North America:
Physical Geography
North America is characterized by clear and well-defined divisions into ten physically homogenous regions called physiographic provinces [naprgl]. They are:
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Each region is marked by a certain degree of uniformity in relief, climate, vegetation, soils and other environmental conditions resulting in a sameness that comes readily to mind. Also, note the north-south alignment of most of these physiographic provinces, especially the continent's mountain backbone (Rocky Mountains) from Alaska to New Mexico and into Mexico.
Coastal Plains
From GEOGRAPHY USA: A Virtual Textbook by Alan
A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
The Gulf Coastal Plain faces the Gulf of Mexico, while the Atlantic Coastal Plain borders the Atlantic Ocean. Together, these two plains wrap around the southern end of the Appalachian Mountains, filling in an area that was underwater up until only 75 million years ago. This is the flattest of the Interior Lowlands subregions, In Texas, these plains are 300 miles wide. Northeastward, from the Florida peninsula, they gradually narrow as more of the plain is under the sea. Near New York, the plain comes to an end - reappearing north of the Hudson only in parts of Cape Cod and the islands off the coast of southern New England.
These plains are also among the youngest in the US. When the Rocky Mountains were first beginning to rise some 70 million years ago, most of the Gulf and Atlantic Coastal Plains were under water. The Coastal Plains were gradually built up by unconsolidated (and easily eroded) sediments washed out from the Appalachian Mountains, the Central Lowlands, and the Great Plains. Successive layers of sedimentation form belts with low scarp edges facing inland, but the general slope of the plain's surface is very gentle and no true coastline is formed. Instead, swamps (wet forests), marshes (wet grasslands), and lagoons are fronted by almost continuous sand bars and reefs which characterize a transition zone between land and sea. Deposits of coastal marine life in these areas is the basis for vast reservoirs of oil and natural gas.
The climate throughout this region is humid subtropical, with summer rainy seasons. Annual rainfall is over 40 inches for the entire coastal plains area, with some areas reaching over 60 inches at the southern extremes. Temperatures are moderate in the winter and hot in the summer. The major air mass which influences the climate of these comes from the south, bring warm, and moist tropical air.
In the spring, this southerly flow may come in the form of hurricanes. This air mass is weaker in the winter and occasional Arctic air masses will reach deep into the south, often destroying the region's fruits and vegetables. The heavy precipitation washes out many of the nutrients in the soils, creating low nutrient ultisols. Vast areas of needleleaf evergreens make the lowland South one of the two major timber producing regions in the US.
Appalachian Highlands
From GEOGRAPHY USA: A Virtual Textbook by
Alan A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
Created when North America first collided with Europe and Africa some 570 million years ago, the Appalachian Mountain Range were once the principal mountain system in North America, rising as high as the Rocky Mountains do today. Starting about 200 million years ago, North America began to break away from Europe and Africa, forming the Atlantic Ocean. Tectonic forces of swirling molten magma inside the earth cause continental and ocean plates to move about on the earth's surface.
The Appalachian Mountain Range is still today the dominant mountain system in the eastern US. It was created by the intense folding of sedimentary rock. In some places, younger rocks were pushed completely over older rock as the Africa and North America shifted and slid against one another. The Ozarks and Ouachita Mountain systems west of the Mississippi formed the southwest end of the Appalachian system. More intrusive igneous rock is located in the northeast portion where the Appalachians continue into New Brunswick and Newfoundland in Canada and across to northern Ireland, Scotland, and Norway (this area was connected to North America prior to 200 million years ago).
The Appalachians generally stopped growing after the continents separated from one another 200 million years ago. Continuous erosion has now brought most of the peaks down to under 5,000 feet in elevation. Narrow valleys, steep hillsides and this soils has resulted in low population densities and has kept cities small. This same scenario holds true for the Ozarks and Ouachitas. Metamorphic metallic minerals (especially copper and iron ore) have been largely mined out of the Appalachians, though coal still remains in abundant supply.
Vast interior plains extends from Hudson Bay to the Gulf of Mexico. The Great Canadian Shield is the geologic core area containing North America’s oldest rocks. The interior lowlands is covered largely by glacial debris laid down by meltwater and wind during the late Cenozoic Glaciation.
