Andes

I

Introduction

Andes, the principal mountain chain of South America, one of the greatest mountain systems of the world, extending in a narrow belt along the western edge of the continent, from Cape Horn nearly to Panama. The chain, about 7,200 km (4,500 approximate mi) long, has an average breadth of 241 km (150 mi) and an average height of about 3,660 m (12,000 ft). As well as being the longest system of mountain ranges on earth, the Andes includes some of the world’s loftiest peaks—only the Himalaya has higher. In addition, the elevated plateaux and high mountain valleys of the Andes contain some of the highest permanent human settlements in the world. The mountains reach into seven countries: Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina. Along its entire length the chain rises abruptly from the Pacific coast.

II

Formation

Uplift and folding of the sedimentary rocks that comprise the Andes began during the Cretaceous period, when the Pacific crustal plate began to subduct (slide beneath) the South American plate (see plate tectonics). Tectonic forces generated by this collision still trigger volcanic eruptions and earthquakes and have lifted parts of the Andes over 1,500 m (5,000 ft) during the past 20 million years.

III

Principal Ranges

The Andes chain is generally about 300 km (200 mi) wide, except in Bolivia, where it expands to twice that width. From north to south the belt can be divided into three regions: a northern section in Venezuela, Colombia, and northern Ecuador; a central section in southern Ecuador, Peru, Bolivia, and the northern regions of Argentina and Chile; and a southern section in the southern regions of Argentina and Chile.

The northern Andes curve in a north-east to south-west arc consisting of three main parallel ranges, known as the Cordillera Occidental (Western Cordillera), the Cordillera Central (Central Cordillera), and the Cordillera Oriental (Eastern Cordillera). The term cordillera (Spanish, “rope”) was coined by early European explorers to describe the roughly parallel formation of many Andean ranges; it is now used to describe mountain ranges in many parts of the world. In the north the ranges fan out: the Cordillera Oriental divides into the Cordillera Mérida, which extends along the east coast of Lake Maracaibo in Venezuela, and the Sierra de Perijá, which extends to the west of Lake Maracaibo along the borders of Colombia and Venezuela. Both of these ranges extend to the Caribbean Sea. Farther south the three major ranges draw closer to one another in Colombia and narrow further in Ecuador to form two main ranges.

The central Andes extend south-east from northern Peru to southern Bolivia and then due south. They include the widest part of the mountain system and some of its highest peaks. The two main ranges here, the Cordillera Occidental and the Cordillera Oriental, enclose the Altiplano, an extensive plateau in Bolivia and southern Peru between 3,700 to 4,300 m (12,000 to 14,000 ft) above sea level. Farther south is another high plateau, called the Puna de Atacama (Atacama Plateau), located mostly in Argentina along the northwest border with Chile and Bolivia.

The southern Andes narrow to form a single range, which curves to the east at its southern end. The highest mountain in the western hemisphere, Aconcagua (6,960 m/22,834 ft), is located in this section of the Andes. South of Aconcagua the mountains diminish gradually in height. Cerro Yogan, the highest peak on Tierra del Fuego, rises to only 2,469 m (8,100 ft).

IV

Mountain Passes

The passes through the Andes, north of Patagonia, are all at a great height (more than 3,000 m (10,000 ft) above sea level) and are narrow, steep, dangerous, and blocked by snow for several months of each year. Even in the south rugged terrain prevents the use of many lower passes. The pass from Arequipa to Puno, in Peru, is 4,468 m (14,660 ft) high; the one from Lima to Tarma and Cerro de Pasco, also in Peru, is 4,804 m (15,760 ft) high. The Uspallata Pass (at 3,900 m (12,795 ft)) is a major transport route between Santiago in Chile and Mendoza in Argentina; a road follows the pass, and the Transandine Railway tunnel runs underneath it. Railways have been built through a number of other passes. They include the railway uniting Mollendo with Puno on Lake Titicaca, and the Central Railway in Peru, which links Lima and La Oroya and is considered one of the most remarkable railway constructions in the world.