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 Depending on their magnitude and location, volcanic eruptions have the potential to become major social and economic disasters (e.g. Tambora, Indonesia, 1815; Vesuvius, Italy, AD 79; Soufriere Hills Volcano, Montserrat, 1995-present).One of the challenges for the volcanology community is to improve our understanding of volcanic processes so as to achieve successful assessments and mitigation of volcanic hazards, which are traditionally based on volcano monitoring and geological records. Geological records are crucial to our understanding of eruptive activity and history of a volcano, but often do not provide a comprehensive picture of the variation of volcanic processes and their effects on the surrounding area. The geological record is also typically biased towards the largest events, as deposits from smaller eruptions are often removed by erosion. Numerical modelling and probability analysis can be used to complement direct observations and to explore a much wider range of possible scenarios. As a result, numerical modelling and probabilistic analysis have become increasingly important in hazard assessment of volcanic hazards (e.g. Barberi et al. 1990 ; Heffter & Stunder 1993 ; Wadge et al. 1994 , 1998 ; Hill et al. 1998 ; Iverson et al. 1998 ; Searcy et al. 1998 ; Canuti et al. 2002 ). Reliable and comprehensive hazard assessments of volcanic This paper offers a detailed review of common approaches for hazard assessments of tephra dispersion. First, the main characteristics of tephra dispersion and tephra hazards are recounted. A critical use of field data for a

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 Dome growth at Soufrière Hills Volcano halted in early March 1998. After dome growth ceased, seismicity reduced significantly, but activity related to dome disintegration and degassing of magma at depth continued. A sustained episode of pyroclastic flows on 3 July 1998 marked the single largest collapse from March 1998 to November 1999. This led to a remarkable episode of dome collapses, low-energy explosions and ash-venting that resulted in the regular production of ash plumes, commonly reaching 1.5–6 km above sea level (a.s.l), but sometimes up to 11 km a.s.l., and the development of a small block-and-ash cone around the explosion crater. During the period of this residual activity, higher levels of activity occurred approximately every five to six weeks. This periodicity was similar to the cycles observed during active dome growth during 1995 to 1998, and probably had a similar cause. The relatively high level of observed activity caused continued concern regarding volcanic hazards and their potential to impact upon the resident population. Vigorous magma extrusion resumed in November 1999. The activity of the intervening period is attributed to the continued cooling and degassing of the dome, conduit and deep magma body, the impact of rising volcanic gases in the volcanic edifice, and limited magma flow in the conduit.

Abstract Four mechanisms caused tephra fallout at Soufrière Hills Volcano, Montserrat, during the 1995–1999 period: explosive activity (mainly of Vulcanian type), dome collapses, ash-venting and phreatic explosions. The first two mechanisms contributed most of the tephra-fallout deposits (minimum total dense-rock equivalent volume of 23 × 10 6 m 3 ), which vary from massive to layered and represent the amalgamation of the deposits from a large numbers of events. The volume of co-pyroclastic-flow fallout tephra is in the range 4-16° of the associated pyroclastic flow deposits. Dome-collapse fallout tephra is characterized by ash particles generated by fragmentation in the pyroclastic flows and by elutriation of fines. Vulcanian fallout tephra is coarser grained, as it is formed by magma fragmentation in the conduit and by elutriation from the fountain-collapse flows and initial surges. Vulcanian fallout tephra is typically polymodal, whereas dome-collapse fallout tephra is predominantly unimodal. Polymodality is attributed to: overlapping of fallout tephra of different types, premature fallout of fine particles, multiple tephra-fallout sources, and differences in density and grain-size distribution of different components. During both dome collapses and explosions, ash fell as aggregates of various sizes and types. Accretionary lapilli grain size is independent of their diameter and is characterized by multiple subpopulations with a main mode at 5ø. Satellite data indicate that very fine ash can stay in a volcanic cloud for several hours and show that exponential thinning rates observed in proximal areas cannot apply in distal areas.

Abstract Hazardous effects of tephra fallout on Montserrat include roof collapse, aviation threats, health hazards from respirable crystalline silica, crop pollution, road safety and lahar generation. An advection-diffusion model was developed to investigate tephra dispersal from dome collapses and Vulcanian explosions, which generated most of the fallout tephra during the 1995–1999 eruptive period of Soufrière Hills Volcano. Wind field, atmospheric diffusion, gravity settling, aggregation and elutriation processes are considered. Computed isomass maps compare well with field observations and require aggregation of fine ash for good agreement. Probability maps were also compiled. Individual probability maps (for individual dome collapses and Vulcanian explosions) are based on the statistics of wind profiles and show that fallout tephra generated by individual eruptive events on a Montserrat scale do not cause serious damage in any area on Montserrat. Cumulative probability maps (for a given scenario of activity) are generated by sampling statistical distributions of wind profiles and eruptive events over an extended period of time. They show that persistent tephra fallout can accumulate enough material to cause roof collapses and serious damage to vegetation in the SW part of the island, and minor damage to vegetation in the north, as also confirmed by field data.