The third largest continent, extending from the narrow isthmus of Central America to the Arctic Archipelago. The physical environments of North America, like the rest of the world, are a reflection of specific combinations of the natural factors such as climate, vegetation, soils, and landforms. Continent

North America covers 9,400,000 mi² (24,440,000 km²) and extends north to south for 5000 mi (8000 km) from Central America to the Arctic. It is bounded by the Pacific Ocean on the west and the Atlantic Ocean on the east. The Gulf of Mexico is a source of moist tropical air, and the frozen Arctic Ocean is a source of polar air. With the major mountain ranges stretching north-south, North America is the only continent providing for direct contact of these polar and tropical air masses, leading to frequent climatically induced natural hazards such as violent spring tornadoes, extreme droughts, subcontinental floods, and winter blizzards, which are seldom found on other continents. Air mass Arctic Ocean Atlantic Ocean Gulf of Mexico Pacific Ocean

The North American continent includes (1) a continuous, broad, north-south-trending western cordilleran belt stretching along the entire Pacific coast; (2) a northeast-southwest-trending belt of low Appalachian Mountains paralleling the Atlantic coast; (3) an extensive rolling region of old eroded crystalline rocks in the north-central and northeastern part of the continent called the Canadian Shield; (4) a large, level interior lowland covered by thick sedimentary rocks and extending from the Arctic Ocean to the Gulf of Mexico; and (5) a narrow coastal plain along the Atlantic Ocean and the Gulf of Mexico. These broad structural geologic regions provide the framework for the natural regions of this continent and affect the location and nature of landform, climatic, vegetation, and soil regions.

Properly referred to as the geological core of the continent, the exposed Canadian Shield extends about 2500 mi (4000 km) from north to south and almost as much from east to west. The rest of it dips under sedimentary rocks that overlap it on the south and west. The Canadian Shield consists of ancient Precambrian rocks, over 500 million years old, predominantly granite and gneiss, with very complex structures indicating several mountain-building episodes. It has been eroded into a rolling surface of low to moderate relief with elevations generally below 2000 ft (600 m). Its surface has been warped into low domes and basins, such as the Hudson Basin, in which lower Paleozoic rocks, including Ordovician limestones, have been preserved. Since the end of the Paleozoic Era, the Shield has been dominated by erosion. Parts of the higher surface remain at about 1500–2000 ft (450–600 m) above sea level, particularly in the Labrador area. The Shield remained as land throughout the Mesozoic Era, but its western margins were covered by a Cretaceous sea and by Tertiary terrestrial sediments derived from the Western Cordillera.

The entire exposed Shield was glaciated during the Pleistocene Epoch, and its surface was intensely eroded by ice and its meltwaters, erasing major surface irregularities and eastward-trending rivers that were there before. The surface is now covered by glacial till, outwash, moraines, eskers, and lake sediments, as well as drumlins formed by advancing ice. A deranged drainage pattern is evolving on this surface with thousands of lakes of various sizes. Drumlin Esker Moraine

The Canadian Shield extends into the United States as Adirondack Mountains in New York State, and Superior Upland west of Lake Superior.

The Southeastern Coastal Plain is geologically the youngest part of the continent, and it is covered by the youngest marine sedimentary rocks. This flat plain, which parallels the Atlantic and Gulf coastline, extends for over 3000 mi (4800 km) from Cape Cod, Massachusetts, to the Yucatán Peninsula in Mexico. It is very narrow in the north but increases in width southward along the Atlantic coast and includes the entire peninsula of Florida. As it continues westward along the Gulf, it widens significantly and includes the lower Mississippi River valley. It is very wide in Texas, narrows again southward in coastal Mexico, and then widens in the Yucatán Peninsula and continues as a wide submerged plain, or a continental shelf, into the sea. Coastal plain

Extending from Cape Cod, Massachusetts, to Mexico and Central America, the Coastal Plain is affected by a variety of climates and associated vegetation. While a humid, cool climate with four seasons affects its northernmost part, subtropical air masses affect the southeastern part, including Florida, and hot and arid climate dominates Texas and northern Mexico; Central America has hot, tropical climates.

