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One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.

We introduce a doubly stochastic method for performing material failure theory based forecasts of volcanic eruptions. The method enhances the well known Failure Forecast Method equation, introducing a new formulation similar to the Hull-White model in financial mathematics. In particular, we incorporate a stochastic noise term in the original equation, and systematically characterize the uncertainty. The model is a stochastic differential equation with mean reverting paths, where the traditional ordinary differential equation defines the mean solution. Our implementation allows the model to make excursions from the classical solutions, by including uncertainty in the estimation. The doubly stochastic formulation is particularly powerful, in that it provides a complete posterior probability distribution, allowing users to determine a worst case scenario with a specified level of confidence. We apply the new method on historical datasets of precursory signals, across a wide range of possible values of convexity in the solutions and amounts of scattering in the observations. The results show the increased forecasting skill of the doubly stochastic formulation of the equations if compared to statistical regression., 27 pages, 13 figures

To better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seismic experiment (Seismic Experiment with Airgun-source - Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufriére Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5-5.5 km s-1) beneath the island, with highest velocities beneath the Soufriére and Centre Hills, corresponding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the offshore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s-1, consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ∼1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0-6.0 km s-1 and is inferred to correspond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ∼5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes. © 2009 The Authors Journal compilation © 2009 RAS.

Strikingly similar examples of edifice collapse and directed blast are the 18 May 1980 eruption of Mount St. Helens (MSH), Washington, USA, and the 30 March 1956 eruption of Bezymianny Volcano (BZ), Kamchatka, Russia. In these cases, flank failures led to near-instantaneous decompression and fragmentation of intra-edifice cryptodome magma, which produced catastrophic, laterally directed blasts. In both instances, the blast products consisted of juvenile material with bimodal density/vesicularity distributions: low- and high-density modes at 1900 and 2400 kg m− 3 for BZ, 1600 and 2300 kg m− 3 for MSH, although the proportion of high-density material is greater at BZ. Blast materials also exhibit striking variety in groundmass crystallinity ( 90 vol.%) despite having fairly uniform pheno-crystallinities, suggesting that degassing-driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes. New bulk-rock H2O and ∂D measurements confirm that progressive open-system outgassing occurred prior to both blasts. The correlations between crystallinity, clast density, and bulk H2O contents suggest that syn-blast magma expansion was modulated both by non-uniform volatile distribution within the cryptodome and rheological controls associated with non-uniform crystal content. Spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin, which probably correlates with timing of magma injection. The greater proportion of high-density material at BZ is speculatively related to lower blast energy compared with MSH.

Emplacement of a debris avalanche in the White River valley of southern Montserrat (Lesser Antilles), on 26 December 1997, was caused by sector failure of the south flank of the active Soufriere Hills volcano. Pre- and post-emplacement surveys of the region indicate a debris avalanche deposit volume of about 40–50 × 106 m³. This avalanche is modeled as the gravitational flow of a homogeneous continuum governed by a basal friction law. Mass and momentum equations are depth-averaged over the slide thickness. Numerical results show that the observed distribution of debris and duration of emplacement is simulated well for a Coulomb-type friction law with a dynamic friction coefficient dependent upon the thickness and the velocity of the flowing mass.

The common occurrence of tree and pole blow-down from pyroclastic currents provides an opportunity to estimate properties of the currents. Blow-down may occur by uprooting (root zone rupture), or flexure or shear at some point on the object. If trees are delimbed before blow-down, each tree or pole can be simulated by a cylinder perpendicular to the current. The force acting on a cylinder is a function of flow dynamic pressure, cylinder geometry, and drag coefficient. Treated as a cantilever of circular cross-section, the strength for the appropriate failure mode (rupture, uprooting or flexure) can then be used to estimate the minimum necessary current dynamic pressure. In some cases, larger or stronger standing objects can provide upper bounds on the dynamic pressure. This analysis was treated in two ways: (1) assuming that the current properties are vertically constant; and (2) allowing current velocity and density to vary vertically according to established models for turbulent boundary layers and stratified flow. The two methods produced similar results for dynamic pressure. The second, along with a method to approximate average whole-current density, offers a means to estimate average velocity and density over the height of the failed objects. The method is applied to several example cases, including Unzen, Mount St. Helens, Lamington, and Merapi volcanoes. Our results compare reasonably well with independent estimates. For several cases, we found that it is possible to use the dynamic pressure equations developed for vertically uniform flow, along with the average cloud density multiplied by a factor of 2–5, to determine average velocity over the height of the failed object.

