THE magma responsible for explosive volcanic eruptions has both a volatile and an inert:phase. Deep in the conduit of an active volcano, bubbles nucleate as the volatiles exsolve(1-3). As the magma rises, the bubbles grow through depressurization and continued exsolution, It is thought that when the pressure in the bubbles exceeds that in the overlying material, the magma undergoes a rapid transformation from a continuous magmatic phase with bubbles to a continuous gas phase with fragmented pyroclastic material(1,2), The fragmentation process is complex and poorly understood. To understand better how the transport of fragmented material is coupled to exsolution and vaporization, We have performed depressurization experiments on a two-phase system, designed to simulate the eruption process. We identify a new explosive vaporization process, in which a fragmentation front propagates downwards through a mixture of volatile liquid and inert particulate material, suppressing the growth of nucleated bubbles by compressing the material ahead of it. This process is distinct from, and may complement, previously identified fragmentation mechanisms such as non-nucleate vaporization(4) and fragmentation induced by an expanding magmatic foam(5).