Andesitic Lava Dome Research Articles

Species Composition of Bryophytic Flora along Altitudinal Gradient in Mt. Musuan, Bukidnon, Philippines

A floristic assessment was conducted in Mt. Musuan Bukidnon, the Philippines – an isolated, low grass-covered andesitic lava dome and tuff cone. Two 50-m transect lines were established in each of the altitudinal gradients (350–400, 401–451, 452–502, 503–553, 554–604, and 605 + masl) in the mountain slope to sample the bryophyte communities. The study compared the bryophyte populations between six altitudinal gradients and analyzed their altitudinal distribution. Results revealed a total of 56 species of bryophytes belonging to 39 genera and 21 families. Of these, 39 are moss species with 29 genera and 16 families. Sixteen (16) species are liverworts with nine genera and four families. One hornwort was encountered. The greatest number of bryophytes were documented between 452–502 masl, whereas the fewest number occurred between 350–400 and 605 masl and near the summit. The two most abundant bryophyte species were Octoblepharum albidum Hedw. and Hyophila involuta (Hook.) A. Jaeger, which are widely distributed in all altitudinal gradients. The findings from this research will offer valuable insights that can promote the implementation of efficient management strategies and initiatives for conserving biodiversity across different altitudinal gradients. This is particularly vital because bryophytes are known to be highly susceptible to human- induced threats and the influences of changing environmental conditions.

The effect of fault systems on volcanic activity: Insights from the subduction-related, Quaternary Villamaría-Termales monogenetic volcanic field in Colombia

The effect of fault systems on volcanic activity: Insights from the subduction-related, Quaternary Villamaría-Termales monogenetic volcanic field in Colombia

Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour

Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments.

Precursors to intra-eruptive reposes during the 1998–2019 lava dome-building eruption at Volcán de Colima, México

The 1998–2019 lava dome-building eruption at Volcan de Colima, Mexico, involved two eruptive cycles of activity during which andesitic lava domes were extruded. The first cycle of extrusion lasted from November 1998 to June 2011 and was followed by a period of intra-eruptive repose from June 2011 to January 2013. The second cycle lasted from January 2013 to March 2017. The second intra-eruptive repose began from March 2017, and continues through the time of writing (April 2019). The reposes were preceded by interruptions in the endogenous regime of growth of the lava domes by extrusion of exogenous shear lobes (before repose 1) and by collapse of the lava dome (before repose 2). We conclude that interruptions in endogenous lava dome growth may herald the onset of intra-eruptive repose periods at this volcano. The nature of such precursors is potentially associated with the process of batch supply from deep magma storage into the shallow system feeding the eruption. During the two cycles of eruption at Volcan de Colima, the equal volumes of products (about 75 × 106 m3) were erupted with a difference in the mean magma discharge rates (approximately 5 × 106 m3 and 18 × 106 m3 per year for cycles 1 and 2 respectively). It seems that a cycle was controlled by a volumetric limit of 75 × 106 m3; once this volume has been erupted, then the cycle ends irrespective of discharge rate. At the same time, a difference in the mean magma discharge rates may be a reason for differences in the type of interruptions.

The Evolution of the Silver Hills Volcanic Center, and Revised 40Ar/39Ar Geochronology of Montserrat, Lesser Antilles, With Implications for Island Arc Volcanism

