DEFINITION OF SAND DUNES

Geometrically regular mounds or ridges of

sand that are found in several geological environments,

including deserts and beaches. Most people think of deserts

as areas covered with numerous big sand dunes and continual

swirling winds of dust storms. Really, dunes and dust

storms are not as common as depicted in popular movies,

and rocky deserts are more common than sandy deserts. For

instance, only about 20 percent of the Sahara Desert is covered

by sand, and the rest is covered by rocky, pebbly, or

gravel surfaces. However, sand dunes are locally very important

in deserts, and wind is one of the most important processes

in shaping deserts worldwide. Shifting sands are one of

the most severe geologic hazards of deserts. In many deserts

and desert border areas the sands are moving into inhabited

areas, covering farmlands, villages, and other useful land

with thick accumulations of sand. This is a global problem,

as deserts are currently expanding worldwide. The Desert

Research Institute in China has recently estimated that in

China alone, 950 square miles (2,500 km2) are encroached

on by migrating sand dunes from the Gobi Desert each year,

costing the country $6.7 billion per year and affecting the

lives of 400 million people.

Wind moves sand by saltation—an arching path, in a

series of bounces or jumps. You can see this often by looking

close to the surface in dunes on beaches or deserts. Wind typically

sorts different sizes of sedimentary particles, forming

elongate small ridges known as sand ripples, very similar to

ripples found in streams. Sand dunes are larger than ripples

(up to 1,500 feet high, or almost 0.5 km) and are composed

of mounds or ridges of sand deposited by wind. These may

form where an obstacle distorts or obstructs the flow of air,

or they may move freely across much of a desert surface.

Dunes have many different forms, but all are asymmetrical.

They have a gentle slope that faces into the wind, and a steep

slope that faces away from the wind. Sand particles move by

saltation up the windward side, and fall out near the top

where the pocket of low-velocity air cannot hold the sand

anymore. The sand avalanches, or slips down the leeward

slope, known as the slip face. This keeps the slope at 30–34°,

the angle of repose. The asymmetry of old dunes is used to

tell the directions ancient winds blew.

The steady movement of sand from one side of the dune

to the other causes the whole dune to migrate slowly downwind

(typically about 80–100 feet per year, or 24–30 m/yr),

burying houses, farmlands, temples, and towns. Rates of

dune migration of up to 350 feet per year (107 m/yr) have

been measured in the Western Desert of Egypt, and the

Ningxia Province of China.

A combination of many different factors led to the formation

of very different types of dunes, each with a distinctive

shape, potential for movement, and hazards. The main variables

that determine a dune’s shape are the amount of sand

that is available for transportation, the strength (and directional

uniformity) of the wind, and the amount of vegetation

that covers the surface. If there is a lot of vegetation and little

wind, no dunes will form. In contrast, if there is very little

vegetation, a lot of sand, and moderate wind strength (conditions

that you might find on a beach) then a group of dunes

known as transverse dunes form, with the dune crests aligned

at right angles to the dominant wind direction.

Barchan dunes have crescent shapes and have horns

pointing downwind, and they form on flat deserts with steady

winds and a limited sand supply. Parabolic dunes have a Ushape

with the U facing upwind. These form where there is

significant vegetation that pins the tails of migrating transverse

dunes, with the dune being warped into a wide U-shape.

These dunes look broadly similar to barchans, except the tails

point in the opposite direction. They can be distinguished

because in both cases, the steep side of the dune points away

from the dominant wind’s direction. Linear dunes are long,

straight ridge-shaped dunes elongate parallel to the wind

direction. These occur in deserts with little sand supply and

strong, slightly variable winds. Star dunes form isolated or

irregular hills where the wind directions are irregular.

See also DESERT.

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