Abstract

Different types of landslides have been observed on Montserrat, a British dependent island in the (Caribbean) West Indies. The Soufrière Hills volcano, situated in the southern part of this island, has been in a state of almost continuous volcanic activity since 1995, after being dormant for about 400 years. The most dramatic, distinct ‘landslides’ occur on the extrusive, expanding, andesitic lava dome within English Crater. Falls, topples and slides are caused by the over-steepening of the dome followed by its gravitational collapse. On occasions, these failures generate incandescent rock avalanches, which may ultimately transgress into pyroclastic flows, surges and block & ash flows. These are highly destructive, hot, density currents and accelerate down-slope, controlled by the topography and radial drainage channels (known as ‘ghauts’), which have been incised on the flanks of the volcano. These flows have deposited huge volumes of incandescent pyroclastic debris on the upper and middle flanks of the volcano. The erosion of the pyroclastic debris and air-fall deposits, during rainstorms, has resulted in the generation of lahars (an Indonesian term to describe a volcanic mudflow). Well over 40 lahars have been observed and recorded, from January 1999 to 2006. These coincided with hurricanes and increased rainfall rates usually between the months of June and November. The lahar deposits, typically consisting of granular material including boulders, have infilled many of the river valleys. The lower reaches of the Belham valley in particular has been infilled, and this has resulted in the choking of drainage catchment areas and the burial of infrastructure and houses. Beyond the area of current volcanic activity landslides have also been observed, although their significance has not always been recognized, as their investigation may often be overlooked because of the more spectacular dramatic events on the volcano itself. The landslides include both coastal and inland landslides, which are generated by a combination of seismic events (such as tectonic and volcano-magmatic earthquakes) and climatic events (such as rainstorm and hurricanes). Landslide generation is facilitated by the presence of thick, tropical weathered laterite soils (regolith) that develop on moderate to steep slopes, in areas of high relief, on intensely weathered volcanic bedrock. Both soils and topography influence drainage and groundwater flows, which in turn affect the stability ofslopes, causing the initiation of first time slope failures and the reactivation of older, more degraded landslides. In addition to natural processes, the activities of man (such as road building, the digging into slopes or the loading of slopes by tipping waste) have also caused landslides to develop. The objectives of this paper are to document and draw attention to the causes and types of landslides on Montserrat with particular reference to the expanding lava dome, the Belham valley lahars and the Montserrat Volcano Observatory (MVO) debris slide. This paper provides an engineering geological perspective on the landslide geohazards that have been observed on Montserrat.

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