Evidences of plug pressurization enhancing magma fragmentation during the September 2016 basaltic eruption at Piton de la Fournaise (La Réunion Island, France)

Co-auteur étrangerInternational audienceIn September 2016, Piton de la Fournaise volcano, well known for its effusive and Hawaiianfountaining activity, produced, at the end of the eruption, an unusual phase of pulsating ash and bombemission. Integration of geophysical data, with textural and petrological analysis of the samples, allowed usto constrain the main factors that controlled this sudden shift in activity, potentially dangerous for the touristpopulation that usually approach these“gentle”eruptive sites. Volcanic tremor, lava discharge rates,fountain heights, and SO2emission changed rapidly during the eruption. Grain size and componentry of thetephra beds evolved from unimodal all along the sequence to bimodal on the last day of the activity,reflecting the contribution of both Hawaiian fountaining at the main vent (Vent A) and transient explosiveactivity at the second vent (Vent B). Hawaiian fountaining produced highly vesicular and almostmicrolite‐free tephra (golden pumice andfluidal scoria) while transient explosive activity emitted denserand crystal‐rich tephra (sideromelane and tachylite scoria) sometimes mingled with vesicular fragments.Permeability measurements on lapilli and bomb‐sized samples reveal that golden pumice andfluidal scoriawere more gas‐permeable than the sideromelane and tachylite ones, while textural and chemical analysesof the ash support the hypothesis that these sideromelane and tachylite components were inherited from thesubsurface crystallization of the initial golden pumice andfluidal scoria components. We thus suggestthat Vent B accumulated a plug of degassed, cooled, and low‐permeable magma, which modulatedoverpressure pulses under the late input of ascending magma