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The Andes is a vast mountain system forming a continuous chain of highland along the western coast of South America. It is roughly 4400 miles long, 100 miles wide in some parts, and of an average height of 13,000 feet.

The connexion of this system with that of the Rocky Mountains, which has been pointed out by many writers, has received much support from the discovery of the extensive eruptions of granite during Tertiary times, extending from the southern extremity of South America to Alaska.

The Andean range is composed of two great principal chains with a deep intermediate depression, in which, and at the sides of the great chains, arise other chains of minor importance, the chief of which is that called the Cordillera de la Costa of Chile. This starts from the southern extremity of the continent, and runs in a northerly direction, parallel with the coast, being broken up at its beginning into a number of islands, and afterwards forming the western boundary of the great central valley of Chile. To the north this coastal chain continues in small ridges or isolated hills along the Pacific Ocean as far as Colombia, always leaving the same valley more or less visible to the west of the western great chain.

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Tierra del Fuego

Of the two principal chains the eastern is generally called Los Andes, and the western La Cordillera, in Colombia, Peru and Bolivia, where the eastern is likewise known as Cordillera Real de los Andes, while to the south of parallel 23°S in Chile and Argentina, the western is called Cordillera de los Andes. The eastern disappears in the centre of Argentina, and it is therefore only the Cordillera de los Andes that is prolonged as far as the south-eastern extremity of the continent.

The Cordillera de la Costa begins near Cape Horn, which is composed principally of crystalline rocks, and its heights are inconsiderable when compared with those of the true Cordillera of the Andes. The latter, as regards its main chain, is on the northern coast of the Beagle Channel, in Tierra del Fuego, bounded on the north by the deep depression of Lake Fagnano and of Admiralty Sound. Staten Island appears to be the termination to the east. The Cordillera of the Andes in Tierra del Fuego is formed of crystalline schists, and culminates in the snow-capped peaks of Mount Darwin and Mount Sarmiento (7200 feet), which contains glaciers of greater extent than those of Mont Blanc. The extent of the glaciers is considerable in this region, which, geographically, is more complex than was formerly supposed. Although, in the explored portion of the Fuegian chain, the volcanoes which have been mentioned from time to time have not been met with, there seem to have existed to the south, on the islands, many neo-volcanic rocks, some of which appear to be contemporaneous with the basaltic sheet that covers a part of eastern Patagonia. The insular region between Mount Sarmiento and the Cordillera de los Andes, properly so called, i.e. that which extends from Magellan Strait northwards, is not fully explored, and all that is known of it is that it is principally composed of the same rocks as the Fuegian section, and that the greater part of its upper valleys is occupied by glaciers that reach down to the sea amid dense forest.