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986‐88, 1992‐93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts 1 microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab63An29Or8) surrounded by alkali feldspar rims (Ab53An5Or42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (PH2 O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992‐93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic. q 2000 Elsevier Science B.V. All rights reserved.

Slope failures resulting from structural instability of andesitic volcanic edifices can generate mobile debris avalanches that travel long distances down or beyond the flanks of volcanoes. More than 20 major slope failures have occurred worldwide over the past 500 years, a rate exceeding that of caldera collapse. Hazards derive from the debris avalanches themselves, from associated explosive activity that ranges from vertical eruptions (often accompanied by pyroclastic currents) to devastating directed blasts, from associated lahars, and from tsunamis. Collapses of growing lava domes are more frequent, are similar, in many ways, to edifice collapse, and can directly generate devastating pyroclastic currents. This paper examines some aspects of current understanding of edifice and lavadome instability. The primary focus of the presentation is on mechanisms and factors associated with collapse, the geometric factors, augmented loading by magma, localized strength reduction by physical and chemical changes (the latter commonly associated with hydrothermal processes), strain weakening, pore-fluid (water or gas) pressure enhancement, retrogressive failure, time-dependent failure, and seismic shaking. Some aspects of material property evaluation, analysis procedures, and implications on monitoring are also discussed. Case examples discussed include edifice instability at Mt St Helens, USA, and Soufriere Hills volcano, Montserrat, the stability of lava spines at Mont Pelee, Martinique, and Lamington, Papua New Guinea, and lavadome stability at Soufriere Hills. The topics bear on understanding hazardous edifice and dome failures, and the measures to anticipate such failures.

An important type of gravitational lava-dome failure involves significant internal gas overpressure that influences the failure process. For the first time, this paper presents a rigorous analysis of the mechanics of gas-pressurized dome failure. We develop diffusion models to calculate gas overpressures in a lava dome, and embed these pressure data into stability analyses to demonstrate how gas pressurization can initiate instability. The results bear on the mechanistic understanding of hazardous dome failures. Gas pressurization can promote deep-seated failure, and the timing of conduit pressurization can control the timing of dome collapse. Theory explains why dome collapse can be delayed for cases of oscillating effusion. Dome collapses and explosive events linked to collapse of lava domes can be hazardous. Monitoring criteria are considered to anticipate whether or not such collapses are likely to be explosive.

Investigation of well-exposed volcaniclastic deposits of Shiveluch volcano indicates that large-scale failures have occurred at least eight times in its history: approximately 10,000, 5700, 3700, 2600, 1600, 1000, 600 14 C BP and 1964 AD. The volcano was stable during the Late Pleistocene, when a large cone was formed (Old Shiveluch), and became unstable in the Holocene when repetitive collapses of a portion of the edifice (Young Shiveluch) generated debris avalanches. The transition in stability was connected with a change in composition of the erupting magma (increased SiO2 from ca. 55-56% to 60-62%) that resulted in an abrupt increase of viscosity and the production of lava domes. Each failure was triggered by a disturbance of the vol- canic edifice related to the ascent of a new batch of vis- cous magma. The failures occurred before magma in- truded into the upper part of the edifice, suggesting that the trigger mechanism was indirectly associated with magma and involved shaking by a moderate to large volcanic earthquake and/or enhancement of edif- ice pore pressure due to pressurised juvenile gas. The failures typically included: (a) a retrogressive landslide involving backward rotation of slide blocks; (b) frag- mentation of the leading blocks and their transforma- tion into a debris avalanche, while the trailing slide blocks decelerate and soon come to rest; and (c) long- distance runout of the avalanche as a transient wave of debris with yield strength that glides on a thin weak layer of mixed facies developed at the avalanche base. All the failures of Young Shiveluch were immediately followed by explosive eruptions that developed along a similar pattern. The slope failure was the first event, followed by a plinian eruption accompanied by partial fountain collapse and the emplacement of pumice flows. In several cases the slope failure depressurised the hydrothermal system to cause phreatic explosions that preceded the magmatic eruption. The collapse-in- duced plinian eruptions were moderate-sized and ordi- nary events in the history of the volcano. No evidence for directed blasts was found associated with any of the slope failures.

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).