AbstractStudying the older volcanic centers on Montserrat, Centre Hills and Silver Hills, may reveal how volcanic activity can change over long time periods (≥1 Myr), and whether the recent activity at the Soufrière Hills is typical of volcanism throughout Montserrat's history. Here we present the first detailed mapping of the Silver Hills, the oldest and arguably least studied volcanic center on Montserrat. Volcanism at the Silver Hills was dominated by episodic andesite lava dome growth and collapse, produced Vulcanian style eruptions, and experienced periodic sector collapse events, similar to the style of volcanic activity that has been documented for the Centre Hills and Soufrière Hills. We also present an updated geochronology of volcanism on Montserrat, by revising existing ages and obtaining new 40Ar/39Ar dates and paleomagnetic ages from marine tephra layers. We show that the centers of the Silver, Centre, and Soufrière Hills were active during at least ∼2.17–1.03 Ma, ∼1.14–0.38 Ma, and ∼0.45 Ma to present, respectively. Combined with timings of volcanism on Basse‐Terre, Guadeloupe, these ages suggest that ∼0.5–1 Ma is a common lifespan for volcanic centers in the Lesser Antilles. These new dates identify a previously unrecognized overlap in activity between the different volcanic centers, which appears to be a common phenomenon in island arcs. We also identify an older stage of Soufrière Hills activity ∼450–290 ka characterized by the eruption of hornblende‐orthopyroxene‐phyric lavas, demonstrating that the petrology of the Soufrière Hills eruptive products has changed at least twice throughout the volcano's development.

B3-10 箱根火山中央火口丘最新期安山岩質溶岩に含まれる集合斑晶の成因(マグマ・岩石学,口頭発表)

B3-10 箱根火山中央火口丘最新期安山岩質溶岩に含まれる集合斑晶の成因(マグマ・岩石学,口頭発表)

Growth of intra-caldera lava domes controlled by various modes of caldera collapse, the Štiavnica volcano–plutonic complex, Western Carpathians

Abstract The Stiavnica volcano–plutonic complex is an erosional relic of Miocene caldera-stratovolcano in the Western Carpathians. The complex exposes a vertical section from the volcano basement through subvolcanic intrusions and a ring fault to volcanic edifice, comprising mostly andesitic lava flows and domes. This paper examines internal structure, magnetic fabric as derived from the anisotropy of magnetic susceptibility (AMS), and emplacement dynamics of three intra-caldera andesite domes (referred to as Domes 1–3) located near the presumed ring fault. Magnetic fabrics, carried by multi-domain titanomagnetite and titanomaghemite, are interpreted as recording various mechanisms of dome growth controlled by active caldera collapse. Dome 1 is explained as a lava coulee, fed by conduits located along the ring fault, with a long lava outflow down the sloping caldera floor. Dome 2 represents an elongated, ring fault-parallel dome wherein the lava flowed a short distance over a flat floor. Dome 3 is interpreted as a composite dome fed from multiple linear fissures opened at a high angle to the ring fault. Subsequently, the dome was intruded by ring fault-parallel dikes that may have potentially fed overlying, now largely eroded lava domes and flows. Finally, we suggest that all domes formed during collapse of the Stiavnica caldera and the various mechanisms of their growth reflect different stages of the caldera evolution from piston (Dome 2) through trap-door (Dome 1) to piecemeal (Dome 3).

The Volcanic Evolution of Cerro Uturuncu: A High-K, Composite Volcano in the Back-Arc of the Central Andes of SW Bolivia

Cerro Uturuncu, southwest Bolivia, is a high-K, calc-alkaline, composite volcano constructed upon extremely thick continental crust approximately 125 km behind the arc-front of the Andean Central Volcanic Zone (CVZ). Eruptive activity occurred between 890 - 271 ka in a single phase of volcanism lasting ~620,000 years. The edifice consists of a central cone and several flank vents where dacitic and andesitic lava flows and domes erupted. Volumes of individual eruptive units range from 0.1 to ~10 km3; the composite volume of Uturuncu is ~89 km3. In this paper, we present new field, petrographic, and geochemical data in an effort to understand the volcanic and magmatic evolution of Uturuncu. Lava flows and domes have a restricted range in whole rock compositions ranging from 61 wt% - 67 wt% SiO2; magmatic inclusions contained within these units have a larger range from 53 wt% - 64 wt% SiO2. Typical phenocryst assemblages are plagioclase > orthopyroxene > biotite >> quartz and Fe-Ti oxides. Pb isotope ratios are characteristic of the southern CVZ by containing high 207Pb/204Pb and 206Pb/204Pb and moderate to high 208Pb/204Pb. Sr and Nd isotope ratios indicate that Uturuncu magmas were modified by high 87Sr/86Sr and low 143Nd/144Nd felsic basement lithology during magma migration and differentiation. In all eruptive units, there is petrographic and geochemical evidence for magma mixing and mingling. In this regard, magma mixing and mingling is considered to be responsible for the small range in lava flow and dome compositions throughout the eruptive history of the center.

