Textural evolution ofmagma during the 9.4-ka trachytic explosive eruption at Kilian Volcano, Chaîne des Puys, France

Co-auteur étrangerInternational audienceTextural parameters such as density, porosity, poreconnectivity, permeability, and vesicle size distributions ofvesiculated and dense pyroclasts from the 9.4-ka eruption ofKilianVolcano, were quantified to constrain conduit and eruptiveprocesses. The eruption generated a sequence of five verticalexplosions of decreasing intensity, producing pyroclasticdensity currents and tephra fallout. The initial and final phasesof the eruption correspond to the fragmentation of a degassedplug, as suggested by the increase of dense juvenile clasts(bimodal density distributions) as well as non-juvenile clasts,resulting from the reaming of a crater. In contrast, the intermediateeruptive phases were the results ofmore open-conduitconditions (unimodal density distributions, decreases in densejuvenile pyroclasts, and non-juvenile clasts). Vesicles withinthe pyroclasts are almost fully connected; however, there are awide range of permeabilities, especially for the dense juvenileclasts. Textural analysis of the juvenile clasts reveals two vesiculationevents: (1) an early nucleation event at lowdecompression rates during slow magma ascent producing apopulation of large bubbles (>1 mm) and (2) a syn-explosivenucleation event, followed by growth and coalescence ofsmall bubbles controlled by high decompression rates immediatelyprior to or during explosive fragmentation. The similaritiesin pyroclast textures between the Kilian explosions andthose at Soufrière Hills Volcano on Montserrat, in 1997, implythat eruptive processes in the two systems were rather similarand probably common to vulcanian eruptions in general