Abstract

The predominantly constructive life cycles of large, long-lived, stratovolcanoes and basaltic shields are punctuated by transitory episodes during which large volumes of material are divested from the flanks. Such shedding typically takes place catastrophically in the form of a lateral collapse, generating a debris avalanche and leaving a scar that may attain caldera dimensions. Collapse may follow instability development arising from a single, discrete, event, such as a crypto-dome intrusion, or may be the end-product of progressive destabilization over a long period of time. Lateral collapses may also occur at persistent slumps, which may have been active over periods as long as 10⁴–10⁵ a prior to catastrophic failure. Collapse velocities may exceed 40 m s⁻¹, leading to completion of the process within a few hundreds of seconds. Collapse volumes span several orders of magnitude, ranging from less than 1 km³ to more than 10 km³ at many continental and subduction-zone volcanoes, to 1000 km³ or more at the great basalt shields of Hawaii. Lateral collapse may be accompanied by a wide range of associated hazards, including atmospheric shock wave, pyroclastic flows and surges, extensive tephra fall, and secondary lahars. For volcanoes in the marine environment, potentially destructive and lethal tsunamis can be added to the inventory. Here, the potential for a lateral collapse at an ocean island volcano to generate a ‘mega-tsunami’ (more than 100 m high at source and destructive at oceanic distances) is discussed and evaluated.