Showing papers in "Bulletin of Volcanology in 2015"
TL;DR: This work classifies Southeast Asian volcanoes into five groups, using their morphology and, where known, their eruptive history and degassing style, and estimates that decadal probabilities of VEI 4–8 eruptions in Southeast Asia are nearly 1.0.
Abstract: There are ~750 active and potentially active volcanoes in Southeast Asia. Ash from eruptions of volcanic explosivity index 3 (VEI 3) and smaller pose mostly local hazards while eruptions of VEI ≥ 4 could disrupt trade, travel, and daily life in large parts of the region. We classify Southeast Asian volcanoes into five groups, using their morphology and, where known, their eruptive history and degassing style. Because the eruptive histories of most volcanoes in Southeast Asia are poorly constrained, we assume that volcanoes with similar morphologies have had similar eruption histories. Eruption histories of well-studied examples of each morphologic class serve as proxy histories for understudied volcanoes in the class. From known and proxy eruptive histories, we estimate that decadal probabilities of VEI 4–8 eruptions in Southeast Asia are nearly 1.0, ~0.6, ~0.15, ~0.012, and ~0.001, respectively.
174 citations
TL;DR: In this article, the authors used geochemical information to construct a temporal and volumetric model for the growth of the Jeju volcano and found that the increased trenchward mantle flow, caused by the greater rollback of the Philippine Sea Plate, activated pre-existing shear weaknesses in the mantle beneath Jeju, resulting in mantle upwelling and decompression melting that caused a change in compositions and an increase in eruption rates at Jeju.
Abstract: The drivers behind the inception of, and the variable, pulsatory eruption rates at distributed intraplate volcanic fields are not well understood. Such broad areas of monogenetic volcanism cover vast areas of the world and are often heavily populated. Reliable models to unravel their behaviour require robust spatio-temporal frameworks within the fields, but an analysis of the potential proximal and distal regional volcano-tectonic processes is also needed. Jeju Island (Republic of Korea) is a volcanic field that has been extensively drilled and dated. It is also located near one of the world’s best-studied tectonic plate boundaries: the subduction zone in southwestern Japan, which generates the Ryukyu and SW Japan arcs. A new set of 40Ar/39Ar ages collected from cores penetrating the entire Jeju eruptive pile, along with geochemical information, is used to construct a temporal and volumetric model for the volcano’s growth. The overall pattern indicates inception of volcanism at ~1.7 Ma, with an initial 1.2 Myr of low-rate activity, followed by over an order of magnitude rise over the last 0.5 Myr. The magma flux at Jeju correlates well with increased extension rates in the arc/backarc region. In particular, we infer that the increased trenchward mantle flow, caused by the greater rollback of the Philippine Sea Plate, activated pre-existing shear weaknesses in the mantle beneath Jeju, resulting in mantle upwelling and decompression melting that caused a change in compositions and an increase in eruption rates at Jeju. Thus, the volcanic activity of an intraplate field system can be modulated by regional subduction processes occurring more than 650 km away. This model may explain the frequent observation of pulsatory behaviour seen in many monogenetic volcanic fields worldwide that lie within 1,000 km of subduction zones.
120 citations
TL;DR: The experiments show that, at shallow depths, both low- and high-porosity lavas dilate and fail by shear fracturing, but deeper in the edifice, the failure of high- porosity lava is compactant and driven by cataclastic pore collapse, and the implication of these data is that sidewall outgassing may be efficient in the shallow Edifice, where rock can fracture, but may be impeded deeper inThe edifice due to compaction.
Abstract: The failure mode of lava—dilatant or compactant—depends on the physical attributes of the lava, primarily the porosity and pore size, and the conditions under which it deforms. The failure mode for edifice host rock has attendant implications for the structural stability of the edifice and the efficiency of the sidewall outgassing of the volcanic conduit. In this contribution, we present a systematic experimental study on the failure mode of edifice-forming andesitic rocks (porosity from 7 to 25 %) from Volcan de Colima, Mexico. The experiments show that, at shallow depths ( 1 km), while low-porosity (<10 %) lava remains dilatant, the failure of high-porosity lava is compactant and driven by cataclastic pore collapse. Although inelastic compaction is typically characterised by the absence of strain localisation, we observe compactive localisation features in our porous andesite lavas manifest as subplanar surfaces of collapsed pores. In terms of volcano stability, faulting in the upper edifice could destabilise the volcano, leading to an increased risk of flank or large-scale dome collapse, while compactant deformation deeper in the edifice may emerge as a viable mechanism driving volcano subsidence, spreading and destabilisation. The failure mode influences the evolution of rock physical properties: permeability measurements demonstrate that a throughgoing tensile fracture increases sample permeability (i.e. equivalent permeability) by about a factor of two, and that inelastic compaction to an axial strain of 4.5 % reduces sample permeability by an order of magnitude. The implication of these data is that sidewall outgassing may therefore be efficient in the shallow edifice, where rock can fracture, but may be impeded deeper in the edifice due to compaction. The explosive potential of a volcano may therefore be subject to increase over time if the progressive compaction and permeability reduction in the lower edifice cannot be offset by the formation of permeable fracture pathways in the upper edifice. The mode of failure of the edifice host rock is therefore likely to be an important factor controlling lateral outgassing and thus eruption style (effusive versus explosive) at stratovolcanoes.
