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Geology and Geography
A geologically young system, the Andes were originally uplifted in the Cretaceous and Tertiary periods. They are still rising; volcanoes and earthquakes are common. The folded ranges are discontinuous—merging and bifurcating within the system—but as a whole they form one of the world's most important mountain masses. They are loftier than any other mountains except the Himalayas, with many snowcapped peaks more than 22,000 ft (6,700 m) high. Andean waters reach the Orinoco, the Amazon, and the Río de la Plata.
Far south in Tierra del Fuego, the mountains run east and west, then turn north between Argentina and Chile. The westernmost of the mountains run into the sea, lining the coast of S Chile with islands. In the Patagonian Andes are high, glacier-fed lakes in both Argentina and Chile.
The highest range of the Andes is on the central and northern Argentine-Chilean border; Aconcagua (22,835 ft/6,960 m; highest mountain of the Western Hemisphere) and Tupuncato are there. Between the peaks is Uspallata Pass, the route of the former Transandine Railway, with the Christ of the Andes. Other major peaks such as Llullaillaco flank the main range, and in N Chile sub-Andean ranges enclose the high, cold Atacama Desert.
The central Andes broaden out in Bolivia and Peru in multiple ranges (c.400 mi/640 km wide) with high plateau country (the altiplano) and many high intermontane valleys, where the great civilization of the Inca had its home. High in the mountains on the Peru-Bolivia border is Lake Titicaca. In Bolivia are the notable volcanoes, Illimani and Illampú, and in S Peru is El Misti. The western or coastal range in Peru has lofty peaks (notably Huascarán) and is crossed by the highest railroad of the Andes (from La Oroya to Lima).
The ranges approach each other again in Ecuador, where the N Andes begin. Between two volcanic cordilleras (including the cloud-capped Chimborazo and Cotopaxi) are rich intermontane basins. In Colombia the Andes divide again, the western range running between the coast and the Cauca River, the central between the Cauca and the Magdalena rivers, and the eastern running north parallel to the Magdalena River, then stretching out on the coast into Venezuela. The Andes continue in some of the islands of the West Indies, and in Panama N Andean spurs connect with the mountains of Central America and thus with the Sierra Madre and the Rocky Mts.
People and Economy
See A. G. Ogilvie, Geography of the Central Andes (1922); C. Arthaud and F. Hébert-Stevens, The Andes: Roof of America (tr. 1956); P. E. James, Latin America (1969, repr. 1988); T. Kazami, The Andes (1972); W. S. Pitcher, Magmatism at a Plate Edge: The Peruvian Andes (1985); D. Murphy, Eight Feet in the Andes (1986); S. Lamb, Devil in the Mountain: A Search for the Origin of the Andes (2004).
the Cordillera de los Andes, the longest (9,000 km) and one of the highest (Mount Aconcagua, 6,960 m) mountain systems, bordering the north and west of all of South America. The Andes are the most important climatic barrier on the continent, isolating the entire east from the influence of the Pacific Ocean and the west from the influence of the Atlantic. The continental divide runs along the Andes, and from the Andes flow the beginnings of the sources and tributaries of the Amazon as well as the tributaries of the Orinoco, the Paraguay, the Paraná, and the rivers of Patagonia. The extent of the Andes and their altitude are the causes of major landscape differences in individual areas. The Andes are a serious obstacle to communication between population points on the Pacific coast and eastern South America; at the same time, a significant portion of the population in the Andean nations is concentrated on their high plateaus.
The geological structure of the Andes is quite diverse. Along the eastern edge of the Andes, between 10° and 30° S lat., stretches the Hercynian folded system, the puna, composed of Paleozoic rock (from the Cambrian to the Permian) and breached by granodiorites. The Andean system itself is one of the major systems of alpine folding. It was formed on a Paleozoic and partially Baikalian (Upper Precambrian) folded basement. The geosynclinal troughs of the Andes were formed in the Triassic period and were filled with thick sedimentary and volcanic beds until the end of the Cretaceous period. At the end of the Cretaceous period, uplifting and folding occurred. Granitoid intrusions were formed, including the enormously long massifs of the coast, of the Maritime Cordillera of Peru, and of the Cordillera Principal of Chile. During the Paleocene and Neocene periods the intermontane troughs and foredeeps were formed, and these were filled with sedimentary beds of great thickness (the Altiplano and Maracaibo). Petroleum deposits have been discovered in some of these. Recent active volcanism and earthquakes, confined chiefly to the western zone, show that orogenesis is continuing.
