Abstract Furnas is the easternmost of the trachytic active central volcanoes of São Miguel. Unlike the other central volcanoes, Sete Cidades and Fogo, Furnas does not have a substantial edifice built up above sea-level. Although not as dominant as the other two volcanoes, Furnas does, however, have an edifice rising from the basal basaltic lavas exposed on the north coast to around 600 m asl on the northern rim of the main caldera. In common with Sete Cidades and Fogo, Furnas had major trachytic explosive eruptions in its volcanic history that emplaced welded ignimbrites. In the last 5 ka Furnas has had 10 moderately explosive trachytic eruptions of sub-Plinian character; two of these have taken place since the island was settled in the mid-fifteenth century. A future eruption of sub-Plinian magnitude is a major hazard posed by Furnas Volcano. Even when not in eruption, Furnas is a hazardous environment. Its fumarolic fields discharge high levels of CO 2 and concentrations in some area of Furnas village present a risk to health; the steep slopes and poorly consolidated volcanic materials are prone to landslides, in particular when triggered by earthquakes or following heavy rain, as was the case in 1997, when landslides caused severe damage and casualties in Ribeira Quente.

Abstract The 1995–present eruption of Soufrière Hills Volcano on Montserrat has produced over a cubic kilometre of andesitic magma, creating a series of lava domes that were successively destroyed, with much of their mass deposited in the sea. There have been five phases of lava extrusion to form these lava domes: November 1995–March 1998; November 1999–July 2003; August 2005–April 2007; July 2008–January 2009; and October 2009–February 2010. It has been one of the most intensively studied volcanoes in the world during this time, and there are long instrumental and observational datasets. From these have sprung major new insights concerning: the cyclicity of magma transport; low-frequency earthquakes associated with conduit magma flow; the dynamics of lateral blasts and Vulcanian explosions; the role that basalt–andesite magma mingling in the mid-crust has in powering the eruption; identification using seismic tomography of the uppermost magma reservoir at a depth of 5.5 > 7.5 km; and many others. Parallel to the research effort, there has been a consistent programme of quantitative risk assessment since 1997 that has both pioneered new methods and provided a solid evidential source for the civil authority to use in mitigating the risks to the people of Montserrat.

Abstract This paper describes ash-venting activity at Soufrière Hills Volcano, Montserrat that was precursory to the onset of three phases of lava extrusion in 2005, 2008 and 2009, and similar ash venting that occurred during the fifth phase of lava extrusion. We describe in detail a style of mild, tephra-generating activity termed ash venting and its associated tephra products. The nature of the seismicity associated with ash venting is compared with that of explosive activity. All explosive events, from small explosions to large Vulcanian explosions, have impulsive, low-frequency onsets. These are absent in ash-venting events, which have subtle, emergent onsets. Microscope and grain-size analyses show that ash-venting events and large Vulcanian explosions generate tephra that is similar in grain size (in medial and distal regions), although phreatic events in 2005 were finer grained. Ash-venting products are either composed of fine-grained, variably altered pre-existing material or juvenile material. There is a general correlation between the length of the pause and the length of the period of precursory activity prior to lava extrusion following it. Syn-extrusive ash venting is frequently associated with short-term increases in extrusion rate and is considered to be related to shear-induced fragmentation at the conduit margin.

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 2.7×10 6 m 3 , with more than half the explosions having volumes greater than 1×10 6 m 3 . 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.

Abstract Extrusion during Phase 5 (8 October 2009–11 February 2010) produced significant volumetric and geomorphic changes to the lava dome and surrounding valleys at the Soufrière Hills Volcano, Montserrat. Approximately 74×10 6 m 3 of lava was extruded at an average rate of 7 m 3 s −1 during the short period of activity. Addition of lava to the pre-existing dome resulted in a net volumetric increase of up to 38×10 6 m 3 . Pyroclastic density current (PDC) and ashfall deposits accounted for the remaining 36×10 6 m 3 . A series of thick, blocky lobes were extruded from a central vent. In addition, several short-lived spines and two large shear lobes were also extruded. Significant PDC activity resulted in substantial valley filling of up to 108 m. The large pre-existing dome significantly influenced the growth of lobes, such that many block-and-ash flows were generated from viscous lobes draped over the summit and upper slopes. Geomorphic changes caused by rapid filling of the surrounding valleys aided in both flow avulsion and the emplacement of deposits up to 6 km from the dome. These geomorphic changes have important consequences for hazards from PDCs.

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 0.3×10 5 –9.2×10 5 m 3 s −1 . This information provided in real time gives important input parameters for modelling the tephra dispersal into the atmosphere.

