II.

The Natural Environment

North America is roughly wedge shaped, with its broadest expanse in the north. Most of its bulk is in the middle latitudes, with a considerable northern section in the Arctic and a narrow part around the tropic of Cancer. The continent sprawls east-west across some 176° of longitude, from about longitude 12° west at Nordost Rundingen (North-east Foreland) in north-eastern Greenland to about longitude 172° east at the western extremity of Attu Island, Alaska. Its northern-southern extent is some 69°, from about latitude 83° north at Cape Morris Jesup in eastern Greenland (the most northerly point of land in the world) to about latitude 14° north in southern Mexico. North America is bordered on the north by the Arctic Ocean; on the east by the Atlantic Ocean; on the south by the Gulf of Mexico, Central America, and the Pacific Ocean; and on the west by the Pacific Ocean. The area of the continent is approximately 23.5 million sq km (9.1 million sq mi).

The outline of North America is exceedingly irregular; some extensive coastal reaches are relatively smooth, but by and large the coastline is broken and embayed, with many prominent offshore islands. The continent has three enormous coastal indentations—Hudson Bay in the north-east, the Gulf of Mexico in the south-east, and the Gulf of Alaska in the north-west. There are many small islands near the eastern and western coasts, but the most prominent islands are in the far north.

A.

Geological History

According to a widely accepted theory, almost all of North America is situated on the North American plate, an enormous platform considered to be one of about a dozen major units constituting the structural mosaic of the Earth's crust. It is thought that North America was once joined to modern-day Europe and Africa and that it began to break away about 170 million years ago, in the Jurassic period, with the process of continental drift (the motor of plate tectonics) accelerating about 95 million years ago, in the Cretaceous period. As North America drifted west at a rate of about 1.25 cm (0.5 in) per year, the plate underlying the Pacific Ocean is believed to have thrust under the North American plate, thereby causing widespread early folding, evident today in a series of high mountains along the western coast. As the Atlantic Ocean widened, it caused extensive faulting along the eastern coast, resulting in the creation of mountains and offshore islands.

B.

Physiographic Regions

North America can be divided into five major physiographic regions. The eastern half of Canada, as well as most of Greenland and sections of Minnesota, Wisconsin, Michigan, and New York in the United States, are part of the Canadian Shield (or Laurentian Plateau), which is a plateau region underlain by ancient crystalline rocks. The region has poor soil, and dense forests cover much of its southern part. A second region is made up of a coastal plain in most of the eastern United States and Mexico. In the United States the coastal plain is bordered on the west by a third region, comprising a relatively narrow cordillera of mountains and hills, notably the rounded Appalachian Mountains. A fourth region consists of the central portion of the continent, from southern Canada to south-western Texas, which encompasses an extensive lowland that has experienced alternating periods of submergence beneath the sea and uplift, with the result that it is deeply covered with layers of sedimentary rock. It is not an uninterrupted flatland, but includes much undulating and even hilly terrain, such as the Ozark Plateau. The western portion is made up of the Great Plains, which slope upwards to the foot of the Rocky Mountains.

The fifth, and westernmost, region of North America, taking in most of Mexico, is an active zone of mountain building; its recent geological history is dominated by crustal movements and volcanic activity. Adjacent to the Great Plains in the United States and Canada are the Rocky Mountains, which are geologically related to the Sierra Madre Oriental range of Mexico. To the west is an area of scattered basins and high plateaux, including the Interior Plateau of British Columbia in Canada, the Colorado Plateau and the Great Basin of the United States, and the vast central plateau of Mexico. Along the Pacific coast are a number of lofty mountain systems, extending from the Alaska Range to the Sierra Madre Occidental and Sierra Madre del Sur of Mexico. In between are such ranges as the Coast Mountains of British Columbia and the Cascade Range, the Coast Ranges, and the Sierra Nevada of the United States. Interspersed are some low-lying areas, notably the fertile Central Valley of California. The highest point in North America, Mount McKinley, or Denali (6,194 m/20,320 ft), is situated in the Alaska Range, and the lowest point, 86 m (282 ft) below sea level, is in Death Valley, California, a part of the Great Basin.

C.

Drainage

The Continental Divide, which mainly runs along the crest of the Rocky Mountains, splits North America into two great drainage basins. To the east of the divide, water flows towards the Arctic Ocean, Hudson Bay, the Atlantic Ocean, and the Gulf of Mexico, and to the west, rivers flow towards the Pacific Ocean.

