Soufriere Hills Research Articles

Topographical Effects on Volcano Deformation Signal Intensity: Implications for GPS Network Configuration

AbstractVolcano GPS networks can capture vital information during volcanic unrest to aid with hazard assessment and eruption forecasting, but can be hindered by their discrete point locations and possibly miss key spatial information. We show how numerical models can reveal controls on spatial deformation signal intensity compared against GPS network design. Using the GPS network at Soufrière Hills Volcano (SHV), Montserrat, and a range of models, we explore expected surface deformation patterns. Peak horizontal deformation is located offshore, highlighting the difficulties with geodetic monitoring on small ocean‐island volcanoes. Onshore areas where the deformation signal is expected to be high are also identified. At SHV, topography plays a greater role in altering the relative distribution of surface displacement patterns than subsurface heterogeneity. Our method, which can be adapted for other volcanoes, highlights spatial areas that can be targeted for effective GPS station placement to help improve deformation monitoring efficiency.

Environmentally associated colour divergence does not coincide with population structure across Lesser Antillean anoles

Abstract Species distributed across heterogeneous environments may undergo local adaptation, which can be limited by the homogenizing effects of gene flow. Lesser Antillean anoles exhibit dorsal colour variation associated with dramatic shifts in environment across small spatial scales, providing an excellent system for studying the maintenance of local adaptation despite ongoing gene flow. The 1995 Soufriere Hills eruption on Montserrat provides an opportunity to understand how natural disasters may influence the evolution of insular species. Thus, our study had two main objectives. First, we collected genetic, phenotypic, and environmental data to investigate whether environmentally associated dorsal coloration reflects underlying population structure in three species of Lesser Antillean anoles. Second, we examined the short-term evolutionary consequences of volcanism by comparing pre- and posteruption samples of Anolis lividus using multilocus sequence data. We found all three species exhibited genetic isolation-by-distance but not isolation-by-environment. Furthermore, phenotypic differences within each species were not predicted by genetic distances, suggesting gene flow across colour morphs. We confirmed the repopulation of A. lividus in the impacted area and found genetic diversity equivalent to pre-eruption sampling. Our findings contribute to our understanding of local adaptation in spatially small-scale systems and the impact of catastrophic natural disasters on population structure.

Declining magma supply to a poroelastic magma mush explains long-term deformation at Soufrière Hills Volcano

Volcano deformation studies traditionally consider melt-dominated magma reservoirs, often overlooking the significant role of poroelastic mush in modifying surface deformation. Here, we analyze the deformation of Soufrière Hills Volcano (SHV) with a focus on a mush-dominated, poroelastic magma reservoir, drawing on temporal deformation data from 14 continuous GPS stations during the ongoing intra-eruptive unrest period. We implemented a 3D Finite Element model and optimization to simulate the observed deformation. Our results reveal that the deformation is likely driven by ongoing, melt injection (Q = 1.1 m³ sec⁻¹) into a low permeability (k = 4.7 × 10⁻¹⁰ m²) reservoir with the injection to the base of the reservoir at 16.5 km depth below sea level. Our findings highlight temporal variations in the melt injection rate to fit the decreasing GPS-recorded deformation rates. An initial injection rate Qi = 1.9 m³ sec⁻¹ inferred at the start of our study period decreases to Qf = 0.3 m³ sec⁻¹ over a 12-year period (2010–2022). Exploratory forward modeling suggests that if current trends continue, the end of magma supply to our modelled reservoir could occur around June 2024 ± 2 years. However, this does not imply the end of associated volcanic hazards at SHV, as the poroelastic diffusion of melt will continue, causing redistribution of melt, surface deformation, and potentially initiating reservoir rupture. While our models offer new insights, inherent limitations in our simulations include interdependencies among our explored model parameters that would benefit from further refinement. Despite these limitations, the study offers crucial guidance on understanding and forecasting volcano deformation dynamics, particularly for volcanoes like SHV with crystal-rich magma reservoirs.

