A reinterpretation of pyroclastic density current deposits at Copahue volcano, Andean Southern Volcanic Zone, Argentina-Chile

摘要

Pyroclastic density currents (PDCs) are one of the most dangerous volcanic phenomena. The correct interpretation and mapping of PDC deposits in the volcano record is important to establish the eruptive style and play a fundamental role in hazard assessment and risk management. The Copahue volcano is an active intermediate volcano of the Southern Volcanic Zone of the Andes (Argentina-Chile) that presents fragmentary evidence of explosive activity during its evolution, with unusual minor PDC deposits. The recorded historic eruptions were mainly phreatomagmatic due to the presence of a crater lake. In this study, four key deposits previously interpreted as PDC products corresponding to different stages of the Copahue volcano evolution (Pleistocene, Holocene, and Historic times) are analyzed and this origin is discussed: (i) A Pleistocene reddish succession located in the northeast flank of the volcano formed by stretched bombs in a coherent lava is interpreted as clastogenic lavas; (ii) a series of proximal bedded volcaniclastics deposits of Pleistocene age are interpreted as redeposition of hyaloclastic fragments from syn-eruptive subglacial meltwater flows associated with subglacial eruptions; (iii) a distal Holocene deposit located similar to 12 km east of the active crater consist mainly in fine-sized clasts forming aggregates is reinterpreted as a sedimentary (lacustrine) deposit with volcaniclastic input; and, (iv) a historic whitish-grey clastic deposit located on the eastern flank is considered a product of a mixed avalanche generated during the 1992-1995 activity. Consequently, the PDC occurrence during the Copahue volcano evolution is less than previously thought. Large PDCs are unlikely in the future and their influence area would be reduced near the active crater as observed in recent eruptions. Flows triggered by the melting of snow/ice during volcanic activity and sudden drainage of the crater lake appear to be a more likely potential hazard that should be considered during risk assessment.