Direct Observation of Spallation Bombs During The 2021 La Palma Eruption, Canary Islands, Spain

Expulsion of incandescent pyroclasts of >64 mm from active volcanoes are known as ‘volcanic bombs’1-3, which can pose significant hazards to life and manmade structures. Most volcanic bombs are considered to fall ballistically, enabling the prediction of a volcanic hazard radius4,5. Here we describe a bomb type that was ejected ballistically, but subsequently travelled downslope a steep volcanic flank, posing an important but hitherto unrecognized impact and fire hazard. The volcanic bombs were observed to fall from the summit of the newly formed La Palma cone in late October 2021 and fell on a soft tephra substrate, travelling downhill for over a kilometer in some instances. The bombs spalled incandescent lava fragments en route, creating a significant fire hazard far beyond the range of ballistic transport. Spallation bomb hazards ought to be considered in volcanic risk assessment, necessitating an increase of hazard radius exceeding a factor of two on steep-sided volcanoes with ballistic bomb activity.