Pre-explosive conduit conditions of the 1997 Vulcanian explosions at Soufrière Hills Volcano, Montserrat: I. Pressure and vesicularity distributions

International audienceAn authoritative case of Vulcanian eruptive dynamics is the series of 88 explosions that occurred between August and October 1997 at Soufrière Hills volcano on Montserrat Island. The state of the magmatic column just before a Vulcanian explosion is still poorly understood, but conditions the eruptive style. This study establishes such a pre-explosive stratigraphy by 1) documenting the textures covering the range of the 1997 products, 2) quantitative analysis of H2O content in interstitial glass measured by Karl–Fischer Titration, and 3) combining these data with a simple model linking pre- and post-explosive vesicularities. The model shows that syn-explosive degassing affects greatly the way porosity evolves by decompression during an explosion. The stratigraphy reconstruction shows a three-part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested. A dense and strongly degassed plug caps the column. It is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 MPa. At higher pressure, up to 80 MPa, lies a more homogeneous zone of relatively dense (10–20 vol.%) magma, which was emplaced under partly open-system degassing. This conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption. Our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse, but rather as stages. The transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the Vulcanian jets at the beginning of the explosions. This contrasts with the nearly constant vesicularities of the deeper part of the pre-explosive magma column, which are propitious to a general, short-lived disruption