26 chapters
52 minute read
Selected Chapters
26 chapters
Deserts: Geology and Resources
Deserts: Geology and Resources
by A. S. Walker Great Sand Dunes National Monument, Colorado ( photograph by John Keith ). Beauty is before me And beauty behind me, Above and below me hovers the beautiful, I am surrounded by it, I am immersed in it. In my youth I am aware of it, And in old age I shall walk quietly The beautiful trail. from a Navajo benedictory chant describing the desert Cacti dominate the Sonoran Desert vegetation near Tucson, Arizona ( photograph by Peter Kresan )....
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What Is a Desert?
What Is a Desert?
Approximately one-third of the Earth’s land surface is desert, arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. Deserts—stark, sometimes mysterious worlds—have been portrayed as fascinating environments of adventure and exploration from narratives such as that of Lawrence of Arabia to movies such as “Dune.” These arid regions are called deserts because they are dry. They may be hot, they may be cold. They may be regions of sand o
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How the Atmosphere Influences Aridity
How the Atmosphere Influences Aridity
We live at the bottom of a gaseous envelope—the atmosphere—that is bound gravitationally to the planet Earth. The circulation of our atmosphere is a complex process because of the Earth’s rotation and the tilt of its axis. The Earth’s axis is inclined 23½° from the ecliptic, the plane of the Earth’s orbit around the Sun. Due to this inclination, vertical rays of the Sun strike 23½° N. latitude, the Tropic of Cancer, at summer solstice in late June. At winter solstice, the vertical rays strike 23
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Where Deserts Form
Where Deserts Form
Dry areas created by global circulation patterns contain most of the deserts on the Earth. The deserts of our world are not restricted by latitude, longitude, or elevation. They occur from areas close to the poles down to areas near the Equator. The People’s Republic of China has both the highest desert, the Qaidam Depression that is 2,600 meters above sea level, and one of the lowest deserts, the Turpan Depression that is 150 meters below sea level. Deserts are not confined to Earth. The atmosp
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Trade wind deserts
Trade wind deserts
The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world’s largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert....
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Midlatitude deserts
Midlatitude deserts
Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical high-pressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert....
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Rain shadow deserts
Rain shadow deserts
Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside “shadow” of the range. The Sahara of Africa is the world’s largest desert. It contains complex linear dunes that are separated by almost 6 kilometers ( Skylab photograph ). A rare rain in the Tengger, a midlatitude des
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Coastal deserts
Coastal deserts
Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmos
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Monsoon deserts
Monsoon deserts
“Monsoon,” derived from an Arabic word for “season,” refers to a wind system with pronounced seasonal reversal. Monsoons develop in response to temperature variations between continents and oceans. The southeast trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon desert
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Polar deserts
Polar deserts
Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10° C. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This “freeze-thaw” alte
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Paleodeserts
Paleodeserts
This aerial photograph of the Nebraska Sand Hills paleodesert shows a well-preserved crescent-shaped dune (or barchan) about 60 to 75 meters high ( photograph by Thomas S. Ahlbrandt ). Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid tha
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Extraterrestrial deserts
Extraterrestrial deserts
Mars is the only other planet on which we have identified wind-shaped ( eolian ) features. Although its surface atmospheric pressure is only about one-hundredth that of Earth, global circulation patterns on Mars have formed a circumpolar sand sea of more than five million square kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest sand sea on our planet. Martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north p
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Soils
Soils
Soils that form in arid climates are predominantly mineral soils with low organic content. The repeated accumulation of water in some soils causes distinct salt layers to form. Calcium carbonate precipitated from solution may cement sand and gravel into hard layers called “calcrete” that form layers up to 50 meters thick. Caliche is a reddish-brown to white layer found in many desert soils. Caliche commonly occurs as nodules or as coatings on mineral grains formed by the complicated interaction
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Plants
Plants
Most desert plants are drought- or salt-tolerant. Some store water in their leaves, roots, and stems. Other desert plants have long tap roots that penetrate the water table, anchor the soil, and control erosion. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Sparse, very dry, single species vegetation in Death Valley, California. Vegetation amidst the desert pavement of the Sonoran Desert ( photograph by John Olsen ). De
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Water
Water
Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous. More people drown in deserts than die of thirst. Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams
