Estimating air ingestion by a dilute pyroclastic density current using the temperature of emplacement of the 40 ka VEI7 Campanian Ignimbrite (Italy)

Abstract The Campanian Ignimbrite was emplaced from pyroclastic density currents that were expanded from the vent to the distal extents (about 75 km runout) in the Apennine Mountains. Thermal remanence analyses of oriented lithic clasts from proximal to distal sites show single components up to > 580 °C, with the characteristic remanent magnetization matching that of the ignimbrite itself. This indicates emplacement temperatures above 580 °C. Incipient welding in proximal deposits and none elsewhere implies emplacement at and below the glass transition temperature (about 690 °C) throughout. Using a magmatic temperature (two-feldspar thermometer) value of 900 °C, we calculate a small heat loss from adiabatic decompression of the available magmatic gas, which leaves the need for another source of heat loss. Significant incorporation of accidental lithic clasts or liquid water is not supported by field evidence. To cool the pyroclastic density current system to the emplacement temperature, 9–40 times more air mass than was provided by the magmatic gas must be incorporated. Using this information, as well as the current’s runout distance, ability to overtop topographic obstacles, and eventual conversion to a co-ignimbrite column, we find that the current becomes thicker (taller) with distance, from 500 m near the caldera to more than 800 m at the first encounter with the Apennine Mountains 40 km from the vent. As the flow thickened, it decelerated from the supercritical (Richardson number Ri < 1) to the subcritical (Ri > 1) regime between 25 and 37 km from the vent, 4–6 min after eruption. Air entrainment probably became less efficient as this regime transition occurred. The data and model presented here provide explanations for how a large dilute pyroclastic density current can erupt and flow long distances while maintaining enough energy and thickness to overtop topographic obstacles.