Breakout of magmatic activity at Soufriere Hills volcano, Montserrat, was preceded by a tenfold increase in rate of earthquake occurrence. A new model of subcritical rock failure shows that this increase is consistent with the growth, possibly episodic, of the magma conduit at a rate controlled by progressive weakening of the host country rock. The preferred weakening mechanism is stress corrosion, by which circulating juvenile and hydrothermal fluids chemically attack the country rock and promote failure at stresses smaller than the rock's theoretical strength. The results illuminate the potential for slow-cracking models to enhance eruption forecasts using the inverse-rate technique combined with traditional monitoring methods.

Telemetered high-resolution tiltmeters were installed in Montserrat in summer of 1995, in December 1996, and in May 1997. The 1995 installations, several km from the Soufriere Hills vent, were too distant to yield useful data. However, the 1996 and 1997 installations on the crater rim revealed 6–14 h inflation cycles caused by magma pressurization at shallow depths (< 0.6 km below the base of dome). The tilt data correlated with seismicity, explosions, and pyroclastic flow activity, and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public.

Varied seismicity has accompanied growth and collapse of the lava dome of the Soufriere Hills Volcano, Montserrat. Earthquakes have been classified as either volcano-tectonic, long-period or hybrid, and daily variations in the numbers of events have mapped changes in the style of eruption. Repetitive hybrid earthquakes were common during the first months of dome growth. In July 1996 the style of seismicity changed and regular, short-lived hybrid earthquake swarms became common. This change was probably caused by an increase in the magma flux. Earthquake swarms have preceded almost all major dome collapses, and have accompanied cyclical deformation, thought to be due to a build-up of pressure in the upper conduit which is later released by magma moving into the dome.

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor-sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved.

[1] The dynamics of the May 18, 1980 lateral blast at Mount St. Helens, Washington (USA), were studied by means of a three-dimensional multiphase flow model. Numerical simulations describe the blast flow as a high-velocity pyroclastic density current generated by a rapid expansion (burst phase, lasting less than 20 s) of a pressurized polydisperse mixture of gas and particles and its subsequent gravitational collapse and propagation over a rugged topography. Model results show good agreement with the observed large-scale behavior of the blast and, in particular, reproduce reasonably well the front advancement velocity and the extent of the inundated area. Detailed analysis of modeled transient and local flow properties supports the view of a blast flow led by a high-speed front (with velocities between 100 and 170 m/s), with a turbulent head relatively depleted in fine particles, and a trailing, sedimenting body. In valleys and topographic lows, pyroclasts accumulate progressively at the base of the current body after the passage of the head, forming a dense basal flow depleted in fines (less than 5 wt.%) with total particle volume fraction exceeding 10−1 in most of the sampled locations. Blocking and diversion of this basal flow by topographic ridges provides the mechanism for progressive current unloading. On ridges, sedimentation occurs in the flow body just behind the current head, but the sedimenting, basal flow is progressively more dilute and enriched in fine particles (up to 40 wt.% in most of the sampled locations). In the regions of intense sedimentation, topographic blocking triggers the elutriation of fine particles through the rise of convective instabilities. Although the model formulation and the numerical vertical accuracy do not allow the direct simulation of the actual deposit compaction, present results provide a consistent, quantitative model able to interpret the observed stratigraphic sequence.

It is widely believed that andesitic magmas erupted at arc-volcanoes are stored in shallow reservoirs prior to eruption, but high-resolution images of focused regions of magma in the shallow crust are rare. We integrate seismic tomography with numerical models of magma chamber growth to constrain the magma chamber beneath Soufriere Hills Volcano, Montserrat. Our approach reveals the characteristics and dynamics of the magmatic system with a level of detail that no single method has yet achieved. The integrated analysis suggests that a magma chamber of 13 km3 with over 30% melt fraction formed between 5.5 and at least 7.5 km depth, a significantly higher melt fraction than inferred from the seismic data alone. The magma chamber may have formed by incremental sill intrusion over a few thousand years and is likely to be a transient, geologically short-lived feature. These volume and geometry estimates are critical parameters to model eruption dynamics, which in turn are key to hazard assessment and eruption forecasting.

Benchmarks installed on the upper eastern flank of Mt. Etna in 1982 have subsided continually since then, with the rate of subsidence twice accelerating prior to eruptions. The first of these eruptions was in December 1985, and the second in September 1989. This pattern of accelerating downslope movement has also been observed prior to landslides, and recent work applying knowledge of the failure of materials has shown that analysis of the inverse rate of these movements can be used to predict the time of failure. Post-eruption analyses of geodetic or seismic data from volcanoes has shown that in several cases, this approach could have been used to forecast eruptions weeks in advance. Applying the same principles to the accelerating subsidence on Mt. Etna's eastern flank prior to the eruptions of 1985 and 1989 shows that rough estimates of eruption dates could have been obtained several months in advance. These observations also suggest a speculative eruption-triggering mechanism involving an interplay between slope creep deformation and extensional weakening over the zone of intrusion.

