Cordilleras


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Cordilleras

(kôrdĭl`ərəz, Span. kōrdēyā`räs) [Span., originally=little string], general name for the entire chain of mountain systems of W North America, extending from N Alaska to Nicaragua. The Cordilleras include the Rocky Mts., the ranges of the Great Basin, the Sierra Nevada, the Coast Ranges, and the Sierra Madre. The name Cordilleras was first applied to the similar systems of W South America, where the mountains stretching from Panama to Cape Horn are known locally as the Cordillera de los Andes (Andes Mts.). Historically geographers used the term cordillera for any extensive group of mountain systems.

Cordilleras

 

the longest mountain system in the world, running along the western borders of North and South America from the Arctic coast of Alaska (66°N) to the southern coasts of Tierra del Fuego (56°S). More than 18,000 km in length, the Cordilleras are found in Canada, the United States, Mexico, Central America, Venezuela, Colombia, Ecuador, Peru, Bolivia, Argentina, and Chile. They form a high barrier between the plains of the eastern part of the continents and the Pacific coast. For almost their entire length the Cordilleras form a drainage divide between the Atlantic and Pacific oceans as well as a pronounced climatic boundary between regions lying on both sides of the mountain uplift. In elevation, they are exceeded only by the Himalayas and the mountain systems of Central Asia. The highest point in North America is Mount McKinley in Alaska (6,193 m), and in South America, Mount Aconcagua (6,960 m). The entire system is divided into two parts—the North American Cordilleras and the South American Cordilleras (Andes)— and consists of numerous parallel ranges rimming a broken belt of inner plateaus and uplands (in North America, the Yukon, Fraser, Columbia, Great Basin, Colorado, and Mexican plateaus; in South America, the Peruvian and central Andes plateaus). Three parallel systems of mountain ranges are found in North America. The first lies to the east of a belt of uplands (the Rockies), the second runs west of this belt (the Alaska Range, the Coast Mountains of Canada, the Cascade Mountains, the Sierra Nevada), and the third extends along the Pacific coast and to some extent the coastal islands. In Central America the Cordilleras descend and divide. One branch runs through the Antilles; the other crosses the Isthmus of Panama into South America. The northern and central parts of the Andes consist of four (and the other regions of the Andes, of two) systems of parallel ranges, divided by deep longitudinal depressions or intermontane plateaus.

The highest ranges are those of the central Andes, where the elevation of certain summits exceeds 6,700 m (Aconcagua, 6,960 m; Ojos del Salado, 6,880 m; Sajama, 6,780 m; Llullaillaco, 6,723 m). The width of the mountain belt in North America reaches 1,600 km, and in South America, 900 km. Basic orogenic processes that led to the emergence of the Cordilleras began in the Jurassic in North America. In South America (where structures of Paleozoic Hercynian folding are important) these processes began at the end of the Cretaceous and were closely related to the formation of mountain systems on other continents. Orogenic movements continued during the Cenozoic. These movements largely determined the principal orographic elements. The folded structures of the Cordilleras are closely related to the mountain systems of northeast Asia and Antarctica. Formation of the Cordilleras has not yet been completed, as is shown by frequent earthquakes and intensive volcanism. There are more than 80 active volcanoes, the most active of which are Katmai, Lassen Peak, Colima, Antisana, Sangay, San Pedro, and the volcanoes of Chile. Quaternary glaciation, especially north of 44°N and south of 40°S, played an important part in the formation of the relief of the Cordilleras.

The Cordilleras contain significant deposits of copper, zinc, lead, molybdenum, tungsten, gold, silver, platinum, tin, and petroleum. The exceptional diversity of natural conditions in the Cordilleras is the result of its great extent from north to south, of the ruggedness of the terrain, and of the great elevation of the mountains. The mountain system encompasses all geographic zones except the antarctic and subantarctic. The climate of the Cordilleras varies greatly, depending on latitude and elevation and exposure of slopes. Outer ranges receive abundant rainfall in temperate and subarctic zones (western slopes) and in equatorial and subequatorial zones (primarily the eastern slopes). Interior uplands have a markedly continental climate, and in subtropical and tropical zones they are extremely arid. Steppes, semideserts, and deserts cover large parts of uplands, interior depressions, and slopes of ranges, primarily in the tropical zones. The very humid outer mountain ranges are covered with thick forests. Coniferous forests (in the north) and mixed forests of evergreen beeches and conifers (in the south) are wide-spread in the temperate zones. Closer to the equator, mixed (deciduous-evergreen) subtropical and tropical forests are found. On the humid slopes of ranges in the equatorial, subequatorial, and subtropical zones there is a complex spectrum of altitude zones, ranging from tropical forests to perennial snows. The snow line in Alaska lies at an altitude of 600 m, and in Tierra del Fuego, 500–700 m; in Bolivia and southern Peru it rises to 6,000–6,500 m. Glaciers descend to sea level in Alaska and southern Chile; in the torrid zone, they cover only the highest peaks.

G. M. INGAT’EV

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