During the first several minutes of an explosive volcanic eruption, the flow in the column above the vent is unsteady even if the discharge from the vent is steady. At a fixed location in the column, parameters such as temperature and ash content change with time until steady flow conditions are established. 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 (low elevation inward flow); (2) ash entrained from the top of the pyroclastic flow by atmospheric inflow back toward the column (higher-elevation inward flow); 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). These temporal variations (if observed, for example, by satellite measurements of temperature pulses or pusles or inferred from field observations of deposit stratigraphy) could be misinterpreted as indicating source variations such as source mass flux, volatile content, or vent diameter, when, in fact, they result from fluid mechanical process. ¿American Geophysical Union 1991