Land and Resources
The United States has an enormous variety of physical features and a wide diversity of animal and plant life. Besides the discussion below, much information is given in the separate articles on the states, as well as those on important mountains, rivers, lakes, and other physical features.
The present-day pattern of the landforms of the United States is the result of a long sequence of collisions and separations of large blocks of the earth's surface crust (seePlate Tectonics). The oldest part of the continent is the Canadian Shield, or Laurentian Plateau, a mass of granite and related rock that underlies eastern Canada and the northeastern United States. The shield was formed during several long periods of crustal convergence in Precambrian time (a period that stretches from the formation of the earth to about 570 million years ago). During these slow but relentless collisions of crustal plates, the rigid surface rocks buckled and cracked. Large pieces of crust were forced downward into the hot interior of the earth, where they warmed and eventually melted. This lighter molten material then moved upward through the crustal cracks, occasionally erupting as a volcano, but more often pushing surface rock upward in a broad bulge, or dome. In time, the molten material cooled and crystallized. The characteristic rock of the shield is granite, a slowly cooled and therefore coarse-grained rock. The margins of the ancient continent are more complex in structure, with zones of granite, darker ocean-bottom rocks, fine-grained volcanic rocks, hardened ocean sediments, and rocks of all types that were altered by heat and pressure during later crustal activity. The iron deposits of Minnesota, Upper Michigan, Wisconsin, and northern New York all occur in contorted rocks near the edges of the ancient shield.
A long period of inactivity in the crust followed the formation of the shield. Erosion reduced the mountainous continent to a low plain, and the adjoining seas were filled with thick beds of sediment. Near the end of this period, great forests covered the land, and the addition of organic material to the sediment formed the vast coal and petroleum layers that stretch in a broad curve from northernPennsylvania through West Virginia to Alabama, then west to Texas and northwest through the Great Plains states and Canadian prairies to Arctic Alaska.
After the Appalachian collision, the continent reversed direction and drifted west. The Atlantic Ocean began to widen, and the eastern United States again became a region of geologic calm. The lofty Appalachians began to erode, and the resulting sediment accumulated on the midcontinental Great Plains and on the Atlantic and Gulf coastal plains. Meanwhile, new ranges of mountains were rising as the western United States collided with the Pacific Plate. Lava erupted onto the surface in many places at different times: in northernNew Mexico, central Arizona, eastern California, and southern Idaho, and especially in the region of the Cascade Range of Oregon and Washington. The sandy sediments of the Great Plains were thrust sharply upward along the Front Range of the Rocky Mountains in Colorado and Montana and around smaller mountain ranges such as the Black Hills of South Dakota. Rock movement along massive faults formed California's Sierra Nevada, Utah's Wasatch Range, Nevada's aligned mountain ranges, and the Teton Range in Wyoming. The land of Arizona and southern Utah was lifted, and rivers cut canyons in the level sedimentary rocks.
The motion of crustal plates thus accounts for the broad panorama of the landforms of the United States. Interpreting the finer details of the present-day landscape requires consideration of the effects of past and present climates. The major past climatic event is the Pleistocene epoch, more commonly known as the Ice Age (seeIce Ages). At least four times in the past 1 million years, great ice sheets formed in eastern Canada and the mountains of the west and spread outward. The moving ice, like a powerful bulldozer, scraped soil and rock from Canada and the northern United States and deposited material farther south. The aligned lakes and exposed rocks of New England and northern Minnesota are results of glacial scouring. Long Island and Cape Cod are huge glacial deposits, characterized by hills composed of rock and soil, with associated swamps and sand outwash plains; similar features are abundant throughout the former glaciated areas, from New England to the Dakotas and in the western mountain valleys. Rivers such as the Hudson, Illinois, Minnesota, Missouri, and Columbia carried huge floods of glacial meltwater and carved valleys much larger than the present-day streams require. Glacial meltwater also formed many large lakes. Today level plains mark the beds and low beach ridges mark the shores of Ice Age lakes on both the eastern and western edges of Vermont, around the Maumee River of northwestern Ohio, in the sand counties of central Wisconsin, around the Red River of Minnesota and the Dakotas, around the Great Salt Lake of Utah, and in the Missoula Basin of Montana, the Central Valley of California, and hundreds of smaller areas in many states. Ice Age dust storms left thick deposits of loess (fine-grained silt or clay) on the undulating plains around the Mississippi River and Missouri River, on the steeper bluffs of western Wisconsin and western Tennessee, and in the Palouse Hills region of eastern Washington. Times of higher sea level built beaches far up on the Gulf Coastal Plain and on slopes overlooking the Pacific Ocean; on the other hand, Chesapeake Bay and many similar drowned river valleys along the Atlantic coast from Georgia to Connecticut are results of periods of lower sea level.