Canadian Shield
From GEOGRAPHY USA: A Virtual Textbook by
Alan A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
Underlying more than one million square miles of the Interior Lowlands of Canada is the Canadian Shield. Important extension exist into northern Michigan, Minnesota, and Wisconsin. In New York, the Adirondack Mountains are an extension of the Shield. The Canadian Shield contains some of the oldest exposed rock on the surface of the earth. It is primarily composed of hard, metamorphic rock created four billion years ago in the Precambrian period. The metamorphic process has created zones rich in metallic mineral deposits. Successive periods of glacial covered have removed most of the sedimentary material that may have once covered the Shield, exposing rich deposits of metallic ores. Canadian Shield iron ores from MN and Canada were one of the main resources allowing for the development of the US and Canadian manufacturing core area around the Great Lakes. Hudson Bay, the largest bay in North America, is situated at the center of the Canadian Shield. During the great ice ages vast sheets of snow extended out of the Hudson Bay area covering much of lowland Canada and the northern US.
The Canadian Shield dips from between 1,000 and 2,000 feet in elevation in the south and west to sea level at Hudson Bay. A string of lakes and other water bodies surrounds the edge of the Shield to the South and west. The Great Lakes, Lake Winnipeg, Great Slave Lake, and Great Bear Lake are the largest lakes in North America. They, along with the St. Lawrence River and Lake Champlain Lowland (NY and VT), and the Mackenzie River in the far north demarcate the boundary between the Canadian Shield and surrounding uplands. The St. Lawrence and Mackenzie Rivers are two of the largest in north America. Unlike the rivers that flow parallel across the Great Plains, those located on the Canadian Shield form a zigzag pattern interspersed with numerous lakes. This pattern reflects the underlying hard rock and is typical of landscapes created by continental glaciation. Most of the Shield is vegetated with skinny spruce and aspen forests, with the exception of the far north which has a treeless tundra vegetation. The tundra line also marks the southern limits of continuous permafrost soils.
The southern two-thirds of the Canada Shield experiences mild summers (50 to 70 degrees F) and cold (below freezing) winters. North of Hudson Bay remaining at freezing and below throughout most of the year, with summer highs in the 40s. (Only interior Greenland averages below freezing all year round). The major air mass influencing this region is the dry and cold, continental polar system. This air mass is weaker in the summer, but comes on strong in the winter months. Annual precipitation is under ten inches for the entire Canadian Shield, except for the areas to the east and south of the Great Lakes.
Great Plains
From GEOGRAPHY USA: A Virtual Textbook by Alan
A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
The Great Plains rise in elevation from under 100 feet at the lower Mississippi River to just over a mile high (1 mi = 5,280 feet) at the foot of the Rocky Mountains. It is composed of layer upon layer of sedimentary rock washed out from the Rocky Mountains to the west. Rivers such as the Platte, Missouri, and Red parallel one another as they flow from west to east down this gentle grade. Much of the Great Plains was a swampy area during the Carboniferous period. Large portions were still under water prior to the uplift of the Rocky Mountains some 70 million years ago. This history has resulted today in large deposits of coal, oil, and natural gas from Texas, through Colorado, Wyoming, and Montana, and into Alberta in Canada.
While most of the Great Plains is flat and undistinguished, it does contain some distinct physical anomalies worthy of mention. In Nebraska, a belt of sand hills have been formed by wind and the outwash of the last continental glaciers. In South Dakotas, however, water erosion has created a vast badlands landscape of deeply cut and barren soils. The one major mountain system that interrupts the evenness of the Great Plains is the Black Hills of South Dakota which rise to over 7,000 feet. Here old crystalline igneous rock breaks throughout the surface in a dome-like swelling believed to be an outlier of the Rocky Mountain system. The oldest and largest gold mine in the US, as well as the famous Mount Rushmore, are both located in the Black Hills.