The various geological structures contain the useful minerals usually associated with them. The major oil deposits—the depressions of Maracaibo, Magdalena, the Andean foothills of Venezuela, Peru, Bolivia, and Argentina—are confined to the foredeeps and intermontane troughs. Tin, silver, lead, zinc, tungsten, vanadium, antimony, bismuth, arsenic, and other ores are located in the eastern Paleozoic structures and in the vents of ancient volcanoes in Peru and Bolivia. The copper ore deposits (Chile) are associated with the intrusions of the Andes. There are also deposits of gold, platinum, and emeralds in Colombia, sodium nitrate in northern Chile, and other minerals.
M. V. MURATOV
Orographically, the Andes consist of parallel, predominantly meridional ranges between which lie the interior highlands or depressions. Geomorphological structural features differ in individual sections of the Andes. The major ones (from north to south) are the Caribbean, northwestern, Ecuadorian, Peruvian, Central, Chilean-Argentinian, and Patagonian Andes.
The Caribbean Andes, located in the northeastern part of the continent along the coast of the Caribbean Sea, consist of two folded-eroded medium-elevation (up to 2,765 m) ranges complicated by faults.
The northwestern Andes, in Colombia and western Venezuela, consist of three basic high and steep folded-block Cordilleras (Oriental, Central, and Occidental). These ranges fan out north of 1° N lat. and are divided by deep and broad depressions that are the valleys of the Magdalena and Cauca-Patia rivers. The northern spurs of the Cordillera Oriental—the Cordillera Mérida and the Sierra de Perijá—enclose the broad depression with Lake Maracaibo. In the extreme north rises the horst massif of Sierra Nevada de Santa Marta (5,800 m). This section of the Andes is characterized by recent faults with uplifts and subsidences, volcanism in the south of the Cordillera Occidental and in the Central Cordillera (the Ruiz volcano, 5,400 m, and others), a glacial alpine relief, and broad peneplains in the Cordillera Occidental. In the north and west the northwestern Andes are bordered by alluvial lowlands, the Caribbean, and the Pacific. The Pacific lowlands become the lowlands of the Atrato River, which divide the coastal chain of the Serranía de Baudó.
In the Ecuadorian Andes (to 4° S lat.) the most striking feature is volcanism. Volcanic cones are located along the interior slopes of the Occidental and Oriental (or Real) cordilleras (Chimborazo, 6,262 m; Cotopaxi; and others). The depression separating the cordilleras has been filled with volcanic products.
In the Peruvian Andes (to 14°30’ S lat.) there are the following major elements: the Cordillera Occidental (Cerro Huascarán, 6,768 m), which has significant contemporary glaciation, an intrusive western wing, and a folded eastern wing represented by plateaus 4,100–4,400 m high; the folded-block Central and southeastern cordilleras (the ranges of Carabaya, Vilcanota, and Vilcabamba), with alpine landforms; and the Cordillera Oriental, made up of lower folded-erosion ranges. Recent volcanism is absent. Deep fragmentation by the upper reaches of the Marañón, Huallaga, and Ucayali rivers is characteristic.
The Central Andes (to 28° S lat.) are the widest (700–800 km) and most complex segment of the Andes. The entire central portion is formed by the broad internal uplands, called the Puna de Atacama (with an elevation of around 4,000 m), with a lower western edge (Altiplano), tectonic and residual lakes (Titicaca, Poopó, Coipasa), and enormous salars. The Puna is enclosed between the ridges of intrusive peaks and volcanoes 5,500–6,700 m high on the west and by high folded-block ranges (the heavily glaciated Cordillera Real and others) and medium-altitude folded subandean chains in the east. From 18°30’ S lat., along the Pacific coast, stretches the intrusive Maritime Cordillera (up to 3,200 m in elevation), to the east of which lies a tectonic longitudinal valley. Arid geomorphological processes predominate in the Central Andes. By 28° S lat. all the eastern structures of the Andes have tapered out, and the Chilean-Argentine Andes show an inherent trimembral structure consisting of the Maritime Cordillera, the longitudinal valley and a dual Cordillera Principal (from 310 S lat.) and Cordillera Frontal (to 35° S lat.) including Mount Aconcagua, with an elevation of 6,960 m. Major intrusions and volcanism are characteristic, as well as the role of glacial and water erosion processes that increase to the south. The Patagonian Andes are the extreme southern section, to the south of 39° S lat., and have an elevation up to 4,035 m. They are marked by active volcanism, great ruggedness, and thick modern glaciation and glacial land forms. The Maritime Cordillera becomes a chain of islands in the Chilean Archipelago, while the longitudinal valley becomes a system of straits.