Magma storage conditions over the past 4 Ma on Martinique Island, Lesser Antilles

We constrain, for the first time, the storage conditions beneath recent (<5 Ma) volcanic edifices in Martinique Island, combining petrography, geochemistry, and thermobarometry. Specifically, we investigate Morne Jacob volcano (5.2–1.5 Ma), Pitons du Carbet complex (998 ka – 322 ka), Trois Îlets Volcanic field (2.35 Ma – 346 ka), and Mt. Conil – Mt. Pelée system (<550 ka). Lava samples range in composition from basalt to dacite, and in age from 4.1 Ma (Morne Jacob) to 1929 CE (Mount Pelée). For the last ∼5 Ma, the volcanoes in Martinique were fed by andesitic to dacitic magma stored in the upper 6 km of the crust, at temperatures between 900 and 1100 °C. These shallow reservoirs were frequently replenished and remobilized by basaltic and basaltic-andesite magmas originating from the lower crust with temperatures as high as 1200 °C. Erupted products contain simple to more complex mineral assemblages, and plagioclase phenocrysts exhibit textures and compositional profiles typical of antecrysts, identified as (1) normal, (2) oscillatory, (3) sieved, (4) resorbed core, and (5) resorbed rim. All complexes also show a crystallization gap in the middle crust (10–18 km). Trois Îlets, a distributed volcanic field, is the only complex that shows continuous crystallization through the entire crust, distinct magma batches and storage regions for each individual volcano. More importantly, we see drastic changes in terms of rock composition, storage conditions, and crystal populations in pre - and post – collapse lavas. Large flank collapse (> 5 km3) can induce decompression as large as 5 MPa in the upper crust and as low as 0.1 MPa down to 10–12 km. Such decompression magnitudes might be sufficient to enhance ascent of basaltic magma from depth, initiate dike intrusions in the mid to upper crust, and remobilize shallow silicic reservoirs. Such events are likely to initiate magma mixing in shallow reservoirs. This noteworthy eruption-triggering mechanism may lead to the incorporation of distinct crystal populations and subsequently contribute to heightened rates of eruption. In summary, the magma storage conditions and plumbing system architecture for the recent volcanoes in Martinique closely resemble those observed in other volcanoes within the Lesser Antilles Island Arc, such as Soufriere Hills Volcano or La Soufriere de Guadeloupe. These newly established correlations offer valuable insights that can aid in the assessment of potential future volcanic eruptions.

Distinguishing shallow from mid-crustal magmatic processes at Soufrière Hills Volcano using Finite Element Modelling and co-analysis of EDM and GPS data

We modelled ground deformation at Soufrière Hills Volcano (SHV), using data collected by the Montserrat Volcano Observatory from 2010–2019. We investigate the combined use of Electronic Distance Measuring (EDM) and Global Positioning System (GPS) to distinguish shallow from mid-crustal magmatic processes and their surface deformation profiles. Our results suggest that the EDM network responds predominantly to changes in the shallow magmatic system, whereas GPS records variation at mid-crustal levels. In addition, we show that the behaviour of the EDM network, and of the GPS site HERM, can be explained by underpressurisation in a shallow dyke conduit orientated NNW–SSE, while the mid-crustal system was still undergoing pressurisation. The modelled dyke may be responding to magma cooling and contraction associated with a previous intrusion. We find that geodetic monitoring coverage of multiple flanks within 1 km of the vent can improve our understanding of shallow magmatic system processes with asymmetric deformation fields.

