Andes

The Andes form the longest continental mountain range on Earth, stretching along the entire western margin of South America. Extending for roughly 7,000 kilometres and reaching widths that commonly exceed 200 kilometres, this immense cordillera spans seven countries—Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina. The Andes mark a continuous highland system that influences climate, geology, ecology, and human settlement across the continent. Their highest peak, Aconcagua in Argentina, reaches about 6,962 metres, making the Andes the tallest mountain range outside Asia. The region also hosts some of the world’s highest active volcanoes and elevated plateaus, including the Altiplano, the second highest in the world after Tibet.

Geography

The Andean chain can be divided into three major sections: the Southern Andes, the Central Andes, and the Northern Andes. Broadly, these regions correspond to different climatic and geological conditions. The Southern Andes lie primarily within Chile and Argentina south of Llullaillaco, the Central Andes cover Peru and Bolivia, and the Northern Andes extend through Ecuador, Colombia, and Venezuela. Smaller related systems, such as the Sierra Nevada de Santa Marta in Colombia, are sometimes included owing to their proximity and geological connections.
The Andes contain numerous high plateaus, several of which host significant urban centres, including Quito, Bogotá, La Paz, Sucre, and Mérida. The Altiplano spans parts of Bolivia, Peru, Chile, and Argentina and is notable for its elevation and extensive basins. The range also exhibits distinct climatic divisions, often described as the Tropical Andes, Dry Andes, and Wet Andes, reflecting substantial north–south variation in precipitation and temperature.
In the far south, the range forms the mountainous backbone of Tierra del Fuego. Across the Drake Passage, the mountains of the Antarctic Peninsula can be viewed as a continuation of the Andean system, completing the southern extent of the American Cordillera.

Etymology

The name “Andes” is widely believed to derive from the Quechua word anti, meaning “east,” as in Antisuyu, one of the traditional regions of the Inca Empire. The Spanish word cordillera—meaning “rope” or “cord”—is used to denote the Andean chain and other long mountain systems, emphasising their appearance as continuous, winding ridges.

Geological Structure and Tectonic Setting

The Andes constitute a major orogenic belt associated with the Pacific Ring of Fire. Their formation results from the long-term subduction of oceanic lithosphere beneath the South American Plate, particularly the Nazca Plate and, farther south, the Antarctic Plate. This convergent boundary has been active since the Mesozoic era, with significant uplift occurring during the Cretaceous and Tertiary periods.
As the western margin of South America rose, it generated a pronounced rain shadow, particularly in northern Chile. Reduced sediment flux into the subduction zone affected frictional conditions along the plate interface, influencing both uplift and seismic behaviour. To the east, the range is bordered by major sedimentary basins including the Amazon, Orinoco, and Gran Chaco basins, which separate the Andes from the older cratonic shields of Brazil and Guiana.
The Andean chain also displays several pronounced bends or oroclines, of which the most prominent is the Bolivian Orocline near 18°S. Here the coastline and the orientation of the mountain range shift markedly, coinciding with areas of extensive crustal shortening and the broadest expanse of the Altiplano. Southward, subtler bends such as the Maipo and Patagonian oroclines reflect variations in tectonic forces and lithospheric properties.

Orogenic Evolution

The Andean orogeny is the latest event in a long history of tectonism along the western edge of Gondwana. Prior to Andean uplift, older orogenic systems shaped the region as terranes and microcontinents accreted onto the continent during the Proterozoic and Paleozoic eras.
The breakup of Pangaea during the Triassic and Jurassic initiated rifting and further deformation. Major uplift in the Cretaceous established the initial configuration of the modern range. Since then, different segments of the Andes have displayed contrasting patterns of uplift, deformation, and erosion. In Patagonia, for example, the transition from an extensional back-arc basin to a contractional foreland basin marks a profound tectonic reorganisation brought about by continued subduction.
Across the Amazonian margin, remnants of older orogenies were overridden and in some cases thrust beneath rising Andean structures. Regions such as the Sierras de Córdoba illustrate how pre-Andean geological frameworks were reactivated and uplifted during the Andean orogeny.

Seismicity and Volcanism

The Andes lie above one of the most active subduction zones in the world, and seismic activity is widespread. The boundary between the Nazca Plate and the South American Plate produces frequent earthquakes, ranging from moderate to extremely powerful events. Notable examples include the 1960 Valdivia earthquake (magnitude 9.5), the most powerful instrumentally recorded earthquake; the 2010 Chile earthquake (magnitude 8.8); and the 2015 Illapel earthquake (magnitude 8.2). Subduction dynamics—including plate angle, convergence rate, buoyancy, and mantle hydration—explain significant variations in seismic behaviour along the margin.
Flat-slab subduction zones, such as those near 33°–35°S, are associated with frequent lower-magnitude events but also pose potential for major earthquakes when accumulated strain is released. In contrast, areas marked by dehydrating oceanic lithosphere may exhibit stronger high-magnitude seismicity.