or parabolic dunes may form in regions of higher moisture content where there is sufficient vegetation to retard the migration of sand. Blount dunes take the mirror image shape of barchan dunes—they are crescent-shaped, but the terminal points of the crescent point windward (into the direction of the prevailing winds). Lineal dunes form parallel to prevailing wind patterns. Lineal dunes may be become the dominant relief feature and dunes may measure several hundred yards or meters high and extend for more than 50 miles (80 km).
Desert dunes migrate downwind from prevailing winds. Relic dunes form as migration slows and vegetation forms on a dune.
Ergs are "dune seas" ("erg" derives from Arabic) or large complexes of dunes. Very large (generally over 100 meters high and at least a kilometer long) complexes of dunes form a drass. Globally, dune fields and seas are common between 20° to 40° N, and 20° to 40° S latitudes.
In contrast to well-sorted dunes, a loess is another form of sedimentary, wind-driven deposit usually associated with glacier movements. Loess formations, however, represent layers of settling dust and are not well-sorted.
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, allows calculation of the strength and direction of the Martian winds and provides insight into Martian atmospheric dynamics. Dunes fields are a significant Martian landform and many have high rates of migration.
Join eNotes
Over 3,500 study guides, question and answer forums, literature criticism, reference content, and much more!