Great Plains Climate and Vegetation
The Great Plains have a considerably drier and more unpredictable climate than does the Midwest and Coastal Plains. Prairie grasses are the natural vegetation of this area. For this reason, early settlers from northern Europe referred to it as the "Great American Desert." Soils that formed under the prairie grasses, however, were actually richer than those of the more forested Midwest, making the region the primary wheat growing area in the US and Canada. The Great Plains become more narrow as it extends into Canada and gradually disappear in northern Alberta. North of Edmonton (Alberta), the prairie grasslands give way to belt of mixed coniferous (pine) deciduous trees. Eventually these become spruce and aspen forests which dominate the cold northern latitudes all the way to Alaska, both on the Canadian Shield to the east and the Rockies to the west.
Rocky Mountains
From GEOGRAPHY USA: A Virtual Textbook by
Alan A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
The Rocky Mountains contains a mix of sedimentary, igneous, and metamorphic rock. Late in Cretaceous times (about 70 million years ago), tremendous mountain building processes disturbed the western half of the continent where a long series of sedimentary beds lay evenly spread over the ancient continental floor. This disturbance was accompanied by volcanic activity and is known as the Laramide Revolution. It resulted in a great uplift of the sedimentary beds, folding and faulting, and metamorphic changes. The uplift was followed by the erosion of the sedimentary layers and exposure the underlying igneous rock. In some places the cover has been preserved, usually in pockets formed by downfaulting. This diversity gives the west its varied and interesting physiography. The Rockies include the Brook Range in Alaska, the Canadian Rockies, the Northern Rockies (ID & MT), the Central Rockies (Co), and the Southern Rockies (NM). The highest peaks rise to over 14,000 feet.
The Rockies are breached in only a few places. In the far north, the Lizard River flows through a break in the Mackenzie Mountains. There is another break between the Northern and Central Rockies where the Wyoming Basin is formed. Here, the old Oregon Trail made use of the route so that wagons could traverse the continental divide at about 7,000 feet.
Climate in the Rocky Mountains is generally cold in the winter and mild in the summer. Highland climates, however, are difficult to classify because of great variation from valley floors to mountain peaks. In addition, south facing slopes which receive much sunshine can be very different from colder north facing slopes in climate and vegetation. Latitude plays a major role as the mountains chains existing further to the north are much colder and have more permanent snow and active alpine glaciers than those to the south.
Intermontain Basins and Plateaus
From GEOGRAPHY USA: A Virtual Textbook by
Alan A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
West of the Rockies lies an area whose astonishing physical features are the product of crustal faulting, volcanic activity, and intense recent erosion and downcutting by rivers. This area is known as the High Plateaus. The basic form of this region is that of a series of plateau steps (some almost as high as the Rockies). In the southwest, the steps descend from the Rockies for about 500 miles to Death Valley. To the north, the plateau province is narrower until it is less than 100 miles wide in British Columbia.
The development of these plateaus, like that of the Rockies, resulted from the Laramide Revolution. Their different levels are the result of faulting. The two main plateaus of the West are the Colorado Plateau and the Columbia Plateau. The Colorado plateau covers portions of CO, UT, AZ and NM. It is composed of relatively flat sedimentary rock that was uplifted. The Colorado River cuts the Grand Canyon through this uplifted sedimentary rock. The Columbia Plateau is located in eastern WA and parts of OR and ID. It is composed of lava flows that combine to be several thousand feet thick. Hells Canyon, on the Snake River between Oregon and Washington, is the deepest gorge in the US and is cut into this plateau.
The climate of the plateaus varies with latitude (those further north are colder), but generally experience a hot summer and mild to cold (depending on elevation) winter. Precipitation is typically under 10 inches a year (although it reaches 20 inches on the Fraser Plateau). Runoff from the surrounding mountain peaks provide a major source of water for irrigated farm fields, which often do well. The Columbia Plateau is one of the richest producing irrigated wheat regions on the continent.
The Fraser Plateau comprises much of central British Columbia, situated between a narrow band of the Rockies and the Canadian Coast Mountains. Like the Columbia Plateau to the south, it is comprised of large areas of extrusive igneous lava. The entire plateau is drained by the Fraser River, which enters the sea at Vancouver. North of here the Plateau region become more narrow and dissected rivers and bisected by mountains composed of a mixture of various rocks of different ages and composition. The older of these rocks is the source of gold, which was first discovered in the Yukon Territory of Canada in the late 1800s. The Klondike gold region sit astride the Yukon Plateau and the Yukon Interior (a northern basin and range-like region in Alaska). The Yukon Interior reaches over 500 miles in width in western Alaska.
