Morfologia e química de cinzas do vulcão Puyehue depositadas na região metropolitana de Porto Alegre em junho de 2011

Abstract

The National Geological Survey of Chile reported a volcanic explosion of order 5 in the Los Ríos area (Volcanic Complex in Puyehue - Cordon Caulle) on June, 2011. This complex comprises Pleistocene volcanoes built in six large evolutionary phases and preserves 131 km³ of the vulcanites. The Puyehue stratovolcano (69 ky) had its last records of the explosive and effusive activity in 1921, 1922, and 1960. The explosion on June, 2011 generated a tephra and gas column of 10 km high under high-temperature conditions. The surrounding air was heated and incorporated reducing the density of the the column, which rises into the atmosphere. The pyroclastic material stops in a balance zone between the densities of the medium and head of the plume, and the wind action determines the dispersion of the ashes. On June 9, 10 and 14, 2011, the Fundação Estadual de Proteção Ambiental (FEPAM-RS) collected Puyehue ashes in its filters at the eighth District of the Meteorology (Porto Alegre). The concentrations remained below the Conselho Nacional do Meio Ambiente - CONAMA standards (150 mg/m³) and Guide Value (WHO-VG = 25 mg/m³). The research by using scanning electron microscope (SEM) indicated that the ashes are thin (2 - 30 microns), with elongated angular and placoid habits. The curved ends, corresponding to the boundaries between vesicles and melt, are vitreous (shards), rich in SiO2 and Al2O3, and show low alkali and FeOt contents. The diffractometer obtained in reworked tephra indicates plagioclase and cristobalite presence, and these minerals were also observed in the fall deposits studied in Argentina.