Conditions of formation of the Mavrokoryfi high-sulfidation epithermal Cu–Ag–Au–Te mineralization (Petrota Graben, NE Greece)

The Mavrokoryfi Cu–Ag–Au–Te prospect, northeastern Greece, consists of atypical, high-sulfidation mineralization where precious metals were introduced contemporaneously with advanced argillic alteration from magmatic vapors. It occurs as veins of massive sulfides in zones of silicic and advanced argillic alteration spatially associated with an andesitic lava dome and hyaloclastites. Mineralogical data demonstrate an unusual ore and gangue mineralogy that is compatible with formation under very oxidizing conditions (logfO2 values of >−31.8) at temperatures of 200°C to 250°C. Oxidizing conditions favored the formation of hypogene lead sulfates (anglesite and barian celestite) instead of galena. Selenian acanthite, cadmian freibergite, and argentian goldfieldite are the main carriers of silver in the deposit and are reported in Greece for the first time. They were deposited at logfS2 of −9 to −7 and logfTe2 values of −9 to −12.5 (250°C). Ag-poor goldfieldite at Mavrokoryfi has up to 3.7 apfu Te and is the most Te-rich goldfieldite yet reported. The mineralization is accompanied by aluminum–phosphate–sulfate minerals of magmatic-hydrothermal origin and an unusual Pb-enrichment. Ore-forming components were likely derived from andesite porphyries.

Hydrothermal system beneath the crater of Tarumai volcano, Japan: 3-D resistivity structure revealed using audio-magnetotellurics and induction vector

Audio-magnetotelluric (AMT) measurements were recorded in the crater area of Tarumai volcano , northeastern Japan. This survey brought the specific structures beneath the lava dome of Tarumai volcano, enabling us to interpret the relationship between the subsurface structure and fumarolic activity in the vicinity of a lava dome. Three-dimensional resistivity modeling was performed to achieve this purpose. The measured induction vectors pointed toward the center of the dome, implying the topographic effect . However, estimation of the topographic effect showed that the measured vector was not explained only by this effect. This suggested that the distribution of induction vectors still held information of the subsurface structure and could be helpful in determining the geometry of 3-D bodies. The 3-D modeling was based on a quasi-one-dimensional layered structure that included topography. The final model revealed that the andesitic lava dome is characterized by comparatively low resistivity (50 Ωm), and that two conductive bodies (50 and 1–5 Ωm) are present beneath the lava dome. The shallower of these conductors is interpreted as an aquifer, such as a buried crater lake . The deeper, extremely conductive body corresponded to a convecting zone containing rising hydrothermal fluid . The shallower aquifer critically controls the temperature and chemical components of the fumarolic gasses. High-temperature gas supplied from deeper part that encounters the shallow aquifer loses its water-soluble components and heat, resulting in weak and low-temperature fumaroles . In contrast, most of the gas, which ascends outside the area of the shallower aquifer, is released as high-temperature fumaroles. This study provides an insight that the shallow aquifer in the crater area plays a significant role in the property of fumaroles at the volcanic surface.

