Textural evolution ofmagma during the 9.4-ka trachytic explosive eruption at Kilian Volcano, Chaîne des Puys, France
Résumé
Textural parameters such as density, porosity, pore
connectivity, permeability, and vesicle size distributions of
vesiculated and dense pyroclasts from the 9.4-ka eruption of
KilianVolcano, were quantified to constrain conduit and eruptive
processes. The eruption generated a sequence of five vertical
explosions of decreasing intensity, producing pyroclastic
density currents and tephra fallout. The initial and final phases
of the eruption correspond to the fragmentation of a degassed
plug, 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 intermediate
eruptive phases were the results ofmore open-conduit
conditions (unimodal density distributions, decreases in dense
juvenile pyroclasts, and non-juvenile clasts). Vesicles within
the pyroclasts are almost fully connected; however, there are a
wide range of permeabilities, especially for the dense juvenile
clasts. Textural analysis of the juvenile clasts reveals two vesiculation
events: (1) an early nucleation event at low
decompression rates during slow magma ascent producing a
population of large bubbles (>1 mm) and (2) a syn-explosive
nucleation event, followed by growth and coalescence of
small bubbles controlled by high decompression rates immediately
prior to or during explosive fragmentation. The similarities
in pyroclast textures between the Kilian explosions and
those at Soufrière Hills Volcano on Montserrat, in 1997, imply
that eruptive processes in the two systems were rather similar
and probably common to vulcanian eruptions in general.