|Present La Soufrière Alert Level:||ORANGE||There is a highly elevated level of seismic and/or fumarolic activity or other unusual activity. An eruption may begin with less than twenty-four hours' notice.|
The La Soufrière Volcano poses many hazards to the island of St. Vincent. Following the effects of past eruptions in the last 300 years and the geological record, scientists have discovered which hazards are most common.
Pyroclastic flows and surges, mudflows, ashfall, and projectiles are the most hazardous events expected compared to lava flows, atmospheric phenomena, earthquakes, and phreatic explosions. Secondary effects such as landslides and events of more remote possibility such as directed blasts and structural collapse are also possible. Future eruptions are likely to be explosive or effusive similar to those experienced in the historical past.
Ballistic Projectiles & Ash Fall
Ballistic projectiles and tephra falls have been a common feature of all explosive eruptions of the volcano in the past. Any future explosive eruption will likely generate ash falls and projectiles. In the past, these phenomena have caused extensive damage to crops, minor damage to buildings and livestock, reduction in visibility, disruption of traffic and minor damage to vehicles, and respiratory illness in the human population.
Periods of phreatic activity have been associated with the Soufrière Volcano’s explosive eruptions in the past, whenever there was a pre-existing crater lake. The early phases of the 1812, 1902, and 1979 eruptions involved violent steam emissions from the volcanic center. The impact of these was restricted to the vicinity of the crater, with plant and animal life on the crater floor, walls, and rim being destroyed.
Lava Flows and Domes
Soufrière lava flows comprise highly viscous ‘aa’ and blocky type lavas that have not surmounted the crater walls during the historical past. Stratigraphic evidence shows that extensive lava flows had reached the lower flanks of the volcano before this period. These flows were most likely formed during the Pre-Caldera/Pre-Somma phase of the volcano’s evolution when the crater may have been breached in some sectors.
Assuming no marked changes in the dimensions of the present crater, it is very unlikely that effusion of magma from the vent would produce lava flows senso strictu or domes that surmount the crater rim. Such effusive activity is more likely to produce basaltic domes that would be confined to the crater floor. Such domes constitute a hazard since they may create conditions that favor explosive eruptions. They act as plugs that facilitate the build-up of pressures in a subsurface conduit. This pressure build-up may eventually cause explosive destruction of the dome.
Should the crater walls be breached in any manner (e.g., by a vertical blast causing the collapse of a section of the crater wall), the potential for lava flows to extend down the volcano sides becomes very high. Also, flank eruptions may provide a direct outlet to the surface and thus bypass the crater walls. Despite the generally high gradient of the volcano flanks, lava flows would not be expected to pose any significant threat to surrounding areas since their high viscosity and yield strength should prevent them from flowing very far.
There is no evidence of directed blasts during historical eruptions of the Soufrière Volcano. Blasts may have occurred in the pre-historic period, particularly during the Transitional to Later Explosive phase of the volcano’s evolution. Structural collapse may have occurred at least once during the pre-historic period at the volcano. Collapse is believed to have involved movement of the southern portion of the volcano towards the southeast, with displacement occurring along the Baleine scarp (Sigurdsson 1981). Such collapses could produce debris avalanches triggered by intrusion of new magma, phreatic explosions, or earthquakes. Therefore, catastrophic edifice failure could occur again at the volcano but is considered a low-frequency event.
Pyroclastic flows and surges have been a common feature of every explosive eruption of the Soufrière Volcano during the Historic Period. Future explosive eruptions are likely to generate pyroclastic flows and surges. Pyroclastic flows have tended to follow the river valleys that radiate from the volcanic center.
The mechanism of flow generation has in all cases been due to either one of the following: (a) partial or complete gravitational collapse of an overloaded vertical eruption column (e.g. 1902 eruption) or (b) fluidised overspill of highly gas-charged magma from an open vent (e.g. 1979 eruption).
Extension of the flow to the lower flanks of the volcano has been largely a function of the violence of the propelling explosion, the height at which collapse occurred, and the morphology of the volcanic cone and the surrounding river valleys. Flows follow paths of least resistance down the volcano’s flanks. In most cases, these have been the principal river valleys, especially where their headwaters have cut back to the crater. The impact of flows in the past has been extremely severe, bringing death and destruction to all areas located within their paths.
Mudflows (lahars) have been generated in most explosive eruptions during the Historic Period. As expected, given the nature of the terrain and the pattern of explosive activity, mudflows have generally followed the same valleys as the pyroclastic flows. The paths of pyroclastic flows can be taken to illustrate mudflows’ paths since they are almost identical. The nature of mudflows has depended largely on the magnitude of the eruption. Given the volcano’s steep gradient and the abundant rainfall expected from the tropical climate, future explosive eruptions are expected to produce mudflows.
Landslides are a common feature in both young and old tephra deposits on St Vincent. The movement has occurred most often in the thick Pleistocene tephra deposits that border the major roads along the east coast and has been largely one of four types: rockslides and rockfalls, debris slides, and debris flows (DeGraff 1988), with the latter type being the most common. Any future volcanic activity generating large amounts of ash would be expected to have associated landslides.
There have been no recorded incidences of volcanic gases causing death or destruction during historical eruptions of the Soufrière. However, the impact of gases in the past has been so closely associated with other hazardous phenomena that no distinction could be made with any certainty.
Future eruptions may be expected to have associated earthquakes, the effect of which is likely to be small to moderate, both in terms of hazardous impact and emergency planning.
Lightning was associated with the 1902 and 1979 eruptions. In both cases the discharges were restricted to areas in the immediate vicinity of the volcano and caused problems to monitoring equipment. In 1902, lightning strikes are believed to have been responsible for some deaths in the villages of Owia and Fancy.
Information was adapted from the Volcanic Hazard Atlas of the Lesser Antilles (UWI SRC).
The U.W.I. Seismic Research Centre is the official source of information for earthquakes and volcanoes in the English-speaking Eastern Caribbean. The SRC is the scientific monitoring agency that supports various local disaster management agencies of the Eastern Caribbean. This includes the monitoring of the La Soufrière Volcano.
The Seismic Research Centre (SRC) updates the National Emergency Management Organization (NEMO) of St. Vincent and the Grenadines which is then responsible for adjusting the alert level of La Soufrière based on the SRC’s information. NEMO is then responsible for coordinating evacuations across the hazard zones based on the alert level and volcanic hazard.