To the western edge of the Colorado Plateau is the great fault at the foot of the Wasatch Mountains. West of this lies the Great Basin which stretches for 500 miles from north to south and east to west. Waters flow into, but not out of the Great Basin. The Great Basin is centered on Nevada and is composed of the caved in remnants of a pre-Rockies mountain system which was formed 80 million years ago. Numerous faulted mountain blocks run north and south through the Great Basin, with areas in between these ranges consisting of unconsolidated alluvium. This basin and range landscape also extends into southern California, Arizona, and New Mexico. The Great Salt Lake occupies some 2,000 square miles of the eastern edge of the Great Basin. It is the remnant of the former Lake Bonneville, which was ten times larger at the end of the last ice age.
Rainfall is generally under 5 inches throughout the basin and range deserts of the American West. Irrigated fields rely on water from the plateaus and higher peaks. Groundwater is another important source of irrigation, although it is becoming more and more difficult to reach as water tables drop. Most of the rain that does fall comes in the summer months, from moisture originating in the Gulf of California and the Gulf of Mexico. Altitudinal zonation on the higher mountains is well pronounced, as one can often travel from Desert to Tundra within a hundred miles.
Pacific Mountains and Valleys
From GEOGRAPHY USA: A Virtual Textbook by
Alan A. Lew, Ph.D., AICP, found online at: http://www.for.nau.edu/%7Ealew/ustxtwlc.html
The Pacific Coast province is very diverse. Within it are found the highest points and the last remaining active volcanoes on the continent. Like the rest of the mountainous West, the Pacific Coast portion was also shaped by plate tectonic activity. The granite peaks of the Sierra Nevada Mountains in CA are an exposed intrusive igneous formation, while the Cascade Range to the north are of extrusive igneous (lava) origin. Further to the north, Canada's Coast Mountains and the Alaska Range (in Alaska) are intrusive igneous formations, similar to the Sierra Nevada. The Coast Mountains are the world's highest coastal mountains system, while the Alaska Range contains the highest mountains on the continent, including McKinley (also known as Denali) at 20,320 feet.
Both the intrusive and extrusive igneous rock comes from Pacific Ocean sea floor plates that have been pushed under the North American plate; a process known a subduction. The ocean plates melt and the molten magma than rises up to form the Alaska Range, Coast Mountains, Sierra Nevada, and Cascade Range. (A similar process pushed up the Rocky Mountains and Colorado Plateau.) As these mountains were uplifted, the area immediately to their west was sunken to form a depression. The depression originates as the Gulf of California in Mexico, continues as the Central Valley in California, the Willamette Valley in Oregon, the Puget Sound lowland in Washington, and the coastal straits of British Columbia, and the Alexander Archipelago in the Alaska Panhandle.
Along the Pacific shoreline of California, Oregon, and Washington, the low-lying Coast Range is formed by the crumpling edge of the North American continent. The continued subduction of a small ocean plate under OR and WA ensures volcanic activity in the Cascades. In CA, however, the North American Plate is sliding sideways against the large Pacific Ocean Plate. This causes lateral or strike-slip faulting throughout the Coast Range of California. The Pacific Plate moves northward at about two inches a year, causing an ever present danger of serious earthquake activity. The Coast Range continues as Baja California in the south and Vancouver Island and Queen Charlotte Island in the north. The Alaska Peninsula, which contains more active volcanoes than anywhere else on the continent, could also be considered an extension of this coastal edge.
Unlike the eastern trailing coastline, the western coasts are all leading coastlines. They are steep and show signs of emergence; i.e., rising up out of the sea with older coastlines now hundreds of feet in the air.
