deserts

deserts Deserts are regions characterized by very low annual rainfall (usually less than 300 mm), sparse vegetation, extensive areas of bare, rocky mountains and plateau, and alluvial plains. Sand dunes cover less than a third of desert regions. Many deserts are regions of high temperature (the hot or tropical deserts), but some polar regions (including most of the Antarctic continent) are also classified as deserts.

A generally accepted definition of a desert is a region in which mean annual potential evapotranspiration (Etp) exceeds mean annual precipitation (P) by a factor of two or more. A world map of desert regions (Fig. 1) identifies three main classes of aridity: (1) hyperarid (P/Etp < 0.03); (2) arid (0.03 < P/Etp >  0.20); and (3) semi-arid (0.20 < P/Etp > 0.50). Defined in this way, deserts cover approximately one-third of the Earth's land surface.

Causes of deserts

Desert climates are characterized by low humidity (except in cool, foggy coastal deserts like the Namib), a high daily range of temperatures, and precipitation that is highly variable in time and space. The most extensive deserts lie astride the tropics. Descending, dry stable air masses in the subtropical anticyclonic belts maintain arid conditions throughout the year. The effects of stable air masses are reinforced by large land masses. Long distances to continental interiors restrict the influence of moist oceanic air masses in summer, as in the central Asian and African deserts. In winter, large continental areas develop strong high-pressure cells, reducing the influence of frontal systems. Mountain barriers block rain-bearing winds and create rain-shadow areas in their lee, especially in the Great Basin Desert of North America and in central Asia, where the Himalaya prevent penetration of the south-west monsoon to the Gobi and Takla Makan deserts. Deserts located on the west coast of South America and southern Africa (Atacama, Namib) owe their hyperarid climates to the influence of cold oceanic currents offshore. These reinforce the subsidence-induced stability of the atmosphere by cooling surface air masses and creating a strong temperature inversion.

Desert landforms

Although no landforms or geomorphological processes are unique to deserts, certain characteristics of desert environments have a significant effect on the operation of the major processes of weathering, erosion, transport, and deposition. A sparse vegetation cover with a high percentage of bare ground results in rapid run-off of water when intense rainfall does occur, and enhances the ability of the wind to erode and transport sand and dust (silt- and clay-sized sediment). Sand accumulates in areas of lower wind velocity and transport capacity to form dunefields and sand seas comprised primarily of crescentic, linear, and star dunes, with sand sheets in marginal areas. Dune form is governed by the availability of sand and the variation in wind direction from season to season. Dust storms transport fine-grained material away from desert regions to be deposited in ocean sediments and desert margin soils. Some may reach the polar ice caps.

The excess of evaporation over precipitation gives rise to physical or mechanical, rather than chemical, weathering of rocks, and to upward movement of soil moisture and near surface groundwater. As a result, water-soluble salts (principally sodium chloride, calcium carbonate, and calcium sulphate) accumulate in desert soils, forming calcic and gypsic horizons in the subsoil. Insolation weathering and salt weathering dominate processes of rock breakdown. On a regional scale, lack of water gives rise to internal drainage and thus to playas and salt lakes.

Table 1. Proportions of landform types in deserts (percentage of area covered) (from Cooke et al. 1993)

Landform type	South-Western USA	Sahara
Desert mountains	38.1	43.0
Playas	1.1	1.0
Desert flats	20.5	10.0
Bedrock fields	0.7	10.0
Regions bordering	1.2	1.0
through flowing
rivers
Ephemeral streams	3.6	1.0
Alluvial fans	31.4	1.0
Sand dunes	0.6	28.0
Badlands	2.6	2.0
Volcanic fields	0.2	3.0

The character of desert landforms is also affected strongly by the regional geological and tectonic environment. Two end-member models can be recognized (Table 1). The tectonically stable Old World shield deserts, such as those in the Arabian Peninsula, Australia, and southern Africa (Fig. 2a), are characterized by low relief and extensive rocky plains and isolated hills (inselbergs), with up to 30 per cents of the land surface occupied by areas of sand dunes or sand seas. Variants of this landscape depend on whether the bedrock is sedimentary, as in the northern and eastern Sahara, or crystalline, as in much of Australia and Namibia. At the other end of the spectrum are the high-relief deserts of the tectonically active areas of the Atacama, the Basin and Range Province of western North America, and parts of central Asia (Fig. 2b). These deserts have only rare and small sand-dune areas, but extensive areas of desert mountains, alluvial fans, and internal drainage.

Deserts in the past

The distribution of desert climates has changed significantly over geological time. Many of the modern areas of aridity originated during late Tertiary times (especially the Mid- to Late Miocene) as the ‘modern’ climate and geography of subtropical regions developed. Aridity in north Africa and Asia intensified with the uplift of the Himalaya and the formation of the Tibetan Plateau, blocking the penetration of the monsoon to central Asia and reinforcing the tropical easterly jet stream that brings dry, stable air masses to the Sahara. Narrowing of the Tethys seaway increased the continentality of northern Africa, culminating in the isolation of the Mediterranean in the Miocene and the first signs of true aridity in the Sahara. The southern African deserts owe their origins to the development of the Antarctic ice sheet and cooling of the Southern Ocean, which led in turn to the formation of the Benguela Current and its upwelling system. Australian deserts developed as the continent ‘drifted’ northward to reach its present latitude in the Miocene. In North America, uplift of the Sierra Nevada and the Transverse Ranges of southern California created a barrier to penetration of moist air masses from the Pacific, giving rise to the Great Basin and Mojave deserts.

During the Quaternary period, climatic changes associated with glacial–interglacial cycles at high latitudes resulted in changes in the extent of deserts and the intensity of aridity. The core hyperarid areas, such as the central Sahara and Namib, were relatively unaffected; instead, desert margin areas (the Sahel and Kalahari) were affect most. For example, there is no evidence to suggest that the Namib has experienced any climate wetter than semi-arid at any time during the Quaternary. By contrast, degraded dune systems, now covered by savannah vegetation, occur in sub-humid areas adjacent to the southern Sahara, the Kalahari, and in Australia, and indicate a expansion of arid conditions during glacial periods. In North America, dunes in the semi-arid High Plains were also mobile as little as 1000 years ago in periods of extended drought. Conversely, lake and spring deposits, as well as archaeological and faunal evidence, show that many parts of the Sahara experienced humid conditions in the period 9000–6000 years ago.

In the more distant past, the rock record of ancient aeolian (i.e. wind-blown) sandstone and evaporite deposits is evidence for extensive desert conditions during the Permian (the Rotliegendes Sandstone of the North Sea Basin) and during the late Late Carboniferous, Permian, and Jurassic (e.g. the Weber, Navajo, and Entrada sandstones of western North America). The Old Red Sandstone (Devonian) of Britain is also thought to have been deposited under somewhat arid conditions.

Nicholas Lancaster

Bibliography

Cooke, R. U.,, Goudie, A. S.,, and and Warren, A. (1993) Desert geomorphology. UCL Press, London.
Thomas, D. S. G. (ed.) (1977) Arid zone geomorphology. John Wiley and Sons, Chichester.