Sources of unsteady column dynamics in pyroclastic flow eruptions

For conditions believed to be typical of fountain-forming silicic eruptions of intermediate volume, numerical simulations show unsteadiness in temperature, plume diameter, mass flux, vertical velocity, and particle concentration. In addition to the steady mass flux from the vent, we have identified three sources of mass and heat flux into the column: (1) recirculation of pyroclastic flow material into the base of the column; (2) ash entrained from the top of the pyroclastic flow by atmospheric inflow back toward the column; and (3) waves reflected within the pyroclastic flow if it encounters topographic obstacles. The thermal and mass flux pulses cause variations in column diameter and fountain height. Changing column diameter would be expected to be manifested in changes in fallout deposits (e.g., by shifts in clast isopleths). -from Authors