Micronutrient Mineral and Nutrient Content of Volcanic Soils and Creeks from the Montserrat Soufriere Hills Volcano. Nature’s Fertilizer: Mineral Nutrient Content of Volcanic Soils

Agricultural soils and potable water supplies provide a major source of minerals known to be essential for plant and animal health. After several hundred years of relative quiescence, the Soufriere Hills Volcano of Montserrat roared back to life in 1995 through 1997, wreaking severe havoc and severe damage to the geography and ecosystems and serious immediate and chronic health risks to the remaining inhabitants of the Island. The pyroclastic ash flow reached temperatures of 1500 °F and progressed down the hillsides at speeds of 80 mph, causing incineration and virtual destruction of everything in its path, while the gaseous and particulate laden plume reached altitudes of 7 miles (10.7 km), impacting air quality and aircraft navigation in the region. A total of 19 individuals became entrapped in the lava flow, and perished quickly as a direct result of the volcanic eruption. Volcanic soils derive from deeper layers of the earths surfaces and are notoriously rich in mineral nutrients representative of those deep mineral deposits. While virtually all combustible objects in the path of the raging ash flow become incinerated within minutes, the aftermath of the eruptions delivers the mineral rich constituents over a wide swath of the surrounding geography often reaching miles from the volcanic origin. Soils and consumable water sources are well known to provide a reliable community source of essential minerals in human nutrition, and in their chronic absence, numerous pathologic sequela may develop. To determine the mineral content of surrounding surface soils, samples of surface ash layers and nearby creeks were obtained from 7 regions and 3 free-flowing creeks and subjected mineral analysis. The surface ash specimens contained abundant amounts of the nutrients Iron, Calcium, Sodium, Magnesium, Sulfur, and non-nutrient Aluminum, intermediate amounts of potassium, Phosphorous, and Manganese, and small amounts of Boron, Copper and Zinc. Creek water runoff originating from higher elevations was consistently acidic with a healthy mean pH of 5.1, and contained smaller proportions of the same minerals, reflective of their relative effective solubility in the acidic water. Thus, the volcanic ash flow while devastating to the environment and public health in the immediate aftermath, provided numerous minerals important to plant and animal nutrition and to human health and health water sources which contributed to the longer-term reinstitution of edible plants and to the recovery of the vibrant lush greenery of the Emerald Isle of the Caribbean.

GEOCHEMISTRY AND MINERAL CONTENT OF ASH FLOW FROM THE MONTSERRAT SOUFRIERE HILLS VOLCANO

The Soufriere Hills Volcano of Montserrat has ancient origins and roared back to life in 1995 wreaking severe havoc and damage to the inhabitants, geography, and ecosystems of the Island. The pyroclastic ash flow reached temperatures of 1500 °F and progressed down the hillsides at speeds of 80 mph, causing incineration and virtual destruction of everything in its path, while the gaseous and particulate laden plume reached altitudes of 35,000 feet, impacting air quality and aircraft navigation. The local health authorities recommended masking and physical protection from any potentially harmful effects from the ash to all residents who remained on Island following the 1995 eruption. Volcanic ash can be an irritant to exposed skin and respiratory surfaces upon contact due in part to the sharp and jagged nature of the microcrystalline particulate matter. In addition, the minute size of the smallest particulate matter can impact deep into the unprotected respiratory tree in humans and animals, potentially causing severe respiratory injury. The composition and physicochemical nature of the ash was largely unknown in 1995 and subsequent years. The current study was designed to determine the geochemical composition of the ash. Samples of ash were collected from 7 locations and water samples from three free flowing creeks in Montserrat and indicated that the creek water was acidic in nature. The mean pH of water was 5.2, while the ash showed abundant levels of Iron, Phosphorus, Calcium, Magnesium, Sodium, Aluminum, and sulfur, in addition to lower concentrations of potassium, boron, copper and zinc and intermediate concentrations of Manganese and was devoid of toxic minerals including lead and mercury. The emergence of acute respiratory issues was not found to exceed similar events in other healthy populations including individuals relocated to the UK. In conclusion, direct human and animal exposure to Montserrat volcanic ash was deemed non-harmful to humans when adequate protective measures including masking and exposure precautions were undertaken, while the rich mineral content of the ash can provide a rich source of micronutrient mineralization to soils and surrounding seawaters and support vigorous revegetation and recovery of affected land and sea zones throughout the island community.

