officially UNITED STATES OF AMERICA, abbreviations U.S., or U.S.A., byname AMERICA, country of North America with 48 contiguous states occupying the mid-latitudes of the continent, together with the state of Alaska at the northwest extreme of North America and the island state of Hawaii lying in the mid-Pacific Ocean. The coterminous United States is bounded to the north by Canada; to the west by the Pacific Ocean; to the south by Mexico, the Gulf of Mexico, and the Straits of Florida; and to the east by the Atlantic Ocean. Its capital is Washington, D.C. Area, including the U.S. share of the Great Lakes, 3,679,192 square miles (9,529,063 square km). Pop. (1993 est.) 258,233,000. [Table] [Map]
A brief treatment of the United States follows. For full treatment, see MACROPAEDIA: United States of America.
For current history and for statistics on society and economy, see BRITANNICA BOOK OF THE YEAR.
The Atlantic Coastal Plain is part of North America's larger eastern- coastal plains, which extend from New England in the United States to Mexico's Yucatán Peninsula. Narrow in the northeastern United States, the plain widens southward, as flooded river valleys such as Chesapeake Bay form major estuaries. Offshore sandbars and barrier beaches stretch intermittently along the length of the eastern seaboard. Turning westward, the plain includes the Florida Peninsula, and the entire Gulf Coast; it extends as much as 500 miles (800 km) inland along the Mississippi River before turning southward into Mexico.
The Appalachian Mountains, trending from south-southwest to north-northeast for about 1,500 miles (2,400 km), rise abruptly to the west of the coastal plain along the Blue Ridge escarpment from the Piedmont. The latter is a well-drained, rolling landscape, contrasting markedly with the low and often swampy Atlantic Plain. The Appalachians are very old, consisting of the eroded stumps of much greater ranges but today rising to only 6,684 feet (2,037 m), at Mount Mitchell. They generally do not exceed 3,500 feet (1,100 m) in elevation and are almost entirely forested.
The Interior Lowlands and their upland fringes, widest in the north, stretch along the Canadian border from Lake Ontario (the easternmost of the Great Lakes) westward to the Rocky Mountains (roughly 400 miles [650 km] from the Pacific Coast). The region narrows to the south, eventually reaching the Atlantic Plain near the Rio Grande. The Interior Lowlands may be further divided into two major subregions: the eastern and more humid Central Lowland corresponds roughly with the country's "Corn Belt"; the western and drier Great Plains continue northward far into Canada and comprise the continent's major wheat-growing region. The Central Lowland is bounded by the Great Lakes and the Canadian Shield to the north and by the Ohio River and the Ozark Plateau (the westernmost extension of the Appalachian system) to the east and south. The Central Lowland merges into the Great Plains roughly 300 miles (500 km) west of the Mississippi River. The Great Plains, though seemingly flat, actually rise continuously from about 2,000 feet elevation in the east to more than 6,000 feet as they approach the Rocky Mountain front, the easternmost limit of the Western Cordillera.
The western United States is dominated by the two branches of the Western Cordillera: the eastern ranges form the Pacific-Gulf of Mexico drainage divide along the Rocky Mountains; the western ranges consist of two smaller systems, with the Sierra Nevada and the Cascades forming one interior chain of mountains and the Coastal Ranges the other. The country's highest peaks lie within the western branches of the Cordillera. Mount McKinley, the continent's highest peak, at 20,320 feet (6,194 m), lies within the Alaska Range (an extension of the Western Cordillera), and Mount Whitney, the highest peak within the coterminous United States, at 14,494 feet (4,418 m), is in the Sierra Nevada.
The western Intermontane Plateau's region is dominated by the Great Basin (a vast interior drainage system), which contains the Great Salt Lake. The west coast of the country is among the most geologically active regions in the world; the explosive eruption of Mount St. Helens during the spring of 1980 was a dramatic reminder of this fact. The Hawaiian Islands, of which the islands of Hawaii and Maui are the largest, were formed almost entirely by volcanic action. Alaska, another geologically active region, consists of an insular and cordilleran area in the south; interior basins, plains, and tablelands; the Brooks Range in the north; and the tundra-covered Arctic Plains in the far north.