Chile-Argentina, 52°-38°S

As Admiralty Sound and Lake Fagnano bound the Cordillera to the north in Tierra del Fuego, so at the eastern side of the Cordillera in the southernmost part of the continent there is a longitudinal depression which separates the Andes from some independent ridges pertaining to a secondary parallel broken chain called the pre-Cordillera. This depression is occupied in great part by a series of lakes, some of these filling transversal breaches in the range, whilst others are remains of glacial reservoirs, bordered by morainic dams, extending as far as the eastern tableland and corresponding in these cases with transversal depressions which reach the Atlantic Ocean. Between the larger lakes, fed by the Andine glaciers of the eastern slope of the Southern Andes, are Lakes Maravilla, 100 square miles, and Sarmiento, 26 square miles, 51°S, which overflow into Last Hope Inlet; Argentino, 570 square miles, 50°S; and Viedma, 450 square miles, 49°30'S, which empty into the river Santa Cruz; the fjordian Lake San Martin, 49°S, and Lakes Nansen, 18 square miles; Azara, eight square miles; and Belgrano, 18 square miles, which are dependents of Lake San Martin (380 square miles), and Lakes Pueyrredon (98 square miles) and Buenos Aires (700 square miles), which now overflow into the Pacific, through one of the remarkable inlets that are found throughout the Cordillera, the Calen Inlet, which is the largest western fjord of Patagonia. To the north of Lake Buenos Aires there is Lake Elizalde, which, while situated on the eastern slope, sends its waters to the Pacific Ocean, and Lakes Fontana (30 square miles) and La Plata (34 square miles), 45°S, which feed the river Senguerr, which flows to the Atlantic. Lake General Paz (66 square miles) on the eastern slope of the Andes, at 44°S, is the principal source of the Palena river, which cuts all the Cordillera, while Lakes Fetalauquen (20 square miles) Menendez (28 square miles), Rivadavia (10 square miles), and other smaller lakes, also situated between 43°30', and 42°30'S on the eastern slope send their waters to the Pacific by the river Fetaleufu which cuts through the Andes by a narrow gorge. The waters of Lake Puelo (18 square miles) likewise flow into the same ocean through the river of that name, which also cuts the Cordillera, and of which the principal affluent likewise drains the waters of a system of small lakes, the largest of which, Lake Mascardi, measures 17 square miles, which in comparatively recent times formed part of the basin of Lake Nahuel Huapi (207 square miles), 41°S. An extensive area of glacial deposits shows that a sheet of ice formerly covered the whole eastern slope to a great distance from the mountains. To the west another sheet reached at the same time the Pacific Ocean.

From the Strait of Magellan[?] up to 52°S, the western slope of the Cordillera does not, properly speaking, exist. Abrupt walls overlook the Pacific, and great longitudinal and transversal channels and fjords run right through the heart of the range, cutting it generally in a direction more or less oblique to its axis, the result of movements of the earth's crust.

The mountains forming the Cordillera between Magellan Strait and 41°S are higher than those previously mentioned in Tierra del Fuego. Generally composed of granite, gneiss and Palaeozoic rocks, covered in many parts by rugged masses of volcanic origin, their general height is not less than 6500 feet, while Mount Geikie is 7500 feet and Mount Stokes 7100 feet. To the north are Mounts Mayo (7600 feet), Agassiz (10,600 feet), and Fitzroy, in 49°S (11,120 feet). The section from 52°to 48°S is a continuous ice-capped mountain range, and some of the glaciers extend from the eastern lakes to the western channels, where they reach the sea-level. The level of the lakes begins at 130 feet at Lake Maravilla[?] and gradually ascends to nearly 700 feet at Lake San Martin[?]. Passing the breach through which Lake San Martin empties itself into Calen Inlet, in 48°S, is found a wide oblique opening in the range, through which flows the river Las Heras, fed by Lake Pueyrredon, which is only 410 feet above the sea-level to the east of the Andes, while Lake Buenos Aires[?], immediately to the north, is 710 feet. The Andes continue to be to the west an enormous rugged mass of ice and snow of an average height of 9000 feet, sending glaciers to all the eastern fjords.

