A sand dune is any hill of loose sand shaped by the wind. Dunes are one of the most common types of eolian landforms. They form where there is a source of sand- for example, a sandstone formation that weathers easily and releases individual grains, or perhaps a beach supplied with abundant sand from a nearby river mouth, or a closed interior basin in an arid region that receives abundant river sand. Active dunes constantly change shape under wind currents, but they must be nearly free of vegetation in order to move. They become inactive when stabilized by vegetation cover or when patterns of wind or sand sources change.
A dune typically begins as a sand drift in the shelter of some obstacle, such as a small hill, rock or clump of brush that lowers wind speed, causing saltating sand to stop moving. Once a sufficient mass of sand has accumulated, it begins to move downwind. Pushed by the prevailing wind, sand blows up the windward dune slope, passes over the dune crest, called the slip face, moving the dune forward. The slip face maintains a more or less constant angle from the horizontal, known as the angle of repose. For loose sand, this angle is about 33° to 34°.
A vast expanse of sand dunes makes up a great desert landscape, called an erg, after the Arabic word for dune field. In the Sahara Desert, enormous quantities of dune sand have been weathered from sandstone to create extensive regions of the erg desert. Also common are regions covered with an aggregate of pebbles that form a vast and semi-impermeable flat surface. This desert pavement surface is called a reg. These two major desert landscapes can be found around the globe.
Types of Sand Dunes:
Sand dunes take a variety of shapes, depending on the supply of sand, the amount and direction of the wind, and the amount of vegetation cover. When sand is abundant, we find sand seas of dunes in constant motion. Where sand is in shorter supply, we find low ridges and rails of sand in parallel lines. Vegetation tends to bind the sand and minimize movement. Some plants make sand their preferred habitat, and stabilize dune forms for long time periods. The wind is needed to shape and move the dunes, and the stronger the prevailing wind, the more the sand moves.
We can recognize five basic types of dunes-
(1) Barchan Dunes- are crescent-shaped, with the points of the crescent directed downwind. They require a moderate sand supply and moderate winds and lack vegetation.
(2) Transverse Dunes- form where the sand supply is abundant and winds are strong and consistent. Their crests are perpendicular to the wind direction, and the dune field resembles a petrified sea of water waves.
(3) Star Dunes- are large, pyramidal hills of sand built from a moderate sand supply when winds blow regularly from several different directions. They are often fixed in position and lack vegetation.
(4) Parabolic Dunes- are often found where shorelines provide a large sand supply. Costal moisture encourages vegetation to take a foothold, anchoring the arms of the dunes in an upwind direction.
(5) Longitudinal Dunes- occur where sand is limited but winds are strong. Vegetation often stabilizes the long dune ridges, which follow the wind direction. They can cover vast areas, especially in interior Australia.