Deserts are the driest places on earth. They are found in the Americas, Africa, Asia and Australia. These deserts and their margins cover over 30 percent of the earth's surface, and the majority of these dry lands are found along two imaginary lines called the Tropic of Cancer and the Tropic of Capricorn which lie north and south of the equator respectively. All desert lands have one common characteristic ╨ they receive less than 10 inches of rainfall per year. Although often thought of as enormous empty expanses of sand or vast areas of barren wasteland, the reality is that desert ecosystems are surprisingly rich in their variety of plant and animal life. The harsh and often hostile climatic conditions has meant that many plants and animals have had to adapt their bodies and behaviours in order to survive. Many people have also made deserts their home.
This CD-ROM takes a look at the world's deserts and shows how varied these arid landscapes can be. Even the most uninhabitable places for humans can be lively environments for wildlife. This CD-ROM also shows that deserts are often areas of unquestioned beauty, some of which are sadly being developed or changed by human activity.
Types of desert
Although deserts are characterised by dry climates, particular deserts are often dry for different reasons. For instance, deserts like the Gobi are dry because the distance from the coast means that moisture bearing winds do not reach them. In contrast, deserts like the Sahara are dry because of the creation of dry high pressure zones fueled by the climatic conditions in the vast equatorial rainforests.
There are four major factors which help the creation of deserts. Tropical high pressure zones, continentality, rain shadows and cold ocean currents. These factors are considered in the following pages of this ZooGuide.
High pressure zones
Deserts which lie to the north and south of the equatorial rain forests are dry because of an atmospheric effect called the ╘Hadley cell╒. The Hadley cell is caused because of rising hot air at the equator which creates low pressure zones at the Earth╒s surface. Moisture-laden air is pulled into these zones creating humid, wet areas in which the tropical rainforests flourish.
The rising hot air moves away from the equator and falls as it cools. The falling air becomes drier and creates zones of high pressure. These high pressure areas prevent incursion of moist air, and the land is dry and arid. Deserts caused by high pressure zones are found in Africa and Australia.
Continentality
Continental deserts are far from the world╒s oceans and the distance from water means that moist winds do not reach these desert areas. Continental deserts are also usually surrounded by mountains which remove moisture from the winds by the ╘rain shadow effect╒ described on the next page.
Continental deserts are the vast deserts of Central Asia, and include the Gobi desert and the Takla Makan desert of western China. These deserts are often further away from the equator than the deserts caused by high pressure zones.
Rain shadows
Rain shadows do not on their own cause deserts, but they often add to the aridity of desert regions. As moisture-laden air moves over mountain ranges, the moisture condenses into clouds and falls as rain. As the air loses its moisture it obviously becomes drier and the area on the other side of the mountain range receives little rainfall.
Cold ocean currents
Like rain shadows, cold ocean currents do not cause deserts, but instead they intensify the other effects which produce arid regions. Cold ocean currents move from the poles towards the equator along the western coasts of continents and as they cool the ocean they lower the rate of evaporation from the ocean╒s surface. Deserts of this type are found in western Africa (Namib Desert) and South America (Atacama desert). Cold ocean currents also dry the southwestern USA and Baja California and northwestern Africa. Fog is quite common in areas of cold ocean currents.
Desert sculpture
There are many different types of desert landscape including seas of sand; vast rocky plains; huge areas of sparse shrub vegetation; dense cactus forests; salt flats; tree-lined oases; deep canyons, salt lakes and barren mountainous regions. Desert scenery can be spectacular: for millions of years the power of wind and water have eroded and changed the desert landscape. Wind blasts sand and gravel against rocks, sculpturing impressive rock and sandstone formations; it also builds and re-shapes huge sand dunes. Freak torrential downpours turn dry beds into raging torrents capable of moving anything in its path and depositing them elsewhere. Water erodes mountain slopes, and cuts out deep gorges. Daily extremes of hot and cold temperatures cause rocks to expand when hot and contract when cold. This weathering fractures rocks and sometimes shatters them to pieces.
Desert sand and rock
Although many desert areas are rocky, most people think of vast landscapes of sand when they think of deserts. Desert sand begins life as rock, which is weathered down over the ages to form fine particles which we call sand. Much of the huge quantity of sand that makes sandy deserts was originally eroded from mountains and highlands, and was carried by rivers or floods to lowland regions, where it was picked up by winds and dropped in desert. Some desert sand also comes from the shores of lakes and seas. In many areas, the sand is blown away by winds so that only solid rock and stone remains. In other regions, the sand builds up into giant sand dunes that may be hundreds of feet high. Some sand is coarse, while the sand from other deserts may be very fine. Desert sands may be black, red, yellow or brown in color. Sandstone is made of grains of sand which cement together under pressure, making the sand hard and rock-like. Wind erosion also smooths the surface of the sandstone.
