Thermal transient PDC behavior induced by topographic drops: A test case at Mt. St. Helens, USA

This study investigates the influence of topography on the internal structure and dynamics of pyroclastic density currents (PDCs), using the 1980 Mt. St. Helens eruption as a case study. By integrating paleomagnetic data and numerical models, we observe significant temperature differences (approximately 100°C) in PDC deposits on different flanks of the volcano. These variations are attributed to local topographic features affecting air entrainment in PDCs. We find that topographic drops induce a transient regime in PDCs, altering their internal temperature, velocity, and concentration. The duration of the transient phase is proportional to the ratio between the drop height and the square root of the current thickness. The influence of topography on PDC dynamics decreases with distance from these drops or when a stationary phase is reached. Taken together, our dataset allows us to estimate a local sedimentation rate of approximately 150 ± 100 mm/s for PDC deposits in a proximal reattachment region. This research emphasizes the importance of transient dynamics in understanding PDC behavior, introduces a new method for measuring sedimentation rates, and highlights the need to consider topographic effects in hazard assessments.