Clastic and core lava components of a silicic lava dome

The formation of a lava dome involves fractionation of the lava into core and clastic components. We show that for three separate, successive andesitic lava domes that grew at Soufriere Hills volcano, Montserrat, between 1999 and 2007, the volumetric proportion of the lava converted to talus or pyroclastic flow deposits was 50%–90% of the lava extruded. Currently, only 8% of the total magma extruded during the 1995–2007 eruption remains as core lava. The equivalent representation in the geological record will probably be even lower. Most of the lava extruded at the surface flowed no further than 150–300 m from the vent before disaggregation, resulting in a lava core whose shape tends to a cylinder. Moderate to high extrusion rates at the Soufriere Hills domes may have contributed to the large clastic fraction observed. Creating talus dissipates much of the energy that would otherwise be stored in the core lava of domes. The extreme hazards from large pyroclastic flows and blasts posed by wholesale collapse of a lava dome depend largely on the size of the lava core, and hence on the aggregate history of the partitioning process, not on the size of the dome.

1989–1995 Earthquake sequences in the Galeras volcano region, SW Colombia, and possible volcano–earthquake interactions

Abstract The Galeras volcanic complex, located in the Narino department, SW Colombia, includes the most historically active volcano in the country, Galeras, a 4276 m high stratovolcano located 9 km west of the city of San Juan de Pasto (400,000 inhabitants). The area is also affected by the continental faulting represented by the Buesaco, Aranda and Pasto faults belonging to one of the most seismically active structures in Colombia, the Romeral fault system. Several moderate to strong shallow earthquakes affected the city of San Juan de Pasto and its neighbouring region since the XVII century. The coexistence of an active volcanic complex and an active fault system complicates the study and interpretation of the different processes taking place in the region as well as the identification of any connection or interaction among them. The reactivation of the volcano in 1989 was characterized by three main magmatic events: (1) a series of Vulcanian eruptions during 4–9 May 1989, with semi-continuous ash emissions from a secondary crater; (2) the emplacement of an andesitic lava dome at the bottom of the main crater from September 1990 until December 1991; and (3) six Vulcanian eruptions from the main crater during 1992–1993, with destruction of most of the dome during the first one on 16 July 1992. During the same period, four earthquake sequences were located in a limited area N and NE of Galeras volcano on August–September 1989 (AUG1989), April–June 1993 (APR1993), November–December 1993 (NOV1993) and March–August 1995 (MAR1995). The last one included a Ml4.7 main shock on 4 March 1995 producing moderate to high damage in the epicentral region (MSK maximum intensity VIII), and in the city of San Juan de Pasto (VI–VII). The last damaging earthquake in the region was a MSK-intensity VIII–IX in 1947. A detailed analysis of the spatio-temporal characteristics of the four earthquake sequences allowed identifying their different origin and suggesting some interrelationship between the reactivated eruptive process and the contemporaneous seismic activity. The AUG1989 sequence presents a typical volcanic swarm-like pattern most probably related with the process of magma intrusion from depth at the beginning of the volcano's reactivation. The APR1993, the NOV1993 and the MAR1995 sequences show a clear tectonic origin with events occurring on rupture planes almost vertical that can be associated to the active faults in the area, The seismogenic process of these three sequences could have been activated or accelerated by the main eruptions during 1992–1993. These results suggest that constrains provided by improved relocations and the detailed analysis of the space–time characteristics of earthquake sequences in Galeras volcanic environment allow to establish the different generation mechanisms involved and to suggest feasible explanations on the possible interrelationships of the magmatic–volcanic processes and the seismicity observed.

Non-explosive magma–water interaction in a continental setting: Miocene examples from the Eastern Cordillera (central Andes; NW Argentina)