The particular features of the topography and river network of the Andes are explained not only by structural differences but also by climatic ones and chiefly by the distribution of precipitation. In the subequatorial Caribbean Andes, precipitation falls only in the summer (500–1,000 mm per year). The rivers are short, with abrupt summer flooding. In the northwestern Andes precipitation is heavy and rather even during the year, both on the western slopes (up to 10,000 mm per year) and on the eastern ones. Moisture-laden easterly winds bring abundant precipitation to the eastern slopes of the Andes (to 28° S lat.), causing deep cutting of the Andes by full rivers as well as determining the position of the continental divide on the Cordillera Occidental. On the other hand, the entire west between 5° and 28° S lat. has a tropical desert climate, with very sparse surface drainage and a broad area of interior drainage in the Central Andes. South of 30° S lat., in the subtropics, the amount of precipitation brought by the western cyclones increases quickly toward the south, but in the temperate latitudes (with constant westerly winds) 5,000–7,000 mm of precipitation falls annually on the western slopes of the Patagonian Andes. The eastern slopes between 28° and 38° S lat. are very arid, and only further south do moisture-laden winds penetrate to the east. In the southern Andes there are many large terminal glacial lakes.
Corresponding to the geographic position and the climate, the snow line in the Andes in the wet north and east lies at an elevation of 4,700–4,900 m (in the Ecuadorian Andes it declines to 4,250 m). In the Central Andes it rises to 5,600–6,100 m (in the Puna to 6,500 m, which is the highest in the world). From there to 35° S lat. it falls to 3,100 m and in the Patagonian Andes to 1,000–1,200 m (in the south of Tierra del Fuego 500–700 m). At 46°30’ S lat., the glaciers reach ocean level.
Location in several climatic zones, contrasts in moisture on the western and eastern slopes, and significant elevations of the Andes cause a great diversity in the soil and vegetation cover as well as a clearly expressed altitude zonality. The windward wet slopes from the northwestern Andes to the southern Central Andes are covered by mountain humid equatorial and tropical forests (the mountain rainforest) on laterite soils in which three altitude zones can be distinguished: tierra caliente, tierra templada, and tierra fria. In the Caribbean Andes there are deciduous (during the winter drought) forests and brush on red mountain soils. In the subtropical Andes of Chile dry evergreen forests and brush are found on brown soils; south of 38° S lat. on both slopes are moist evergreen and mixed forests on brown forest soils and in the south on podzolized soils. The high uplands that are part of the tierra fria and the tierra helada have distinctive alpine types of vegetation. In the north there are the equatorial meadows, or paramos, and in the Peruvian Andes and in the wetter northeastern Puna there are dry tropical steppes, or jalcas. In the southwestern Puna and along the entire Pacific west between 5° and 28° S lat. there are desert types of soil and vegetation. The Andes are the homeland of coca, the cinchona, the potato, and other valuable plants.
The animal world of the Andes belongs to the Brazilian zoogeographic subregion and is similar to the rich fauna of the adjacent plains. South of 5° S. lat. the fauna is related to the Chilean-Patagonian subregion, which in the Andes is characterized by the llama, the relict spectacled bear, the endemic deer called the pudu and guemal, the Azara’s fox, the bush dog, the South American bush rat, the chinchilla—which has almost become extinct because of its valuable fur—and other animals. There are many endemic birds, including the condor.
REFERENCESGerth, H. Geologiia And. Moscow, 1959. (Translated from German.)
King, Lester. Morfologiia Zemli. Moscow, 1967. (Translated from English.)
Kordil’ery Ameriki. Moscow, 1967. (Translated from English.)
Lukashova, E. N. luzhnaia Amerika. Moscow, 1958.
Machatschek, F. Rel’ef Zemli, vol. 2. Moscow, 1961. (Translated from German.)
Rel’ef Zemli. Moscow, 1967.
E. N. LUKASHOVA