[1] We use time-series of magma efflux and GPS-derived surface deformation observations to constrain the transfer of compressible magma within the crustal plumbing of the Soufriere Hills volcano for three cycles of effusion and pause. Our system model has two vertically-stacked spherical chambers. Deep melt supply to the system is constrained to be continuous and steady, yielding a rate of 1.2 m3/s, which fixes the geometry of the dual interconnected chambers to depths of about 5 and 19 km. The eruptive volume change of the shallow chamber is in-phase and an order of magnitude smaller than the deep chamber. Significantly, the shallow chamber seems to control the periodic system behavior: surface magma efflux resumes when the shallow chamber reinflates to its initial threshold pre-eruptive volume (triggering re-opening of an eruptive feeder dike), and ceases when it has lost 14–22 Mm3 (106 m3) of its volume (sealing the conduit and staunching magma flow). These observations are consistent with eruption re-initiation and re-cessation controlled by magma overpressure thresholds.

Volcanic lateral blasts are among the most spectacular and devastating of natural phenomena, but their dynamics are still poorly understood. Here we investigate the best documented and most controversial blast at Mount St. Helens (Washington State, United States), on 18 May 1980. By means of three-dimensional multiphase numerical simulations we demonstrate that the blast front propagation, final runout, and damage can be explained by the emplacement of an unsteady, stratified pyroclastic density current, controlled by gravity and terrain morphology. Such an interpretation is quantitatively supported by large-scale observations at Mount St. Helens and will influence the definition and predictive mapping of hazards on blast-dangerous volcanoes worldwide. Published Version

[1] Vulcanian explosions with plumes to 12 km occurred at Soufriere Hills volcano (SHV) between July 2008 and January 2009. We report strainmeter and barometric data, featuring quasi-linear strain changes that correlate with explosive evacuation of the conduit at rates of ∼0.9−2 × 107 kg s−1. July and January explosion-generated strains were similar, ∼20 nanostrain at ∼5 km, and interpreted as contractions of a quasi-cylindrical conduit, with release of magmastatic pressure, and exsolution-generated overpressure of order 10 MPa. The 3 December 2008 event was distinctive with larger signals (∼140–200 nanostrain at 5–6 km) indicating that a rapid pressurization preceded and triggered the explosion. Modeling suggests a dike with ENE trend, implying that feeder dikes at SHV had diverse attitudes at different times during the eruption. All explosions were associated with acoustic pulses and remarkable atmospheric gravity waves.

[1] We explore volcanotectonic earthquake activity during the early stages of volcanic activity (1995–96) at Soufriere Hills volcano. We focus on several zones of temporally-confined seismic activity that comprise assemblages of small scale structural elements, >2–4 km distant from the volcano, at depths 2–4 km below sea level, and consider seismicity in relation to regional tectonics and heterogeneity of stiffness and strength as revealed by the SEA-CALIPSO tomography experiment. The clustered seismicity and relatively aseismic zones are interpreted to reflect a broad weakened tectonic zone of ESE trend that crosses Montserrat, and the ascent of a magmatic dike of NNE trend, which altered the stress distribution to promote localized fault movements and caused localized dilatation with changes in pore-fluid pressures, to either weaken or strengthen the rock mass depending on the local polarity of strain.

[1] The SEA-CALIPSO experiment was designed to image structure related to active volcanism beneath the island of Montserrat in the Caribbean. As part of that experiment, over 200 Texan recorders with 5 Hz geophones were deployed in 3 linear arrays at a nominal spacing of 100m, primarily to record an airgun source towed offshore around the island. Because the recorders were operating in continuous mode for three days, a number of shallow microearthquakes under the active summit of Soufriere Hills Volcano (SHV) were also recorded. 20 events were sufficiently well recorded and located that they could be used to identify and map reflections from deep subsurface structure. Here we report on the processing of those recordings as multichannel CMP reflection sources, with emphasis on careful statics correction and coherency enhancement. The resulting reflection images indicate subhorizontal layering at depths between 6 and 19km which we interpret as sills.

