Large explosive eruptions may be dominated by pyroclastic flows instead of buoyant plumes: insights from a global data compilation

The dynamics of Earth’s largest, most voluminous eruptions (≥ 100 km3 ejecta; VEI 7) are poorly understood. Here, we explore the question of whether these very large eruptions can be treated as scaled-up versions of moderate volume historical eruptions (0.1–10 km3 ejecta; VEI 4–6), or whether they should be treated as fundamentally different Earth system phenomena. To examine this, we compile fall deposit and ignimbrite volume data for 74 explosive eruptions worldwide that are magnitude 4 or greater, and use this dataset to assess how material is partitioned into buoyant plumes versus pyroclastic density currents as a function of eruption magnitude. Importantly, we filter our results by overall distance from seas/oceans, so that we can focus in on the eruptions for which preservation of the deposits is as reliable as possible. After filtering, we find that the largest eruptions are dominated by ignimbrites and not fall deposits, implying that, co-ignimbrite plumes notwithstanding, there may be little or no buoyant plume component to eruptions of the highest known magnitudes. This result is consistent with model simulations showing that the pyroclastic materials produced during larger events can be emplaced in density currents alone, and highlights important considerations for contemporary eruption simulations, the fate of volcanic gases relative to solid mass, and subsequent appraisals of the climatological and environmental impacts of explosive volcanism on Earth.