South America has two major mountain systems of contrasting nature. Bordering the Pacific Ocean to the west, the geologically young cordilleras of the Andes extend along the entire continent from north to south. Stretching along the continent’s northern and eastern sides are the ancient Guiana and Brazilian highlands, which are much lower in elevation and slope gently to the west; farther south are the Patagonian plateaus. Lowlands—the basins of the Orinoco, Amazon, and Paraguay-Paraná rivers and the plains of the Pampas—separate the highlands from one another. Taken as a whole, the relief of the continent shows a great imbalance: the major drainage divide is far to the west along the crest of the Andes. Thus, rain that falls only 100 miles (160 kilometres) east of the Pacific may flow to the Atlantic, 2,500 miles away.
The ranges of the Andes Mountains, about 5,500 miles long and second only to the Himalayas in average elevation, constitute a formidable and continuous barrier, with many summits exceeding 20,000 feet (6,100 metres). The Venezuelan Andes—the northernmost range of the system—run parallel to the Caribbean coast west of Caracas, before turning to the southwest and entering Colombia. In Colombia the Andes—which trend generally to the north and south—form three distinct ranges: the Cordilleras Oriental, Central, and Occidental. The valley of the Magdalena River, between the Oriental and the Central ranges, and the valley of the Cauca River, between the Central and the Occidental ranges, are huge rift valleys formed by faulting rather than by erosion. An aerial view of the Andes in Colombia shows, within relatively short distances, a succession of hot lowlands interspersed with high ranges with snowcapped peaks.
In Ecuador the Andes form two parallel cordilleras, one facing the Pacific and the other descending abruptly eastward toward the Amazon basin, crowned by towering peaks. Between the ranges lies a series of high basins. Three cordilleras run through Peru and are known by Peruvians as the Eastern Cordillera; the Central, or Blanca (“White”), Cordillera, named for the glaciated summit of Mount Huascarán, the country’s highest peak, which rises to 22,025 feet; and the Western, or Negra (“Black”), Cordillera, which has no snowcapped summits. South of Lima and extending through western Bolivia, the Andes branch into two distinct ranges. Between them lies the Altiplano, a vast complex of high plateaus between 12,000 and 15,000 feet in elevation and as much as 125 miles wide.
The Altiplano forms a maze of depressions, hills, and vast plains without equivalent except in Tibet. Water accumulates in closed basins to form marshes and lakes, the largest of which is Lake Titicaca on the border of Peru and Bolivia. This central region of the Andes has been dissected by several rivers, all of which have cut spectacular gorges down the eastern slopes.
Farther to the south—along the border between Chile and Argentina—the Andes form a single but lofty chain with many of the system’s highest peaks, including Mount Aconcagua; south of Aconcagua, elevations gradually diminish. In southern Chile part of the cordillera descends beneath the sea, forming innumerable islands with steep slopes. The Andes have been deeply carved by glaciers, particularly in the south. Ice masses still occupy some 1,900 square miles (4,900 square kilometres), constituting a huge ice cap with long terminal tongues running into piedmont lakes or into the sea. The Andes are studded with numerous volcanoes that are part of the Circum-Pacific volcanic chain, often called the Ring of Fire. Earthquakes are frequent. Almost every major city has been devastated at least once by earthquake, even along the coastal plains, where clear signs of recent vertical movement are visible. "South America" © Emmanuel BUCHOT, Encarta, Wikipedia
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