Antuco, Stratovolcano in Bio Bio Region, Chile

Antuco is a mixed stratovolcano of basaltic and andesitic-basaltic composition in Laguna del Laja National Park, on the southwest shore of Laja Lagoon. It consists of two units: the older Laja cone, truncated by a 4 km-wide horseshoe-shaped amphitheater open to the west, and the younger Antuco 2 cone built inside it. A small pyroclastic cone called Sombrerito sits in the main crater.

Antuco's activity began about 130,000 years ago in the Upper Pleistocene. Its first building, Antuco 1 or volcán Laja, collapsed around 9,700 years ago in a violent phreatomagmatic eruption that left a 4 km horseshoe amphitheatre open to the west, where the present cone, Antuco 2, later grew. Historical records list at least seventeen eruptions between 1739 and 1911, including the cold lahar of 1 February 1820 and the 1853 fissure eruption that raised Laguna del Laja by twenty metres.

The name Antuco comes from Mapudungun: antu (sun) and ko (water), "water of the sun". The volcano stood beside the Antuco pass, used to move goods to and from the Argentine pampas; Luis de la Cruz crossed it from Concepción to Buenos Aires in 1806. The first ascent, in 1829 by Eduard Poeppig and an unnamed Chilean muleteer, is considered the start of mountaineering in Chile.

The volcano lies in Laguna del Laja National Park, 90 km east of Los Ángeles, on the southwest shore of the Laja lagoon; the Antuco ski centre sits on its slopes. The international road to Pichachén crosses the 1752 lava field on the north flank. Access is from the park side, and the summit crater holds the small Sombrerito cone.

The crater's current inner cone is called Sombrerito and emits only weak fumarolic activity. The 1820 eruption produced a cold lahar that ran down the Laja valley, and in 1853 a fissure eruption dammed the Laja lagoon's outlet, raising its level by twenty metres.