The climate of a place is the seasonal pattern of its inputs of solar energy, wind, and precipitation. With the exception of the principal islands of Hawaii, no place in the United States ever sees the sun directly overhead. In general, sun intensity and, consequently, temperatures decrease from south to north; in summer, however, the decrease in intensity is partly offset by longer days in the north. Montana,North Dakota, and Minnesota actually have higher record temperatures than New Mexico and Alabama. In winter, on the other hand, the short days in the north exaggerate the effect of low sun angles, creating wide temperature differences from south to north. Forests use much solar energy to evaporate water, and therefore the humid states of the eastern United States do not get as warm as the dry western deserts. Oceans and lakes moderate temperatures; this is especially true of the western coast, where the ocean is cool and the wind is usually onshore. Finally, mountains are somewhat cooler by day and much colder at night than surrounding lowlands. Effects of each of these factors can be seen in the accompanying table of temperatures.
The pattern of precipitation is largely a consequence of the interaction of wind and topography. The wind system of the earth behaves with one simple goal: to equalize temperatures on earth by taking heat from the equator and carrying it to the poles. The process is grand in scope and complex in detail. Two features of global atmospheric circulation are particularly significant for the United States. One is a current of sinking air, a gentle, but persistent, downward movement of air from the upper atmosphere. This subsidence is part of the global convection cycle and starts with updrafts of warm and humid air near the equator; the air loses moisture as it rises to the upper atmosphere and begins to move poleward. At about latitude 30° north the air begins to sink, bringing hot and dry conditions to the southwestern United States, especially in summer.
The other significant part of atmospheric circulation is the jet stream, a shifting zone of fast winds blowing generally from west to east high above the ground. The path of the jet stream on any given day is a key to surface weather. In summer, the jet stream is usually near the Canadian border, although on any given day it may loop as far north as Alaska or as far south asLouisiana. The jet brings wet Pacific air onshore in Washington and Alaska, but in other western states dry air masses from Mexico and Canada dominate. In the east, by contrast, the jet can pull moist air masses northward from the Gulf of Mexico all the way to Canada. In winter, the entire wind system follows the sun southward. Pacific air masses now bring clouds and rain to the coastal mountains from California to southern Alaska. The jet usually crosses the country at the latitude of Oklahoma, and cold, dry Canadian air covers the northern half of the country; however, day-to-day shifts of the jet may pull warm, moist Gulf air as far north as Illinois or bring Canadian air to Florida.
Regional weather hazards are intimately associated with the seasonal position of the jet stream and associated fronts. Torrential rains are most common in the United States near the Gulf of Mexico, which is the major source of moisture for the country. Tornadoes occur in the center of the United States, where Canadian and Gulf air masses often collide violently. Southern California has smog episodes and forest fires in late summer, when the jet stream and its associated rain are far to the north and the hot, dry sinking air from the tropical circulation dominates the weather. Hurricanes arise out of the late-summer warmth of the Atlantic Ocean and drift toward the southeastern states in the autumn.
Heavy winter snows in the eastern United States are caused by the rapid cooling of the Gulf air, amplified in the Great Lakes region by local lake breezes. December and March are the major snow months in Minnesota and the Dakotas; January there is a time of intense cold and little snowfall, because Gulf air cannot penetrate this far north in midwinter. Finally, the occasional kona (west coast) storms of Hawaii are wintertime incursions of North Pacific air that occur when the jet stream curves far to the south. Normal weather consists of trade winds that cause rain only on the northeastern slopes of each island.
