c. 10 Million BCE

The Andes

One of the longest continuous tectonic plate-collision zones under continental crust occurs along the western edge of the South American plate. For more than 90 million years, new oceanic crust being created at submerged spreading centers has been riding on a conveyor belt and crashing into South America. The collision has gone through a number of distinct episodes and orientations. Starting around 10 million years ago, a piece of the former Farallon plate called the Nazca plate began to collide into western South America, forming the high peaks and volcanoes of the modern Andes Mountains.

Just like along the Cascade Volcanoes stretch of the subduction zone of former Farallon plate fragments in North America, subduction of the Nazca plate under South America has led to extensive mountain-building and explosive volcanism. The Andes volcanic zone extends along a belt running through Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina, from north to south; the zone arguably even includes volcanoes in Antarctica, which was once attached to South America. Hundreds of active and extinct volcanoes occur along the belt.

The Andes also have a special place in the hearts of petrologists, geologists who study the composition, mineralogy, and formation conditions of rocks. Because of the long history of oceanic plate subduction along the western South American plate boundary, the continental crust has thickened considerably, and thus magma rising from oceanic slabs through the overlying continental crust melts and assimilates significant amounts of silica and alkaline elements that are more common in continental than in ocean crust. The resulting ash and lava that makes it to the surface has a distinctly different composition than mid-ocean ridge basalts. In honor of the mountains in which they occur, petrologists call this composition andesite.

The active volcanoes of the Andes form the southeastern section of the Pacific Ring of Fire, a hemispheric-scale zone of intense seismic and volcanic activity that includes the Pacific coasts of Central and North America (including the Cascade Volcanoes), up through the Aleutian Islands, and back around through Japan, Southeast Asia, Indonesia, and New Zealand. The Ring of Fire is one of the largest continuous geologic structures on Earth.

SEE ALSO Plate Tectonics (c. 4–3 Billion BCE?), The Sierra Nevada (c. 155 Million BCE), Cascade Volcanoes (c. 30–10 Million BCE), Island Arcs (1949), Seafloor Spreading (1973)

Aerial view of Carbajal Valley in the Southern Andes, Tierra del Fuego, Argentina.