The processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology. Observations of explosive eruptions(1-3), as well as theoretical analyses of the underlying processes : have raised the question of whether the rapid decompression of highly viscous, vesicular magma that results from the collapse of a lava dome or volcanic edifice can, in itself, produce explosive magma fragmentation and pyroclast formation. Here we report the results of a laboratory investigation of pyroclast formation following rapid decompression. Samples of magma from the 1980 eruption of Mount St Helens, when rapidly depressurized from initial pressures of up to 12 MPa (and at temperatures in the range 750-825 degrees C), fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions. Moreover, we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes, suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events.