Abstract The Soufrière Hills Volcano (SHV) crystallizes cristobalite (crystalline silica) in its lava domes, and inhalation of cristobalite-rich ash may pose a chronic respiratory hazard. We investigate the causes of variation in cristobalite abundance (measured by X-ray diffraction) in ash from dome collapses, explosions and ash venting from 1997 to 2010. Cristobalite abundance in bulk dome-collapse ash varies between 4 and 23 wt%. During periods of slow lava extrusion (<5 m 3 s −1 ), cristobalite is abundant (7–23 wt%), which we attribute to extensive devitrification in slow-cooling lava; it can also form rapidly (15 wt% in 2 months), but we find no correlation between cristobalite abundance and dome residence time (DRT). By contrast, during rapid extrusion (>5 m 3 s −1 ), cristobalite abundance is low (4–7 wt%, similar to that associated with Vulcanian explosions), and correlates strongly with DRT. We attribute this correlation to progressive vapour-phase mineralization or devitrification, and the lack of contamination by older lava. Cristobalite abundance is expected to be >7 wt% for collapse of slowly extruded lava, for ash venting through a dome or for incorporation of hydrothermally altered edifice during explosions; cristobalite abundance is expected to be <7 wt% for collapse of rapidly extruded lava, for ash venting without dome incorporation and from Vulcanian explosions at SHV.

Abstract Long-term volcanic activity at Soufrière Hills Volcano (SHV), Montserrat (1995–ongoing) has created challenges for society and the resilience of the essential services (infrastructure) that support it. This paper explores the consequences, adaptations and resilience of essential services through interviews with their staff. We find that quick fixes for essential service reinstatement in the north of Montserrat have prevailed. Yet, the legacy of this approach inhibits functionality through inadequate facilities and the perception of sites as temporary, stalling investment. Emigration resulted in staff shortages, retraining requirements and challenges for the viability of specialist services. Low-impact hazards exacerbate shortcomings in essential services, causing power cuts, corrosion, and temporary closures of schools, clinics and the airport. Adaptations developed over time include changes to roofing materials, the addition of back-up systems, collaborative working and the development of contingency plans. Resilience of essential services has improved through decentralization, adaptations, and via strong community networks and tolerance of disruptions. Barriers to increasing resilience include the expense of some adaptations and the current reluctance to invest in essential services, hindering development. We offer some lessons for policy and practice to guide post-crisis redevelopment, through engagement with the community and by complementing community-level adaptations with investment to address long-term needs.

Abstract On 25 June 1997, an unsteady, retrogressive, partial collapse of the lava dome at Soufrière Hills Volcano lasted 25 minutes and generated a major pulsatory block-and-ash flow, associated pyroclastic surges and a surge-derived pyroclastic flow that inundated an area of 4 km 2 on the north and NE flanks of the volcano. Three main pulses are estimated to have involved 0.78, 2.36 and 2.36 × 10 6 m 3 of debris and the average velocities of the fronts of the related block-and-ash flow pulses were calculated to be 15 m s −1 , 16.1 m s −1 and 21.9 m s −1 respectively. Deposits of block-and-ash flow pulses 1 and 2 partially filled the main drainage channel so that material of the third pulse spilled out of the channel at several places, inundating villages on the eastern coastal plain. Bends and constrictions in the main drainage channel, together with depositional filling of the channel, assisted detachment of pyroclastic surges from the pulsatory block-and-ash flow. The most extensive pyroclastic surge detached at an early stage from the third block-and-ash flow pulse, swept down the north flank of the volcano and then climbed 70 m in elevation before dissipating. Rapid sedimentation from this surge generated a high-concentration granular flow (surge-derived pyroclastic flow) that drained westwards into a valley not anticipated to be at high risk. Observations support the hypothesis that the interior of the Soufrière Hills Volcano lava dome was pressurized and that pyroclastic surge development became more substantial as deeper, more highly pressurized parts of the dome were incorporated into the pyroclastic flow. Surge development was at times so violent that expanded clouds detached from the block-and-ash flow within a few tens of metres of the lava dome.

Abstract Pyroclastic flows were formed at Soufrière Hills Volcano by lava-dome collapse and by fountain collapse associated with Vulcanian explosions. Major episodes of dome collapse, lasting tens of minutes to a few hours, followed escalating patterns of progressively larger flows with longer runouts. Block-and-ash flow deposit volumes range from <0.1 to 25 x 10 6 m 3 with runouts of 1-7 km. The flows formed coarse-grained block-and-ash flow deposits, with associated fine-grained pyroclastic surge deposits and ashfall deposits. Small flows commonly formed lobate channelized deposits. Large block-and-ash flows in unconfined areas produced sheet-like deposits with tapering margins. the development of pyroclastic surges was variable depending on topography and dome pore pressure. Pyroclastic surge deposits commonly had a lower layer poor in fine ash that was formed at the current front and an upper layer rich in fine ash. Block-and-ash flows were erosive, producing striated and scoured bedrock surfaces and forming channels, many metres deep, in earlier deposits. Abundant accidental material was incorporated. Pyroclastic flow deposits formed by fountain collapse were pumiceous, with narrow sinuous, lobate morphologies and well developed levees and snouts. Two coastal fans formed where pyroclastic flows entered the sea. Their seaward extent was limited by a submarine slope break.

