EXPLOSIVE VOLCANIC-ERUPTIONS .7. THE RANGES OF PYROCLASTS EJECTED IN TRANSIENT VOLCANIC EXPLOSIONS

An earlier treatment of the motions of pyroclasts produced in discrete volcanic explosions assumed that clasts were ejected into a stationary atmosphere. We argue here that the air in the vicinity of an explosion site will itself be moving radially away from the source with a speed comparable to that of the clasts, especially at early times after the start of the motion. As a result, the initial relative velocity of clasts and air will be small. In the earlier treatment this relative velocity was inevitably large, leading to an overestimate of the atmospheric drag force at early times and a consequent underestimate of the final clast range. We set up a simple model of the explosion process which allows us to link the initial clast and air velocities and to find an approximation to the subsequent air flow field. Integration of the equations of motion for clasts with a wide range of sizes, densities, initial speeds and initial elevation angles allows the ranges of these clasts to be found. Clast range data from three documented explosive eruptions are re-analysed to yield initial velocities, and the implications of these velocities for pre-explosion pressure conditions are discussed.