Rivers respond to the quantity and seasonal pattern of precipitation in a region. The streams of the eastern United States, principal among which are the Hudson, Delaware, Susquehanna, Potomac, and Savannah, receive rainfall in every month and therefore are reliable avenues for waterborne commerce. Rivers of the interior, such as the Ohio, Tennessee, Illinois, and Mississippi, often flood in spring and decrease in size during the hot weeks of late summer and the snow months of winter. Some flow regulation and flood control have been achieved on these rivers through a costly and controversial system of dams and levees. Argument over water projects is even more heated in the western United States, where mountain snowmelt is the principal source of water for the eastward-flowing Missouri,Platte, Arkansas, and Rio Grande rivers and the westward-flowing Colorado, Sacramento, Snake, and Columbia rivers. Most of these rivers shrink in volume as they flow away from their mountain sources; some, like the Colorado, are dammed and diverted for so many urban or agricultural uses that they no longer carry water to the sea. Along the Pacific seaboard are a few coastal streams that have their seasonal peaks during the winter rainy season. In Alaska the drainage system is dominated by the Yukon, a river as long as the Rio Grande but considerably greater in volume.
The surface water sources of the nation also include the five Great Lakes, which occupy an interconnected set of glacially scoured basins and together serve as a major transportation artery. Glaciers also left tens of thousands of smaller lakes throughout the northeastern United States, the upper Midwest, and much of Alaska. Among the larger of these areChamplain, Winnipesaukee, and Cayuga in the northeast and Winnebago, Red, and Mille Lacs in the Midwest. The Great Salt Lake of Utah and many smaller salt basins of the Mountain states are remnants of much larger Ice Age lakes. Many groundwater aquifers, especially those of the Great Plains, are also relics of wetter climates of the past.
Soils, like landforms, are formed by the action of climate on surface material. The central Interior Plains are just far enough south to have adequate heat for rock decomposition and plant production during the growing season. The moisture balance here is also generally favorable: enough rainfall for crops but not so much that vital nutrients will be leached from the soil. Farther south, excess heat accelerates the chemical weathering and stimulates the growth of microorganisms, which consume organic residues and leave a less fertile red or yellow mineral soil. In the humid east and northeast, surplus water seeps down through the soil and removes valuable nutrient salts. At the other extreme, soils in the dry climates of the west and southwest can accumulate salts, often to levels that are toxic to plants.
During the Ice Age, glaciers scoured material from the already thin and poor soils of the north and moved a substantial amount of earth southward.Illinois, Indiana, Iowa, and Ohio were the principal beneficiaries; many parts of these states had more than 30 m (100 ft) of new earth laid down as gently undulating till plain. Parts of New England, New York, Michigan, Wisconsin, and Minnesota also gained material, but often of a more coarse and rocky nature. Meltwater floods filled many valleys in the Midwest and West with rich sediment, and upland soils were often enriched by fine windblown dust.
The United States can thus be divided into five major soil regions that are a result of these continuing formative forces: (1) the deep, black mollisols of the midcontinent (the country's most fertile), in an area stretching from Illinois west toNebraska and the Dakotas; (2) the leached red ultisols of the southeastern and western coast mountains, which are considerably less fertile; (3) the coarse and acid spodosols in the northeastern United States and in the high western mountains; (4) the moderately fertile gray or brown alfisols found around the Great Lakes, on the southern Great Plains, and in the Central Valley of California; and (5) the dry, salty aridsols of the desert Southwest.
Superimposed on these broad patterns are local geologic exceptions, such as the black clay vertisols of Texas and the organic histosols, which are peat and muck soils found in swampy places such as the FloridaEverglades or the Minnesota bogs. Younger soils, such as entisols and inceptisols, occur on shifting sand dunes, recent volcanoes, river floodplains, and steep slopes, where disturbances such as floods or landslides prevent the development of mature soil profiles. These immature soils range from barren rock outcrops in Arizona and grassy sandhills in Nebraska to fertile volcanic ash soils in Hawaii and the extraordinarily productive floodplain of the Mississippi River.
Because plants respond directly to local climate and to a lesser degree to local topography, the indigenous vegetation of the United States is marked by great diversity. In regions other than those dominated by forests are a wide variety of plants that are adapted to colder or drier conditions.
At the time European settlement began, about one-half of the United States was covered by forests. Today, because of extensive human modification, about 30 percent of the country's land area is forested. Similarly, grasslands and other natural vegetative cover decreased in extent as the continent was settled.
