An Ice-Capped Cone of Fire
Lanín Volcano is a near-perfectly cone-shaped stratovolcano that rises 3,747 meters (12,293 feet) above sea level on the border of Argentina and Chile. It is the easternmost volcano in a 60-kilometer-long chain that also includes Quetrupillán and Villarrica. The volcano's steep upper flanks are covered by glaciers, a permanent feature that has immense potential energy. Geologically, Lanín is a compound stratocone built from layers of erupted material over several hundred thousand years. The oldest rocks of the volcano proper date back at least 200,000 years. Its composition is primarily basaltic and andesitic lavas, with some dacitic flows. This bimodal nature, meaning it erupts both primitive (basaltic) and evolved (dacitic) magmas, is a distinct feature among its neighbors. The entire volcanic structure has an estimated volume of 180 cubic kilometers. Although no eruptions have been recorded in historical times, geological evidence shows activity within the last 10,000 years, including a block-and-ash flow dated to around 2,170 years ago and a lava flow as recent as 1,650 years ago.
Rivers of Rock and Mud
The primary hazard at Lanín stems from the direct interaction of its volcanic heat and its frozen water cap. An eruption, even a small one, can rapidly melt large volumes of snow and glacial ice. This meltwater swiftly mixes with loose volcanic ash, soil, and rock fragments on the volcano's steep slopes, generating fast-moving mudflows called lahars. These debris flows are dense, powerful, and capable of traveling many kilometers down river valleys, burying everything in their path. The geological record around Lanín documents at least eight to ten significant lahar events during the Holocene, the geologic epoch that began about 11,700 years ago. The volcano's structure is divided by geologists into four main units. The two most recent, Lanín units 3 and 4, were formed after the last major glacial period and contain deposits from these post-glacial lahars and pyroclastic flows. The steep morphology of the volcano, combined with the readily available source material from neoglacial moraines, makes downstream alluvial fans to the north and south particularly vulnerable to future flows.