The volcanic ash record of shallow magma intrusion and dome emplacement at Nevados de Chillán Volcanic complex, Chile

Volcanoes are often monitored by geophysical and geochemical instruments that aim to track and anticipate their eruptive activity. However, assessment of the state of the volcano at any given time, and its evolution towards eruption or change in eruptive activity is notoriously difficult. Once explosive activity has begun, the study of volcanic ash can provide crucial insights on whether the activity is mainly driven by the hydrothermal system, shallow gas accumulation, or/and a new stalled intrusion close to the surface. We present the results of a study of ash componentry from part of the current volcanic crisis that started in December 2015 at Nevados de Chillán Volcanic complex (Chile) which we integrate with seismic and visual data. We identified three main stages: (i) an early one that lasted for about a year and includes two months of increased seismicity and significant amount of juvenile ash fragments, and thus suggesting some explosions were fed by a shallow magma intrusion. (ii) A second one which lasted for about six months with cycles of quiescence and explosions, and a predominance of lithic particles in the ash, suggesting that the explosions were probably driven by the dynamics in the upper part of the system, including shallow gas accumulation or/and the hydrothermal system, rather than by fresh magma intrusion. (iii) Finally, after about two years of unrest and intermittent explosions, seismicity increased again and the ash became dominated by juvenile particles, and led to the extrusion of a dome. The timing and sequence of events that we report is broadly similar to other volcanoes that have produced dome eruptions such as Soufriere Hills (Montserrat), Unzen (Japan) and Sinabung (Indonesia). Our study highlights the usefulness of integration of volcanic ash studies with other monitoring data and importance of integration of many case studies to gain a more comprehensive understanding of the processes and evolution of dome-forming eruptions.

Integration of Medical and Graduate Education with the USAT Secure SSPOC Instructional Model

Emergence of the 2019 Coronavirus pandemic posed new challenges for the delivery of medical and graduate education by Institutions throughout most of the world, as travel restrictions and regional lockdowns placed new demands never previously imposed on higher education. The University of Science Arts and Technology [USAT], located on the volcanic island of Montserrat, British West Indies began to prepare for such an eventuality since 2007 due to new volcanic eruptions. Despite 24/7 monitoring from the nearby state-of-the-art Montserrat volcano observatory, with little warning, the Soufriere Hills Volcano, located a distant 5 miles Southeast of the Campus spontaneously roared back to life in January 2007, venting clouds of volcanic ash and gas 8 kilometers (5 miles) into the sky with overflow lava and debris reaching the sea to the East and West; the volcano's lava dome continued to grow as it had since 2006, and local volcano experts worried that the rapidly growing lava dome, the mounting pressure building inside the dome could cause a catastrophic explosive eruption, and the renewed volcanic activity indicates that the volcano is now predicted to remain active well into the future. The volcanic activity caused the partial evacuation of the Island, including areas bordering the campus, resulting in the establishment of the USAT Campus satellite transmitter system [CST] in 2007. The CST enabled USAT the establishment of the first secure network on Island for the Secure Small Platform Online Course delivery network [SSPOC] and which enabled USAT to continue lecture operations uninterrupted with faculty that are highly experienced in the delivery of high quality Medical and Graduate education from both on-Island and remote locations in an efficient, timely, cost effective and highly productive and timely manner, while enabling students an on-island campus experience during periods of volcanicactivity including options for lectures, public health, epidemiology and environmental research, and selected experiences in areas of family and tropical medicine. During the current COVID pandemic and travel restriction imposed we further enhanced our on-line teaching system and secure SSPOC model, expanded International options, and as professors and students could not travel to the campus but all lectures were delivered online with absolutely no disruption or compromise in delivery of exceptional quality lectures and teaching experiences continued as if they occurred on campus.