California is famous for its Mediterranean climate. This climate exhibits a distinct summer dry and winter wet pattern, which is different from that found anywhere else in the US or Canada. This climate gradually changes to the Marine West Coast climate as one moves north through Oregon and into Washington. The Marine West Coast climate is wet all year round, although it too has a winter peak. The main air masses creating these climates are a north Pacific low pressure system (which bring cool and moisture storms) and a mid-Pacific high pressure system that brings dry air to the coast of California. These two air masses move north in the summer and south in the winter. Because the moisture hitting the west coast comes off the ocean, it has a moderating maritime influence, with coastal snow being uncommon except in northern British Columbia and Alaska.
The two north-south mountain system on the west coast create two rainshadow patterns. The first, smaller rainshadow occurs in the inland valleys of California, Oregon and Washington, and the coastal channels of British Columbia. Thus, Sacramento, Portland, Seattle, and Vancouver (BC) all lie in a warmer and less rainy rainshadow area. A more significant rainshadow exists east of the Sierra Nevada, Cascade Range, and Canada's Coast Mountains. This rainshadow extends the naturally occurring deserts of the southwestern US far north to the Canadian border. In Canada and Alaska, the dry polar desert is extended southward by the same rainshadow process behind the Coast Mountains. Dry rainshadow conditions also extend into Alaska where the north-south mountains shift to an east-west moisture barrier in the Alaska Range.
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In general temperature varies by latitude, the farther poleward one goes the colder the average temperature [naclimat]. Local land and water heating differentials distort this broad pattern. Because land surfaces heat and cool far more rapidly than water bodies, yearly temperature ranges are much larger where continentally is greatest.
The dryness [napreces] of most of the western U.S. is a result of this relationship between climate and physiography. Despite the prevailing wind direction from west to east, moisture-laden air moving onshore from the Pacific is unable to penetrate the center of the continent because high mountains stand in the way. When eastward Pacific air reaches the foot of the Cascade Mountains and other North-South aligned ranges, it is forced to rise up the windward slope in order to surmount the topographic barrier. As the air rises toward the summit level, it steadily cools since cooler air is less able to hold moisture, it precipitates out as rain fall and snow fall. The mountains squeeze out the Pacific moisture. This latitudinal induced precipitation is called orographic rainfall (precipitation).
Robbed of its moisture content by the time it is pushed across the summit the eastward moving air now rushes down the leeward (downwind) slopes of the mountain barrier. As air warms, its capacity to hold moisture increases and the result is a warm dry wind that can blow strongly for hundreds of miles in kind. This widespread existence of semi arid environmental conditions is known as the rain shadow effect.
Air masses continue to move eastward across the Intermountain and Rocky Mountain provinces subjected to additional orographic uplift – East of the Rockies in the Great Plains farmers depend on moisture from the Gulf of Mexico from winds often undependable. Precipitation in Humid America is far more regular. The prevailing westerly winds pick up considerable moisture over the Interior lowlands distributing it throughout eastern North America. A large number of storms develop here on the highly active weather front between tropical Gulf air to the south and polar air to the north. Even if major storms do not materialize, local weather disturbances created by sharply contrasting temperature differences are always a danger. There are more tornadoes in the central U.S. each year than anywhere else on earth. In the winter, the northern half of this region receives large amounts of snow especially around the Great Lakes.
The broad environmental partitioning into Humid and Arid America [napreces] [naclimat] is also reflected in the distribution of the realm’s soils and vegetation. For farming purposes there is usually sufficient soil moisture to support crops where annual precipitation exceeds 20 inches – where it is less (Great Plains) [ soils may still be fertile but irrigation is often necessary to achieve their full agricultural potential. Natural vegetation of areas receiving more than 20 inches of water annually is forest, drier climates give rise to a grassland cover.
Surface water patterns in North America are dominated by the two major drainage systems [nariver] that lie between the Rockies and the Appalachians. For example the Five Great Lakes (Superior, Michigan, Huron, Erie and Ontario) and the Ohio – Mississippi – Missouri Network. Both are products of the last episode of late Cenozoic glaciation and together they amount to nothing less than the best natural inland waterway system in the world. Colorado and Columbia Rivers are important as suppliers of drinking water. Rivers of the northeastern United States are known as fall line cities, cities located at the limit of tidewater navigation.
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