PLATEAU

extensive area of flat upland usually bounded by an escarpment on all sides but sometimes enclosed by mountains. The essential criteria for plateaus are low relative relief and some altitude. Plateaus are extensive, and together with enclosed basins they cover about 45 percent of the Earth's land surface. Classifications of different plateaus reflect their origins and subsequent erosional history. Tectonic plateaus are the most common. Most of Africa is such an uplifted continental block. The Arabian Peninsula and the Deccan Plateau of the Indian subcontinent are similar landforms. On a rather smaller scale, fault blocks and horsts are plateaus uplifted along marginal fault systems or left standing high when neighbouring blocks have sunk. Horsts are usually more distinctive than the larger fault block. A variation is the tilted block, in which the plateau has one steeper edge and a gently tilted surface. Plateaus enclosed within mountain systems are termed intermontane plateaus. In the United States the term basin and range is often used for this type of landscape, which extends between the Sierra Nevada and the Rockies. Basin-and-range topography, however, has a worldwide distribution. It includes much of Central Asia, Tibet, part of Szechwan, and Mongolia. Anatolia, Armenia, and Iran consist of intermontane plateaus, and some writers have gone so far as to class the Mediterranean, Aegean, and Black seas as intermontane basins. Intermontane plateaus are also common in the Andes; Lake Titicaca, the highest navigable water in the world at 12,500 feet (3,810 m), lies on such a plateau. Intermontane plateaus, horsts, and fault blocks are usually associated with young fold mountains. Other plateau types consist of resistant rocks. Volcanoes pouring out extensive areas of basaltic lava have created a number of plateaus. Examples include the Antrim basaltic plateau in northern Ireland, the Columbia-Snake basin in the northwestern United States, Ethiopia, and the northwestern part of the Deccan in India. Low relief distinguishes plateaus from mountains, although modes of origin may be the same. Plateau areas, however, are subject to erosion and dissection by streams and glaciers. Ancient fold systems, once eroded, have been uplifted as plateaus and begin to undergo erosion once again. The Ardennes of Belgium, the Fjeld-plateaus of Scandinavia, and the Allegheny-Cumberland plateaus in the Appalachians are examples of this. In other places dissection has been more thorough and little remains of the original plateau surface. Such surfaces are inferred from accordant summits and have aroused debate among geomorphologists as to the existence of such erosional surfaces. The Highlands of Scotland are one area where surfaces have been postulated but where the existence of particular levels is often disputed. Apart from presenting problems in sampling surfaces, uplifted areas may have undergone substantial warping, and one cannot be sure that a former low-lying plain has been uplifted. In arid and semiarid areas dissection often leads to the formation of tabular masses, especially if there is a resistant caprock. This is the typical frontier scenery of the southwestern United States, where surfaces have been eroded into mesas (tablelands) and buttes (smaller flat-topped hills). Plateaus, being high, often create their own local climates. In basin-and-range topography, height together with the rain-shadow effect created by surrounding mountains work to produce arid and semiarid conditions. extensive area of flat upland usually bounded by an escarpment on all sides but sometimes enclosed by mountains. The essential criteria for plateaus are low relative relief and some altitude. Plateaus are extensive, and together with enclosed basins they cover about 45 percent of the Earth's land surface. Although plateaus stand at higher elevation than surrounding terrain, they differ from mountain ranges in that they are remarkably flat. Some plateaus, like the Altiplano in southern Peru and western Bolivia, are integral parts of mountain belts. Others, such as the Colorado Plateau (across which the Colorado River has cut the Grand Canyon), were produced by processes very different from those that built neighbouring mountain ranges. Some plateaus, as, for example, the Deccan Plateau of central India, occur far from mountain ranges. The differences among plateaus can be ascribed to the different geologic processes that have created them. Peter H. Molnar