The Middle-Upper Miocene Las Burras–Almagro-El Toro (BAT) igneous complex within the Eastern Cordillera of the central Andes (∼24°S; NW Argentina) has revealed evidence of non-explosive interaction of andesitic magma with water or wet clastic sediments in a continental setting, including peperite generation. We describe and interpret lithofacies and emplacement mechanisms in three case studies. The Las Cuevas member (11.8 Ma) comprises facies related to: (i) andesite extruded in a subaqueous setting and generating lobe-hyaloclastite lava; and (ii) marginal parts of subaerial andesite lava dome(s) in contact with surface water, comprising fluidal lava lobes, hyaloclastite, and juvenile clasts with glassy rims. The Lampazar member (7.8 Ma) is represented by a syn-volcanic andesite intrusion and related peperite that formed within unconsolidated, water-saturated, coarse-grained volcaniclastic conglomerate and breccia. The andesite intrusion is finger-shaped and grades into intrusive pillows. Pillows are up to 2 m wide, tightly packed near the intrusion fingers, and gradually become dispersed in the host sediment ≥50 m from the parent intrusion. The Almagro A member (7.2 Ma) shows evidence of mingling between water-saturated, coarse-grained, volcaniclastic alluvial breccia and intruding andesite magma. The resulting intrusive pillows are characterized by ellipsoidal and tubular shape and concentric structure. The high-level penetration of magma in this coarse sediment was unconfined and irregular. Magma was detached in apophyses and lobes with sharp contacts and fluidal shapes, and without quench fragmentation and formation of a hyaloclastite envelope. The presence of peperite and magma–water contact facies in the BAT volcanic sequence indicates the possible availability of water in the system between 11–7 Ma and suggests a depositional setting in this part of the foreland basin of the central Andes characterized by an overall topographically low coastal floodplain that included extensive wetlands.

Broadband Seismic Noise Analysis of the Soufriere Hills Volcano Network

Research Article| July 01, 2008 Broadband Seismic Noise Analysis of the Soufrière Hills Volcano Network Silvio De Angelis Silvio De Angelis Montserrat Volcano Observatory Flemmings, Montserrat, West Indies silvio@mvo.ms Search for other works by this author on: GSW Google Scholar Author and Article Information Silvio De Angelis Montserrat Volcano Observatory Flemmings, Montserrat, West Indies silvio@mvo.ms Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2008 by the Seismological Society of America Seismological Research Letters (2008) 79 (4): 504–509. https://doi.org/10.1785/gssrl.79.4.504 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Silvio De Angelis; Broadband Seismic Noise Analysis of the Soufrière Hills Volcano Network. Seismological Research Letters 2008;; 79 (4): 504–509. doi: https://doi.org/10.1785/gssrl.79.4.504 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The Soufrière Hills Volcano (SHV), Montserrat, West Indies, is an active andesitic dome-building volcano at the northern end of the Lesser Antilles volcanic arc. The ongoing eruption, characterized by cyclic growth and collapse of an andesite lava dome with associated pyroclastic flows (PF), vulcanian explosions, and debris flows, began on 18 July 1995 (Young et al. 1998; Aspinall et al. 1998) after a three-year period of precursor seismic activity. Prior to this eruption, volcanic activity in Montserrat was monitored by the Seismic Research Unit (SRU), based at the University of West Indies, St. Augustine, Trinidad, as part... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Pre-1991 sulfur transfer between mafic injections and dacite magma in the Mt. Pinatubo reservoir

