Volcanic and Associated Hazards in the Lesser Antilles
- Category I:
Events that occur every few decades and affect only the parent volcano’s immediate flanks.
- Category II:
Events that occur every few hundreds to thousands of years with effects extending beyond the flanks of the parent volcano, often over large parts of the island.
- Category III:
Events which occur every few tens to hundreds of thousands of years, and affect several islands; these may include non-volcanic activity such as sector collapse of the volcano’s flanks with associated earthquakes and tsunamis.
Category II hazards are typified by large Plinian-type eruptions which have generated widespread air falls, valley-fill pumice and ash flows (high aspect ratio ignimbrites, HARIs) and extensive ash hurricanes (low aspect ratio ignimbrites, LARIs). Examples of such activity were the eruption of Mt. Pelée around 2000 years ago, and the eruption that produced the Roseau tuff on Dominica approximately 30,000 years ago. Although this type of activity has not been witnessed by Europeans, several such eruptions occurred during the 3000-year occupation of the islands by pre-Columbian peoples. Category II hazard is also exemplified by the older geology of the Quill, St. Eustatius (Statia), where, some 30,000 years ago extrusion of rhyolitic magma onto a shallow marine shelf (about 3 km offshore from an older island) generated pyroclastic activity which became partially phreatomagmatic with increased energy release. The adjacent island was engulfed by surge deposits that eventually linked the early Quill to the older island.
Category III hazards involve major events during which, in addition to large new geological structures appearing on one island, associated seismic effects and tsunamis are likely to be experienced on adjacent islands. Examples of this type are illustrated by the geology of St. Lucia, Dominica and St. Vincent, where large arcuate depressions occur on the back-arc coasts. These structures have been interpreted as either calderas or as resulting from the gravity collapse of the steeper back-arc flanks of the islands. Whatever their origin, up to 12 km3 of land may be consumed.
At present the reactions of the governments of the many small islands to volcanic hazards vary considerably. Most are struggling with variable success (according to their level of affluence/impoverishment) to consider and plan for the short-term or category I hazard. For Saba and St. Eustatius where, and possibly because, we have prepared detailed category I and II hazard assessment reports, these reports are “lost” in the offices of the Netherlands Antilles government in Curaçao “awaiting approval” for publication and for 6 years have been unavailable to the administration and populations of these volcanic islands.
The mitigation of volcanic disasters requires not only hazard assessment reports but a wholehearted effort by the local government including education of the population at risk. Interaction between the international scientific community and local governments in developing countries is complicated by political-social-economic factors. The whole complex provides a new subject of geohazard management, without which the hazard assessment reports are unlikely to mitigate natural disaster.
KeywordsPyroclastic Flow Volcanic Hazard Eruptive Style Surge Deposit Submarine Eruption
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