The Andes are the longest continental mountain range on Earth, running approximately 7,000 kilometres along the western edge of South America from the Caribbean coast of Venezuela in the north to Cape Horn at the southern tip of Tierra del Fuego. The range passes through seven South American countries — Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina — and forms the geological backbone of the continent. The highest peak, Aconcagua in Argentina, reaches 6,961 metres, making it the highest mountain in the Americas and the highest mountain outside Asia. The Andes are the product of an ongoing subduction zone where oceanic plates from the Pacific (principally the Nazca Plate) are descending beneath the South American continental plate. The subduction process has been ongoing for around 200 million years and is still very actively producing earthquakes, volcanic eruptions, and continued uplift across the entire range. The geography, climate, and human history of South America are all shaped by the Andes in ways that no other single landform shapes any other continent.
The Subduction Zone
The Andes are the type example of a non-collisional active margin range — produced by oceanic-continental subduction rather than continent-continent collision. The Nazca Plate, in the southeastern Pacific, is descending eastward beneath South America at approximately 7-8 centimetres per year. The descent is steep along much of the range's length but flatter in a few sections (notably central Peru and central Chile, where the subducting slab is unusually horizontal), with consequences for the volcanic and seismic patterns above. The compression and uplift associated with subduction has built up the entire range over the past 200 million years, with the most rapid uplift in the past 30 million years. The plate boundary along the Andean margin is the longest active subduction zone on Earth and is responsible for some of the largest earthquakes ever recorded, including the 1960 Valdivia earthquake in southern Chile (magnitude 9.5, the largest earthquake instrumentally recorded).
The Volcanic Belt
The Andes contain hundreds of volcanoes, both active and dormant, distributed across the range. The volcanic belt is not continuous — it is broken into four distinct segments separated by "volcanic gaps" where the subducting slab is shallow enough that the slab does not produce the melting required for volcanism. The Northern Volcanic Zone in Colombia and Ecuador includes Cotopaxi (one of the world's highest active volcanoes), Tungurahua, and Nevado del Ruiz (whose 1985 eruption killed approximately 23,000 in the disaster of Armero). The Central Volcanic Zone in southern Peru, Bolivia, northern Chile, and northwestern Argentina includes Ojos del Salado (the highest volcano in the world at 6,893 metres), Misti, and many others. The Southern Volcanic Zone in central Chile and Argentina includes Villarrica, Llaima, and Calbuco. The Austral Volcanic Zone in southern Chile and Argentina is the southernmost segment and is less continuously active.
The Altiplano
The widest and highest part of the Andes is the Altiplano, a vast high-elevation plateau between two parallel cordilleras in southern Peru, Bolivia, northern Chile, and northwestern Argentina. The Altiplano sits at around 3,500-4,200 metres in elevation and covers approximately 170,000 square kilometres. It is the second-largest high plateau on Earth after the Tibetan Plateau. Lake Titicaca, at 3,812 metres elevation on the Peru-Bolivia border, is the highest navigable lake in the world and one of the major water bodies of the Altiplano. The Salar de Uyuni, a vast salt flat in southwestern Bolivia at 3,656 metres elevation, is the largest salt flat on Earth and contains a substantial fraction of the world's lithium reserves. The Altiplano was the heartland of the pre-Columbian Tiwanaku civilization (~500-1000 CE) and later of the Inca Empire (~1438-1533 CE). Modern Bolivia's major cities (La Paz, El Alto, Oruro) sit on or around the Altiplano, with La Paz being the highest capital city in the world (depending on how one measures — the seat of government is at around 3,650 metres).
The Inca Heartland
The Andes were the home of the largest pre-Columbian empire in the Americas, the Inca Empire (Tawantinsuyu, "Land of the Four Quarters"), which at its peak around 1525 controlled approximately 4,000 kilometres of Andean territory from northern Ecuador to central Chile and Argentina. The Inca capital was at Cusco in the Peruvian Andes at around 3,400 metres elevation. The empire's remarkable engineering — including a road network of approximately 40,000 kilometres of paved trails across the most rugged terrain in the world, the agricultural terraces that converted steep mountain slopes into productive farmland, the irrigation systems that channeled glacial meltwater through complex distribution networks, and the architectural achievements at Machu Picchu, Ollantaytambo, and other sites — depended on a sophisticated understanding of mountain geography and engineering. The Spanish conquest in 1532-33 effectively destroyed the empire's political structure but the underlying Quechua and Aymara linguistic and cultural traditions survive across the modern Andean countries.
