The Andes are the longest continuous mountain chain in the world. The present mountain formation started to build up early in the Mesozoic Era, when a subduction zone came to lie beneath the margin of South America. Enormous volumes of igneous rock have since risen from the subducted oceanic plate and been added to the Andean crust, thickening it in some places to more than 70 kilometers. We know that for about the last 200 million years the Andean crust has been thickened by the addition of igneous material below it. Throughout that period, it has also been bobbing up isostatically. At the same time, volcanic rocks have been piled on top.

Probably because today the subducted plate descends at a low angle, the zone of plutonism and volcanism is now centered about 200 kilometers inland from the coast. Earlier, the subducted plate probably descended at a steeper angle, thus reaching the depth of partial melting nearer the coast. The Andes are a highly active volcanic zone with hundreds of volcanoes. Chile contains the highest land volcanoes with the Llullaillaco (6723 m) and Ojos del Salado (6880 m). In addition, a high elevated plateau, the Altiplano, has been built up where the Andes reach the largest width (about 800 km). Towards the southern end of South America, the elevation of the Andes diminishes drastically, but the region is extensively glaciated due to high moisture availability.

1 - Cross-section from the coast to the high-elevated altiplano with abundant active volcanoes.