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Eolian erosion
Eolian erosion
Wind erodes the Earth’s surface by deflation, the removal of loose, fine-grained particles by the turbulent eddy action of the wind, and by abrasion, the wearing down of surfaces by the grinding action and sand blasting of windborne particles. Most eolian deflation zones are composed of desert pavement , a sheetlike surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of the Earth’s desert surfaces are stony deflation zones. The rock mantle in
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Eolian transportation
Eolian transportation
Particles are transported by winds through suspension, saltation, and creep. Small particles may be held in the atmosphere in suspension . Upward currents of air support the weight of suspended particles and hold them indefinitely in the surrounding air. Typical winds near the Earth’s surface suspend particles less than 0.2 millimeters in diameter and scatter them aloft as dust or haze. Saltation is downwind movement of particles in a series of jumps or skips. Saltation normally lifts sand-size
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Eolian deposition
Eolian deposition
Wind-deposited materials hold clues to past as well as to present wind directions and intensities. These features help us understand the present climate and the forces that molded it. Wind-deposited sand bodies occur as sand sheets, ripples, and dunes. Sand sheets are flat, gently undulating sandy plots of sand surfaced by grains that may be too large for saltation. They form approximately 40 percent of eolian depositional surfaces. The Selima Sand Sheet, which occupies 60,000 square kilometers
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Types of Dunes
Types of Dunes
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 . 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
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Remote Sensing of Arid Lands
Remote Sensing of Arid Lands
The world’s deserts are generally remote, inaccessible, and inhospitable. Hidden among them, however, are hydrocarbon reservoirs, evaporites, and other mineral deposits, as well as human artifacts preserved for centuries by the arid climate. In these harsh environments, the information and perspective required to increase our understanding of arid-land geology and resources often depends on remote-sensing methods. Remote sensing is the collection of information about an object without being in d
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Mineral Resources in Deserts
Mineral Resources in Deserts
Some mineral deposits are formed, improved, or preserved by geologic processes that occur in arid lands as a consequence of climate. Ground water leaches ore minerals and redeposits them in zones near the water table. This leaching process concentrates these minerals as ore that can be mined. Of the 15 major types of mineral deposits in the Western Hemisphere formed by action of ground water, 13 occur in deserts. Evaporation in arid lands enriches mineral accumulation in their lakes. Playas may
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Problem
Problem
Desertification became well known in the 1930’s, when parts of the Great Plains in the United States turned into the “Dust Bowl” as a result of drought and poor practices in farming, although the term itself was not used until almost 1950. During the dust bowl period, millions of people were forced to abandon their farms and livelihoods. Greatly improved methods of agriculture and land and water management in the Great Plains have prevented that disaster from recurring, but desertification prese
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Global monitoring
Global monitoring
In the last 25 years, satellites have begun to provide the global monitoring necessary for improving our understanding of desertification. Landsat images of the same area, taken several years apart but during the same point in the growing season, may indicate changes in the susceptibility of land to desertification. Studies using Landsat data help demonstrate the impact of people and animals on the Earth. However, other types of remote-sensing systems, land-monitoring networks, and global data b
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Local remedies
Local remedies
At the local level, individuals and governments can help to reclaim and protect their lands. In areas of sand dunes, covering the dunes with large boulders or petroleum will interrupt the wind regime near the face of the dunes and prevent the sand from moving. Sand fences are used throughout the Middle East and the United States, in the same way snow fences are used in the north. Placement of straw grids, each up to a square meter in area, will also decrease the surface wind velocity. Shrubs and
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Temperature
Temperature
Landsat image shows complex linear and crescentic dunes in the northeastern Taklimakan Desert of China. In this desert there are a great many evil spirits and also hot winds; those who encounter them perish to a man. There are neither birds above nor beasts below. Gazing on all sides as far as the eye can reach in order to mark the track, no guidance is to be obtained save from the rotting bones of dead men, which point the way. Explorer Fa Xian describing the Taklimakan Desert of China about 40
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Cover Photographs:
Cover Photographs:
Granite Mountain in the Great Basin Desert ( photograph by Terrence Moore ). Sonoran Desert ( photograph by Peter Kresan ). Zabriskie Point in Death Valley, California ( photograph by Peter Kresan ). Artists Point in Monument Valley ( photograph by Peter Kresan ). Death Valley, California ( photograph by Cecil Stoughton ). Cacti in the Sonoran Desert ( photograph by John Olson )....
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