In flight fragmentation reduces bomb size range and hazard during explosive volcanic eruptions

Coarse, molten fragments of low-viscosity magma (volcanic bombs) that are ejected during explosive volcanic eruptions represent a source of hazard and a record of past eruptions. After ejection, bombs tend to break up during flight, but how much this affects their dispersal is unclear. Here, we use high speed and high-definition imaging of three recent explosive eruptions to parameterise the in-flight fragmentation of bombs. We estimate that in-flight fragmentation involves 73% of bombs coarser than 0.2 m, with bomb-to-bomb collisions and aerodynamic frictional (drag) forces being the main drivers of in-flight fragmentation, depending on eruption style. Drag force increases with increasing bomb velocity and size, selectively fragmenting the coarsest and fastest bombs, acting as a self limiting factor for the range and energy of falling bombs. These findings pose a quantitative basis for incorporating the in-flight fragmentation processes into the interpretation of volcanic deposits and for modelling hazards from falling bombs.