98 citations
TL;DR: The Puyehue-Cordon Caulle Volcano (Chile) erupted in June 2011 following a month of continuously increasing seismic activity as discussed by the authors, and the resulting tephra deposits consist of 13 main layers grouped into four units.
Abstract: After decades of repose, Puyehue-Cordon Caulle Volcano (Chile) erupted in June 2011 following a month of continuously increasing seismic activity. The eruption dispersed a large volume of rhyolitic tephra over a wide area and was characterized by complex dynamics. During the initial climactic phase of the eruption (24–30 h on 4–5 June), 11–14-km-high plumes dispersed most of the erupted tephra eastward towards Argentina, reaching as far as the Atlantic Ocean. This first eruptive phase was followed by activity of lower intensity, leading to the development of a complex stratigraphic sequence, mainly due to rapid shifts in wind direction and eruptive style. The resulting tephra deposits consist of 13 main layers grouped into four units. Each layer was characterized based on its dispersal direction, sedimentological features, and on the main characteristics of the juvenile fraction (texture, density, petrography, chemistry). The lowest part of the eruptive sequence (Unit I), corresponding to the tephra emitted between 4 and 5 June, is composed of alternating lapilli layers with a total estimated volume of ca. 0.75 km3; these layers record the highest intensity phase, during which a bent-over plume dispersed tephra towards the southeast-east, with negligible up-wind sedimentation. Products emitted during 5–6 June (Unit II) signaled an abrupt shift in wind direction towards the north, leading to the deposition of a coarse ash deposit in the northern sector (ca. 0.21 km3 in volume), followed by a resumption of easterly directed winds. A third phase (Unit III) began on 7 June and resulted in tephra deposits in the eastern sector and ballistic bombs around the vent area. A final phase (Unit IV) started after 15 June and was characterized by the emission of fine-grained white tephra from ash-charged plumes during low-level activity and the extrusion of a viscous lava flow. Timing and duration of the first eruptive phases were constrained based on comparison of the dispersal of the main tephra layers with satellite images, showing that most of the tephra was emitted during the first 72 h of the event. The analyzed juvenile material tightly clusters within the rhyolitic field, with negligible chemical variations through the eruptive sequence. Textural observations reveal that changes in eruption intensity (and consequently in magma ascent velocity within the conduit) and complex interactions between gas-rich and gas-depleted magma portions during ascent resulted in vesicular clasts with variable degrees of shear localization, and possibly in the large heterogeneity of the juvenile material.
93 citations
TL;DR: The authors used small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations, extending earlier experimental results that changed explosion locations only along a vertical axis.
Abstract: We present results of experiments that use small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations, extending earlier experimental results that changed explosion locations only along a vertical axis. The focus is on the resulting crater size and shape and subcrater structures. Final crater shapes tend to be roughly circular if subsurface explosion epicenters occur within each other’s footprints (defined as the plan view area of reference crater produced by a single explosion of a given energy, as predicted by an empirical relationship). Craters are elongate if an epicenter lies somewhat beyond the footprint of the previous explosion, such that their footprints overlap, but if epicenters are too far apart, the footprints do not overlap and separate craters result. Explosions beneath crater walls formed by previous blasts tend to produce inclined (laterally directed) ejecta jets, while those beneath crater centers are vertically focused. Lateral shifting of explosion sites results in mixing of subcrater materials by development of multiple subvertical domains of otherwise pure materials, which progressively break down with repeated blasts, and by ejection and fallback of deeper-seated material that had experienced net upward displacement to very shallow levels by previous explosions. A variably developed collar of material that experienced net downward displacement surrounds the subvertical domains. The results demonstrate key processes related to mixing and ejection of materials from different depths during an eruptive episode at a maar-diatreme volcano as well as at other phreatomagmatic and hydrothermal explosion sites.