Mount San Lorenzo[?], detached from the main chain in the Pre-Cordillera, is 11,800 feet high. Mount San Valentin[?] (12,700 feet) is the culminating point of the Andes in the region extending from 49° to 46°S, a little north of which is the river Huemules which is followed by the breach of the river Aisen. These two rivers have emptied a large system of lakes, which in pre-Glacial times occupied the eastern zone, thus forming a region suitable for colonization in the broad valleys and hollows, where the rivers, as in the case with those in the north, cut through the Andes by narrow gaps, forming cataracts and rapids between the snowy peaks. Volcanic action is still going on in these latitudes, as the glaciers are at times covered by ashes, but the predominant rocks to the east are the Tertiary granite, while to the west gneiss, older granite and Palaeozoic rocks prevail. The highest peaks, however, seem to be of volcanic origin. Farther north, up to 41°S, the water gaps are situated at a lesser distance one from the other, owing mainly to more continuous erosion, this section of the continent being the region of the maximum rainfall on the western coast to the south of the equator. Between the gaps of the river Aisen and river Cisnes or Frias, which also pierces the chain, is found a huge mountain mass, in which is situated Mount la Torre (7150 feet). These form the continental watershed, but in this region erosion is taking place so rapidly that the day is not far distant when Lakes La Plata and Fontana, situated to the east at a height of 3000 feet and now tributaries of the Atlantic, may become tributaries of the Pacific. Already filtrations from the former go to feed western affluents through the granitic masses. To the north of Mount la Torre flows in the river Cisnes, 44°48'S, across another water gap, continuing the range to the north with high peaks, as Alto Nevado (7350 feet) and Cacique (7000 feet). The glaciers reach almost the western channels, as is the case at the river Quelal. The northern glaciers, descending nearly to sea-level, are situated at 43°40'S. To the north, a well-defined western longitudinal valley, at some recent time occupied by lakes and rivers, divides the Cordillera into two chains, the eastern being the main chain, to which belong Mounts Alto Nevado, Cacique, Dentista, Maldonado, Serrano, each over 7000 feet high; and Torrecillas (7400 feet), Ventisquero (7500 feet), and Tronador (11,180 feet); while the western chain, broken into imposing blocks, contains several high volcanic peaks such as Mounts Tanteles, Corcovado, Minchimahuida, Hornopiren and Yates. The rivers Palena, with its two branches, Pico and Carrenleufu, Fetaleufu, Puelo and Manso cut the two chains, while the rivers Renihue, Bodadahue and Cochamo have their sources in the main eastern ridge. Mention has been made of active volcanoes in 51°, 49°, and 47°S, but these have not been properly located. The active volcanoes south of 41°, concerning which no doubt exists, are the Huequen, in 43°, and the Calbuco, both of which have been in eruption in modern times.

The surroundings of Mount Tronador, consisting of Tertiary granite and basalt, form one of the most interesting regions in the Pataronian Andes for the mountaineers of the future. To the east extends the large and picturesque lake of Nahuel-Huapi, to the west is Lake Todos Los Santos (50 square miles), to which the access is easy and of which the scenery is of surpassing beauty. Between 41°and 38°S, among other smaller lakes, are Lakes Traful (45 square miles), Lacar (32 square miles), which, properly belonging to the system of Atlantic lakes, empties itself by the only water gap that occurs in this zone of the Cordillera into the river Valdivia, a tributary of the Pacific, Lake Lolog (15 square miles), Huechu-lafquen (45 square miles), and Lake Alumine (21 square miles). The volcanoes of Lanin (12,140 feet), Quetropillan (9180 feet), Villarica (10,400 feet), Yaimas and Tolhuaca are all more or less active; the first is in the main chain, while the others are on the western slope. The scenery in the neighbourhood is magnificent, the snowy cones rising from amidst woods of araucaria, and being surrounded by blue lakes. While the scenery of the western slope of the Andes is exceedingly grand, with its deep fjords, glaciers and woods, yet the severity of its climate detracts considerably from its charm. The climate of the eastern slope, however, is milder, the landscapes are magnificent, with wooded valleys and beautiful lakes. The valleys are already partly settled by colonists. Between 52°and 40°S erosion has carried the watershed of the continent from the summit of the Cordillera to the eastern plains of Patagonia.

From 40°S. southward the Chile-Argentine Boundary Commission under Sir T. H. Holdich carried out important investigations in 1902; and between 38°and 33°S the Andes were somewhat extensively explored about the close of the 19th century by Argentine and Chilean Commissions. The highest peaks in the latter section are volcanic and their eruptions have sensibly modified the character of the primitive ridges. Outflows of lava and tufa cover the mountain sides and fill up the valleys. The Jurassic and Cretaceous formations, which in the Southern Cordillera are situated outside of the range to the east, form to a considerable extent the mass of the great range, together with quartz porphyry, the Tertiary, granite and other eruptive rocks, which have been observed along all the chain in South America up to Alaska in the north. Gneiss is seldom met with, but there are crystalline rocks, belonging chiefly to the pre-Cordillera of the eastern and to the Cordillera de la Costa on the western side.

