The geography of deserts involves landforms, elevation, soil, mineral resources, and water resources.
Desert terrain may consist of mountains, a basin surrounded by mountains, or a high plain. Many desert areas were once lake beds that show the effect of erosion and soil deposits carried there by rivers. Wind also helps shape the desert terrain by blowing great clouds of dust and sand that bite into rock, sometimes sculpting it into strange and magnificent shapes. In the Australian Desert, unusual pinnacles (tall mountain shapes) of limestone rock formed over thousands of years by the wind, stand on the flat desert floor.
Unless anchored by grass or other vegetation, sand dunes migrate constantly. Their rate of movement depends upon their size—smaller dunes move faster—and the speed of the wind. Some dunes may move over 100 feet (30 meters) in a year and can bury entire villages. In the Sahara, dunes created by strong winds may achieve heights of 1,000 feet (305 meters). Larger dunes may become permanent. Scientists estimate that dunes in the Namib Desert may be as much as 40,000,000 years old.
Dunes take different shapes, depending upon how they lie in respect to prevailing winds. When the wind tends to blow in one direction, dunes often form ridges. The ridges may lie parallel to the wind, forming seif (SAFE), or longitudinal dunes, or at right angles to it forming transverse dunes. Seif dunes are the largest, with some in the Sahara approaching 250 miles (400 kilometers) in length. At desert margins, where there is less sand, dunes may assume crescent shapes having pointed ends. These are called barchan (bahr-KAN) dunes, and the wind blows in the direction of their "points." In the Sahara, stellar (star-shaped) dunes are commonly found. Stellar dunes are formed when the wind shifts often, blowing from several directions.
Deserts exist at many altitudes. North American deserts are partly mountainous, but Death Valley, a large basin in California, is 282 feet (86 meters) below sea level at its lowest point. The main plateau of the Gobi is 3,500 feet (1,067 meters) above sea level, and the Sahara extends from 436 feet (133 meters) below sea level to 11,204 feet (3,361 meters) above. Temperature, plant, and animal life, are all influenced by elevation.
Desert soils tend to be coarse, light colored, and high in mineral content. They contain little organic matter because there is so little vegetation. If the area is a basin or a catch-all for flash-flood waters, mineral salts may be carried to the center where concentrations in the soil become heavy. If the area was once an inland sea, like the Kalahari (kah-lah-HAHR-ee) Desert of Botswana, eastern Namibia, and northern South Africa, exposed bottom sediments (matter deposited by water or wind) are very high in salt.
Most desert sand is made of tiny particles of the mineral quartz. Placed under pressure for long periods, grains of sand may stick together, forming a type of rock called sandstone.
Some deserts have little soil, exposing bare, wind-polished, pebbly rock, called "desert pavement." Rocks are often broken due to contraction and expansion caused by extreme temperature variations. Basin areas scoured by winds often show surfaces of gravel and boulders, and on steep slopes, whipping winds may leave little soil.
Dust devils, columns of dust that spin over the desert landscape, are carried by whirlwinds. Some dust storms produce clouds thousands of feet high. In the Sahara, up to 200,000,000 tons (180,000,000 metric tons) of dust is created each year. Red Saharan dust has been found on rooftops as far away as Paris, France, more than 1,000 miles (1,600 kilometers) from the Sahara.
Two long-lasting chemical reactions also affect desert rocks and soil. One, called desert varnish, gives rocks, sand, and gravel a dark sheen. Desert varnish is believed to be caused by the reaction between the moisture from overnight dew and minerals in the soil. The second reaction is the formation of duricrusts—hard, rocklike crusts that form on ridges when dew and minerals such as limestone combine, creating a type of cement.
Desert soils offer little help to plant life because they lack the nutrients provided by decaying vegetation and are easily blown away, exposing plant roots to the dry air. Some deep-rooted plants can exist on rock, however, where moisture accumulates in cracks. Other plants remain dormant during the driest periods, thriving and blooming after brief rains.
Soil also reveals much about a desert's geological history. In Jordan, a Middle-Eastern country, for example, the Black Desert takes its name from black basalt, a rock formed from volcanic lava.
Valuable minerals like gold and oil (petroleum) are often found in desert regions. In the Great Sandy Desert of Australia, miners hunt for gold nuggets. "Black gold," as oil is often called, is found beneath the desert regions of the Middle East, where it was formed over time from the sediment of prehistoric oceans. (Countries such as Saudi Arabia have become wealthy from the sale of their oil reserves.) Iron ore is mined in portions of the Sahara. And, borax—a white salt used in the manufacture of such products as glass and detergent—was once mined in Death Valley, California.
Water sources in the desert include underground reserves and surface water.
In addition to occasional rainfall, deserts may have reserves of underground water. These reserves, often trapped in layers of porous rock called aquifers, were formed over thousands of years when rainwater seeped underground. Reserves close to the surface may create an oasis, a green, fertile haven where trees and plants thrive. The presence of water may allow a completely different biome to form like an island in the desert.
Desert peoples often dig wells into aquifers and other underground water sources to irrigate crops and water their animals. As desert populations grow, however, water sources shrink and cannot be replaced fast enough. There is a real danger that groundwater reserves will one day be depleted.
Water may also be found in desert areas in the form of rivers or streams. Some streams form only after a rain, when water sweeps along a dry river bed in a torrent (violent stream) then quickly sinks into the ground or evaporates. However, moisture sometimes remains under the surface, for plants can be seen growing along the path of streams.
Permanent rivers are also found in desert regions. The Colorado River is one example. Over a million years ago, the Colorado began to cut a path into the plateau of limestone and sandstone rock in northern Arizona, ultimately forming the Grand Canyon, which is 1.2 miles (1.9 kilometers) deep and 277 miles (446 kilometers) long. Perhaps the most famous desert river is the Nile, which bisects Egypt. Since ancient times, Nile floods have brought enough rich soil from countries farther south to turn Egypt's river valley into fertile country well known for its agricultural products, such as cotton.
Permanent lakes, however, rarely occur in desert regions. Two exceptions are the Great Salt Lake of Utah, which is all that remains of what once was a great inland sea, and the Dead Sea of Israel and Jordan. The Dead Sea is actually a salt lake that was once part of the Mediterranean.
Source: "Desert." U·X·L Encyclopedia of Biomes, Vol. 1. U·X·L, Detroit: 2000.