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Hydrothermal sealing and erratic explosive behaviour at Rincón de la Vieja, Turrialba, and Poás volcanoes (Costa Rica)

Michael Heap1, Geoffroy Avard2, Claire Harnett3, Albert Gilg4, Thierry Reuschlé1, and Patrick Baud1

  • Affiliations: 1Université de Strasbourg, CNRS, Institut Terre et Environnement de Strasbourg, UMR 7063, 5 rue Descartes, Strasbourg F-67084, France; 2Observatorio Volcanológico y Sismológico de Costa Rica (OVSICORI), Universidad Nacional, Campus Omar Dengo, Apartado Postal 2386-3000 Heredia, Costa Rica; 3UCD School of Earth Sciences, University College Dublin, Dublin, Ireland; 4Department of Civil, Geo and Environmental Engineering, Technical University of Munich, Arcisstrasse 21, 80333 Munich, Germany

  • Presentation type: Poster

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

  • Poster Board Number: 29

  • Programme No: 3.8.13

  • Theme 3 > Session 8

Abstract

Reductions to the permeability of a volcanic system can increase pore fluid pressure and, in turn, promote volcanic hazards. Hydrothermal alteration, ubiquitous at volcanoes worldwide, is one mechanism thought to reduce permeability and therefore increase volcanic hazard potential. Turrialba, Poás, and Rincón de Vieja, active stratovolcanoes in Costa Rica, are characterised by challenging-to-forecast phreatic and phreatomagmatic eruptions that are thought to be the result of the formation, maturation, and rupture of hydrothermal seals that clog the conduit. To better understand this process, we present here a systematic study in which we assessed the textural, mineralogical, and physical properties of hydrothermal seals from Turrialba, Poás, and Rincon de Vieja volcanoes, ejected as ballistics following recent explosive activity. We first documented the type and intensity of the hydrothermal alteration preserved in the collected ballistics using scanning electron microscopy and X-ray powder diffraction. We then measured their porosity, permeability, specific surface area, and tensile strength. We found that, despite their typically high porosities, the materials forming the hydrothermal seal are characterised by low values of permeability (as low as 10−18 m2). Microstructural observations show that this low-permeability is due to pervasive hydrothermal alteration. Numerical modelling using COMSOL Multiphysics shows that permeability reductions resulting from hydrothermal alteration are capable of generating pore pressures beneath the hydrothermal seal that exceed the tensile strength of the materials forming the seal. Our laboratory data and modelling, therefore, highlight how hydrothermal alteration can create a low-permeability hydrothermal seal that promotes cyclic, but erratic, explosive volcanic behaviour.