Effects of external water on volcanic column height and collapse

Volcanic plumes are important because they spread volcanic material, can impact climate, and pose hazards to aviation. Among eruption processes, column collapse is arguably the most consequential in terms of direct impacts, as it marks the onset of ground-hugging pyroclastic density currents that pose the greatest immediate threat to life and infrastructure. This study inves- tigates how external water, such as is incorporated during subaqueous eruptions, affects the critical condition (including mass eruption rate, temperature, and gas fraction) at which column collapse occurs in the atmosphere, referred to as “the column collapse condition”. We use the 1-D plume model, Plumeria, to explore how variations in external water (0–60 wt.%), vent exit velocity (75–125 m s⁻¹), and initial magma temperature (700–1100 °C) affect the column collapse condition. We find that the occurrence of column collapse is highly sensitive to the amount of external water. Small amounts of external water (⪅ 25 wt.%) suppress column collapse, whereas higher amounts of water encourage collapse. Using more than 150,000 simulations, we map out the highly non-linear shape of the column collapse condition as a function of mass eruption rates and external water contents. The Richardson number, the ratio of buoyancy to shear forces, offers a useful framework for defining the column collapse condition for eruptions involving external water. This work enhances our understanding of how external water ingestion affects volcanic plume dynamics, including height and column collapse conditions. Recent shallow submarine eruptions that produced very tall atmospheric plumes and partial column collapse (e.g. the 2021 Fukutoku-Oka-no-Ba eruption in Japan and the 2022 Hunga eruption in Tonga) demonstrate the importance of understanding how external water influences the collapse of eruption columns.