gravity waves propagated at ∼30 m s −1 . Eruption volumes estimated from plume height and strain data are 0.32– 0.42 × 10 6 ,0 .26–0.49 × 10 6 , and 0.81–0.84 × 10 6 m 3 ,f or Explosions 3–5 respectively, consistent with quasi‐cylindrical conduit drawdown

[1] The sudden ejection of material during an explosive eruption generates a broad spectrum of pressure oscillations, from infrasonic to gravity waves. An infrasonic array, installed at 3.5 km from the Soufriere Hills Volcano has successfully detected and located, in real-time, the infrasound generated by several pyroclastic flows (PF) estimating mean flow speeds of 30–75 m/s. On July 29 and December 3, 2008, two differential pressure transducers, co-located with the array, recorded ultra long-period (ULP) oscillations at frequencies of 0.97 and 3.5 mHz, typical of atmospheric gravity waves, associated with explosive eruptions. The observation of gravity waves in the near-field (

[1] Campaign and continuous GPS geodetic measurements on Soufriere Hills Volcano, Montserrat are reported from 1995 to 2009, spanning three dome growth and repose episodes. Uniform elastic half-space inversions were used to examine how crustal pressure sources evolved by inverting subsets of all available 3D site data for any given episode using a single Mogi-source. Changes in network topology were also examined. The average best-fitting single Mogi model yields X = 0.3 ± 0.5 km, Y = 0.8 ± 0.4 km, and a depth of Z = 10.4 ± 2.1 km (1-σ, Z positive down), relative to the center of the model domain. The mean Mogi depth for effusive (deflation) versus repose (inflation) is different and significant at >90% confidence, yielding Z = 11.4 ± 2.0 km and 9.3 ± 1.6 km, respectively. A vertical, prolate ellipsoid improves the fit at >95% confidence over a single Mogi source or stacked, two-source model for the 2003–2005 repose and 2005–2007 dome growth episodes.

[1] Most inversions of geodetic data for volcanic systems assume homogeneous media. Here we examine the effect of inhomogeneous media using axisymmetric finite element models of Soufriere Hills Volcano (SHV), to show that homogeneous elastic crustal models underestimate both the depth and excess pressure of the deformation source. Continuous GPS data from 1999 to 2009 are then used to infer that the SHV system has multiple crustal pressure sources. For a dual reservoir volcanic model, a deep reservoir located within the mid crust undergoes volume changes approximately an order of magnitude larger than those of the shallow reservoir.

[1] Noritic anorthosite, gabbroic anorthosite and hornblende-gabbro xenoliths are ubiquitous in the host andesite at Montserrat. Other xenoliths include quartz diorite, metamorphosed biotite-gabbro, plagioclase-hornblendite and plagioclase-clinopyroxenite. Mineral compositions suggest a majority of the xenoliths are cognate. Cumulate, hypabyssal and crescumulate textures are present. A majority of the xenoliths are estimated to have seismic velocities of 6.7–7.0 km/s for pore-free assemblages. These estimates are used in conjunction with petrological models to constrain the SEA CALIPSO seismic data and the structure of the crust beneath Montserrat. Andesitic upper crust is interpreted to overlie a lower crust dominated by amphibole and plagioclase. Xenolith textures and seismic data indicate the presence of hypabyssal intrusions in the shallow crust. The structure of the crust is consistent with petrological models indicating that fractionation is the dominant process producing andesite at Montserrat.

[1] The CALIPSO collaborative volcano monitoring system on the Caribbean island of Montserrat includes observations of strain at depths ∼200 m using Sacks-Evertson strainmeters. Strain data for the March 2004 explosion of the Soufriere Hills Volcano are characterized by large, roughly equal but opposite polarity changes at the two near sites and much smaller changes at a more distant site. The strain amplitudes eliminate a spherical pressure (Mogi-type) source as the sole contributor. The initial changes are followed by smaller recoveries, but with differing relative recovery magnitudes. This dissimilarity requires a minimum of two pressure sources, which we model as a deep spherical pressure source and a shallow dike. The spherical source is fixed at the location derived from data for the massive dome collapse in July 2003. We solve for the best fitting dike plus sphere source combination. The dike geometry is consistent with earlier interpretations of dikes based on GPS data and other lines of evidence.