U-series histories of magmatic volatile phase and enclave development at Soufrière Hills Volcano, Montserrat

Injection of volatile-rich mafic magma prior to an eruption may trigger episodes of volcanism and can act to transfer metals from depth. However, petrologic knowledge of the timescales from mafic injection to eruption have thus far been focussed on mineral-scale studies of chemical zoning patterns. The study of mafic enclaves dispersed within eruption products can provide insights into the interaction between deep and shallow reservoirs. We combine 238U-230Th-226Ra-210Pb isotope data with trace element concentrations across the interface of two contrasting mafic enclaves in contact with their host andesite from the 2010 eruption at Soufrière Hills Volcano (SHV), Montserrat to investigate the history of mass exchange between the mafic enclave and the andesite host. The application of these time-sensitive isotopes highlights complexities in the transfer of volatiles and metal elements between magmas and the enclaves' potential as eruption triggers. The enclaves exhibit (210Pb/226Ra)0 ratios >1 consistent with volatile input to the subsurface plumbing system a few decades prior to eruption. Samples of the andesitic host, however, which make up the bulk of the eruptive products, have (210Pb/226Ra)0 ≤ 1 suggesting no net volatile gain in the decades leading up to eruption, or that melt-volatile interaction is on a timescale unresolvable by 210Pb226Ra systematics (i.e. <2 years). Variations in trace elements such as Cu, Pb and Ba show loss of a magmatic volatile phase and transport of metals within the deeper part of the plumbing system during differentiation of magmas feeding SHV. Our results do not support that volatile transfer into the andesite via enclaves is a direct trigger of explosive eruptions although the enclaves are likely syn-eruptively formed. 238U-230Th-226Ra-210Pb and trace element systematics at SHV support a role for fresh magma influx during periods of unrest, but long-term accumulation of the andesite.

The 1951 eruption of Mount Lamington, Papua New Guinea: Devastating directed blast triggered by small-scale edifice failure

The catastrophic explosion of Mount Lamington volcano, Papua New Guinea on January 21, 1951 produced a devastating pyroclastic density current (PDC) that knocked down dense tropical rainforest over an area of 230 km2 and killed approximately 3000 people. We present results of a field reinvestigation of the 1951 PDC deposit combined with an analysis of the available photographs and eyewitness accounts of the eruption first published in the fundamental work of G. A. M. Taylor (1958).We have concluded that the six-days-long pre-climactic activity before the 1951 eruption (which included felt local seismicity, frequent ash-laden explosions of vulcanian type, bulging of the volcano slope accompanied with landslides) was associated with shallow-level intrusion of a highly viscous magma body (cryptodome/dome) of andesitic composition with a volume of approximately 0.01 km3. This intrusion destabilized Mount Lamington's prehistoric intra-crater lava dome.On January 21 the destabilized dome gravitationally collapsed and produced a relatively small-volume debris avalanche, the deposit of which was not recognized during Taylor's original investigation. The debris avalanche had a volume of approximately 0.02–0.04 km3, travelled a distance (L) of 8.5 km and had the ratio of vertical drop (H) to runout (L) of 0.14. The edifice collapse decompressed the intruding cryptodome and triggered its explosive fragmentation.Photographs of the climactic explosion show that the eruptive cloud initially rose vertically but subsequently collapsed upon the terrain around the vent, and formed a PDC which flowed radially outward. The enhanced northward propagation of the PDC to a maximum distance of 13 km reveals that the northern breach in the ancient crater's high walls influenced the distribution of the deposit. In the studied NE-N-NW sector of the devastated area, in the zone proximal to the volcano, the PDC emplaced a normally graded layer of coarse ash and lapilli mixed in the base with picked-up soil and plant fragments. The layer gradually becomes thinner and finer-grained with distance from the volcano. The PDC deposit has a volume of approximately 0.025 km3 and consists of approximately 80% juvenile rock fragments derived from the explosively fragmented cryptodome. The remaining 20% consists of accidental clasts derived from the old volcanic edifice. The juvenile material is crystal-rich andesite with a unimodal vesicularity distribution (4 to 36%). The reconstructed eruption sequence, the PDC tree blowdown pattern and characteristics of the PDC deposit are similar to those of catastrophic laterally-directed blasts of volcanoes Bezymianny in 1956, Mount St.Helens in 1980, and Soufriere Hills, Montserrat in 1997. In contrast to the cases of these “classic” lateral blasts, the blast cloud of Lamington was initially vertically-directed before collapsing to produce a PDC. We speculate that the climactic explosion of Mount Lamington was initially vertical because the rupture surface of the triggering sector collapse intersected the apex of the intruding cryptodome (it exposed a subhorizontal surface of the cryptodome apex), while at Bezymianny, Mount St.Helens, and Soufriere Hills the rupture intersected the main body of the cryptodome/dome, and exposed their steeply inclined surfaces.

