Suzanne R. L. Young

Growth patterns and emplacement of the andesitic lava dome at Soufriere Hills Volcano, Montserrat

abstract Eruption of the Soufrière Hills Volcano on Montserrat allowed the detailed documentation of a Pélean dome-forming eruption. Dome growth between November 1995 and March 1998 produced over 0.3 km3 of crystal-rich andesitic lava. Discharge rates gradually accelerated from >1 m3 s-1 during the first few months to >5 m3 s-1 in the later stages. Early dome growth (November 1995 to September 1996) was dominated by the diffuse extrusion of large spines and mounds of blocky lava. A major dome collapse (17 September 1996) culminated in a magmatic explosive eruption, which unroofed the main conduit. Subsequent dome growth was dominated by the extrusion of broad lobes, here termed shear lobes. These lobes developed through a combination of exogenous and endogenous growth over many weeks, with movement accommodated along curved shear faults within the dome interior. Growth cycles were recognized, with each cycle initiated by the slow emplacement of a large shear lobe, constructing a steep flank on one sector of the dome. A growth spurt, heralded by the onset of intense hybrid seismicity, pushed the lobe rapidly out, triggering dome collapse. Extrusion of another lobe within the resulting collapse scar reconstructed the steep dome flanks prior to the next cycle.

The 1995–1998 eruption of the Soufriére Hills volcano, Montserrat, WI

Eruption of the Soufriere Hills volcano began on 18 July 1995 after three years of elevated seismic activity. Four months of increasingly vigorous phreatic activity culminated in mid-November 1995 with the initiation of dome growth. Growth rates increased unevenly through early March 1996, with fluctuations on time-scales from hours to months. Since March 1996, gravitational collapse of the unstable dome flank has affected an ever-increasing area with pyroclastic flows, surges and ashfalls. Major collapse of the eastern flank on 17 September 1996 resulted in a sub-Plinian explosive eruption later that day. By February 1997, the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys. A large collapse on 25 June 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption. In August, September and October 1997, vulcanian explosions followed further collapses on the western and northern flanks. The largest event of the eruption occurred on 26 December 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome-collapse pyroclastic flows. Dome growth ceased in early March 1998, but residual volcanic activity has continued and consists of ash venting, mild explosions and dome-collapse pyroclastic flows.

Unique and remarkable dilatometer measurements of pyroclastic flow–generated tsunamis

Pyroclastic flows entering the sea may cause tsunamis at coastal volcanoes worldwide, but geophysically monitored field occurrences are rare. We document the process of tsunami generation during a prolonged gigantic collapse of the Soufriere Hills volcano lava dome on Montserrat on 12–13 July 2003. Tsunamis were initiated by large-volume pyroclastic flows entering the ocean. We reconstruct the collapse from seismic records and report unique and remarkable borehole dilatometer observations, which recorded clearly the passage of wave packets at periods of 250–500 s over several hours. Strain signals are consistent in period and amplitude with water loading from passing tsunamis; each wave packet can be correlated with individual pyroclastic flow packages recorded by seismic data, proving that multiple tsunamis were initiated by pyroclastic flows. Any volcano within a few kilometers of water and capable of generating hot pyroclastic flows or cold debris flows with volumes greater than 5 × 10 6 m 3 may generate significant and possibly damaging tsunamis during future eruptions.

Hazard implications of small-scale edifice instability and sector collapse: a case history from Soufrière Hills Volcano, Montserrat

Abstract During the 1995 to 1998 phase of dome growth at Soufrière Hills Volcano on Montserrat, we documented instability of the steep southern rim of Englishs Crater, known as Galways Wall. The horseshoe-shaped Englishs Crater provided good evidence for previous sector collapses, and assessments undertaken in late 1996 anticipated the possibility of a partial sector collapse and a SW-directed explosion, hazards previously unrecognized on Montserrat. A change from predominantly endogenous to exogenous growth of the lava dome at the end of 1996 eased the stress on the southern sector. However, rapid dome growth in November and December 1997 led to severe reloading and eventual sector failure at the base of the buried Galways Wall and in the adjacent hot-spring area. This failure resulted in the debris avalanche and lateral blast of 26 December 1997. Similar sector collapses at a number of small volcanoes in the Caribbean, as well as worldwide, are evidence that edifice instability develops commonly in dome-forming eruptions. The hazards from a sector collapse and a consequent lateral blast are extreme, and monitoring operations and disaster planning at such volcanoes should focus on these, as well as on the more common hazards of conventional pyroclastic flows associated with dome growth.

The SEA-CALIPSO volcano imaging experiment at Montserrat: plans, campaigns at sea and on land, scientific results, and lessons learned

Abstract Since 1995 the eruption of the andesitic Soufrière Hills Volcano (SHV), Montserrat, has been studied in substantial detail. As an important contribution to this effort, the Seismic Experiment with Airgunsource-Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory (SEA-CALIPSO) experiment was devised to image the arc crust underlying Montserrat, and, if possible, the magma system at SHV using tomography and reflection seismology. Field operations were carried out in October–December 2007, with deployment of 238 seismometers on land supplementing seven volcano observatory stations, and with an array of 10 ocean-bottom seismometers deployed offshore. The RRS James Cook on NERC cruise JC19 towed a tuned airgun array plus a digital 48-channel streamer on encircling and radial tracks for 77 h about Montserrat during December 2007, firing 4414 airgun shots and yielding about 47 Gb of data. The main objecctives of the experiment were achieved. Preliminary analyses of these data published in 2010 generated images of heterogeneous high-velocity bodies representing the cores of volcanoes and subjacent intrusions, and shallow areas of low velocity on the flanks of the island that reflect volcaniclastic deposits and hydrothermal alteration. The resolution of this preliminary work did not extend beyond 5 km depth. An improved three-dimensional (3D) seismic velocity model was then obtained by inversion of 181 665 first-arrival travel times from a more-complete sampling of the dataset, yielding clear images to 7.5 km depth of a low-velocity volume that was interpreted as the magma chamber which feeds the current eruption, with an estimated volume 13 km3. Coupled thermal and seismic modelling revealed properties of the partly crystallized magma. Seismic reflection analyses aimed at imaging structures under southern Montserrat had limited success, and suggest subhorizontal layering interpreted as sills at a depth of between 6 and 19 km. Seismic reflection profiles collected offshore reveal deep fans of volcaniclastic debris and fault offsets, leading to new tectonic interpretations. This chapter presents the project goals and planning concepts, describes in detail the campaigns at sea and on land, summarizes the major results, and identifies the key lessons learned.

Vaporization-Induced Overpressures as a Trigger for the Hazardous Collapse of Lava Domes

Abstract Rainfall-triggered collapses of hot lava domes are described relative to the fluid overpressurization that accompanies the infiltration of rainfall into the carapace. Similar rainfall-triggered modes of failure are apparent for both hemispherical (endogenous) and lobate (exogenous) domes. Failure modes for these disparate geometries are linked by the necessary incursion of rainfall into the hot dome and the resulting impeded drainage of interior gas pressures – either self-generated by the vaporizing infiltration front, or the impeded drainage of interior pressurized gases. Limit equilibrium models are developed to assess the stability of idealized hemispherical or lobate domes, with driving forces generated by interior gas overpressures. For endogenous domes, shallow failures may develop in the absence of gas pressurization from the core, but observed deep-seated failures require that the free escape of conduit gases be impeded. For exogenous domes, failure is driven to the base of the lobe by the desire to maximize overpressures.