Estimate of pyroclastic flow velocities resulting from explosive decompression of lava domes

Abstract

APPARENTLY benign silicic domes or lava flows can travel for several kilometres and then suddenly collapse to generate pyroclastic phenomena capable of causing widespread destruction, as happened recently at Mount Unzen in Japan¹. Two sources have been proposed for the energy that propels such 'Peléan' or 'Merapi'-type² pyroclastic flows: gravitational col-lapse (supplemented by heating and expansion of air) and sudden 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 will bear a simple relationship to the part of the lava flow from which they issued. But if gas pressure adds a significant contribution, hazard assessment becomes more difficult because gas decompression adds velocities beyond those acquired by gravitational forces, putting much larger areas at risk and forming pyroclastic deposits that are much more difficult to relate to their source. Here we estimate the initial velocities of pyroclastic flows generated by dome disintegration for a range of lava compositions and volatile contents, and offer a conceptual framework for correlating the dynamics of dome-front collapse with the resulting sediment record. Our results indicate that explosive decompression at distal portions of domes can cause velocities comparable to gravitational collapse, especially in cases where volatiles become locally concentrated above equilibrium values.