Characterization of the ejected material during the 2012-2014 eruptive cycle of the Copahue Volcano.

SOMMER, CARLOS A.; CASELLI, ALBERTO T.; AUGUSTO, MARIANO; DAGA, ROMINA; ALBITE, JUAN MANUEL; ARCE, MARTÍN; VILLA, STEFANIA; BÁEZ, ALEJANDRO D.

Sao Paulo

Simposio; Simpósio de Vulcanismo e Ambientes Associados; 2015

Sociedade Brasileira de Geologia - USP

Copahue volcano (CV) (37.8°S, 71.2°W, 2997 m.a.s.l.) is a basaltic-andesitic stratovolcano located in the SouthVolcanic Zone (33° - 46° S) of Andes Cordillera. The active crater currently hosts a hot and hyperacidic craterlake (30-40°C, pH~1) with a diameter of approximately 250 m and 40 m depth. CV has a record of historicaleruptions, the most recent ones occurred in 1960, 1992, 1994, 1995, 2000 and 2012-2014.This contribution describes the main characteristics of the eruptive activity in the period 2012-2014 basedon field observations and petrographic characterization of the erupted material. The 2012-2014 eruption isdivided into two pulses occurred during July and December, 2012, followed by low-intensity magmaticphreatomagmaticactivity during 2013 and 2014, ending with a final event in October, 2014. The first 2012pulse generated a phreatomagmatic eruption with emission of pyroclastic material through a small plumewhich was dispersed 18 km in E-SE direction. The erupted material mainly consisted of elongated greenyellowishsulfur particles, highly vesicular planar vitreous shards accompanied by low-vesicularity scoriaceousfragments and white argillaceous fragments. The second 2012 pulse consisted of a phreaticphreatomagmaticeruption which generated pyroclastic fall material and ballistic projection of incandescentjuvenile material. The fall material was mainly lapilli composed of low-vesicularity dark grey scoria and darkbrown vitreous shards with vesicular textures along with white argillaceous fragments in lesser proportion.Dark brown highly-vesicular pumice fragments were ejected up to 14 km by the plume. They presentporphyritic texture with euhedral phenocrystals and crystal fragments representing a modal percentage of1% (90% oligoclase-andesine, 10% clinopiroxene). The ejected juvenile material consisted of elongated blackvolcanic bombs, with sub-rounded and elongated vesicles. They present a porphyritic to glomeroporphyritictexture composed of plagioclase phenocrystals (85% modal), clinopiroxene phenocrystals (10% modal) andolivine phenocrystals (5% modal). Matrix presents hyalopilitic texture composed of glass and microlithicplagioclase, clinopiroxene and olivine. Geochemical data indicates an andesitic-basaltic composition forpumice fragments and andesitic-basaltic to trachyandesitic-basaltic composition for bombs. In October 2014,occurred the last and more important event of the cycle, which emitted volcanic ashes and vesiculatedspatters linked to the main eruption. Similarly to material from previous eruption, geochemical data from2014 scoriaceous fragments indicates an andesitic-basaltic composition. The pyroclasts ejected throughoutthe 2012-2014 CV eruptive cycle does not evidence marked compositional changes among them, as wellas they does not show compositional differences respect to material from 2000 eruption and post-glacial lavalithological unit. This eruptive cycle has not ended yet and, based on seismological data, it is concluded thatthe system is still unstable.