Potential hazards at Soufriere Hills Volcano, Montserrat: Northwards-directed dome-collapses and major explosive eruptions
Ogburn, Sarah Elizabeth
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Soufrière Hills Volcano, Montserrat, West Indies, has been erupting since 1995. The current eruption has produced numerous dome and column collapse pyroclastic flows, and many explosions with a Volcanic Explosivity Index (VEI) of up to 3. This thesis involves the investigation of two issues pertinent to continued hazard analysis of Montserrat: (1) the potential, although deemed to be small, of a major explosive eruption (VEI ≥ 4) associated with the current dome growth; and (2) the potential hazard posed by large pyroclastic flows in the Belham Valley. For determining the association of large explosive eruptions with lava dome growth, two databases were compiled: one focused on dome-forming eruptions and one focused on large explosive eruptions 1000-2006. The results indicate that in most cases where large explosive activity has been associated with dome growth, the eruptions occurred at andesitic volcanoes (because these are the most common type of dome-forming eruption). For andesitic dome eruptions, however, only around 11% are associated with major explosive activity, and in no cases did the explosion occur after the end of the dome eruption. Short (< 1yr) dome growth durations were most commonly associated with large explosive activity. Large explosive activity, when associated with dome growth (at volcanoes of any composition), is more likely before dome growth rather than during or at the end of the dome eruptions. The evolution of the eruption at Soufrière Hills Volcano evolving into a more explosive phase during or after dome growth is considered unlikely. New Digital Terrain Models (DTMs) of Montserrat and a new dome-slicing tool are used for TITAN2D modeling of dome-collapse pyroclastic flows. Mobility measurements taken from real flows are used for model calibration. Comparisons of flows modeled with different internal and basal friction angles showed that only very low basal friction angles (5°) were in keeping with previous pyroclastic flows on Montserrat. The dome-slicing tool is useful in that it can realistically model pyroclastic flows that start from the dome summit; collapsing a pile from an empty crater underestimates potential flow volumes in the Belham Valley because of the morphology of the crater rim. For collapse volumes up to 200 x 10 6 m3, the maximum flow volume capable of entering the Belham Valley is around 27-32 x 10 6 m3. Depending on pyroclastic surge extents, the villages of Old Towne and Frith are most at risk, but with large surge clouds, Olveston, Salem, and the MVO could be impacted.