R. Stewart

Insights into processes and deposits of hazardous vulcanian explosions at Soufrière Hills Volcano during 2008 and 2009 (Montserrat, West Indies)

During the Soufriere Hills eruption, vulcanian explosions have generally occurred 1) in episodic cycles; 2) isolated during pauses in extrusion, and 3) after major collapses of the dome. In a different eruptive context, significant vulcanian explosions occurred on 29 July 2008, 3 December 2008, and 3 January 2009. Deposits are pumiceous except for the 3 December event. We reconstructed the dispersal pattern of the deposits and their textural characteristics to evaluate erupted volume and vesicularity of the magma at fragmentation. We discuss the implications of these explosions in terms of eruptive processes and chronology, and the hazards posed by their sudden and often unheralded occurrence. We suggest that overpressurization of the conduit can develop over time-scales of months to weeks by a process of self-sealing of conduit walls and/or the cooling dome by silica polymorphs. This work provides new insights for understanding the generation of hazardous vulcanian explosions at andesitic volcanoes.

Chapter 5 Vulcanian explosions at Soufrière Hills Volcano, Montserrat between 2008 and 2010

Abstract Vulcanian explosions generated at Soufrière Hills Volcano between 2008 and 2010 varied from simple events involving minimal pyroclastic density currents (PDCs) to complex events involving more than one explosion. Calculated volumes for the deposits of the PDCs formed by these explosions ranged up to 2.7×106 m3, with more than half the explosions having volumes greater than 1×106 m3. The deposits formed by the explosions varied in lithology, with some explosions generating pumice-rich PDCs (e.g. 29 July 2008 and 11 February 2010) showing development of sinuous lobes. These explosions are similar to those formed in 1997, with gas-rich, conduit-derived magma being the dominant driving mechanism. Other explosions were pumice-poor (c. 5 wt% pumice) and generated morphologically distinct PDC deposits. Many of the pumice-poor explosions were associated with lower tephra plumes of <8 km, but were some of the largest volume events in terms of PDC production and suggest a generation mechanism involving destruction of significant quantities of the lava dome. Analysis of video footage shows that PDC formation was pulsatory, probably related to destabilization of portions of the lava dome during the initial phases of the explosion.

Chapter 7 The 11 February 2010 partial dome collapse at Soufrière Hills Volcano, Montserrat

Abstract On 11 February 2010, a partial dome collapse, the largest since 20 May 2006, occurred at Soufrière Hills Volcano (SHV), Montserrat. The collapse is also the largest generated on the northern flank of SHV since the eruption began in 1995. Approximately 50×106 m3 was removed from the dome, resulting in widespread pyroclastic density currents (PDCs). Mapping revealed a complex stratigraphy that varied widely across the northern and NE flanks, and reflected the complex evolution of the collapse. The deposits included a range of fine-grained ash-rich and pumice-rich units deposited by dilute PDCs, and several types of coarse-grained, blocky deposits from dense PDCs. Several previously unaffected areas, including Bugby Hole, Farm River Valley, the village of Harris and Trants, suffered significant damage to the natural and built environments. The collapse lasted 107 min but the bulk of the activity occurred in a 15 min period that included five of the six peaks in PDC generation and two Vulcanian explosions. Although powerful, the PDCs generated were not associated with a lateral blast. The likely cause was the piecemeal collapse of a series of large, unstable lobes that had been extruded on the northern flank of the pre-existing dome.

Cyclic phenomena at the Soufriere Hills Volcano, Montserrat

Abstract Cycles of eruptive activity are generally interpreted as evidence of one or more mechanisms operating in equilibrium. Modulation of cycle characteristics thus reflects changes in the conditions affecting those mechanisms. This kind of semi-deterministic behaviour at the Soufrière Hills Volcano has occurred on multiple timescales and with a range of eruptive intensity. By documenting cyclic phenomena, it is possible to investigate the mechanisms that modulate the state of the eruption and examine conceptual models. Pattern recognition and model development allows some degree of short-term forecasting ability for volcanic activity. We report the cyclic eruptive phenomena that have occurred on Montserrat on scales of hours to centuries. We identify four dominant types of cyclicity: sub-daily variations in lava flux; sub-annual cycles in eruption intensity; multi-annual ‘on–off’ switching of lava extrusion; and multi-decadal recurrence of seismic crises. We exploit a wealth of multi-parameter datasets (including seismic, geodetic, thermal, archive and visual observations), and present the evidence and observations for each type of cyclicity, some of which are documented here for the first time. Wavelet time-series analysis is used to constrain cycle characteristics, where appropriate. We discuss the implications of these observations in understanding the eruptive mechanisms of the Soufrière Hills Volcano. Supplementary material: Details of all sub-daily cycles are listed in a supplementary table that is available at http://www.geolsoc.org.uk/SUP18700.