Chile-Argentina from 38°S northward

About 38°S the Andes take a great transversal extension; there are no wide intermediate valleys between the different ridges but the main ridge is perfectly defined. Volcanic cones continue to predominate, the old crystalline rocks almost disappear, while the Mesozoic rocks are most common. The higher peaks are in the main chain, while the Domuyo (15,317 feet) belongs to a lateral eastern ridge. The principal peaks between this and Mount Tupungato at 33°S are: Mount Cochico (8255 feet), Campanario, (13,140 feet), Peteroa (13,297 feet), Tinguiririca, Castillo (16,535 feet), Volcano Maipu (17,576 feet), Alvarado (14,600 feet), Amarillo (15,321 feet), Volcano San Jose (19,849 feet), Piuquenes (17,815 feet), and Volcano Bravard (19,619 feet).

North of Maipu volcano, ascended by R. P. Gussfeldt in 1883, the Cordillera is composed of two huge principal ridges which unite and terminate in the neighbourhood of Mount Tupungato. The valley between them is 9000 feet high; and in that part of the Cordillera are situated the highest passes south of 33°S, one of which, the Piuquenes Pass, reaches 13,333 feet, whilst the easiest of transit and almost the lowest is that of Pichachen (6505 feet), which is the most frequented during winter. Mount Tupungato reaches 22,329 feet, according to Argentine measurement. To the north of this mountain, situated at the watershed of the Andes, extends a lofty region comprising peaks such as Chimbote (18,645 feet) and Mount Polleras (20,266 feet). The Pircas Pass is situated at a height of 16,962 feet. The gaps of Bermejo and Iglesia, in the Uspallata road, the best known of all the passes between Argentina and Chile, are at 13,025 feet and 13,412 feet altitude respectively, while the nearest peaks, those of Juncal and Tolorsa, are 19,358 and 20,140 feet high.

Mounts Tupungato, Aconcagua (23,393 feet) and Mercedario (21,982 feet) are the highest peaks of the central Argentine-Chilean Andes. These three peaks are formed of eruptive rocks, surrounded by Jurassic beds which have undergone a thorough metamorphosis. While in the west of the Andes, from the latitude of Aconcagua, the central valley of Chile runs without any notable interruption to the south end of the continent, a valley which almost disappears to the north, leaving only some rare inflexions which are considered by Chilean geographers and geologists to be a continuation of the same valley; to the east in Argentina a longitudinal valley, perfectly characterized, runs along the eastern foot of the Cordillera, separating this from the pre- Cordillera, which is parallel to the Cordillera de la Costa of Chile. Between Aconcagua and Mercedario are the passes of Espinacito (14,803 feet) and Los Patos or Valle Hermoso (11,736 feet), chosen by the Argentine General San Martin, when he made his memorable passage across the chain during the War of Independence. North of Valle Hermoso the Andean ridges, while very high, are not abrupt, and the passes are more numerous than in the south; some of them descending 10,000 feet, but most of them between 13,000 and 14,000 feet. The pass of Quebrada Grande is 12,468 feet in altitude; Cencerro, 12,944 feet; Mercedario, 13,206 feet; Ojota, 14,304 feet; Pachon, 14,485 feet; while Gordito is 10,318 feet. Farther north the passes are higher. Barahona Pass is 15,092 feet; Ternera, 15,912 feet; San Lorenzo, 16,420 feet, while the peak of the volcano reaches 18,143 feet; Mount Olivares, 20,472 feet; Porongos, 19,488 feet; Tortolas, 20,121 feet; and Potro, 19,357 feet.


