Richard A. Herd

SO2 emissions from Soufrière Hills Volcano and their relationship to conduit permeability, hydrothermal interaction and degassing regime

The time series of sulphur dioxide (SO2) emissions during the continuing eruption of Soufriere Hills Volcano, Montserrat, yields insights into conduit permeability and driving pressures, the role of the hydrothermal system and changes in magma flux both at depth and to the surface. On a time scale of years, an effectively constant supply of sulphur from a more mafic magma at depth permits evaluation of changes in the permeability of the plumbing system between 1995 and 2002 (due to magma rheology changes and hydrothermal sealing), most of which take place in the upper few hundreds of metres (dome and upper conduit). A broadly increasing SO2 emission rate from 1995 to 1997 can be attributed to a constant or increasing supply of exsolving sulphur from depth, combined with a broadly increasing magma discharge rate at the surface. Decreases in SO2 flux over three orders of magnitude, from July 1998 to November 1999, were due to a corresponding decrease in permeability of the upper conduit and dome due to cooling and ‘sealing’ by the precipitation of hydrothermal minerals and the closure of fracture and bubble networks. The second phase of dome growth, from November 1999 to the present, April 2002, has been associated with a similar range of SO2 fluxes to the first phase. Large dome collapses in 1997 and during a period of zero magma flux in 1998 were associated with instantaneous SO2 emissions of >10 kt, which indicate a capacity for significant SO2 storage in the conduit and dome prior to the collapses. SO2 data suggest that the second phase of dome building, despite a similar sulphur budget in terms of supply from depth and mean SO2 emission rate at the surface (around 500 t/d), is characterised by a higher bulk permeability at shallow depths and is a more ‘open’ system with respect to fluid through-flow than the first phase of dome building from 1995 to 1998. The lack of large SO2 emissions after large dome collapses, in 2000 and 2001, suggests limited storage of SO2 in the conduit system. The data suggest that the likelihood of a switch to explosive activity after a large collapse is more unlikely now than during the first phase of dome building. Over shorter time scales, permeability changes may be recognised from the SO2 flux data prior to the onset of dome growth and during cycles of small explosions in 1999. On time scales of minutes to hours, pulses of SO2-rich gas emissions occur after rockfalls and pyroclastic flows, due to the release of a SO2-rich fluid phase stored in closed fractures and pore spaces within the dome. Long period and hybrid seismic events may be associated with changes in SO2 emission rate at the surface at various times of the eruption, although only when the temporal resolution of SO2 monitoring is improved, will it be possible for these short-term changes to be correlated and evaluated effectively. Monitoring SO2 emission rates from Soufriere Hills Volcano is, at this stage, of primary value in the long run, on the time scale of years, where the relationships between deep supply and surface emissions can be used to evaluate whether the eruption might be waning, or has merely paused, which is of considerable value for hazard assessment.

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.

Episodes of cyclic Vulcanian explosive activity with fountain collapse at Soufrière Hills Volcano, Montserrat

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 x 105m3 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.

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.

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.

Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes

Analysis of topography and new swath bathymetry as well as geophysical data provides information about aerial and submarine morphological features and mass transfer processes on Montserrat. The island has a characteristic shallow (<100 m) submarine shelf, interpreted as having been formed through erosion with a depth controlled by glacio-eustatic sea-level variation. Several debris avalanche deposits are identified on the lower submarine flanks of Soufrière Hills volcano, and there is evidence of lateral collapses at the older volcanic centres. The morphological evolution of Montserrat is interpreted in terms of three stages. The first stage comprises submarine growth. The second stage, subaerial growth, is represented by the active South Soufrière Hills–Soufrière Hills volcanic centre. During the current eruption of Soufrière Hills volcano (1995–2002) more than half of the lava erupted was transported into the sea. Flank collapses occurred several times during this stage, such as the Englishs Crater event (c. 4000 years ago) or the Boxing Day event during the current eruption (26 December 1997). Montserrats older volcanic centres, the Centre Hills and Silver Hills, illustrate the third stage of evolution, extinction and erosion. Magma production, long-term erosion and total sedimentation rates on Montserrat have been estimated as 0.17 km3 ka−1, 0.0125 km3 ka−1 and 0.11 km3 ka−1 (i.e. 1.1 cm ka−1), respectively.

Implications of magma transfer between multiple reservoirs on eruption cycling

Volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate. We used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the Soufrière Hills volcano on Montserrat, West Indies. For three cycles of effusion followed by discrete pauses, supply of the system from the deep crust and mantle was continuous. During periods of reinitiated high surface efflux, magma rose quickly and synchronously from a deflating mid-crustal reservoir (at about 12 kilometers) augmented from depth. During repose, the lower reservoir refilled from the deep supply, with only minor discharge transiting the upper chamber to surface. These observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid-crustal reservoir, episodically valved below a shallow reservoir (at about 6 kilometers).

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.

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.

Excess volatiles supplied by mingling of mafic magma at an andesite arc volcano

We present the results of a study of volcanic gases at Soufriere Hills Volcano, Montserrat, which includes the first spectroscopic measurements of the major gas species CO2 and H2S at this volcano using a Multisensor Gas Analyzer System (MultiGAS) sensor. The fluxes of CO2 and H2S were 640.2750 t/d and 84.266 t/d, respectively, during July 2008, during a prolonged eruptive pause. The flux of CO2 is similar to estimates for the entire arc from previous geochemical studies, while the measured H2S flux significantly alters our interpretation of the sulphur budget for this volcano. The fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma. We demonstrate, using thermodynamic models and published constraints on preeruptive volatile concentrations, that the gas composition and fluxes are best modeled by mixing between (1) gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and (2) gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite. The escape of deep-derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system. These results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption. From a broader perspective, this study illustrates the importance and role of underplating mafic magmas in arc settings. These magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes. Copyright