Abstract

Lascar Volcano in the Central Andes, is an ESE–WNW elongate composite stratocone. Stage I (<43 ka) formed a mafic andesite stratocone and culminated in three coarse-grained pyroclastic flow eruptions. Stage II switched to the west and magmas evolved to silicic andesite and dacite of a lava dome complex and hot rock avalanche apron. Stage II culminated with a major explosive eruption (volume 15 km3) at 26.45 ka producing the Soncor ignimbrite. No caldera formed. Laharic and alluvial fan deposits and a debris avalanche formed from 26.45 to 19.2 ka, indicating a humid climatic period. The dacitic and andesitic Western Stratocone (Stage III) developed over the Soncor vent. Stage III culminated in the 9.2 ka Tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater. Stage IV activity switched to the east forming an andesite lava, three nested summit craters, which repeatedly filled with lava and collapsed. Absence of significant erosion since 9.2 ka is due to the much drier Holocene climate. Activity since 1984 displays cycles of lava dome formation in the summit crater, lava dome subsidence with crater collapse, vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19/20 April 1993. The total volume of magma erupted from Lascar over a 43 ka period is estimated at 30 to 40 km3.

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