Capturing, Preserving, and Digitizing Legacy Seismic Data from the Montserrat Volcano Observatory Analog Seismic Network, July 1995–December 2004

Abstract An eruption of the Soufrière Hills Volcano (SHV) on the eastern Caribbean island of Montserrat began on 18 July 1995 and continued until February 2010. Within nine days of the eruption onset, an existing four-station analog seismic network (ASN) was expanded to 10 sites. Telemetered data from this network were recorded, processed, and archived locally using a system developed by scientists from the U.S. Geological Survey (USGS) Volcano Disaster Assistance Program (VDAP). In October 1996, a digital seismic network (DSN) was deployed with the ability to capture larger amplitude signals across a broader frequency range. These two networks operated in parallel until December 2004, with separate telemetry and acquisition systems (analysis systems were merged in March 2001). Although the DSN provided better quality data for research, the ASN featured superior real-time monitoring tools and captured valuable data including the only seismic data from the first 15 months of the eruption. These successes of the ASN have been rather overlooked. This article documents the evolution of the ASN, the VDAP system, the original data captured, and the recovery and conversion of more than 230,000 seismic events from legacy SUDS, Hypo71, and Seislog formats into Seisan database with waveform data in miniSEED format. No digital catalog existed for these events, but students at the University of South Florida have classified two-thirds of the 40,000 events that were captured between July 1995 and October 1996. Locations and magnitudes were recovered for ∼10,000 of these events. Real-time seismic amplitude measurement, seismic spectral amplitude measurement, and tiltmeter data were also captured. The result is that the ASN seismic dataset is now more discoverable, accessible, and reusable, in accordance with FAIR data principles. These efforts could catalyze new research on the 1995–2010 SHV eruption. Furthermore, many observatories have data in these same legacy data formats and might benefit from procedures and codes documented here.

Vertically Extensive Magma Reservoir Revealed From Joint Inversion and Quantitative Interpretation of Seismic and Gravity Data

AbstractRecent advances in our understanding of arc magmatic systems indicate that melt is stored for long periods in low‐melt fraction crystal mushes and that eruptible magma reservoirs are short‐lived and are assembled rapidly by buoyancy‐induced instabilities and draining of the crystal mush. Many aspects of their architecture remain unclear, particularly in relation to their geometry and shallow melt distribution. We investigate the storage of melt below the active Soufrière Hills Volcano (SHV), Montserrat, using joint geophysical inversion combined with a quantitative interpretation approach based on rock physics. We jointly inverted active‐source P‐wave traveltimes and gravity anomalies to derive coincident 3‐D models of P‐wave velocity and density to a depth of 8 km. Comparative analysis of the active SHV and extinct Centre Hills volcano and effective elastic medium computations allow us to constrain temperature, melt fraction, and melt geometry. A continuous column of partial melt is inferred beneath SHV, at 4–8 km depth. Melt fraction is ~6% (ranging from 3 to 13% depending on melt geometry) and is maximum at 5–6 km depth. When under‐recovery of the low‐vP volume is taken into account, the melt fraction is revised to ~17% (ranging from 11 to 28%). Analysis of vP/density cross plots indicates that the melt distribution is best represented by low‐aspect ratio geometries. These likely span a multiscale spectrum ranging from grain‐scale inclusions and fractures to 100‐m‐scale dykes and sills. Our results confirm the concept of vertically extensive crystal mush including one or multiple more melt‐rich layers.