The Climate Variation
The Andes span an enormous climatic range. The northern (equatorial) Andes have a tropical climate at lower elevations and progressively cooler vertical zones up to the snow line at around 4,800 metres. The central Andes (Peru, Bolivia, northern Chile) span from coastal desert (the Atacama, one of the driest places on Earth) through the Altiplano to the tropical rainforest of the eastern slopes. The southern Andes (central Chile, Argentina) have a temperate climate similar to the Mediterranean basin or the Pacific Northwest, with Patagonia's ice fields and steppe to the south. The range therefore contains essentially every climate type from tropical jungle to polar desert, often within a single country and sometimes within a single day's travel.
The Glaciers and Water
The Andes contain the largest glacier system in the tropics. The Cordillera Blanca in central Peru contains over 700 glaciers across an area of approximately 700 square kilometres. The Quelccaya Ice Cap in southern Peru is the largest tropical ice mass in the world. The glaciers feed major river systems including the Amazon (which rises in the Peruvian Andes), the Orinoco, the Paraná, and many smaller rivers serving the coastal Pacific cities of Peru and Chile. Climate change has been retreating the tropical Andean glaciers rapidly. The Chacaltaya Glacier in Bolivia, which used to host a ski resort, disappeared completely in 2009. The Quelccaya is losing mass at unprecedented rates. The implications for water supply in the coastal Peruvian cities (Lima, in particular, is exposed) and the agricultural systems of the Altiplano are substantial and ongoing.
The Mining Economy
The Andean countries have substantial mineral wealth, much of it tied directly to the geological processes that built the range. Chile produces approximately 25 percent of world copper, mostly from the central and northern Andes. Peru produces around 10 percent of world silver and substantial copper, gold, and zinc. Bolivia's salt flats contain enormous lithium reserves. The Cerro Rico mountain at Potosí, Bolivia, was the most productive silver mine in the world for several centuries after its discovery in 1545 and underwrote substantial portions of the Spanish Empire's economic strength. Modern Andean mining is large-scale, capital-intensive, environmentally contentious, and often the centre of conflicts between mining companies, indigenous communities, and national governments. The economic dependence of Chile, Peru, and Bolivia on mineral exports has shaped their political and economic trajectories in major ways.
The Climbing Tradition
Andean mountaineering has its own distinct tradition separate from Himalayan climbing. The peaks are lower than the eight-thousanders but technically demanding, and the higher Andean peaks were among the first major mountains in the world to be summited (Mount Chimborazo in Ecuador, once thought to be the highest mountain in the world, was first climbed by Edward Whymper in 1880). Aconcagua, the highest peak, is technically a relatively easy climb on its normal route and is climbed by approximately 3,000 people per year. The Cordillera Blanca in Peru and the Cordillera Real in Bolivia host many of the most technically challenging climbs in the Americas. Patagonia's granite spires — Cerro Torre, Fitz Roy, and others — are among the most difficult rock-and-ice climbs in the world and were not fully climbed until the late 20th century.
The Living Range
The Andes are not a static feature. The range continues to rise, the volcanoes continue to erupt, and the seismic patterns continue to produce regular major earthquakes. The 2010 Maule earthquake in Chile (magnitude 8.8) and the 2014 Iquique earthquake (magnitude 8.2) are recent reminders of the seismic intensity of the margin. The volcanic eruptions of Cotopaxi, Sangay, Villarrica, and many others have produced ongoing impacts in the 21st century. The glaciers are visibly retreating. The political situations of the seven Andean countries continue to evolve. The Andes are in this sense a living range whose interaction with the human populations along its length is constantly being renegotiated, and whose underlying geological processes are unlikely to stop within any timeframe that matters for those populations.