EXPLOSIVE FRAGMENTATION OF ERUPTING MAGMA

THE magma responsible for explosive volcanic eruptions has both a vola tile and an inert:phase. Deep in the conduit of an active volcano, bub bles nucleate as the volatiles exsolve(1-3). As the magma rises, the b ubbles grow through depressurization and continued exsolution, It is t hought that when the pressure in the bubbles exceeds that in the overl ying material, the magma undergoes a rapid transformation from a conti nuous magmatic phase with bubbles to a continuous gas phase with fragm ented pyroclastic material(1,2), The fragmentation process is complex and poorly understood. To understand better how the transport of fragm ented material is coupled to exsolution and vaporization, We have perf ormed depressurization experiments on a two-phase system, designed to simulate the eruption process. We identify a new explosive vaporizatio n process, in which a fragmentation front propagates downwards through a mixture of volatile liquid and inert particulate material, suppress ing the growth of nucleated bubbles by compressing the material ahead of it. This process is distinct from, and may complement, previously i dentified fragmentation mechanisms such as non-nucleate vaporization(4 ) and fragmentation induced by an expanding magmatic foam(5).