In Brief

Volcanic bouncing bombs

An exploration of a previously undescribed volcanic hazard, 'bouncing spallation bombs', observed during the 2021 La Palma eruption in the Canary Islands, Spain

Words by Sade Agard
30 November 2023

The 2021 eruption at La Palma, the Canary Islands, Spain. Ballistic bombs bounced down the volcanic cone travelling large distances and presenting a previously underappreciated volcanic hazard

The growing human presence near active volcanoes has raised concerns about volcanic hazards. Currently, more than one billion people live within a 100 km radius of active volcanoes, and eruptions over the past decade have led to the loss of more than a thousand lives.

The La Palma eruption in the Canary Islands, Spain, which began on 19 September 2021 and lasted until 13 December that year, stands as the largest and most extended in the island’s recorded history. James Day at the University of California, San Diego, USA, and colleagues, use photographic and video observations made during this eruption to explore a previously undescribed volcanic hazard that they term ‘bouncing spallation bombs’. These projectiles comprise pyroclasts or incandescent lava fragments that exceed 64 mm in size.

Conventionally, volcanic hazard assessments focus on the fall of such projectiles, using the bomb location and associated impact crater to estimate the intensity of the hazard. However, the 2021 La Palma eruption unveils a unique behaviour exhibited by the ejected bombs. These projectiles not only traverse the air but also roll and bounce upon the steep, tephra-dominated volcanic cone, enabling them to travel distances of more than 1 km from their initial impact sites, effectively increasing their total travel distance by up to 100%. The bombs create numerous impact craters and, by shedding incandescent fragments as they go, can present fire hazards to nearby trees and structures far beyond the range of ballistic transport.

While the official exclusion zone encompasses this hazard at La Palma, the team underscore the critical importance of considering bouncing spallation bombs in risk assessments in other areas – particularly steep-sided volcanoes.

Sade Agard