4 Types Of Sand Dunes
Most people think of sand dunes when they hear the word dune. However, a dune is any landmass created by deposited material that forms a low mound or ridge of sediment. Dunes can be formed by sediments of different sizes or even snow. They are formed when wind blows sediments, mostly from the same direction, until it encounters an obstruction of some sort. The wind slows and drops the heavier particles, which then build up and add to the size of the original obstruction. This becomes a cycle, with the blockage getting larger and more sand gathering.
The formation of a sand dune is progressive. The wind causes a layering effect. This is called a slip face movement. As the wind builds a mound, it tends to keep blowing some sand over the top in a leading edge. This leading edge eventually gets too heavy, creates an unstable slope, and slides down the other side in a layer. The wind continues to blow and builds another leading edge that eventually slips into a new layer. See Fig. 10-3 for the slope layering process and growth of sand dunes.
There are five main types of sand dunes. These are the barchan, transverse, blowout, linear, and composite dunes. Although it is sometimes easier to see different dune types from the air, some deserts have only one predominant type.
The barchan dune is a horseshoe-shaped dune with the front curve facing into the wind. Barchan dunes are often found in groups, but occasionally alone. They often move over a flat surface of pebbles or bedrock. The slip face of a barchan is away from the wind.
Transverse dunes form long lines of ridged dunes that are perpendicular to the wind direction. They have steep slip faces at the back sides of the ridges. These wavy dunes form in areas with plenty of sand and not much plant life. The sand dunes found behind beaches are often transverse dunes formed by strong ocean winds. The farther inland you travel, the more plant life you encounter.
Blowout dunes are basically opposite in shape from barchan dunes. In a blowout dune, the horseshoe-shaped curve faces away from the wind. The slip face is away from the wind. Blowout dunes have vegetation that stabilized the sand at one point, but has since become covered by sand to form a mound, creating the dune.
Linear dunes are mostly parallel to the wind and form long, straight ridges. These dunes can be over 100 meters tall and go on for many kilometers. Geologists think linear dunes are caused by winds that blow from one direction in one part of the year (northwest), and then shift and blow from a different direction (southwest) during another part of the year. The overall dune movement is easterly with a long, thin shape. The sand bedding is criss-crossed and the slopes are unequal. the different types of sand dunes most commonly seen by geologists.
There are four main types of sand dune formations
LA last dune type is really a combination of two or more types. Composite dunes form extremely large, tall, hilly forms known as draas. They are a mix of mostly transverse and linear dunes that get to be over 400 meters high. These giant dunes move much more slowly—sometimes only z-2 meter per year—than their smaller cousins. They form wide dune fields called ergs. Some ergs can cover as much 500,000 km2. That is one big sandbox!
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