Counterfactual Analysis of Runaway Volcanic Explosions

If your volcano is erupting, “The past is a nice place to visit, but certainly not a good place to stay…”. One of the major challenges in volcano crisis decision-making arises when unrest escalates and the threat could exist of a massive sudden eruption, despite information suggesting past eruptive activity had been more limited. Even a small chance of this happening may warrant an urgent call for evacuation. For most active volcanoes, there is only a very limited geological and historical record upon which to base an estimate of the chance of a massive eruption. However, this database may be expanded by stochastic modelling of past notable volcanic crises that had the dangerous potential but did not ultimately result in a massive eruption. The conceptual framework for the counterfactual analysis of runaway volcanic explosions is presented here, with reference to other extreme geohazards and georisks. This innovative type of probabilistic analysis has widespread application and is illustrated with the example of the well-documented 1997 Montserrat Vulcanian explosions sequence. An alternative possible mode of eruptive behaviour might have substituted this sequence with fewer but larger explosive eruptions or even by a single runaway extreme event. This latter contingency was considered at the time, and motivated a brief, temporary evacuation of the building then housing the Montserrat Volcano Observatory; in response to the escalating violence of explosions, the base for observatory operations was subsequently re-located much further away from the volcano.

Livelihoods, Wellbeing and the Risk to Life During Volcanic Eruptions

A forensic analysis of fatalities and displacements from recent volcanic eruptions (1986-2015) provides insights into factors that influence actions to protect life in high-risk environments. Unlike many other geophysical hazard events, volcanic eruptions may be prolonged, and of variable intensity. This is reflected in patterns of volcanic fatalities. A global survey reveals that 63% of primary volcanic deaths occur after the first week of activity, with >44% of these deaths associated with citizens returning to an established high-hazard zone. Evacuations during volcanic eruptions are protracted and this allows time for competing pressures to arise. Examination of detailed data from three volcanic crises (Soufriere St. Vincent, Soufriere Hills, Montserrat and Tungurahua, Ecuador) suggests that the need to preserve livelihoods plays a strong role in protecting life. A dynamic, associated with pull (e.g. protecting assets, place attachment) and push factors (e.g. poor shelter conditions), can draw evacuees to return during high-risk periods. Similar considerations can restrain people with previous experience of volcanic hazards and displacement, from evacuating. Our global analysis shows that these pressures, when coupled with forecasting uncertainties and the rapid landscape change associated with volcanic eruptions, mean that the physical and social vulnerability of populations change significantly during the course of an eruption. Ongoing risk to life is shaped by hazard experience and action; timescales of hazard escalation and their relationship to warning and action; and the timescales over which evacuation conditions are tolerable to livelihood and asset preservation, and mental and physical wellbeing in shelters.