[1] We use well-documented time histories of episodic GPS surface deformation and efflux of compressible magma to resolve apparent magma budget anomalies at Soufriere Hills volcano (SHV) on Montserrat, WI. We focus on data from 2003 to 2007, for an inflation succeeded by an episode of eruption-plus-deflation. We examine Mogi-type and vertical prolate ellipsoidal chamber geometries to accommodate both mineralogical constraints indicating a relatively shallow pre-eruption storage, and geodetic constraints inferring a deeper mean-pressure source. An exsolved phase involving several gas species greatly increases andesite magma compressibility to depths >10 km (i.e., for water content >4 wt%, crystallinity ∼40%), and this property supports the concept that much of the magma transferred into or out of the crustal reservoir could be accommodated by compression or decompression of stored reservoir magma (i.e., the “magma-sponge”). Our results suggest quasi-steady deep, mainly mafic magma influx of the order of 2 m3s−1, and we conclude that magma released in eruptive episodes is approximately balanced by cumulative deep influx during the eruptive episode and the preceding inflation. Our magma-sponge model predicts that between 2003 and 2007 there was no evident depletion of magma reservoir volume at SHV, which comprises tens of km3 with radial dimensions of order ∼1–2 km, in turn implying a long-lived eruption.

Characterizing internal structures of active volcanoes remains an enigmatic issue in geosciences. Yet studies of such structures can greatly improve hazard assessments, helping scientists to better monitor seismic signatures, geodetic deformation, and gas emissions, data that can be used to improve models and forecasts of future eruptions. Several passive seismic tomography experiments—which use travel times of seismic waves from natural earthquakes to image underground structures—have been conducted at active volcanoes (Hawaii's Kilauea, Washington's Mount St. Helens, Italy's Etna, and Japan's Unzen), but an inhomogeneous distribution of earthquakes compromises resolution. Further, if volcanic earthquakes are dominantly shallow at a given location, passive methods are limited to studying only shallow features. Thus, active source experiments—where seismic waves from the explosion of deliberately set charges are used to image below the surface—hold great potential to illuminate structures not readily seen through passive measures. Published version

THE 'materials science' method for eruption prediction1–3 arises from the application of a general law governing the failure of materials: Ω⊙−α Ω¨ − A = 0, where A and α are empirical constants, and Ω is an observable quantity such as ground deformation, seismicity or gas emission. This law leads to the idea of the 'inverse-rate' plot, in which the time of failure can be estimated by extrapolation of the curve of Ω−1 versus time to a pre-deter-mined intercept. Here we suggest that this method can be combined with real-time seismic amplitude monitoring to provide a tool for near-real-time eruption prediction, and we demonstrate how it might have been used to predict two dome-growth episodes at Mount St Helens volcano in 1985 and 1986, and two explosive eruptions at Redoubt volcano in 1989–90.

Volatiles dissolved in silicic magma at depth exsolve as the magma nears the surface and cause an increase in viscosity of the magma. A model of a volcanic conduit within an elastic medium and a viscosity dependent on the volatile content of the magma produces oscillatory magma flow for a critical range of steady input flow rates. Oscillatory flow is recognized as a fundamental mode of behavior at silicic volcanoes, and understanding it allows improved short-term forecasting of timing and eruption style.

(1) Lava dome eruptions are commonly characterized by large fluctuations in discharge rate with cyclic behaviour on time-scalesranging from hours to decades. Examples include Bezymianny volcano (Russia), Merapi (Java), Santiaguito (Guatemala), Mt St Helens (USA), Mt Unzen (Japan), and Soufriere Hills volcano (Montserrat). Previous models have assumed simple cylindrical conduits for magma transport, but extrusions are mainly fed by dykes, with cylindrical geometries developing only at shallow levels. The widths of dykes embedded in an elastic medium are influenced by local magma pressure, affecting flow rates and system dynamics strongly. We develop a model for magma flow in dykes, which predicts intense pulsations of magma extrusion for the case of a constant source pressure. The period time scale is determined by the elastic deformation of the dyke walls and the length-to-width ratio of the dyke. The dyke acts like a volumetric capacitor, storing magma as pressure increases and then releasing magma in a pulse of extrusion. For the Soufriere Hills volcano, cyclic extrusions with time-scales of a few weeks are predicted for dykes 300-500 m long and 3-6 m wide, matching observations. The model explains the sharp onset of tilt pulsations and seismic swarms. Citation: Costa, A., O. Melnik, R. S. J. Sparks, and B. Voight (2007), Control of magma flow in dykes on cyclic lava dome extrusion, Geophys. Res. Lett., 34, L02303, doi:10.1029/ 2006GL027466.