Varied soils characterize the Coastal Plain, including the fertile alluvial soils of the Mississippi Valley. Broadleaf forests are present in the northeast, citrus fruits grow in Florida, grasslands dominate the dry southwest, and tropical vegetation is present on Central American coastal plains.

Between the Southeastern Coastal Plain and the extensive interior provinces lies a belt of mountains that, by their height and pattern, create a significant barrier between the eastern seaboard and the interior of North America. These mountains consist of the Adirondack Mountains and the New England Highlands.

The Adirondack Mountains are a domal extension of the Canadian Shield, about 100 mi (160 km) in diameter, composed of complex Precambrian rocks. The New England Highlands consist of a north-south belt of mountains east of the Hudson Valley, including the Taconic mountains in the south and the Green mountains in the north, and continuing as the Notre Dame Mountains along the St. Lawrence Valley and the Chic-Choc Mountains of the Gaspé Peninsula. The large area of New England east of these mountains is an eroded surface of old crystalline rocks culminating in the center as the White Mountains, with their highest peak of the Presidential Range, Mount Washington, reaching over 6200 ft (1880 m). This area has been intensely glaciated, and it meets the sea in a rugged shoreline. Nova Scotia and Newfoundland have a similar terrain.

New England is a hilly to mountainous region carved out of ancient rocks, eroded by glaciers, and covered by glacial moraines, eskers, kames, erratics, and drumlins, with hundreds of lakes scattered everywhere. It has a cool and moist climate with four seasons, thin and acid soils, and mixed coniferous and broadleaf forests.

The Appalachian Highlands are traditionally considered to consist of four parts: the Piedmont, the Blue Ridge Mountains, the Ridge and Valley Section, and the Appalachian Plateau. These subregions are all characterized by different geologic structures and rock types, as well as different geomorphologies.

The northern boundary of the entire Appalachian System is an escarpment of Paleozoic rocks trending eastward along Lake Erie, Lake Ontario, and the Mohawk Valley. The boundary then swings south along Hudson River Valley and continues southwestward along the Fall Line to Montgomery, Alabama. The western boundary trends northeastward through Cumberland Plateau in Tennessee, and up to Cleveland, Ohio, where it joins the northern boundary. Together with New England, this region forms the largest mountainous province in eastern United States.

The southwestern part of the Appalachian Plateau, overlain mainly by the Mississippian and Pennsylvanian sedimentary rocks, has been warped into two low structural domes called the Blue Grass and Nashville Basins, and a structural basin, drained by the Green River; its southern fringe is called the Pennyroyal Region. The Interior Dome and Basin Province is contained roughly between the Tennessee River in the south and west and the Ohio River in the north.

There is no boundary on the east, because the domes are part of the same surface as the Appalachian Plateau. However, erosional escarpments, forming a belt of hills called knobs, clearly mark the topographic domes and basins. The northern dome, called the Blue Grass Basin or Lexington Plain, has been eroded to form a basin surrounded by a series of inward-facing cuesta escarpments. The westernmost cuesta reaches about 600 ft (180 m) elevation while the central part of the basin lies about 1000 ft (300 m) above sea level, which is higher than the surrounding hills. This gently rolling surface with deep and fertile soils exhibits some solutional karst topography. Fluvial erosion landforms

The Paleozoic rocks of the Pennyroyal Region continue Westward across southern Illinois to form another dome of predominantly Ordovician rocks, called the Ozark Plateau. This dome, located mainly in Missouri and Arkansas, has an abrupt east side, and a gently sloping west side, called the Springfield Plateau. Its surface is stream eroded into hilly and often rugged topography that is developed mainly on limestones, although shales, sandstone, and chert are present. Much residual chert, eroded out of limestone, is present on the surface. There are some karst features, such as caverns and springs. In the northeast, Precambrian igneous rocks protrude to form the St. Francois Mountains, which reach an elevation of 1700 ft (515 m).

