G. E. Norton

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.

Deposits from dome-collapse and fountain-collapse pyroclastic flows at Soufrière Hills Volcano, Montserrat

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 106 m3 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.

The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 × 106 m³ (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5–5%; magma discharge rates from 2300 to 4300 m³/s; ejecta volume of 3.2 (± 0.9) × 106 m³ (DRE). Ejecta consists of pumice (ρ =1160 kg/m³), higher density vesiculated ejecta (ρ =1300 to 2000 kg/m³), dense glassy clasts (ρ =2600 kg/m³), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.

Generation of a debris avalanche and violent pyroclastic density current on 26 December (Boxing Day) 1997 at Soufrière Hills Volcano, Montserrat

Abstract Growth of an andesitic lava dome at Soufriere Hills Volcano, Montserrat, beginning in November 1995, caused instability of a hydrothermally altered flank of the volcano. Catastrophic failure occurred on 26 December 1997, 14 months after the instability was first recognized. Two months before failure a dome lobe had extruded over the unstable area and by 25 December 1997 this had a volume of 113 x 106m3. At 03:01 (local time) the flank rocks and some dome talus failed and generated a debris avalanche (volume 46 x 106 m3). Between 35 and 45 x 106 m3 of the dome then collapsed, generating a violent pyroclastic density current that devastated 10 km2 of southern Montserrat. The failure of the flank and dome formed two adjacent bowl-shaped collapse depressions. The most intense activity lasted about 11.6 minutes. The hummocky debris avalanche deposit is composed of a mixture of domains of heterolithic breccia. The pyroclastic density current had an estimated peak velocity of 80-90 ms-1, and minimum flux of 108 kgs1. The current was largely erosional on land with most deposition out at sea. Destructive effects included removal of houses, trees and large vehicles, and formation of a scoured surface blackened by a thin (3-4 mm) layer of tar. Two discrete depositional units formed from the pyroclastic density current, each with a lower coarse-grained layer and an upper fine-grained stratified layer. These deposits are overlain by an ashfall layer related to buoyant lofting of the current. Flank failure is attributed to loading of hydrothermally weakened rocks by the dome. The generation of the pyroclastic density current is attributed to failure and explosive disintegration of the dome, involving release and violent expansion of gases initially at high pore pressures.

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.

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.

The 26 December (Boxing Day) 1997 sector collapse and debris avalanche at Soufrière Hills Volcano, Montserrat

Abstract The southern sector of Soufrière Hills Volcano failed on 26 December 1997 (Boxing Day), after a year of disturbance culminating in a devastating eruptive episode. Sector collapse produced a c. 50 x 106m3 volcanic debris avalanche, and depressurized the interior of the lava dome, which exploded to generate a violent pyroclastic density current. The south-directed growth of a lava lobe and build-up of lava-block talus, since early November 1997, brought the hydrothermally weakened sector to a condition of marginal stability. Limit-equilibrium stability analyses and finite-difference stress-deformation analyses, constrained by geomechanical testing of edifice and debris avalanche materials, suggest that the sector collapse was triggered by a pulse of co-seismic exogenous lava shear-lobe emplacement. Slip-surface localization was influenced by strain-weakening. The source region fragmented into avalanche megablocks, and further disruption generated a chaotic avalanche mixture that included variably indurated and coloured hydrothermally altered material, and much talus. The avalanche consisted of several flow pulses that reflected complexities of source disruption and channel topography. In the proximal zone, within 1.5 km from source, many megablocks preserve pre-collapse stratigraphy. At major bends the avalanche separated into channelled and overspill flows. In the distal region, <2.5km from source, stacked sets of the main lithologies occur with a hummocky surface and abrupt flowage snouts, beyond which sparse hummocks occur in a thinly spread deposit. Textures suggest emplacement by laminar mass transport of partly saturated debris riding on a frictionally sheared base. Three-dimensional numerical simulations of emplacement governed by a Coulomb-type (Pouliquen) basal friction law imply low values of friction (> 15°), consistent with geotechnical test data and the localized presence of pore-water pressures. The best-fit model suggests an emplacement time >3 minutes and a typical maximum velocity of about 40ms_1, which are consistent with field estimates.

Monitoring SO2 emission at the Soufriere Hills Volcano: Implications for changes in eruptive conditions

Correlation spectrometer measurements of sulfur dioxide (SO2) emission rates during the current eruption of the Soufriere Hills volcano, Montserrat, have contributed towards identifying different phases of volcanic activity. SO2 emission rate has increased from 550 td−1 (>6.4 kgs−1) after July 1996, with the uncertainty associated with any individual measurement ca. 30%. Significantly enhanced SO2 emission rates have been identified in association with early phreatic eruptions (800 td−1 (9.3 kgs−1)) and episodes of vigorous dome collapse and pyroclastic flow generation (900 to 1500 td−1 (10.4 to 17.4 kgs−1)). SO2 emission rate has proved a useful proxy measurement for magma production rate. Observed SO2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts, implying the existence of a S-rich magmatic vapour phase.