Two prominent drainage systems—the Great Lakes-St Lawrence system and the Mississippi-Missouri system—dominate the hydrography of eastern and central North America. The five Great Lakes (Superior, Michigan, Huron, Erie, and Ontario) drain north-east to the Atlantic Ocean via the relatively short St Lawrence River. Most of the central part of the United States and a small part of southern Canada are drained south to the Gulf of Mexico by the Mississippi and its tributaries, notably the Missouri, the longest river in North America. A great many short, but often voluminous, rivers flow to the Atlantic and Gulf of Mexico along the well-watered eastern coasts of Canada, the United States, and Mexico. The northern interior of the continent is drained by the great Mackenzie River system of western Canada and by the numerous rivers that flow into Hudson Bay. To the west of the Continental Divide are relatively few major rivers (notably the Colorado, Columbia, Fraser, and Yukon) and many short but large-volume streams.

The southern half of North America contains only a few large natural lakes, but Canada and the northern United States have a vast number of sizeable lakes. Lake Superior, the world's largest freshwater lake in area, and 10 of the next 25 largest natural lakes are found in this region. Lake Mead, on the Colorado River in the United States, is a large artificial lake, and Great Salt Lake, in Utah, is noted for its highly saline water.

D.

Climate

Although North America has considerable climatic variety, five principal climatic regions can be identified. The northern two-thirds of Canada and Alaska, as well as all of Greenland, have subarctic and arctic climates, in which long, dark, bitterly cold winters alternate with brief, mild summers. Most of the region, which receives relatively little precipitation, is covered with snow and ice during much of the year. A second climatic region is made up of the eastern two-thirds of the United States and southern Canada. It is characterized by a humid climate in which all four seasons are evident, and weather changes are frequent. The southern part of this region has a warmer average temperature. A third region includes the western interior of the United States and much of northern Mexico. It is mostly mountain and desert country, generally receiving small amounts of precipitation, but with significant local variations due to altitude and exposure. A fourth climatic region is made up of a narrow zone along the Pacific Ocean from southern Alaska to southern California. It has relatively mild but wet winters and almost rainless summers. Most of southern Mexico has a tropical climate, with year-round warmth and considerable precipitation, especially in summer.

E.

Vegetation

The natural vegetation of North America has been significantly modified by human activity, but its general nature is still apparent over much of the continent. The most notable forest is the taiga, or boreal forest, an enormous expanse of mostly coniferous trees (especially spruce, fir, hemlock, and larch) that covers most of southern and central Canada and extends into Alaska. In the eastern United States a mixed forest, dominated by deciduous trees in the north and by various species of yellow pine in the south-east, has mostly been cleared or cut over, but a considerable area has regrown since the 1940s. In the western portion of the continent, forests are primarily associated with mountain ranges, and coniferous trees are dominant. In California, the redwood and giant sequoia grow to enormous size. A great mixture of species characterizes the tropical forests of Mexico.

The vegetation cover in the drier parts of the continent is made up mainly of grassland and shrub land. The central plains and prairies of the United States and southern Canada were originally grass covered, but much of the natural flora has been replaced by commercial crops. The dry lands of the western United States and northern Mexico are sparsely covered with a variety of shrubs and many kinds of cactus. Beyond the tree line in the far north is a region of tundra, containing a mixture of low-growing sedges, grasses, mosses, and lichens.

F.

Animal Life

The native wildlife of North America was once numerous and diverse, but the spread of human settlement has resulted in contracting habitats and diminishing numbers. In general, the fauna of North America is similar to that of the northern areas of Europe and Asia. Notable large mammals include several kinds of bear, the largest being the grizzly; bighorn sheep; bison, now only in protected herds; caribou; moose, called elk in Europe; musk ox; and wapiti. Large carnivores include the puma and, in southernmost regions, the jaguar; the wolf and its smaller relative, the coyote; and, in the far north, the polar bear. One species of marsupial, the common opossum, is indigenous to the continent. A few of the many reptiles are poisonous, including the coral snake, pit vipers such as the rattlesnake and copperhead, and the Gila monster and beaded lizard of the south-western United States and Mexico, the only poisonous lizards in the world. A great variety of finfish and shellfish live in the marine waters off North America, and many kinds of fish are found in its freshwater rivers and lakes.

G.

Mineral Resources

North America has large deposits of many important minerals. Petroleum and natural gas are found in great quantity in northern Alaska, western Canada, the southern and western conterminous United States, and eastern Mexico; huge beds of coal are in eastern and western Canada and the United States; and great iron-ore deposits are in eastern Canada, the northern United States, and central Mexico. Canada also has major deposits of copper, nickel, uranium, zinc, asbestos, and potash; the United States contains great amounts of copper, molybdenum, nickel, phosphate rock, and uranium; and Mexico has large reserves of barite, copper, fluorite, lead, zinc, manganese, and sulphur. All three countries have significant deposits of gold and silver.