Volatile behaviour in the 1995-2010 eruption of the Soufrière Hills Volcano, Montserrat recorded by U-series disequilibria in mafic enclaves and andesite host

Mafic enclaves brought to the surface in volcanic eruptions offer insight into the deeper sections of a volcanic plumbing system, where the degree of interaction between deeper, mafic magma and the magma reservoir directly feeding a volcanic edifice is not well-constrained. At Soufrière Hills Volcano (SHV) on the island of Montserrat, Lesser Antilles, mafic enclaves have been a ubiquitous feature of the andesitic eruptions during the five phases of the 1995-2010 eruption. We use the short-lived isotopes 210Pb and 226Ra within the Uranium series decay chain, which are sensitive to volatile transfer and loss (due to the loss or gain of the intermediary daughter 222Rn), in both the enclaves and the host andesite to reveal significant time-related information. The sequentially erupted andesites are almost entirely in equilibrium or have deficits of 210Pb with the deficits becoming more pronounced over several eruptive phases. We model that the andesitic reservoirs involved were subject to continuous volatile loss both before and during the eruption. The majority of enclaves, however, have excesses of 210Pb, showing volatile enrichment that lasted over a decade. The highest (210Pb/226Ra)0 ratios are from enclaves in Phase II, indicating that the deeper mafic system was closed to fresh gas influx from Phase III onwards. Enclaves are modelled as being sporadically entrained in the andesite prior to eruption, suggesting that enclave formation is not a triggering mechanism for each eruptive phase.

Using microlites to gain insights into ascent conditions of differing styles of volcanism at Soufrière Hills Volcano

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn-extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufrière Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008).Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn-extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn-extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn-extrusive ash venting compared with the lava dome.Calculated magma ascent rates for Vulcanian explosions and syn-extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth).

Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints

Abstract Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR) experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s−1 slip rate and 1 MPa normal stress) using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1) an amphibole-bearing andesite (Soufriere Hills Volcano, Montserrat); (2) an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3) an amphibole-free andesite (Volcan de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1) at the onset of melting, (2) upon formation of a steady-state melt layer, and (3) after 5 m, (4) 10 m, and (5) 15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the control of melt composition on slip behaviour. Amphiboles melted preferentially, leading to consistently lower shear stress (∼1 MPa less than amphibole-poor samples) and pronounced shear-weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblages on fault slip, including conduit flow processes, which may influence the style of eruptions, and run-out distances of rapid granular flows.

Investigation of surge-derived pyroclastic flow formation by numerical modelling of the 25 June 1997 dome collapse at Soufrière Hills Volcano, Montserrat

Deposits from ash-cloud surges associated with dome collapse can, under certain conditions, be remobilised to form surge-derived pyroclastic flows (SDPFs). Using numerical modelling, we reproduce the emplacement of these flows and investigate the conditions that favour their genesis. We use the new version of the numerical model VolcFlow, which simulates the two components of a pyroclastic flow: the basal avalanche and the overriding ash-cloud surge. The basal avalanche (primary block-and-ash flows and SDPFs) are simulated using three previously published rheological laws: plastic, frictional and frictional velocity-weakening rheologies. Applied to the 25 June 1997 dome collapse at Soufriere Hills Volcano, the models reproduce to different degrees the deposit footprints formed by the block-and-ash flows, the ash-cloud surges and the SDPFs. In the plastic model, SDPFs occur if the ash-cloud surge deposit exceeds a threshold thickness that allows it to remobilise and flow. In the frictional models, SDPFs occur only if ash-cloud surge deposition takes place on a slope exceeding the friction angle of the ash. Results also highlight that SDPFs appeared so clearly in 1997 at Montserrat due to a combination of topographic factors: (i) a bend in the Mosquito Ghaut drainage that allowed the ash-cloud surges to detach, (ii) a depositional area on the watershed between the eastern and western drainage channels and (iii) a network of tributaries that drained all the remobilised mass into Dyer’s River to form a single, large SDPF. Our model could be a promising tool for the future forecasting of hazards posed by surge-derived pyroclastic flows.

Role of Feldspar and Pyroxene Minerals in the Ice Nucleating Ability of Three Volcanic Ashes

The mineralogical and immersion-mode freezing properties of volcanic ashes from three volcanoes, Volcan de Fuego and Santiaguito in Guatemala, and Soufriere Hills Volcano, in Montserrat, were exami...