As far as 28°S the Cordillera de los Andes has been principally formed by two well-defined ridges, but to the north, recent volcanic action has greatly modified its orography. Only a single line of passes characterizes the main ridge, and amongst them are the passes of Ollita (15,026 feet), Penas Negras (14,435 feet), Pircas Negras (13,615 feet), La Gallina (16,240 feet), Tres Quebradas (15,535 feet), and Aguita (15,485 feet). To the north of Mount Potro the peaks in the Cordillera are not very prominent as far as the great mass of Tres Quebradas, but here are to be met with some that may he considered as amongst the highest of the whole range. Mount Aguita is 20,600 feet, and the culminating peak of those of Tres Cruces reaches 226,58 feet. To the east of the eastern longitudinal valley, at 27°S, begins a high volcanic plateau between the Cordillera and the southern prolongation of the Bolivian Cordillera Real, which contains lofty summits, such as Mount Veladero ( 20,998 feet), Mount Bonete (21,980), Mount Reclus (20,670), Mount Pissis (22,146), Mount Ojo del Salado (21,653), and Incahuasi (21,719). To the north of Tres Cruces is a transversal depression in the Cordillera, which is considered to be the southern termination of the high plateau of the Puna de Atacama. The Cordillera of the Andes borders the Puna to the west, while the Bolivian Cordillera Real bounds it to the east. In that region the Cordillera of the Andes is of comparatively recent origin, being principally constituted by a line of high volcanoes, the chief summits being those of Juncal, Panteon de Aliste, Azufre or Listarria (18,636 feet), Llullaillaco (21,720), Miniques (19,357), Socompa (19,948), Licancaur (19,685), Viscachuelas (20,605), Tapaquilcha (19,520), Oyahua (19,242), Ancaquilcha (20,275), Olca (19,150), Mino (20,112), Sillilica (21,100), Perinacota (20,918), Sagama (22,339), Tacona (19,740), Misti (19,029); to the east closes in the intermediary high plateau which begins at 28°S in Argentina. The principal peaks of the Bolivian Andes and its prolongation from south to north, are Famatina, in the centre of Argentina, (20,340 feet), Languna Blanca (18,307), Diamante (18,045), (Cachi (20,000), Granadas, Lipez (19,680), Guadalupe (18,910), Chorolque (18,480), Cuzco (17,930), Enriaca (18,716), Junari (16,200), Michiga (17,410), Quimza-Cruz (18,280), Illimani (21,190) and Sorata (21,490).

While the western range of the Cordillera is principally formed by volcanic rocks, the eastern (to the east of the range is Cerro Potosi, 15,400 feet) Andes of Bolivia are chiefly composed of old crystalline rocks. Between the ranges in the high plateau north to 27° are numerous isolated volcanoes which have been in activity in recent times, such as Peinado (18,898 feet), San Pedro (18,701), Antuco (19,029), Antofalla (20,014), Rincon (17,881), Pastos Grandes (17,553), Zapalegui (17,553), Suniguira (19,258), Tahue (17,458); volcanoes which have been elevated from a lncustrine basin, which very recently occupied the whole extension, and the remains of which are, in the south, the Laguna Verde, at 28°, and in the north Lake Titicaca. The discovery of great Pampean mammals in the Pleistocene beds of that region shows that this upheaval of the latter is very recent, for in the heart of the Cordillera, as well as on the west coast of Bolivia and Peru, there have been discovered, in very recent deposits, the remains of some mammals which cannot have crossed the high range as it now exists.


The two Cordilleras that formed the Andes to the north of 28°S are continued in Peru. The western, which reaches an altitude of about 10,000 feet, then ceases to exist as a continuous chain, there remaining only a short, high ridge, called by Edward Whymper the "Pacific range of the equator," and between this ridge and the crystalline Andean axis, the "avenue of volcanoes," to use his words, arises amidst majestic scenery. Chimborazo, which is not in the main chain, reaches 20,517 feet; Cotopaxi (19,580), Antisana (19,260), Coyambo (19,200) are in the eastern range, with many other peaks over 16,000 feet which still contain glaciers. Sangay (17,380 feet), under the equator, according to Wolff, appears to be the most active volcano in the world. Pichincha (15,804 feet) and Cotocachi (16,297 feet) are the loftiest volcanoes of the western range. In Colombia the three principal chains are continuations of those under the equator, and show very slight traces of volcanic action,


