Susan C. Loughlin

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is < 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.

Operational prediction of ash concentrations in the distal volcanic cloud from the 2010 Eyjafjallajökull eruption

[1] During the 2010 eruption of Eyjafjallajokull, improvements were made to the modeling procedure at the Met Office, UK, enabling peak ash concentrations within the volcanic cloud to be estimated. In this paper we describe the ash concentration forecasting method, its rationale and how it evolved over time in response to new information and user requirements. The change from solely forecasting regions of ash to also estimating peak ash concentrations required consideration of volcanic ash emission rates, the fraction of ash surviving near-source fall-out, and the relationship between predicted mean and local peak ash concentrations unresolved by the model. To validate the modeling procedure, predicted peak ash concentrations are compared against observations obtained by ground-based and research aircraft instrumentation. This comparison between modeled and observed peak concentrations highlights the many sources of error and the uncertainties involved. Despite the challenges of predicting ash concentrations, the ash forecasting method employed here is found to give useful guidance on likely ash concentrations. Predicted peak ash concentrations lie within about one and a half orders of magnitude of the observed peak concentrations. A significant improvement in the agreement between modeled and observed values is seen if a buffer zone, accounting for positional errors in the predicted ash cloud, is used. Sensitivity of the predicted ash concentrations to the source properties (e.g., the plume height and the vertical distribution of ash at the source) is assessed and in some cases, seemingly minor uncertainties in the source specification have a large effect on predicted ash concentrations.

Global database on large magnitude explosive volcanic eruptions (LaMEVE)

To facilitate the assessment of hazards and risk from volcanoes, we have created a comprehensive global database of Quaternary Large Magnitude Explosive Volcanic Eruptions (LaMEVE). This forms part of the larger Volcanic Global Risk Identification and Analysis Project (VOGRIPA), and also forms part of the Global Volcano Model (GVM) initiative (http://www.globalvolcanomodel.org). A flexible search tool allows users to select data on a global, regional or local scale; the selected data can be downloaded into a spreadsheet. The database is publically available online at http://www.bgs.ac.uk/vogripa and currently contains information on nearly 3,000 volcanoes and over 1,800 Quaternary eruption records. Not all volcanoes currently have eruptions associated with them but have been included to allow for easy expansion of the database as more data are found. Data fields include: magnitude, Volcanic Explosivity Index (VEI), deposit volumes, eruption dates, and rock type. The scientific community is invited to contribute new data and also alert the database manager to potentially incorrect data. Whilst the database currently focuses only on large magnitude eruptions, it will be expanded to include data specifically relating to the principal volcanic hazards (e.g. pyroclastic flows, tephra fall, lahars, debris avalanches, ballistics), as well as vulnerability (e.g. population figures, building type) to facilitate risk assessments of future eruptions.

Tephra fallout in the eruption of Soufrière Hills Volcano, Montserrat

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 x 106 m3), 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.

Distal deposition of tephra from the Eyjafjallajökull 2010 summit eruption

The 2010 Eyjafjallajokull lasted 39 days and had 4 different phases, of which the first and third (14-18 April and 5-6 May) were most intense. Most of this period was dominated by winds with a northerly component that carried tephra toward Europe, where it was deposited in a number of locations and was sampled by rain gauges or buckets, surface swabs, sticky-tape samples and air filtering. In the UK, tephra was collected from each of the Phases 1-3 with a combined range of latitudes spanning the length of the country. The modal grain size of tephra in the rain gauge samples was 25 mu m, but the largest grains were 100 mu m in diameter and highly vesicular. The mass loading was equivalent to 8-218 shards cm(-2), which is comparable to tephra layers from much larger past eruptions. Falling tephra was collected on sticky tape in the English Midlands on 19, 20 and 21st April (Phase 2), and was dominated by aggregate clasts (mean diameter 85 mu m, component grains <10 mu m). SEM-EDS spectra for aggregate grains contained an extra peak for sulphur, when compared to control samples from the volcano, indicating that they were cemented by sulphur-rich minerals e. g. gypsum (CaSO4 center dot H2O). Air quality monitoring stations did not record fluctuations in hourly PM10 concentrations outside the normal range of variability during the eruption, but there was a small increase in 24-hour running mean concentration from 21-24 April (Phase 2). Deposition of tephra from Phase 2 in the UK indicates that transport of tephra from Iceland is possible even for small eruption plumes given suitable wind conditions. The presence of relatively coarse grains adds uncertainty to concentration estimates from air quality sensors, which are most sensitive to grain sizes <10 mu m. Elsewhere, tephra was collected from roofs and vehicles in the Faroe Islands (mean grain size 40 mu m, but 100 mu m common), from rainwater in Bergen in Norway (23-91 mu m) and in air filters in Budapest, Hungary (2-6 mu m). A map is presented summarizing these and other recently published examples of distal tephra deposition from the Eyjafjallajokull eruption. It demonstrates that most tephra deposited on mainland Europe was produced in the highly explosive Phase 1 and was carried there in 2-3 days.

The Montserrat Volcano Observatory: its evolution, organization, role and activities

Abstract The Montserrat Volcano Observatory (MVO) is a statutory body of the Government of Montserrat and is the organization responsible for volcano monitoring operations on the island. It was formed shortly after the first phreatic explosions from Soufrière Hills Volcano occurred on 18 July 1995, and evolved from a hastily created, interim entity to a fully established volcano monitoring operation. Participating scientific teams have been drawn mainly from the Seismic Research Unit of the University of the West Indies, the US Geological Survey, the British Geological Survey and universities from various countries including the USA, UK, France and Puerto Rico. Despite its hurried inception, the MVO has been able to provide timely, high quality hazard advice to the civil authorities and has maintained an exceptional documentary record of all scientific aspects of the eruption. Its public education and information efforts have been extensive and there have been unusually high levels of interaction between scientists and the civil authorities, and between scientists and the public, both within Montserrat and outside in the wider world. The experience of setting up and running the MVO, under difficult and stressful conditions, has exemplified the advantages of teamwork and flexibility within monitoring operations and the benefits of openness and clarity in public interactions. Novel techniques have been applied to the appraisal of hazards and advances in scientific understanding have proved invaluable for risk assessment and management.

The relationship between degassing and ground deformation at Soufriere Hills Volcano, Montserrat

We examine the correlations between SO2 emission rate, seismicity and ground deformation in the month prior to the 25 June 1997 dome collapse of the Soufriere Hills Volcano, Montserrat. During this period, the volcano exhibited a pattern of cyclic inflation and deflation with an 8‐14 h period. We find that SO 2 emission rates, measured by COSPEC, correlate with the amplitude of these tilt cycles, and that higher rates of SO 2 emission were associated with stronger ground deformation and enhanced hybrid seismicity. Within tilt cycles, degassing peaks coincide with maximum deformation gradients. Increases in the amount of gas in the magma conduit feeding the dome, probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude, hybrid seismicity and SO2 emission rate. q 2000 Elsevier Science B.V. All rights reserved.

Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surges at Soufrière Hills Volcano, Montserrat: an important volcanic hazard

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 104m3), 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.

Pyroclastic flows and surges generated by the 25 June 1997 dome collapse, Soufrière Hills Volcano, Montserrat

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 km2 on the north and NE flanks of the volcano. Three main pulses are estimated to have involved 0.78, 2.36 and 2.36 x 106m3 of debris and the average velocities of the fronts of the related block-and-ash flow pulses were calculated to be 15 ms-1, 16.1 ms-1 and 21.9 ms-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.

Chapter 1 An overview of the eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010

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.