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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

AYALEW, D., DELL'ERBA, F., DI VITO, M.A., ORSI, G., YIRGU, G., DINGWELL, D.B., KUEPPERS, U., MANGIACAPRA, A., POTUZAK, M., RICHARD, D. (2006) - The Da'ure' eruption at the Boyna Volcanic Complex during the September 2005 Afar extension episode: volcanology. AGU Fall Metting, San Francisco, USA, 11 - 15 de Dezembro (Comunicação Oral).


The September 2005 Afar extension episode generated many fractures in the Da'ure' area. On September 26, a volcanic eruption began along one of these fractures and, according to local accounts, lasted 3 days. The fracture, trending around N10W, was 400 m long. A detailed geological survey of the area, carried out during the following month of October, permitted to reconstruct the evolution of the Da'urè eruption, including the geometry of the vent. The stratigraphic sequences of natural and digged sections were described and measured, and samples were collected for sedimentological and petrological analyses and experiments. The deposits are successions of pyroclastic beds and one lava dome. The pyroclastic successions are alternations of fallout layers composed prevailingly of fine- to coarse-ash, and subordinately of cm- to dm-sized fragments. Few fine-ash pyroclastic-current beds occur only in very proximal sections. The coarse juvenile fragments are mainly pumice clasts at variable degrees of vesiculation, and dense glass. Lithic clasts are dominantly rhyolitic lavas, similar to the surrounding exposed rocks. A lava dome overlies the pyroclastic sequence in the central part of the fissure. Pyroclastic deposits with a thickness in excess of 1 mm were dispersed over an area of about 2 square chilometers. Their dispersal area is rather irregular due to the overlapping of fallout and rare pyroclastic-current deposits produced by different vents. Lithic and juvenile clasts larger than 30 cm, impacted onto the ground after following ballistic trajectories, can be found over an area within about 250 m from the vent. The volume of the pyroclastic deposits, calculated using a GIS assisted method, is about 400,000 cubic meters, while the volume of erupted magma has been evaluated to about 200,000 cubic meters. The erupted rocks are peralkaline rhyolites, extremely enriched in volatile elements. Petrological, geochemical and isotopical data indicate that the rhyolite derives from the middle alkaline Boyna basalts by fractional crystallization. Sedimentological and compositional characteristics of the deposits, geometry of the fissure vent, and volume of emitted magma suggest that the Da'ure' volcanic eruption was fed by a small batch of peralkaline magma, located at shallow depth and decompressed by activation of faults and fractures formed in relation to the Afar extension episode. The volcanic activity migrated along the fracture from North to South with formation of at least 8 vents of diameters smaller than 100 m. The eruption was dominated by explosive activity with only one effusive phase. The explosive phases were generated by very efficient fragmentation dynamics which produced pyroclastic deposits containing a high amount of fine ash.