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(詳細はNorth America is the third largest continent, and is also a portion of the second largest supercontinent if North and South America are combined into the Americas and Africa, Europe, and Asia are considered to be part of one supercontinent called Afro-Eurasia. With an estimated population of 460 million and an area of 24,346,000 km² (9,400,000 mi²), the northernmost of the two continents of the Western Hemisphere〔(【引用サイトリンク】title=Geographic Guide - Images of North America )〕 is bounded by the Pacific Ocean on the west; the Atlantic Ocean on the east; the Caribbean Sea, Atlantic and Pacific oceans, and South America on the south; and the Arctic Ocean on the north. The northern half of North America is sparsely populated and covered mostly by Canada, except for the northeastern portion, which is occupied by Greenland, and the northwestern portion, which is occupied by Alaska, the largest state of the U.S. The central and southern portions of the continent are represented by the United States, Mexico, and numerous smaller states primarily in Central America and in the Caribbean. The continent is delimited on the southeast by most geographers at the Darién watershed along the Colombia-Panama border, placing all of Panama within North America.〔("Americas" ) ''Standard Country and Area Codes Classifications (M49)'', United Nations Statistics Division〕〔("North America" ) ''Atlas of Canada''〕〔(North America Atlas ) National Geographic〕 Alternatively, a less common view would end North America at the man-made Panama Canal. Islands generally associated with North America include Greenland, the world's largest island, and archipelagos and islands in the Caribbean. The terminology of the Americas is complex, but "Anglo-America" can describe Canada and the U.S., while "Latin America" comprises Mexico and the countries of Central America and the Caribbean, as well as the entire continent of South America. Natural features of North America include the northern portion of the American Cordillera, represented by the geologically new Rocky Mountains in the west; and the considerably older Appalachian Mountains to the east. The north hosts an abundance of glacial lakes formed during the last glacial period, including the Great Lakes. North America's major continental divide is the Great Divide, which runs north and south down through Rocky Mountains. The major watersheds all drain to the east: The Mississippi/Missouri and Rio Grande into the Gulf of Mexico, and St. Lawrence into the Atlantic. Climate is determined to a large extent by the latitude, ranging from Arctic cold in the north to tropical heat in the south. The western half of North America tends to have milder and wetter climate than other areas with equivalent latitude, although there are steppes (known as "prairies") in the central and western portions, and deserts in the Southwestern United States of Arizona, Colorado, California, Nevada, New Mexico, Utah, Oklahoma, and Texas; along with the Mexican states of Baja California, Baja California Sur, Sonora, Chihuahua, Coahuila, Nuevo Leon and Tamaulipas. ==Paleogeography== Seventy percent of North America is underlain by the Laurentia craton,〔("United Plates of America" ). Smithsonian Museum of Natural History. Retrieved on 2009-01-31.〕 which is exposed as the Canadian Shield in much of central and eastern Canada around the Hudson Bay, and as far south as the U.S. states of Michigan, Wisconsin, and Minnesota. The continental crust started to form 4 billion years ago (Ga), and six of the microcontinents collided to form the craton about 2 Ga. This core has been enlarged by plate tectonics, most notably between 1.8 and 1.65 Ga when a piece currently stretching from Arizona to Missouri fused with the south and west portion of the craton. The craton started to rift about 1.1 Ga, and the fissure (now the Midcontinent Rift System) ran between Kansas and Lake Superior before stopping, perhaps due to the Grenville collision in the east. Otherwise the craton has remained relatively stable, with some rocks dating from 2.5 to 4 Ga, including what may be the world's oldest known rock: Specimens from the Nuvvuagittuq greenstone belt on the coast of the Hudson Bay have been dated to 4.38 Ga,〔Thompson, Andrea (2008-09-25). ("Oldest rocks on Earth found" ). MSNBC. Retrieved 2009-02-01.〕〔O'Neil, Jonathan; Carlson, Richard W.; Francis, Don; Stevenson, Ross K. (2008-09-26). "Neodymium-142 Evidence for Hadean Mafic Crust". ''Science'' 321 (5897): 1828–1831. .〕 though the dating methods are disputed.〔Brahic, Catherine (2008-09-26). ("Discovery of world's oldest rocks challenged" ). NewScientist. Retrieved 2009-02-01.〕 Periodic flooding by inland seas, most recently the Western Interior Seaway during the Cretaceous, caused the layer of sedimentary rock over the remainder of the craton. The Laurentia craton is the center of the Proterozoic supercontinent Rodinia in most models,〔("Rodinia" ) (2005). Palæos. Retrieved on 2009-02-01.〕 and was also part of the later Laurussia, Pangea, and Laurasia supercontinents. Roughly 3 million years ago (Ma), the volcanic Isthmus of Panama formed between the North and South American continents creating a bridge over what was the Central American Seaway and allowing the migration of flora and fauna between the two landmasses in the Great American Interchange. Starting 2.58 Ma, the Quaternary glaciation covered much of the continent with ice, centered west of Hudson Bay, the floor of which is slowly rebounding after being depressed by the great weight of the ice. Glaciers descended the slopes of the Rocky Mountains and those of the Pacific Margin. Extensive glacial lakes, such as Glacial Lake Missoula, Bonneville, Lahontan, Agassiz, and Algonquin, formed by glacial melt water. "Remnants of them are still visible in the Great Basin and along the edge of the Canadian Shield in the form of the Great Salt Lake, the Great Lakes, and the large lakes of west central Canada."〔 The last glacial period of the current ice age caused a lowering of the sea level, exposing the Bering Land Bridge between Alaska and Siberia, which resulted in human migration from Asia to the Americas between 40,000 and 15,000 years ago.〔("Historical & Cultural Significance" ) (1995). Bering Land Bridge Natural Preserve.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geography of North America」の詳細全文を読む スポンサード リンク
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