Abstract Gravitational collapses of the lava dome at Soufrière Hills Volcano on 25 June and 26 December 1997 generated pyroclastic surges that spread out over broad sectors of the landscape and laid down thin, bipartite deposits. In each case, part of the settling material continued to move upon reaching the ground and drained into valleys as high-concentration granular flows of hot (120–410°C) ash and lapilli. These surge-derived pyroclastic flows travelled at no more than 10 m s −1 but extended significantly beyond the limits of the parent surge clouds (by 3 km on 25 June and by 1 km on 26 December). The front of the 25 June flow terminated in a valley about 50 m below a small town that was occupied at the time. Despite their small deposit volumes (5–9 x 10 4 m 3 ), the surge-derived pyroclastic flows travelled as far as many of the Soufrière Hills block-and-ash flows on slopes as low as a few degrees, reflecting a high degree of mobility. An analysis of the deposits from 26 December suggests that sediment accumulation rates of at least several millimetres per second were sufficient to generate pyroclastic flows by suspended-load fallout from pyroclastic surges on Montserrat. Surge-derived pyroclastic flows are an important, and hitherto underestimated, hazard around active lava domes. At Montserrat they formed by sedimentation over large catchment areas and drained into valleys different from those affected by the primary block-and-ash flows and pyroclastic surges, thereby impacting areas not anticipated to be vulnerable in prior hazards analyses. The deposits are finer-grained than those of other types of pyroclastic flow at Soufrière Hills Volcano; this may aid their recognition in ancient volcanic successions but, along with valley-bottom confinement, reduces the preservation potential.

Abstract In 1997 Soufriére Hills Volcano on Montserrat produced 88 Vulcanian explosions: 13 between 4 and 12 August and 75 between 22 September and 21 October. Each episode was preceded by a large dome collapse that decompressed the conduit and led to the conditions for explosive fragmentation. The explosions, which occurred at intervals of 2.5 to 63 hours, with a mean of 10 hours, were transient events, with an initial high-intensity phase lasting a few tens of seconds and a lower-intensity, waning phase lasting 1 to 3 hours. In all but one explosion, fountain collapse during the first 10–20 seconds generated pyroclastic surges that swept out to 1–2 km before lofting, as well as high-concentration pumiceous pyroclastic flows that travelled up to 6 km down all major drainages around the dome. Buoyant plumes ascended 3–15 km into the atmosphere, where they spread out as umbrella clouds. Most umbrella clouds were blown to the north or NW by high-level (8–18 km) winds, whereas the lower, waning plumes were dispersed to the west or NW by low-level (<5 km) winds. Exit velocities measured from videos ranged from 40 to 140 ms −1 and ballistic blocks were thrown as far as 1.7 km from the dome. Each explosion discharged on average 3 × 10 5 m 3 of magma, about one-third forming fallout and two-thirds forming pyroclastic flows and surges, and emptied the conduit to a depth of 0.5–2 km or more. Two overlapping components were distinguished in the explosion seismic signals: a low-frequency (c. 1 Hz) one due to the explosion itself, and a high-frequency (>2 Hz) one due to fountain collapse, ballistic impact and pyroclastic flow. In many explosions a delay between the explosion onset and start of the pyroclastic flow signal (typically 10–20 seconds) recorded the time necessary for ballistics and the collapsing fountain to hit the ground. The explosions in August were accompanied by cyclic patterns of seismicity and edifice deformation due to repeated pressurization of the upper conduit. The angular, tabular forms of many fallout pumices show that they preserve vesicularities and shapes acquired upon fragmentation, and suggest that the explosions were driven by brittle fragmentation of overpressured magmatic foam with at least 55 vol% bubbles present in the upper conduit prior to each event.

Abstract At 03:01 (local time) on 26 December 1997, major sector collapse followed by collapse of the andesitic lava dome occurred at Soufrière Hills Volcano, Montserrat. The collapse of the dome involved explosive disintegration and formation of a highly energetic pyroclastic density current (PDC), which was dispersed principally to the SW and devastated an area of 10 km 2 . The deposits of the PDC are divisible into valley-confined and unconfined facies. The latter is characterized by two bipartite units (Units I and II), both of which are composed of a fines-poor layer (layer 1) typically overlain by a finer-grained, fines-rich layer (layer 2). The sequence is interpreted as recording strongly pulsatory (unsteady) flow and is capped by Unit III, an accretionary lapilli-rich fallout layer. There are pronounced variations of lithofacies, thickness, grain size and sedimentary structures related to local topography. The PDC was highly erosive: it sculpted isolated mounds of deposit and heavily scoured the pre-existing substrate. Lithofacies are granulometrically distinct, with median diameter (Mdø) increasing as sorting coefficient (Ãø) decreases. Lithofacies characteristics depend strongly on azimuth over a 70° sector, with major lateral (cross-flow) changes at similar radial distances from the dome. The deposits are similar to those produced in the blast eruptions of Mont Pelée in 1902 and Mount St Helens in 1980. We infer that particle size sorting occurred during explosive expansion of the collapsing lava dome, such that the resulting PDC was initially stratified in both grain size and density. The marked lateral and vertical variations in grain size of the deposits indicate efficient further development of density stratification and grain-size sorting during transport, due to air entrainment and sedimentation.