Abstract

During the Late Pleistocene-to-Holocene, the mafic Planch & oacute;n volcano (35.2 degrees S, Southern Andes) experienced two important destructive events: a sector collapse to the west and a multiphase explosive eruption transforming the east summit area. We provide new field and laboratory evidence, including geochemical, geochronologic, and geological-morphological analysis, to reconstruct the evolution, triggering mechanisms, and physical parameters of these events. The lateral collapse (48 ka BP) was mainly predisposed by a tectonically westward-inclined substratum and rapid edifice growth rates (0.3-0.48 km3 3 ka-1). - 1 ). The resulting Planch & oacute;n-Teno debris avalanche became valley-confined traveling at c. 260 km h - 1 up to 95 km distance and forming an 8.6 +/- 1.3 km3 3 deposit. The resulting 4.1 km wide amphitheater was later destroyed at c. 7 ka BP by the multiphase Valenzuela phreatomagmatic eruptions, forming a c. 2.5 km diameter caldera. The case of the Planch & oacute;n volcano warns that rapidly growing mafic volcanoes imply a substantial catastrophic hazard increase for the surrounding areas.