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Kim Deeming

The May 1980 destruction of Mount St Helens represents one of the few well documented examples of a catastrophic lateral collapse event. Since then, both ancient and recent mass wasting events have been recognized at volcanoes world-wide, many on a far greater scale. The Valle del Bove amphitheatre, cut into the eastern flank of Mount Etna, provides evidence of one such example. Previous estimates of the age of this lateral collapse scar have ranged from 5,000 to 80,000 years old, but now, using cosmic ray exposure dating, we have been able to directly constrain the timing of the event.

High energy cosmic rays enter the atmosphere, inducing nuclear reactions in the Earth's surface creating new cosmogenic isotopes, in particular a rare isotope of helium, 3He. This accumulates wherever a rock is exposed, so 3He abundance records the duration of exposure. For the Valle del Bove, the exposure ages of abandoned fluvial features on the outer flanks of the amphitheatre provide an insight into its controversial age. These valleys were fed by ice and snow fields that capped the ancient Trifoglietto and Ellittico centres - the upper flanks of which were removed by the collapse - during glacial times. Dating the cessation of water flow and subsequent abandonment of the palaeovalleys (upon collapse) constrains the timing of this single collapse to 6791 ± 610 years BP.

Notably, this coincides with the end of the period of most rapid post-glacial Holocene sea level rise, when water tables in the edifice are likely to have been substantially elevated. A combination of a water-saturated, incipiently unstable edifice, destabilization due to progressive dyke-induced rifting, and deep stress changes due to sea-water loading at the base of the eastern flank, are implicated in the timing of the Valle del Bove lateral collapse.

The addition of this event to the inventory of Holocene island and coastal volcano collapses also supports a role for dramatic post-glacial environmental change - including rapid sea level rise and climate amelioration - in promoting volcano lateral collapse events.