Northern Alaska, located in the northernmost part of the United States, is characterized by a windswept tundra, a region of lichens, mosses, hardy low shrubs, and flowering plants. Inland and to the south, the growing season lengthens and certain trees can survive. A few species of needle-leaf trees, notably spruces and firs, dominate a vast but slow-growing evergreen forest, interspersed with lichen-covered rocky areas, grassy swamps, and aspen-choked fire scars. This forest, known as the taiga, stretches southeast from interior Alaska and has small outliers in northern New England and the Great Lakes region. South of the taiga the growing season is still longer and more tree species can survive. Here is found a mixed forest containing both needle-leaf and broadleaf trees, and including pines, maples, elms, birches, and oaks, as well as hickory, beech, and sycamore. This type of mixed forest covered the region around the Great Lakes and most of the New England and Middle Atlantic states when European settlers arrived.
Still farther south, the frost-free season exceeds six months, rainfall becomes more reliable, and the forest reaches its maximum diversity. TheGreat Smoky Mountains National Park in western North Carolina and eastern Tennessee contains more tree species than the entire continent of Europe. The Gulf of Mexico coast is even warmer than this mountainous area, but its plains and low hills do not support as complex a forest. Moreover, the sandy soils and hot summers encourage fires, which suppress oaks and other hardwoods and favor the fast-growing pines that now represent the major forest resource of the nation. Other species found here include southern magnolia, pecan, red gum, and black gum (tupelo). A number of subtropical and tropical trees flourish in southern Florida. Along the coast of the Gulf of Mexico, salt marshes and groves of cypress and mangrove help to armor the shore against the eroding forces of wind and water.
The diversity of the forest also decreases west of the Appalachian Mountains. First, the mountaintop spruces, firs, and mountain ashes disappear. Then, rainfall decreases in quantity and reliability, and fires become more frequent. The lush bottomland hardwood forests of the Mississippi Valley slowly dwindle in size and complexity. Oak-hickory forests give way to isolated stands of oak and then to tall grass prairies, which, before cultivation, occupied the present Corn Belt from Indiana to the eastern Great Plains.
Farther west, the climate becomes still drier, and the tall bluestem grasses yield to shorter grama and wheatgrass ranges. The grasses of the northern Great Plains grow only during the short summer and flower in late summer or early autumn. By contrast, the grasses of the southern Great Plains grow rapidly in spring, flower early, and then go dormant during the hot, dry summers. Both kinds of grass become less productive as rainfall continues to diminish toward the west. Shrubby sagebrush (in the north) and mesquite and juniper (in Texas) are typical invaders on poorer grasslands that have been overgrazed or protected from fires.
A gradual transition to true desert vegetation is interrupted by the Rocky Mountains and other ranges, the elevation of which both increases rainfall and decreases temperature and evaporation. Trees become prominent on the lower and middle slopes. Hardy pines and junipers dominate at lower elevations, giving way to aspens, firs, and spruces at higher elevations. Still higher, the spruces and firs become stunted and widely spaced. Above this zone is treeless tundra, outwardly similar to Arctic vegetation, although the two kinds of tundra have different patterns of solar radiation, day length, and diurnal (day-to-night) temperature changes. Shrubby low-lying deserts alternate with forested (and occasionally tundra- or ice-capped) mountains across all of the Mountain states and into the Pacific states. This region is agriculturally productive only when massive investments are made in irrigation. Death Valley, which lies below sea level, is but one of the many nearly barren lowlands. Vegetation in these regions includes species such as sagebrush, juniper, piсon, rabbitbrush, mesquite, creosote bush, and yucca; the cactus ⌠forests■ that form a popular image of deserts are actually found on the slopes of mountain ranges in theMojave Desert of southern Arizona and California. On the higher but still relatively dry Colorado Plateau are found ponderosa and piсon pines.
The hot, dry summers and mild, moist winters of coastal southern California produce a distinctive shrub vegetation known as chaparral. The plants here have adapted to rainy winters, dry summers, autumn fires, and thin soils. Farther north on the western slopes of the coastal hills andSierra Nevada, the winter rainy season is longer; forests of giant sequoia and redwood occupy this favored location, where there is enough rain to permit rapid growth but a long enough dry season to discourage competition from numerous species. Still farther north, in western Oregon and Washington, a true rain forest appears as the dry periods shrink to less than a few weeks in midsummer. This luxuriant forest consists primarily of a great variety of needle-leaf trees: Douglas firs, true firs, hemlocks, cedars, spruces, and pines, each occupying its own preferred elevation zone here, and together constituting the second richest forest resource for the nation. The coastal forests of Alaska have fewer species than the rich rain forest to the south but a faster growth than the taiga to the north.