One of the largest subdivisions of North America is the Central Lowlands province which is located between the Appalachian Plateau on the east, the Interior Domes and Basins Province and the Ozark Plateau on the south, and the Great Plains on the west. It includes the Great Lakes section and the Manitoba Lowland in Canada. This huge lowland in the heart of the continent (whose elevations vary from about 900 ft or 270 m above sea level in the east and nearly 2000 ft or 600 m in the west) is underlain by Paleozoic rocks that continue from the Appalachian Plateau and dip south under the recent coastal plain sediments; meet the Cretaceous rocks on the west; and overlap the crystalline rocks of the Canadian Shield on the northeast.

The present surface of nearly the entire Central Lowlands, roughly north of the Ohio River and east of the Missouri River, is the creation of the Pleistocene ice sheets. When the ice formed and spread over Canada, and southward to the Ohio and Missouri rivers, it eroded much of the preexisting surface. During deglaciation, it left its deposits over the Canadian Shield and the Central Lowlands.

The Central Lowlands are drained by the third longest river system in the world, the Missouri-Mississippi, which is 3740 mi (6000 km) long. This mighty river system, together with the Ohio and the Tennessee, drains not only the Central Lowlands but also parts of the Appalachian Plateau and the Great Plains, before it crosses the Coastal Plain and ends in the huge delta of the Mississippi. The river carries an enormous amount of water and alluvium and continues to extend its delta into the Gulf. In 1993 it reached a catastrophic level of a hundred-year flood, claimed an enormous extent of land and many lives, and created an unprecedented destruction of property. This flood again alerted the population to the extreme risk of occupying a river floodplain. Floodplain River

The Great Plains, which lie west of the Central Lowlands, extend from the Rio Grande and the Balcones Escarpment in Texas to central Alberta in Canada. On the east, they are bounded by a series of escarpments, such as the Côteau du Missouri in the Dakotas. The dry climate with less than 20 in. (50 cm) of precipitation, and steppe grass vegetation growing on calcareous soils, help to determine the eastern boundary of the Great Plains. On the west, the Great Plains meet the abrupt front of the Rocky Mountains, except where the Colorado Piedmont and the lower Pecos River Valley separate them from the mountains.

The Great Plains region shows distinct differences between its subsections from south to north. The southernmost part, called the High Plains or Llano Estacado, and Edwards Plateaus are the flattest. While Edwards Plateau, underlain by limestones of the Cretaceous age, reveals solutional karst features, the High Plains have the typical Tertiary bare cap rock surface, devoid of relief and streams.

The central part of the Great Plains has a recent depositional surface of loess and sand. The Sand Hills of Nebraska form the most extensive sand dunes area in North America, covering about 24,000 mi² (62,400 km²). They are overgrown by grass and have numerous small lakes. The loess region to the south provides spectacular small canyon topography. Dune

The northern Great Plains, stretching north of Pine Ridge and called the Missouri Plateau, have been intensly eroded by the western tributaries of the Missouri River into river breaks and interfluves. In extreme cases, badlands were formed, such as those of the White River and the Little Missouri.

The terrain of the Canadian Great Plains consists of three surfaces rising from east to west: the Manitoba, Saskatchewan, and Alberta Prairies developed on level Creteceous and Tertiary rocks. Climatic differences between the arid and warm southern part and the cold and moist northern part have resulted in regional differences. The eastern boundary of the Saskatchewan Plain is the segmented Manitoba Escarpment, which extends for 500 mi (800 km) northwestward, and in places rises 1500 ft (455 m) above the Manitoba Lowland. Côteau du Missouri marks the eastern edge of the higher Alberta Plain.

The mighty and rugged Western Cordilleras stretch along the Pacific coast from Alaska to Mexico. There are three north-south-trending belts: (1) Brooks Range, Mackenzie Mountains, and the Rocky Mountains to the north and Sierra Madre Oriental in Mexico; (2) Interior Plateaus, including the Yukon Plains, Canadian Central Plateaus and Ranges, Columbia Plateau, Colorado Plateau, and Basin and Range Province stretching into central Mexico; and (3) Coastal Mountains from Alaska Range to California, Baja California, and Sierra Madre Occidental in Mexico.