Abstract Before the 1991–1992 activity, a large andesite lava dome belonging to the penultimate Pinatubo eruptive period (Buag ∼ 500 BP) formed the volcano summit. Buag porphyritic andesite contains abundant amphibole-bearing microgranular enclaves of basaltic–andesite composition. Buag enclaves have lower K2O and incompatible trace element (LREE, U, Th) contents than mafic pulses injected in the Pinatubo reservoir during the 1991–1992 eruptive cycle. This study shows that Buag andesite formed by mingling of a hot, water-poor and reduced mafic magma with cold, hydrous and oxidized dacite. Depending on their size, enclaves experienced variable re-equilibration during mixing/mingling. Re-equilibration resulted in hydration, oxidation and transfer of mobile elements (LILE, Cu) from the dacite to the mafic melts and prompted massive amphibole crystallization. In Buag enclaves, S-bearing phases (sulfides, apatite) and melt inclusions in amphibole and plagioclase record the evolution of sulfur partition among melt, crystal and fluid phases during magma cooling and oxidation. At high temperature, sulfur is partitioned between andesitic melt and sulfides (Ni-pyrrhotite). Magma cooling, oxidation and hydration resulted in exsolution of a S–Cl–H2O vapor phase at the S-solubility minimum near the sulfide–sulfate redox boundary. Primary magmatic sulfide (pyrrhotite) and xenocrystic sulfide grains (pyrite), recycled together with olivines and pyroxenes from old mafic intrusives, were replaced by Cu-rich phases (chalcopyrite, cubanite) and, partially, by Ba–Sr sulfate. Sulfides degassed and transformed into residual spongy magnetite in response to fS2 drop during final magma ascent and decompression. Our research suggests that a complete evaluation of the sulfur budget at Pinatubo must take into account the en route S assimilation from the country rocks. Moreover, this study shows that the efficiency of sulfur transfer between mafic recharges and injected magmas is controlled by the extent and rate of mingling, hydrous flushing and melt oxidation. Vigorous mixing/mingling and transformation of the magmatic recharge into a spray of small enclaves is required in order to efficiently strip their primary S-content that otherwise remains locked in the sulfides. Hydrous flushing increases the magma oxidation state of the recharges and modifies their primary volatile concentrations that cannot be recovered by the study of late-formed mineral phases and melt inclusions. Conversely, S stored in both late-formed Cu-rich sulfides and interstitial rhyolitic melt represents the pre-eruptive sulfur budget immediately available for release from mafic enclaves during their decompression.

Anatomy of a submarine pyroclastic flow and associated turbidity current: July 2003 dome collapse, Soufrière Hills volcano, Montserrat, West Indies

AbstractThe 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub‐aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water‐saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (∼60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (∼40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far‐reaching turbidity current.

Engineering geology of landslides on the volcanic island of Montserrat, West Indies

Different types of landslides have been observed on Montserrat, a British dependent island in the (Caribbean) West Indies. The Soufrière Hills volcano, situated in the southern part of this island, has been in a state of almost continuous volcanic activity since 1995, after being dormant for about 400 years. The most dramatic, distinct ‘landslides’ occur on the extrusive, expanding, andesitic lava dome within English Crater. Falls, topples and slides are caused by the over-steepening of the dome followed by its gravitational collapse. On occasions, these failures generate incandescent rock avalanches, which may ultimately transgress into pyroclastic flows, surges and block & ash flows. These are highly destructive, hot, density currents and accelerate down-slope, controlled by the topography and radial drainage channels (known as ‘ghauts’), which have been incised on the flanks of the volcano. These flows have deposited huge volumes of incandescent pyroclastic debris on the upper and middle flanks of the volcano. The erosion of the pyroclastic debris and air-fall deposits, during rainstorms, has resulted in the generation of lahars (an Indonesian term to describe a volcanic mudflow). Well over 40 lahars have been observed and recorded, from January 1999 to 2006. These coincided with hurricanes and increased rainfall rates usually between the months of June and November. The lahar deposits, typically consisting of granular material including boulders, have infilled many of the river valleys. The lower reaches of the Belham valley in particular has been infilled, and this has resulted in the choking of drainage catchment areas and the burial of infrastructure and houses. Beyond the area of current volcanic activity landslides have also been observed, although their significance has not always been recognized, as their investigation may often be overlooked because of the more spectacular dramatic events on the volcano itself. The landslides include both coastal and inland landslides, which are generated by a combination of seismic events (such as tectonic and volcano-magmatic earthquakes) and climatic events (such as rainstorm and hurricanes). Landslide generation is facilitated by the presence of thick, tropical weathered laterite soils (regolith) that develop on moderate to steep slopes, in areas of high relief, on intensely weathered volcanic bedrock. Both soils and topography influence drainage and groundwater flows, which in turn affect the stability ofslopes, causing the initiation of first time slope failures and the reactivation of older, more degraded landslides. In addition to natural processes, the activities of man (such as road building, the digging into slopes or the loading of slopes by tipping waste) have also caused landslides to develop. The objectives of this paper are to document and draw attention to the causes and types of landslides on Montserrat with particular reference to the expanding lava dome, the Belham valley lahars and the Montserrat Volcano Observatory (MVO) debris slide. This paper provides an engineering geological perspective on the landslide geohazards that have been observed on Montserrat.

