TRANSITIONS BETWEEN EXPLOSIVE AND EFFUSIVE ERUPTIONS OF SILICIC MAGMAS

WATER-RICH silicic magmas may erupt explosively, giving rise to massiv e columns of fragmented ash(1-5), or effusively as viscous, bubbly lav as to form lava domes(6-8). Complex cycles of explosive and effusive e ruptions are often observed(8-12). Although these two eruption mechani sms have been modelled independently(1,8), the conditions under which a volcano exhibits a particular style of suption are not known and the transitions between eruption styles are poorly understood. By modelli ng the ascent of magma along a permeable conduit, we show here that, f or small magma-chamber overpressures, the style of eruption-explosive or effusive-is dependent on the magma flow rate. We also identify a cr itical overpressure above which only explosive eruptions occur. Comple x eruption sequences follow naturally. For example, if a shallow magma tic system is supplied with magma at an intermediate flow rate, the er uption will be slow and effusive until the chamber over-pressure becom es too large. An explosive eruption then relieves the overpressure and an effusive eruption style resumes. We suggest that monitoring of tem poral variations in chamber overpressure (for example, by measuring gr ound deformation) can be used to assess whether a passively effusing v olcano has the potential to erupt explosively.