In the western chain, which is remarkable for its regularity, the highest peak is 11,150 feet, and the lowest pass 6725 feet. The central chain, separated from the western chain by the valley of the Cauca and from the eastern by the valley of the Magdalena, is unbroken; it is the more important owing to its greater altitudes and is of volcanic character. To the south, near the equator, are Mounts Arapul (13,360 feet) and Chumbul (15,720 feet). The volcanoes Campainero (12,470 feet) and Pasto (14,000 feet) are also in that zone. Farther north is the volcano Purace, which presents a height of 16,000 feet; then come Huila (18,000), Santa Catalina (16,170), and Tolima (18,400), Santa Isabel (16,760), Ruiz (17,390) and Hervas (18,340). The eastern chain begins north of the equator at 6000 feet, gradually rises to the height of Nevado (14,146 feet), Pan de Azucar (12,140 feet), and in the Sierra Nevada de Cochi attains to peaks of 16,700 feet.

The snow-line of the Andes is highest in parts of Peru where it lies at about 16,500 feet. Its general range from the extreme north to Patagonia is 14,000 to 15,500 feet, but along the Patagonian frontier it sinks rapidly, until in Tierra del Fuego it lies at about 4900 feet.


The structure of the Andes is least complex in the southern portion of the range. Between 33° and 36°S the chain consists broadly of a series of simple folds of Jurassic and Cretaceous beds. It is probably separated on the east from the recent deposits of the pampas by a great fault, which, however, is always concealed by an enormous mass of scree material. The Cretaceous beds lie in a broad synclinal upon the eastern flank, but the greater part of the chain is formed of Jurassic beds, through which, on the western margin, rise the numerous andesitic volcanic centres. There is no continuous band of ancient gneiss, nor indeed of any beds older than the Jurassic. There is very little over-folding or faulting, and the structure is that of the Jura mountains rather than of the Alps. The inner or eastern ridge farther north of Argentina consists of crystalline rocks with infolded Ordovician and Cambrian beds, often overlaid unconformably by a sandstone with plant-remains (chiefly Rhaetic). In Bolivia this eastern ridge, separated from the western Cordillera by the longitudinal valley in which Lake Titicaca lies, is formed chiefly of Archaean and Palaeozoic rocks. All the geological systems, from the Cambrian to the Carboniferous, are represented and they are all strongly folded, the folds leaning over towards the west. West of the great valley the range is composed of Mesozoic beds, together with Tertiary volcanic rocks. (The Cordillera of Argentina and Chile is clearly the continuation of the western chain alone.) In Ecuador there is still an inner chain of ancient gneisses and schists and an outer chain composed of Mesozoic beds. The longitudinal valley which separates them is occupied mainly by volcanic deposits. North of Ecuador the structure becomes more complex. Of the three main chains into which the mountains are now divided, the western branch is formed mostly of Cretaceous beds; but the inner chains no longer consist exclusively of the older rocks, and Cretaceous beds take a considerable share in their formation.

The great volcanoes, active and extinct, are not confined to any one zone. Sometimes they rise from the Mesozoic zone of the western Cordillera, sometimes from the ancient rocks of the eastern zone. But they all he within the range itself and do not, as in the Carpathians and the Apennines, form a fringe upon the inner border of the chain.

The curvature of the range around the Brazilian massif, and the position of the zone of older rocks upon the eastern flank, led Suess to the conclusion that the Andes owe their origin to an overthrust from east to west, and that the Vorland lies beneath the Pacific. In the south Wehrli and Burckhardt maintain that the thrust came from the west, and they look upon the ancient rocks of Argentina as the Vorland. In this part of the chain, however, there is but little evidence of overthrusting of any kind.

Original entry came from 1911 encyclopedia

The geology in this article apparently reflects views from the first decade of the 20th Century. It is a good example of the descriptive geology of that time, but worthless when it comes to why these ranges exist and some of the structural commentary. It is a pre Plate tectonics view and no longer valid.
ANDES is an acronym for an architecture with non-sequential dynamic execution scheduling.

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