Volcano Instability and Lateral Collapse

Active volcanoes are dynamically evolving structures, the life-cycles of which are punctuated by episodes of flank instability and lateral failure. Such behaviour is now recognised as ubiquitous and lateral collapses are estimated to have occurred at least four times a century over the past 500 years. In the Andes, three quarters of the large volcanic edifices have experienced collapse, while over a hundred debris avalanche deposits around the Japanese Quaternary volcanoes testify to repeated episodes of instability and collapse over the last million years. A range of both internal (endogenetic) and external (exogenetic) factors contribute to the destabilisation of a volcanic edifice. The former include gravitational spreading, the development of steep slopes and a mechanically unsound structure, surface loading by erupted materials, and oversteepening or incremental lateral displacement due to magma intrusion. External factors include basement uplift or subsidence, fault activity, climatic effects, and changing sea levels. The triggering of lateral collapse at a destabilised volcano typically occurs in response to a short-lived dynamic event such as the intrusion of a body of fresh magma (magmagenic), or to volcanic or tectonic earthquakes (seismogenic). In the former case, collapse may be due to either mechanical push or to pore-pressure changes in ground water, while in the latter increased ground accelerations are the probable cause. Volcanic edifices may become unstable and experience structural failure at any scale. This ranges from relatively small rock falls involving a few hundred to a few thousand cubic kilometres of debris, to the giant ocean-island megaslides, such as those identified around the Hawaiian and Canary archipelagoes, that may incorporate up to 5000 cubic kilometres of material. While the lowest volume collapses probably occur at one active volcano or another every few weeks, the largest events have frequencies of tens to hundreds of thousands of years. Such major lateral collapses are primarily confined to large, long-lived polygenetic volcanoes, located on either continental (eg Etna, Mount Rainier, Colima) or oceanic (eg Mauna Loa, Kilauea, Piton de la Fournaise) crust. Edifice instability may develop rapidly in response to a discrete event, such as the emplacement of the Mount St. Helens cryptodome in 1980. Alternatively, the volcano may become destabilised incrementally over many thousands or tens of thousands of years due, for example, to rift-related lateral displacement associated with persistent dyke intrusion. The hazard implications of volcano lateral collapse are significant at both the local and regional scales. In addition to the production of debris avalanches that may exceed 100km in length, the scale of tsunami generated by the largest collapses of ocean island volcanoes are sufficient to cause ocean-wide destruction. In the latter context, candidate volcanoes for future major collapse and tsunami generation include the Cumbre Vieja (La Palma, Canary Islands) and Fogo (Cape Verde Islands).