We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufriere Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition. The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5, 2.5, and 0.05 km3. The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase, and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern 3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC. This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly viscous, phenocryst-rich, variable microlite content) determined the bimodal distributions of juvenile clast density and vesicularity. Thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater, and laterally from the axial region. The proximal stratigraphy of the blast deposits comprises four layers named A, B, C, D from bottom to top. Layer A is represented by very poorly sorted debris with admixtures of vegetation and soil, with a strongly erosive ground contact; its appearance varies at different sites due to different ground conditions at the time of the blasts. The layer reflects intense turbulent boundary shear between the basal part of the energetic head of the PDC and the substrate. Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; its deposition occurred by rapid suspension sedimentation in rapidly waning, high-concentration conditions. Layer C is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated, created by rapid sedimentation under moderate-concentration, weakly tractive conditions, with the uppermost laminated part reflecting a dilute depositional regime with grain-by-grain traction deposition. By analogy to laboratory experiments, mixing at the flow head of the PDC created a turbulent dilute wake above the body of a gravity current, with layer B deposited by the flow body and layer C by the wake. The uppermost layer D of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high-rising buoyant thermal plume derived from the sediment-depleted pyroclastic density current. The strong similarity among these eruptions and their deposits suggests that these cases represent similar source, transport and depositional phenomena.

[1] We explore physical mechanisms controlling formation of a confining conduit plug using 1D, steady-state numerical models of magma ascent. Model results for the well-documented 1997 Vulcanian explosions at Soufriere Hills volcano were compared against subsurface conditions constrained by geophysical and petrologic analysis. We suggest that, if magma is permeable and overpressured and rock surrounding the conduit is permeable, degassing occurs both vertically and through conduit walls. This outgassing creates a region of low-vesicularity, dense magma near the surface (magma plug) which eventually seals the conduit and promotes system overpressure. Driving pressure increases with increasing magma flow rate, hindering volatile exsolution and shifting open-system degassing to shallower levels of the conduit. As a result, increasing magma flow rate for a fixed conduit width creates a vertically thinner plug and increases the magnitude and vertical extent of conduit overpressure. Plug thickness and density are also controlled by magma and edifice permeability.

[1] The collapse of the Soufriere Hills Volcano lava dome on Montserrat in July 2003 is the largest such event worldwide in the historical record. Here we report on borehole dilatometer data recording a remarkable and unprecedented rapid (∼600s) pressurisation of a magma chamber, triggered by this surface collapse. The chamber expansion is indicated by an expansive offset at the near dilatometer sites coupled with contraction at the far site. By analyzing the strain data and using added constraints from experimental petrology and long-term edifice deformation from GPS geodesy, we prefer a source centered at approximately 6 km depth below the crater for an oblate spheroid with overpressure increase of order 1 MPa and average radius ∼1 km. Pressurisation is attributed to growth of 1–3% of gas bubbles in supersaturated magma, triggered by the dynamics of surface unloading. Recent simulations demonstrate that pressure recovery from bubble growth can exceed initial pressure drop by nearly an order of magnitude.

[1] Ground surface tilts recorded proximal to a growing lava dome, and sustained through repeated dome collapses, provide unique geodetic constraints on the depth, size, and overpressures of the shallow magmatic system at Soufriere Hills Volcano. Tilt records identify a single pressure-source inflating with periods ∼3–30 h. Sustained tilt amplitude ratios for paired stations identify the pressure-source at a centroid depth between ∼740 and 970 m. This pressure-source and its robust oscillatory response survived a dome collapse that removed the upper ∼200 m of the magmatic system. Shallow overpressures of 1–4 MPa are confirmed over a radius of 200–340 m; this size suggests pressurization of a fluid-saturated fractured rock mass annulus surrounding a narrow magma conduit.

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. © 2002 The Geological Society of London.