This subcontinental-size mountain belt has the highest mountains, greatest relief, roughest terrain, and most beautiful scenery of the entire continent. It has been formed by earth movements resulting from the westward shift of the North American lithospheric plate. The present movements, and the resulting devastating earthquakes along the San Andreas fault system paralleling the Pacific Ocean, are part of this process.

This very high, deeply eroded and rugged Rocky Mountains region comprises several distinct parts: Southern, Middle, and Northern Rockies, plus the Wyoming Basin in the United States, and the Canadian Rockies. The Southern Rockies, extending from Wyoming to New Mexico, include the Laramie Range, the Front Range, and Spanish Peaks with radiating dikes on the east; Medicine Bow, Park, and Sangre de Cristo ranges in the center; and complex granite Sawatch Mountains and volcanic San Juan Mountains of Tertiary age on the west. Most of the ranges are elongated anticlines with exposed Precambrian granite core, and overlapping Paleozoic and younger sedimentary rocks which form spectacular hogbacks along the eastern front. There are about 50 peaks over 14,000 ft (4200 m) high, while the Front Range alone has about 300 peaks over 13,000 ft (3940 m) high. The southern Rocky Mountains, heavily glaciated into a beautiful and rugged scenery with permanent snow and small glaciers, form a major part of the Continental Divide.

The interior Plateaus and Ranges Province of the Western Cordillera lies between the Rocky Mountains and the Coastal Mountains. It is an extensive and complex region. It begins in the north with the wide Yukon Plains and Uplands; narrows into the Canadian Central Plateaus and Ranges; widens again into the Columbia Plateau, Basin and Range Province, and Colorado Plateau; and finally narrows into the Mexican Plateau and the Central American isthmus.

The coastal Lowlands and Ranges extend along the entire length of North America and include Alaskan Coast Ranges, Aleutian Islands, Alaska Range, Canadian Coast Ranges, and a double chain of the Cascade Mountains and Sierra Nevada on the east, and Coast Ranges on the west, separated by Puget Sound, Willamette Valley, and Great Valley of California. These ranges continue southward as Lower California Peninsula, Baja California, and Sierra Madre Occidental in Mexico.

The basin-and-range type of terrain of the southwest United States continues into northern Mexico and forms its largest physiographic region, the Mexican Plateau. This huge tilted block stands more than a mile above sea level—from about 4000 ft (1200 m) in the north, it rises to about 8000 ft (2400 m) in the south. The Mexican Plateau is separated from the Southern Mexican Highlands (Sierra Madre del Sur) by a low, hot and dry Balsas Lowland drained by the Balsas River. To the east of the Southern Highlands lies a lowland, the Isthmus of Tehuantepec, which is considered the divide between North and Central America. Here the Pacific and Gulf coasts are only 125 mi (200 km) apart. The lowlands of Mexico are the coastal plains. The Gulf Coastal Plain trends southward for 850 mi from the Rio Grande to the Yucat'an Peninsula. It is about 100 mi (160 km) wide in the north, just a few miles wide in the center, and very wide in the Yucatan Peninsula. Barrier beaches, lagoons, and swamps occur along this coast. The Pacific Coastal Plains are much narrower and more hilly. North-south-trending ridges of granite characterize the northern part, and islands are present offshore. Toward the south, sandbars, lagoons, and deltaic deposits are common.

East of the Isthmus of Tehuantepec begins Central America with its complex physiographic and tectonic regions. This narrow, mountainous isthmus is geologically connected with the large, mountainous islands of the Greater Antilles in the Carribean. They are all characterized by east-west-trending rugged mountain ranges, with deep depressions between them. One such mountain system begins in Mexico and continues in southern Cuba, Puerto Rico, and the Virgin Islands. North of this system, called the Old Antillia, lies the Antillian Foreland, consisting of the Yucatán Peninsula and the Bahama Islands. Central American mountains are bordered on both sides by active volcanic belts. Along the Pacific, a belt of young volcanoes extends for 800 mi (1280 km) from Mexico to Costa Rica. Costa Rica and Panama are mainly a volcanic chain of mountains extending to South America. Nicaragua is dominated by a major crustal fracture trending northwest-southeast.