Size matters: a new view of the relationship between shape and size for molten volcanic ballistics

Abstract

Volcanic ballistic projectiles (VBPs) are a common hazard near volcanic vents and often threaten volcano tourists, especially at accessible volcanoes with Strombolian eruption style. Current ballistic hazard models used to estimate potential VBP impact zones often assume round and solid (fixed shape) projectiles, though the validity of these assumptions remains uncertain. In this study, we use high-speed video observations to examine the shape and size distribution of molten VBPs, termed “bombs” from Strombolian eruptions at Stromboli (in 2014 and 2017) and Etna (in 2014) volcanoes (Italy). We provide a framework for describing in-flight bomb shapes, defining three shape classes (rounded, elongate, and bilobate) and subclass end members. The ratio of rounded to total VBPs decreases with size; most bombs (71% of the total catalogued) are smaller than 0.16 m and tend to be rounded (53% at all sizes and 62% for < 0.16 m); however, 80% of the bombs larger than 0.32 m tend to be elongate or bilobate. This trend is generally consistent across Strombolian eruptions in both this study and previous studies. However, video datasets of bomb-bearing eruptions at Batu Tara (Indonesia, 2014, Strombolian to Vulcanian) and Cumbre Vieja (Canary Islands, 2021, strong Strombolian and high fire-fountaining) show fewer elongate and bilobate bombs, especially at larger sizes. This size-shape relationship presented here has not been previously accounted for in ballistic models but has the potential to provide a size-based drag coefficient within ballistic models, leading to more accurate modelling results.

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