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Insights into fragmentation and densification processes from a fossilised shallow conduit on the flank of the Nevados de Chillán Volcanic Complex

Flavia Rojas1,2, John Browning1,2, Hugh Tuffen3, José Cembrano1,2, Javier Espinosa-Leal1,2, Holly E. Unwin4, Thomas M. Mitchell5, Karin Hofer-Apostolidis1,2, Philip G. Meredith5

  • Affiliations: 1Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile; 2Centro de Excelencia en Geotermia de los Andes (CEGA), Chile; 3Lancaster Environment Centre, Lancaster University, Lancaster, UK; 4British Geological Survey, Keyworth, Nottingham, UK; 5Department of Earth Sciences, University College London, London, UK. 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 224

  • Programme No: 3.6.28

  • Theme 3 > Session 6

Abstract

Eruptive style transitions are common in silicic volcanoes and an improved understanding of transitional controls is necessary for hazard forecasting. Examples of hybrid eruptions where explosive and effusive behaviours occur simultaneously have led to a re-examination of models used to understand these complex processes. We present a conceptual model of the evolution of a narrow (2.5 m wide) conduit located on the SW flank of the Nevados de Chillán Volcanic Complex, Chile. This conduit records evidence of fragmentation and densification processes through intercalated and juxtaposed banded, porous and dense domains. We combined qualitative textural analyses at different scales, vesicle size and shape, connected porosity using helium pycnometer and total water content using Fourier transform infrared spectroscopy. The results allow us to identify five principal phases of the conduit evolution: (I) an explosive phase where the conduit is filled with pyroclastic material, (II) a cyclic process of fragmentation and densification within the conduit that generates intercalation of the porous and dense domains, and leads to a hybrid explosive-effusive phase, (III) the formation of a dense magma plug that eventually seals the conduit and deforms vesicles and bands, (IV) the compaction of the pyroclastic domain due to the ascent of the plug, driving porosity reduction (to as little as 3% in the densest bands), with micro-folds and glassy fiamme, and (V) a final phase of post-sintering vesicle relaxation, yielding mainly rounded shapes. We compare our results with other exposed conduits to propose a model of conduit evolution during small-volume, short-lived silicic eruptions.