Chapter 2 Cyclic phenomena at the Soufrière Hills Volcano, Montserrat

Abstract Cycles of eruptive activity are generally interpreted as evidence of one or more mechanisms operating in equilibrium. Modulation of cycle characteristics thus reflects changes in the conditions affecting those mechanisms. This kind of semi-deterministic behaviour at the Soufrière Hills Volcano has occurred on multiple timescales and with a range of eruptive intensity. By documenting cyclic phenomena, it is possible to investigate the mechanisms that modulate the state of the eruption and examine conceptual models. Pattern recognition and model development allows some degree of short-term forecasting ability for volcanic activity. We report the cyclic eruptive phenomena that have occurred on Montserrat on scales of hours to centuries. We identify four dominant types of cyclicity: sub-daily variations in lava flux; sub-annual cycles in eruption intensity; multi-annual ‘on–off’ switching of lava extrusion; and multi-decadal recurrence of seismic crises. We exploit a wealth of multi-parameter datasets (including seismic, geodetic, thermal, archive and visual observations), and present the evidence and observations for each type of cyclicity, some of which are documented here for the first time. Wavelet time-series analysis is used to constrain cycle characteristics, where appropriate. We discuss the implications of these observations in understanding the eruptive mechanisms of the Soufrière Hills Volcano. Supplementary material: Details of all sub-daily cycles are listed in a supplementary table that is available at http://www.geolsoc.org.uk/SUP18700.

Chapter 9 Thermal, acoustic and seismic signals from pyroclastic density currents and Vulcanian explosions at Soufrière Hills Volcano, Montserrat

Abstract We show two examples of how integrated analysis of thermal and infrasound signal can be used to obtain, in real time, information on volcanic activity. Soufrière Hills Volcano (SHV) on Montserrat offers the opportunity to study a large variety of processes related to lava-dome activity, such as pyroclastic density currents (PDCs) and large Vulcanian eruptions. Infrasound and thermal analysis are used to constrain the propagation of PDCs and their velocities, which are calculated here to range between 15 and 75 m s−1. During the Vulcanian eruption of 5 February 2010, infrasound and thermal records allow us to identify an approximately 13 s seismic precursor possibly related to the pressurization of the conduit before the explosion onset. The associated very long period (VLP) seismic signal is correlated with the gas-thrust phase detected by thermal imagery, and may reflect a change in the upward momentum induced by the mass discharge. Moreover, from infrasound and thermal analysis, we estimate a gas-thrust phase lasting 22 s, with an initial plume velocity of approximately 170 m s−1 and a mean volumetric discharge rate of 0.3×105–9.2×105 m3 s−1. This information provided in real time gives important input parameters for modelling the tephra dispersal into the atmosphere.

An inclined Vulcanian explosion and associated products

Vulcanian explosions generate some of the most hazardous types of volcanic phenomena, including pyroclastic density currents. Non-vertical directionality of an explosion promotes asymmetrical distribution of proximal hazards around the volcano. Although critical, such behaviour is relatively uncommon and has been seldom documented. Here we present, for the first time, evidence both from geophysical monitoring and field survey data that records the occurrence of such an event. Thermal imagery captures a Vulcanian explosion at Soufrière Hills Volcano, Montserrat, which occurred during a large partial lava dome collapse in February 2010, and was inclined at about 25° from the vertical in a northerly direction. Pyroclastic products were preferentially distributed to the north and included: an unusual pumice boulder deposit that we propose was formed by a dilute pyroclastic density current; pumice flow deposits; and a proximal lapilli and block fallout lobe. The inclined nature of the explosion is attributed to the asymmetric geometry around the vent. The explosion-derived pyroclastic density currents had notably lower velocities than those associated with lateral blasts, which, we suggest, result from a separate and distinct mechanism. These inclined explosions present an additional mechanism that is able to generate directed pyroclastic density currents, with consequent implications for hazard assessment.