The hydrology of the central and east-central United States is dominated by the Mississippi River basin (including its two major tributaries, the Missouri and the Ohio rivers), the waters of which drain southward into the Gulf of Mexico. The Mississippi basin is roughly coincident with the Interior Lowlands region and covers about two-fifths of the coterminous United States. The Mississippi and several of its tributaries make up one of the world's great navigable inland-waterway systems. The country's other major network of inland waterways consists of the Great Lakes (constituting by far the largest freshwater-lake group in the world) and the St. Lawrence River. West of the Rockies, nearly all of the rivers are strongly affected by aridity. In the deserts and steppes of the intermontane basins, most of the scanty runoff disappears into interior basins, only one of which, the Great Salt Lake, holds any substantial volume of surface water. Aside from lesser coastal streams, only three large river systems reach the Pacific Ocean--the Columbia, the Colorado, and the San Joaquin- Sacramento system of California's Central Valley.
Although the United States experiences wide climatic variation, the precipitation pattern may be depicted as two comparatively humid coasts separated by a progressively more humid (west to east) interior. Rainfall generally declines westward from the humid eastern zone (lying roughly along and southeastward of the Appalachians), where precipitation is usually greater than 40 inches (1,000 mm). It ranges between 30 and 40 inches (760 to 1,000 mm) throughout much of the Central Lowland and between 10 and 30 inches (250 and 760 mm) westward in the Great Plains. The Great Plains are separated from the Pacific coast by the extremely arid landscape of the intermontane basins, much of which receives less than 4 inches (100 mm) of precipitation annually. The far northwest coast (or Pacific Northwest) is the wettest part of the country, often receiving 70 inches (1,780 mm) or more of rainfall annually. Both Alaska and Hawaii are very humid, precipitation ranging from 60 to 200 inches (1,500 to 5,100 mm) in Alaska and from as little as 10 to more than 480 inches (12,200 mm; at Mount Waialeale) in Hawaii.
Temperatures throughout the coterminous United States vary
seasonally. The greatest extremes occur in the vast north-central
plains; Chicago's average temperature ranges from 27
F (-3 C) in January
to 75 F (24
C) in July, while along the Gulf of Mexico, Mobile's average
January and July temperatures are 52
F (11 C) and 82
F (28 C), respectively.
The frost-free period decreases northward, from more than 240
days along the Gulf of Mexico to less than 120 days along most
of the country's border with Canada. The climate is generally
milder along both oceanic coasts than it is in the interior.
About one-fifth of the territory of the United States is arable; the area sown to cereal grains by itself is approximately equivalent to the combined areas of France and the United Kingdom. The Central Lowland and the Great Plains, now largely devoted to agriculture, originally supported tall grasslands (prairie) and short grasslands (steppe), respectively. The country also possesses vast rangelands and pasturelands, and forests (covering roughly one-fourth and one-third of total land area, respectively). Prior to modern settlement, forests of such hardwoods as oak, hickory, and walnut, interspersed with softwood species, stretched unbroken through much of the east and north-central United States. The western-coastal forests are known for their enormous redwoods and Douglas firs. The western mountains support typically alpine vegetation, while the intermontane region, for the most part, contains desert shrub and a variety of cactus species.
Animal life in the United States is composed of both a range of indigenous wildlife and an extensive list of introduced species, such as the English sparrow. Among the best-known species of indigenous wildlife are the American bison, black bear, puma (mountain lion), white-tailed deer, pronghorn antelope, coyote, beaver, bobcat, porcupine, skunk, opossum, and prairie dog.
The United States is rich in both metallic and energy minerals and is among the leading producers of many: two-fifths of the world's molybdenum reserves; about one-fifth of its lead; one-sixth of its copper, silver, and zinc; one-tenth of its gold; and somewhat smaller amounts of its iron ore and cobalt, to mention a few of the more important. The country's extensive energy resources include about one-fifth of the world's recoverable reserves of high-quality coal (anthracite and bituminous), more than one-eighth of the world's recoverable reserves of lignite, and substantial reserves of petroleum and natural gas. Such critical metals as chromium, manganese, and platinum, however, supply only a fraction of domestic needs.
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