On the geophysical fingerprint of Vulcanian explosions

Extrusion of viscous magma and the subsequent formation of a lava dome is often interspersed by short-lived vigorous (Vulcanian) explosions. The causes for and the timing of the transition from effusive to explosive activity during dome formation are poorly understood and forecasting this transition remains a challenge. Here, we describe and interpret a robust and unique multi-parameter data set documenting the subsurface processes associated with Vulcanian explosions at Soufrière Hills Volcano, Montserrat (W.I.) in July and December 2008. We quantify explosion priming by processes in either the shallow (< 2 km depth) or the deep magmatic system and quantify syn-eruptive processes. The July 29 explosion has a signature related exclusively to shallow dynamics including conduit destabilisation, syn-eruptive decompression and magma fragmentation, conduit emptying and expulsion of juvenile pumice. In contrast, the December 3 explosion was triggered by unprecedented sudden pressurisation of the entire plumbing system from depths of about 10 km (including the magma chambers) resulting in surficial dome carapace failure, a violent cannon-like explosion, propagation of pressure waves and pronounced ballistic ejection of dome fragments. With timescales for explosion priming on the order of a few minutes, the precursory geophysical signatures are indicative of the nature of ensuing Vulcanian explosions. The short precursory phases characterise Vulcanian explosions as freak events triggered by abrupt rather than gradual changes in subsurface dynamics. Our findings provide important constraints for theoretical and experimental investigations of the effusive to explosive transition, forecasting of Vulcanian explosions and volcanic risk mitigation.