Deserts: Wind Deposits (Dunes)
Table of Contents
Deposition in deserts is mainly caused by:
- A disturbance in the wind speed causing material to be deposited
- Obstacles such as shrubs, rocks, dead camels and bushes
- Material moving onto a flat surface such as in alluvial fans
Perhaps the most picturesque wind depositional features in deserts are sand dunes. Dunes can vary in size from small ripples of several millimetres to mega sized dunes of 500m or more. Dune formation is bleak and attempts to explain them is flawed or not very clear, although some explanations are widely accepted.
Free dunes are are mobile and are not stopped or hindered by any obstacles. Most free dunes are formed when the wind speed lowers enough to allow sand accumulations.
Barchan /Crescent Dunes
Barchan dunes are most common types of dunes in most sand seas. They are formed:
- along flat surfaces
- under limited sand supply
- unidirectional wind
As saltating grains drops on a incoherent sandy surface, they are trapped and the sand piles up. The upwind facing sand is blown up the dune and a gently sloping surface called stoss (nose) is formed. On the leeward side (behind the dune) winds are weak and material from the upslope is deposited forming a slip face, which is fairly steep than the stoss side.
Wind from the same direction blows along the sand mound sides entraining the sand into horns and a crescent shape is formed. The area between the two horns is called a court.
The dune moves in the direction the horns are facing, which is also the direction the wind will be blowing.
A barchanoid is a merged barchan dune with horns barely present.
They’re the merged type of bachanoids. They are also formed when wind prevails in one direction. The crests lie perpendicular to the wind and the slip face lies on the leeward side.
Linear Dunes/ Seif
A type of dune formed when wind blows in from two directions or parallel sand mounds resulting in a triangular cross section of sand accumulation. The crest (top of dune) has a sharp outline resembling the edge of a sword.
Seifs have slips faces on both sides and the one side where winds are dominate will be active. Linear dunes can reach 100m or more or several centimeters and run for several kilometers along the desert surface.
These dunes have an interesting shape, a star. Star dunes form when wind blows from different directions (north, south, west and east) resulting in a mound of sand piling in the middle with horns extending outwards like a star. They can be several meters in height.
Dome dunes are circular mounds of sand without any horns
Reverse dunes have an irregular pattern with winds blowing erratically from opposite directions. The side with strong winds will be most active.
Some dunes are formed from obstacles along the desert surface such as shrubs, rocks or dead camels. These type of dunes are called nebhka in the Aribian world (John Hugget Fundamentals of Geomorphology).
These obstacles act as accumulation points for sand restricting them from moving. The sand can pile up for several metres in height. In addition, some dunes can climb atop a ridge or scarp or conversely fall down from a scarp as falling dunes.
These dunes are mostly found along the coasts of desert where wind prevailing in one direction and wave water blows out mound of sand. Parabolic dunes have a U shape appearance and resembles barchan dunes but are different in that, unlike barchan dunes, the dune migrates in the direction of the nose with the horns trailing behind. The slip face migrates in the direction of the wind.
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