The symmetrical, glacier-clad Osorno volcano forms a renowned landmark that towers over Todos los Santos and Llanquihué lakes. Osorno was constructed over a roughly 250,000-year-old eroded stratovolcano, La Picada, that has a mostly buried 6-km-wide caldera. La Picada underlies Osorno on the NE and has postglacial maars and scoria cones. The 2652-m-high dominantly basaltic to basaltic-andesite Osorno is one of the most active volcanoes of the southern Chilean Andes. The conical volcano contains two small dacitic lava domes on the NW and SSE flanks. Flank scoria cones and fissure vents, primarily on the west and SW sides, have produced lava flows that reached Lago Llanquihué. Frequent explosive eruptions including pyroclastic flows and surges have occurred during the past 14,000 years. Historical eruptions at Osorno have originated from both summit and flank vents and have produced basaltic and andesitic lava flows that have entered both Llanquihué and Todos los Santos lakes.