ESTIMATE OF PYROCLASTIC FLOW VELOCITIES RESULTING FROM EXPLOSIVE DECOMPRESSION OF LAVA DOMES

APPARENTLY benign silicic domes or lava flows can travel for several k ilometres and then suddenly collapse to generate pyroclastic phenomena capable of causing widespread destruction, as happened recently at Mo unt Unzen in Japan1. Two sources have been proposed for the energy tha t propels such 'Pelean' or 'Merapi'-type2 pyroclastic flows: gravitati onal collapse (supplemented by heating and expansion of air) and sudde n expansion of pressurized gases from inside the lava flow. If gravity controls the energy transfer, then areas likely to be affected can be predicted on the basis of topography3, and the resulting deposits wil l bear a simple relationship to the part of the lava flow from which t hey issued. But if gas pressure adds a significant contribution, hazar d assessment becomes more difficult because gas decompression adds vel ocities beyond those acquired by gravitational forces, putting much la rger areas at risk and forming pyroclastic deposits that are much more difficult to relate to their source. Here we estimate the initial vel ocities of pyroclastic flows generated by dome disintegration for a ra nge of lava compositions and volatile contents, and offer a conceptual framework for correlating the dynamics of dome-front collapse with th e resulting sediment record. Our results indicate that explosive decom pression at distal portions of domes can cause velocities comparable t o gravitational collapse, especially in cases where volatiles become l ocally concentrated above equilibrium values.