Monitoring landslides in hazardous terrain using terrestrial LiDAR: an example from Montserrat

It is important to monitor unstable slopes to determine where adverse changes in morphology are indicating the likelihood of failure and a consequent risk to life and property. However, where highly unstable slopes need to be monitored, their very nature precludes the use of direct measurement by surveyors and remote means must be used to protect the safety of the survey team. The use of LiDAR (Light Detection and Ranging) enables the accurate location of a network of points that can be used to create a detailed 3D terrain model, or DEM (Digital Elevation Model), of greater coverage and accuracy than conventional methods, with almost complete safety of the operators. There can be few more hazardous situation than that of monitoring a volcanic andesite lava dome for signs of an impending collapse. Partial collapse of a lava dome generates hot, fast-moving pyroclastic density currents, which are categorized as pyroclastic surge or block-and-ash flow deposits based on sedimentary structures. Monitoring in such circumstances requires that measurements are taken from a distance that minimizes the threat from eruption and from asphyxiation by volcanic gases. The method also needs to be rapid to minimize the time spent by the monitoring team in the hazardous zone. The techniques described here are applicable to the monitoring of any unstable slopes and active landslide where hazardous conditions threaten the survey team. Montserrat is a volcanic island of the Leeward Islands in the Caribbean Sea situated 43 km SW of Antigua. The survey was carried out from Perches Mountain, on the margin of the Soufriere Hills volcanic crater (Fig. 1). The survey covered …

Imaging a growing lava dome with a portable radar

The tiny Caribbean island of Montserrat has been in a state of crisis since the Soufriere Hills Volcano (SHV) began its current eruption in July 1995. With its main town, Plymouth, destroyed by pyroclastic flows in 1997, the islanders who have remained have had to rebuild their society on the northern half of the island under varying degrees of threat from the volcano to the south. During this time, the Montserrat government continues to receive advice on the volcanic hazards from the Montserrat Volcano Observatory (MVO).The continuing eruption has provided a wealth of research opportunities for many international groups who sought to study the growth and repeated partial destruction of a Peleean andesitic lava dome. There have been three episodes of dome growth: November 1995 through March 1998, November 1999 through July 2003, and August 2005 to present. Pyroclastic flows and explosions have been the main source of hazard.The pyroclastic flows have been generated mainly by gravitational collapse of the lava dome, but also by collapse of explosive ash columns. The dome collapses tend to occur from the area of the dome where new lava is being added. Similarly collapses are more likely when the rate of lava extrusion varies. Also, the propensity of explosive evacuation of the magma in the conduit is partly controlled by the magma supply rate, with high rates favoring explosions.

Seismic quantification of the explosions that destroyed the dome of Volcán de Colima, Mexico, in July–August 2003

In July–August 2003, the andesitic lava dome at Volcan de Colima, Mexico, was destroyed by a sequence of explosions that replaced the 2×106 m3 dome with a crater 200 m across and 30 m deep. The two strongest explosions occurred on July 17 and August 28. The initial low-frequency impulses that they produced, which were recorded on broadband seismic records, allowed an estimation of the counter forces of the initiating process as being equal to 0.3×1011 N and 1×1011 N for the July and August events, respectively. The seismic characteristics follow the Nishimura-Hamaguchi scaling law for volcanic explosions, reflecting self-similarity in the processes initiating explosive events. The results also show that counter forces can discriminate between the sizes of explosive eruptions that are assigned the same magnitude by conventional methods of classification such as the Volcanic Explosivity Index. The increasing use of broadband seismometers may therefore provide the basis for using counter forces to determine the magnitude of explosive eruptions.