If the wind were a homogeneous stream of air blowing from one constant direction, long straight dune ridges oriented at right angles to the wind would result. Most dunes, however, are neither straight nor at right angles to the wind, and this indicates that the winds are not a uniform stream or that they blow from different directions. The fairly uniform geometric shapes of several basic types of dunes can be recognized from desert to desert on Earth, and some of the same types have been identified on Mars as well.

Barchan dunes are common to both the Earth and Mars. These small crescent-shaped sand bodies occur in areas where the regional wind blows consistently from one direction. Their crescentic shape must be due to spatial variations in wind velocity, and the regular repetition of dune shapes and spacings when they are close together indicate that the variations in the wind are also regular. This is a property common to all bed forms. It is thought that the flow of a fluid arranges itself in long spiral vortices parallel to the direction of flow, which, with zones of faster and slower velocities arranged transverse to the flow, gives a regular sinuous pattern on the bed.

Where there is a continuous sand cover, a varied dune pattern results from the pattern of flow. The main forms are transverse ridges composed of alternating crescentic elements, like barchans, facing downwind, and other crescentic elements facing upwind. These enclose between them a regular pattern of small hollows. Superimposed on this are small straight ridges parallel with the flow. These elements form a network pattern that is extremely common in the great sand seas. The dunes commonly reach a height of nearly 200 metres and are spaced hundreds of metres to more than two kilometres apart.

One of the important features of sandy terrains is that their forms occur in a number of distinct sizes. Large features are covered with smaller ones, and the smaller ones are covered with ripples. In most of the larger sand seas there is usually a network pattern of very large dunes known as compound dunes, mega-dunes, or draa. These are sometimes arranged parallel to the apparent flow, in long ridges, and occasionally transverse to it in great sand waves. The compound dunes are usually covered with a smaller, secondary dune pattern, and the smaller dunes with ordinary sand ripples in most cases. Within each of the size groups of the hierarchy (ripples, dunes, or compound dunes) there are variations in size depending on the grain size of the sand and wind velocity; for example, whereas most ripples are spaced only a few centimetres apart, “mega-ripples,” built in very coarse sand, are spaced almost as far apart as small dunes; and whereas most dunes are about 100 metres apart, the low undulations of coarse sand on sand sheets are up to 500 metres apart. The relation between sand grain size and the shape of a dune is not, however, one of simple cause and effect, for the relation is not constant in all dunes of a given shape or in all localities.

Some dune forms can be related to variations in the overall wind direction, usually on a seasonal cycle. In some areas, winds from opposed directions blow during different seasons, so that “reversing dunes” are formed, in which the slip faces face first in one direction and then in the other. Distinct dunes are formed around topographic obstructions and in sheltered zones on the lee of small hills into which the sand migrates. If the wind meets a high scarp or large hill massif, a so-called echo dune is deposited on the upwind side separated from the scarp by a rolling eddy of air that keeps a corridor free of sand. Many oases and routeways are found in this kind of corridor. Echo dunes are among the largest dunes in the desert, sometimes reaching a height of more than 400 metres.

Mt. Elbrusvolcano, Western Caucasus mountain range, Russia. (dormant Russia)
Britannica Quiz
Natural Wonders

Fixed dunes in semiarid regions

Dunes also form around plants in the desert where groundwater is available for vegetation. The usual dune forms that occur in such instances are isolated mounds around individual plants. These forms are known as coppice dunes, or nebkha. Further, in many regions that are now subhumid or humid, one finds areas of older dunes fixed by vegetation, providing undeniable evidence that these regions were once more arid than they are today. On the North American high plains, in Hungary, and in Mongolia, the fixed sands have a cover of rich grassland. In Poland they are covered with coniferous forests. The dune patterns on these fixed sands bear a close resemblance to those in active sand seas, except that their forms are rounded and subdued.

Andrew Warren William J. Breed C.S. Breed

desert, any large, extremely dry area of land with sparse vegetation. It is one of Earth’s major types of ecosystems, supporting a community of distinctive plants and animals specially adapted to the harsh environment. For a list of selected deserts of the world, see below.

Desert environments are so dry that they support only extremely sparse vegetation; trees are usually absent and, under normal climatic conditions, shrubs or herbaceous plants provide only very incomplete ground cover. Extreme aridity renders some deserts virtually devoid of plants; however, this barrenness is believed to be due in part to the effects of human disturbance, such as heavy grazing of cattle, on an already stressed environment.

According to some definitions, any environment that is almost completely free of plants is considered desert, including regions too cold to support vegetation—i.e., “frigid deserts.” Other definitions use the term to apply only to hot and temperate deserts, a restriction followed in this account.

Origin

The desert environments of the present are, in geologic terms, relatively recent in origin. They represent the most extreme result of the progressive cooling and consequent aridification of global climates during the Cenozoic Era (65.5 million years ago to the present), which also led to the development of savannas and scrublands in the less arid regions near the tropical and temperate margins of the developing deserts. It has been suggested that many typical modern desert plant families, particularly those with an Asian centre of diversity such as the chenopod and tamarisk families, first appeared in the Miocene (23 to 5.3 million years ago), evolving in the salty, drying environment of the disappearing Tethys Sea along what is now the Mediterranean–Central Asian axis.

Deserts also probably existed much earlier, during former periods of global arid climate in the lee of mountain ranges that sheltered them from rain or in the centre of extensive continental regions. However, this would have been primarily before the evolution of angiosperms (flowering plants, the group to which most present-day plants, including those of deserts, belong). Only a few primitive plants, which may have been part of the ancient desert vegetation, occur in present-day deserts. One example is the bizarre conifer relative welwitschia in the Namib Desert of southwestern Africa. Welwitschia has only two leaves, which are leathery, straplike organs that emanate from the middle of a massive, mainly subterranean woody stem. These leaves grow perpetually from their bases and erode progressively at their ends. This desert also harbours several other plants and animals peculiarly adapted to the arid environment, suggesting that it might have a longer continuous history of arid conditions than most other deserts.

Arabian Camel (Camelus dromedarius) in the Sahara Desert sand dunes. (pack animal; sand; Morocco; Africa; African desert; mammal; dromedary; drought)
Britannica Quiz
Quick Quiz: Deserts

Desert floras and faunas initially evolved from ancestors in moister habitats, an evolution that occurred independently on each continent. However, a significant degree of commonality exists among the plant families that dominate different desert vegetations. This is due in part to intrinsic physiologic characteristics in some widespread desert families that preadapt the plants to an arid environment; it also is a result of plant migration occurring through chance seed dispersal among desert regions.

Such migration was particularly easy between northern and southern desert regions in Africa and in the Americas during intervals of drier climate that have occurred in the past two million years. This migration is reflected in close floristic similarities currently observed in these places. For example, the creosote bush (Larrea tridentata), although now widespread and common in North American hot deserts, was probably a natural immigrant from South America as recently as the end of the last Ice Age about 11,700 years ago.

Are you a student?
Get a special academic rate on Britannica Premium.

Migration between discrete desert regions also has been relatively easier for those plants adapted to survival in saline soils because such conditions occur not only in deserts but also in coastal habitats. Coasts can therefore provide migration corridors for salt-tolerant plants, and in some cases the drifting of buoyant seeds in ocean currents can provide a transport mechanism between coasts. For example, it is thought that the saltbush or chenopod family of plants reached Australia in this way, initially colonizing coastal habitats and later spreading into the inland deserts.