The natural vegetation of Hawaii is conditioned by its isolation, which has served to limit the number of species, and by the interplay of its mountains and the moist trade winds. Forests dominated by guava trees on the windward (northeast) coasts of the islands grade upward to a rich but swampy rain forest at moderate elevations, where the annual rainfall may exceed about 10,000 mm (about 400 in). The high mountains support shrub forest, and patches of tundra are found on the summits of the highest peaks,Mauna Loa and Mauna Kea. The dry leeward (southwest) coasts are virtual deserts, with spiny koa and kiawe shrubs growing on the slightly wetter slopes.
Animals depend, either directly or indirectly, on plants for their survival. Each major vegetation region thus has its own characteristic kinds of animal life. In the Arctic areas and regions of mountain tundra are found burrowing marmots, ground squirrels, cold-water fish such as grayling and trout, and an occasional bear. Alaskan coastal waters are the habitat of a number of large mammals, including walrus and fur seals. Caribou and elk spend summers on the tundra but migrate into the conifer forest for winter. Many birds migrate even farther, going from the polar regions to the Tropics each winter. The hardwood forests of the eastern United States contain moose, black bears, deer, foxes, raccoons, skunks, squirrels, and a diversity of small birds. Along the Gulf of Mexico coast live larger and more colorful birds such as pelicans, flamingos, and green kingfishers, as well as alligators and warmwater fish such as catfish. Several varieties of venomous snakes are also found here.
Bison (buffalo) are popularly associated with the grasslands, although in fact they once ranged over most of eastern North America before being nearly exterminated by hunting; they now exist only in captivity or in protected areas. Gophers, rabbits, prairie dogs, ferrets, ants, and other burrowing creatures are best suited to the grasslands, which were once swept by fires. The mountainous western states, especially Alaska, are the last refuges in the United States of most big-game animals. Here may be found elk, pronghorn, moose, deer, bighorn sheep, mountain goats, timber wolves, and, in a few remote areas, brown bears. The Kodiak bear, the largest carnivore in North America, is found in Alaska. The deserts have few plants, fewer small animals, and almost no large animals. Kangaroo rats, lizards, and wide-ranging birds are typical animals in this harsh region. The animals of Hawaii include many endemic species (those not found elsewhere), but many of these have been driven to near extinction by human alteration of the habitat and by competition from introduced animals. Hawaii's only indigenous mammal is the bat.
The United States is exceedingly rich in natural resources. The country's climate is favorable for a diversity of crops and forest products. The Corn Belt, a region stretching from western Ohio to central Nebraska, is the largest expanse of prime farmland in the world. Significant deposits of many important minerals, including more than a fifth of the world's coal, are present within the country's borders. Despite this wealth, the United States cannot produce all the resources needed by its large industrial economy. The country imports more than 80 percent of its asbestos, chromium, cobalt, magnesium, platinum, tin, and tungsten. The United States now produces about 65 percent of its petroleum needs, but new domestic discoveries tend to be small and costly to recover, especially in the geologically complex mountain regions and the deep waters of the continental shelf.
Agricultural exports have helped compensate for mineral imports, but the price is high. More than a third of the nation's topsoil has been lost to erosion. Gullies and other obvious scars common in the 1930s can now be repaired by modern heavy machinery, but soil losses caused by less noticeable sheet erosion are still high on more than half the nation's farmland. Many observers have also expressed concern about the rapid conversion of prime cropland to urban and other nonfarm uses.
Despite these trends, the environmental picture of the United States is not all bleak. Because of government regulations, water pollution diminished considerably during the 1970s; forests are growing more rapidly than they are being cut in most regions; and substantial areas have been set aside as wilderness preserves and national parks. The federal government and most of the states have adopted laws that require investigation and public disclosure of the environmental impact of proposed projects. These procedures have helped to increase public awareness of the environmental costs of certain activities. In the future the questions of acid rain, toxic waste disposal, water supply, and climate change will continue to be major national environmental issues.