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Variability of ash deposits at Piton de la Fournaise (La Reunion Island): insights into fragmentation processes at basaltic shield volcanoes

Abstract : 25 It is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic 26 shield volcanoes. At Piton de La Fournaise (La Réunion Island, France), eruptive activity occurs mostly within the 27 non-populated Enclos Fouqué caldera and generally does not pose any risk to the population. However, historical 28 observations, recent monitoring data and field work on tephra deposits suggest that some eruptions have produced 29 unusual and unexpected explosive phases. A comprehensive sampling of tephra from major historical and recent 30 eruptions on this volcano allowed us to perform systematic componentry, grain size, chemical and morphological 31 analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions. This 32 integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation 33 efficiencies and multiple associated mechanisms. Primary ductile and partially brittle fragmentation of various 34 intensity of juvenile magma emitted during Hawaiian fountaining or mild Strombolian explosions were identified 35 as the most common fragmentation mechanisms, in particular during the 2014-2018 dominantly effusive eruptive 36 sequence. In parallel, we distinguished more efficient short-lived fragmentation events related to (i) plug 37 pressurization and brittle fragmentation enhanced by syn-eruptive crystallization, (ii) magma/water interactions, 38 (iii) rare phreatic explosions and (iv) secondary fragmentation producing fine ash during caldera collapse. We 39 conclude that textural, geochemical and morphological analysis make it possible to identify and characterize the 40 variability in eruptive processes, with the grain size and the componentry of the ash particles being probably the 41 most important parameters to quantify both the efficiency and the nature of the fragmentation. 42 43
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Simon Thivet, Lucia Gurioli, Andrea Muro, Julia Eychenne, Pascale Besson, et al.. Variability of ash deposits at Piton de la Fournaise (La Reunion Island): insights into fragmentation processes at basaltic shield volcanoes. Bulletin of Volcanology, Springer Verlag, 2020, 82 (63), ⟨10.1007/s00445-020-01398-0⟩. ⟨hal-02930441⟩



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