A contribution to the hazards assessment at Copahue volcano (Argentina- Chile) by facies analysis of a recent pyroclastic density current deposit

BALBIS, C.; PETRINOVIC, I. A.; GUZMÁN, S.

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016

0377-0273

We recognised and interpreted a recent pyroclastic density current (PDC) deposit at theCopahue volcano (Southern Andes), through a field survey and a sedimentologicalstudy. The relationships between the behaviour of the PDCs, the morphology of the RíoAgrio valley and the eruptive dynamics were interpreted. We identified two lithofaciesin the deposit that indicate variations in the eruptive dynamics: i) the opening of theconduit and the formation of a highly explosive eruption that formed a diluted PDCthrough the immediate collapse of the eruptive column; ii) a continued eruption whichfollowed immediately and records the widening of the conduit, producing a dense PDC.The eruption occurred in 2000 AD, was phreatomagmatic (VEI ≤ 2), with a vesiculationlevel above 4000 m depth and fragmentation driven by the interaction of magma withan hydrothermal system at ca. 1500 m depth. As deduced from the comparison betweenthe accessory lithics of this deposit and those of the 2012 AD eruption, the depth ofonset of vesiculation and fragmentation level in this volcano is constant in depth. Inorder to reproduce the distribution pattern of this PDC?s deposit and to simulatepotential PDC?s forming-processes, we made several computational modelling from?denser? to ?more diluted? conditions. The latter fairly reproduces the distribution ofthe studied deposit and represents perhaps one of the most dangerous possible scenariosof the Copahue volcanic activity. PDCs occurrence has been considered in the lastvolcanic hazards map as a low probability process; evidences found in this contributionsuggest instead to include them as more probable and thus very important for thehazards assessment of the Copahue volcano.