Strong winds are characteristic of deserts, because there are few plants or buildings to provide resistance. Dry sand is whipped into dust or sandstorms: the whirling sand can be so thick that the sky darkens, making it hard to see more than few paces ahead. The Sahara Desert often produces over a hundred million tons of sand each year ╨ this desert may be the world╒s largest source of dust. Some regions of Iran and the south of the CIS (formerly called the Soviet Union) have duststorms on more than 100 days each year.
Sand dunes
Sand dunes cover around 20% of the world╒s deserts. The dunes are often called ╘seas of sand╒, an appropriate name because the winds move and shape the sand so that the terrain changes constantly. The largest continuous expanse of sand is the Empty Quarter or Rub Al Khali, part of the Arabian desert covering over 250,000 sq miles. The largest dune areas in North America are west of Yuma, Arizona and at White Sands National Monument in New Mexico.
Sand seas consist of two distinct areas: flat, duneless regions and the dunes themselves which may be anything from 3 feet to 1,200 feet high. Winds blow dunes into variety of shapes; the best-known forms are horseshoe-shaped barchans, linear dunes and star dunes. The animation on this page shows how the different shaped dunes are formed. Star dunes are quite stable, while barchan dunes may move as much as 66 feet each year.
Weathered rocks
Deserts are also well-known as areas in which strange rock formations can be seen. In deserts, rocks are weathered or worn away by wind, extreme temps, and rare but heavy rains. The strong winds pick up sand which then scours away at the rock, in a similar way to the sandblasting process used to clean dirt from old stone and brick buildings.
The desert landforms were originally formed when rocks thrust upward millions of years ago when the Earth was shaped. Over millions of years, rocks have gradually been weathered by heat, cold, wind, rain and chemical processes. In deserts, scarce rainfall means that few plants are present to protect the soil from erosion. When it does rain, torrents and floods are important in shaping the landscape: boulders and stones are carried down temporary rivers gouging out the bedrock. Wind-borne sand scours away at the rocks, sculpting strange-looking formations. Rocks crack due to extreme variation in daily temperatures ╨ they may expand when heated, only to contract during cold nights. As they cool, rocks something split with a loud bang.
Water in the desert
Although deserts are dry places, they receive up to 4 inches of rain each year: if this rainfall is received as a once-a-year downpour, a desert may temporarily become a very wet place indeed!
Water is one of desert's great sculptors. The rain falling during brief storms in the mountains rushes into dried out stream beds or washes. With nothing to stop it, the water gains speed and roars onward to the plains between the mountains, producing one of nature's spectacular dangers - the flash flood. Flooding can produce a wall of water several feet high, which pushes anything in its way, including boulders, bushes and mud.
Flood waters eventually soak into the ground or may enter a river. In some cases the water may reach an undrained basin where it becomes a pool that quickly dries up or evaporates, leaving a dry lake called a playa. The extreme temperatures of desert regions quickly evaporates water in basins, and clay and salts are deposited. The southwestern part of the US features several playas including the Great Salt Lake Desert and part of the Death Valley National Monument.
Mountains can also be changed by storms. The storm water carries rocks, gravel, soil and other materials down the sides of the mountains. Although larger rocks are only carried a short distance, the water continues its descent and leaves the finer material farther down the mountainside. This material gradually forms a land delta, known as a bajada, at the base of the mountain. Bajadas are made up of sand and clay, ranging from coarse to fine-textured descending onto the plain. Near the large dry stream beds or wahses in center of the plains, there may be water below the surface, encouraging growth of different types of plants.
Some desert areas have large rivers running through them: these include the Indus river in India, the Rio Grande in the US and the Nile in Africa. These rivers have their sources outside of the the desert region, and they provide essential water for the peoples of these areas.
Water from deep layers of rock comes to the surface at oases, where crops such as date palms are cultivated. Groundwater is stored in spaces within rocks ╨ water bearing rocks are known as aquifers, and they include sandstone and limestones. Water from aquifers may emerge at springs or it may be necessary to drill wells to tap the water source.
Today, vast areas of deserts have been irrigated by water pumped from deep bore wells.
Salts and minerals
Many desert areas of the world have high concentrations of salt and other minerals. High rates of evaporation result in salt flats or pans that are either completely evaporated or have some surface water.
The Great Salt Lake in the USA and the Kavir in central Iran are examples of salt pans.
Water flowing from mountain ranges may bring other salts into desert regions, like chlorides, sulfates and carbonates. When the water evaporates, the mineral crystals precipitate out.
Desert temperatures
Desert temperatures can vary enormously ╨ some regions endure searing heat during the day all year round, accompanied by freezing night time temperatures. In other desert areas the extremes of temperature are not quite so pronounced. Deserts like the Asian Gobi and the American Great Basin Desert are cold during the winter and may be covered in snow. These deserts can also experience very high temperatures in the middle of summer. The Arctic and Antarctic regions are classified as deserts because they both receive very little precipitation. These areas are freezing all year round and are sometimes called ╘polar deserts╒.