The 8-10 May 1997 eruption of Bezymianny volcano began with extrusion of a crystallized plug from the vent in the upper part of the dome. Progressive gravi- tational collapses of the plug caused decompression of highly crystalline magma in the upper conduit, leading at 13:12 local time on 9 May to a powerful, vertical Vulcanian explosion. The dense pyroclastic mixture col- lapsed in boil-over style to generate a pyroclastic surge which was focused toward the southeast by the steep- walled, 1956 horseshoe-shaped crater. This surge, with a temperature 30 km 2 with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised mas- sive to vaguely laminated, gravelly sand (Md -1.2 to 3.7φ; sorting 1.2 to 3φ) of poorly vesiculated andesite (mean density 1.82 g cm -3 ; vesicularity 30 vol%; SiO2 content ~58.0 wt%). The deposits, with a volume of 5-15×106 m3, became finer grained and better sorted with distance; the maximal diameter of juvenile clasts decreased from 46 to 4 cm. The transport and deposition of the surge over a snowy landscape generated extensive lahars which traveled >30 km. Immediately following the surge, semi-vesiculated block-and-ash flows were emplaced as far as 4.7 km from the vent. Over time the juvenile lava in clasts of these flows became progres- sively less crystallized, apparently more silicic (59.0 to 59.9 wt% SiO 2 ) and more vesiculated (density 1.64 to 1.12 g cm-3; vesicularity 37 to 57 vol%). At this stage the eruption showed transitional behavior, with mass di- vided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column ~14 km high, from which platy fallout clasts were deposited (~59.7% SiO 2 ; density 1.09 g cm-3; vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a to- tal of ~0.026 km 3 dense rock equivalent of magma, with an average discharge of ~1.2×104 m3 s-1. A lava flow ~200 m long terminated the eruption. The evolutionary succession of different eruptive styles during the explo- sive eruption was caused by vertical gradients in crystal- lization and volatile content of the conduit magma, which produced significant changes in the properties of the erupting mixture.

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 SO2 emission rates, measured by COSPEC, correlate with the amplitude of these tilt cycles, and that higher rates of SO2 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. (C) 2000 Elsevier Science B.V. All rights reserved.

Nuées ardentes associated with dome collapse on 22 November 1994, at Merapi volcano traveled to the south-southwest as far as 6.5 km, and collectively accumulated roughly 2.5-3 million cubic meters of deposits. The damaged area comprises 9.5 km2 and is covered by two nuée ardente facies, a conventional "Merapi-type", valley-fill block-and-ash flow facies and a pyroclastic surge facies. The proximal deposits reflect the accumulation of dozens of nuées ardentes, with many subsidiary flow units. The distal deposits are more simply organized, as only a few individual events reached to distances >3.5 km. The stratigraphic relationships north of Turgo hill indicate that the surge deposits are a facies of particularly mobile nuées ardentes that also deposited channeled block-and-ash flow facies. They further suggest that the surge facies beyond the channel margins correlate laterally with a finer-grained sublayer locally developed at the base of the block-and-ash flow facies. Eyewitness reports suggest that the emplacement of the block-and-ash flow facies in the distal part of the Boyong river may have followed, by a short time interval, the destruction and deposition of the surge facies at Turgo village. The stratigraphy is in accord with the eyewitness reports. The surge facies was emplaced by a dilute surge current, detached from the same dome-collapse nuée ardente that, as a separate flow unit, subsequently emplaced the distal block-and-ash deposit in the Boyong valley. The detachment occurred at higher elevations, likely at or above the slope break at about 2000 m elevation. This flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block-and-ash flow continued its tortuous southward movement in the Boyong channel. Dome-collapse nuée ardente activity formed the bulk of the eruption, which was accompanied by virtually no significant vertical summit explosive activity.

—The 1996 subaquatic explosive eruption near the northern shore of Karymskoye Lake in Kamchatka, Russia, generated multiple tsunamis. We document the explosive process that produced the tsunamis, and describe the tsunami effects and runup around the 4-km diameter lake. These data enable the determination of an attenuation relation of runup (wave) height for these “explosive” tsunamis, which is compared with theoretical models of wave height distributions. For the proximal zone, involving radial distances (r) up to 1.3 km from the source, the runup height (R) shows rapid attenuation (from > 30 m to 8 m) with distance as log R = −1.98 log[r] + 2.6. For the distal zone, r > 1.3 km, involving mainly wave travel southeastwards along the body of the lake away from the explosion source, R decays more slowly (from 8 m to 3 m) as log R = −0.56 log[r] + 1.9. Rapid decay in the proximal zone suggests that near the source of the explosion, the tsunami propagated radially as a collapsing wave (bore) with discontinuous change in height. The break-in-slope of the runup plot at 1.3 km suggests that beyond this distance the tsunami propagated approximately as a decaying one-dimensional wave in a channel of approximately constant width.

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.

Volcanology became a modern, interdisciplinary science after two deadly eruptions in 1902 killed 36,000 people and researchers realized the need for a better understanding of volcano behavior. In a Perspective, the staff of the Montserrat Volcano Observatory give an overview of what is known about a current hazard: the Soufriere Hills volcano.