A worldwide inventory of deserts has been developed using images from
the Landsat satellites and from space and aerial photography. It defines
five basic types of dunes: _crescentic_, _linear_, _star_, _dome_, and
_parabolic_.
[Illustration: These crescentic dunes of coastal Peru are migrating
toward the left (photograph by John McCauley).]
The most common dune form on Earth and on Mars is the _crescentic_.
Crescent-shaped mounds generally are wider than long. The slipface is on
the dune’s concave side. These dunes form under winds that blow from one
direction, and they also are known as barchans, or transverse dunes.
Some types of crescentic dunes move faster over desert surfaces than any
other type of dune. A group of dunes moved more than 100 meters per year
between 1954 and 1959 in China’s Ningxia Province; similar rates have
been recorded in the Western Desert of Egypt. The largest crescentic
dunes on Earth, with mean crest-to-crest widths of more than 3
kilometers, are in China’s Taklimakan Desert.
Straight or slightly sinuous sand ridges typically much longer than they
are wide are known as _linear dunes_. They may be more than 160
kilometers long. Linear dunes may occur as isolated ridges, but they
generally form sets of parallel ridges separated by miles of sand,
gravel, or rocky interdune corridors. Some linear dunes merge to form
Y-shaped compound dunes. Many form in bidirectional wind regimes. The
long axes of these dunes extend in the resultant direction of sand
movement.
[Illustration: Star dunes, such as these of the Namib, indicate the
winds that formed them blew from many directions (photograph by Georg
Gerster).]
Radially symmetrical, _star dunes_ are pyramidal sand mounds with
slipfaces on three or more arms that radiate from the high center of the
mound. They tend to accumulate in areas with multidirectional wind
regimes. Star dunes grow upward rather than laterally. They dominate the
Grand Erg Oriental of the Sahara. In other deserts, they occur around
the margins of the sand seas, particularly near topographic barriers. In
the southeast Badain Jaran Desert of China, the star dunes are up to 500
meters tall and may be the tallest dunes on Earth.
[Illustration: Linear dunes advance on small playas east of Lake Eyre in
the Simpson Desert of central Australia (photograph by C. Twidale).]
[Illustration: Linear dunes in the Western Desert of Egypt (photograph
by Carol Breed).]
Oval or circular mounds that generally lack a slipface, _dome dunes_ are
rare and occur at the far upwind margins of sand seas.
U-shaped mounds of sand with convex noses trailed by elongated arms are
_parabolic dunes_. Sometimes these dunes are called U-shaped, blowout,
or hairpin dunes, and they are well known in coastal deserts. Unlike
crescentic dunes, their crests point upwind. The elongated arms of
parabolic dunes follow rather than lead because they have been fixed by
vegetation, while the bulk of the sand in the dune migrates forward. The
longest known parabolic dune has a trailing arm 12 kilometers long.
[Illustration: Small crescentic dunes occur on the crests of these
complex dome dunes of Saudi Arabia’s Empty Quarter (photograph by Elwood
Friesen).]
[Illustration: Ripples and horns of this crescentic dune in Egypt
indicate that the dune is moving right to left (photograph by John
Olsen).]
Occurring wherever winds periodically reverse direction, _reversing
dunes_ are varieties of any of the above types. These dunes typically
have major and minor slipfaces oriented in opposite directions.
All these dune types may occur in three forms: _simple_, _compound_, and
_complex_. Simple dunes are basic forms with a minimum number of
slipfaces that define the geometric type. Compound dunes are large dunes
on which smaller dunes of similar type and slipface orientation are
superimposed, and complex dunes are combinations of two or more dune
types. A crescentic dune with a star dune superimposed on its crest is
the most common complex dune. Simple dunes represent a wind regime that
has not changed in intensity or direction since the formation of the
dune, while compound and complex dunes suggest that the intensity and
direction of the wind has changed.
[Illustration: The northern Mojave Desert. The Landsat Thematic Mapper
(TM) acquires data in seven bands of the electromagnetic spectrum. On
this image, white and yellow colors indicate rocks rich in clay minerals
and limonite in rocks of red and yellow hues. The large concentrations
of limonite and clays may indicate mineral deposits exposed at the
surface or buried up to several thousand feet below it (photograph
courtesy of Melvin Podwysocki).]