74 citations
TL;DR: In this paper, the emplacement temperatures of three ignimbrites belonging to the 4.6-ka Fogo A plinian eruption sequence in Sao Miguel Island (Azores, Portugal) were determined using partial thermal remanent magnetization (pTRM) of lithic clasts and reflectance (Ro%) of charcoal fragments embedded within the deposits and collected at the same localities close to each other.
Abstract: The emplacement temperatures of three ignimbrites belonging to the 4.6-ka Fogo A plinian eruption sequence in Sao Miguel Island (Azores, Portugal) were determined using partial thermal remanent magnetization (pTRM) of lithic clasts and reflectance (Ro%) of charcoal fragments embedded within the deposits and collected at the same localities close to each other. The Fogo A sequence is characterised by a complex stratigraphy consisting of a thick plinian deposit interbedded with two intraplinian ignimbrites (here named “pink” and “black” intraplinian ignimbrite, respectively) and capped by a final ignimbrite (here named “dark brown” ignimbrite). A total of 140 oriented lithic clasts from the three ignimbrites were collected from 15 localities distributed along the northern and southern flanks of the volcano. The pTRM analyses show different paleomagnetic behaviours, which correspond to different emplacement temperatures of the ignimbrites. The emplacement temperatures of the pink and black intraplinian ignimbrites inferred from pTRM analysis were respectively ≥400 and ≥600 °C; the temperatures of the dark brown ignimbrite are lower, estimated between 300 and 350 °C. Thermal estimations of three key sites were compared with the results of the analysis of reflectance (Ro%) measured on eight specimens derived from charcoal fragments collected from the pink intraplinian ignimbrite and the dark brown ignimbrite. Results indicate Ro% values between 1.61 and 1.37 for the pink intraplinian ignimbrite, whereas fragments collected from the dark brown ignimbrite show Ro% values between 0.85 and 0.50. No charred wood was found in the black intraplinian ignimbrite. Ro% values indicate that charcoal fragments in the pink intraplinian ignimbrite reached temperatures of 380–460 °C, whereas the Ro% values of the dark brown ignimbrite indicate slightly lower temperatures of 330–350 °C. TRM and Ro% results are comparable and validate the use of both methods. Greatest accuracy in the determination of emplacement temperatures of ignimbrites is achieved when both methods can be applied at the same locations.
72 citations
TL;DR: This paper investigated the relationship between large explosions and lava dome growth from 1000 AD to present and developed the DomeHaz database, which contains information from 397 dome-forming episodes, including duration of dome growth, duration of pauses in extrusion, extrusion rates, and the timing and magnitude (VEI) of associated large explosions.
Abstract: Investigation of the global eruptive records of particular types of volcanoes is a fundamental and valuable method of understanding what style of activity can be anticipated in the future and can highlight what might be expected or unusual in particular settings. This paper investigates the relationship between large explosions (volcanic explosivity index, VEI ≥ 4) and lava dome growth from 1000 AD to present and develops the DomeHaz database. DomeHaz contains information from 397 dome-forming episodes, including duration of dome growth, duration of pauses in extrusion, extrusion rates, and the timing and magnitude (VEI) of associated large explosions. Major explosive activity, when associated with dome growth, is more likely to occur before dome growth rather than during, or at the end of, dome-forming eruptions. In most cases where major explosive activity has been associated with dome growth, the eruptions occurred at basaltic andesite to andesitic volcanoes (the most common type of dome-forming volcano), but a greater proportion of dacitic and rhyolitic dome growth episodes were associated with large explosions. High extrusion rates (>10 m3 s−1) seem to be associated with large explosions and may inhibit degassing or destabilize existing domes, leading to explosive decompression. Large explosions may, alternatively, be followed by dome growth, which represents the clearing of residual magma from the conduit. Relationships extracted from the global record can be used to construct probability trees for new and ongoing dome-forming eruptions or can be used in conjunction with other types of event trees to aid in forecasting volcanic hazards during a crisis, especially for volcanoes where data are sparse.
66 citations
TL;DR: In this article, a detailed analysis of grain-size variations of tephra deposits with distance from vent and total grain size distribution (TGSD) was performed, and preferential breakage of coarse pumices on ground impact was inferred from the study of particle terminal velocity.
Abstract: Sedimentation processes and fragmentation mechanisms during explosive volcanic eruptions can be constrained based on detailed analysis of grain-size variations of tephra deposits with distance from vent and total grain-size distribution (TGSD). Grain-size studies strongly rely on deposit exposure and, in case of long-lasting eruptions, can be complicated by the intricate interplay between eruptive style, atmospheric conditions, particle accumulation, and deposit erosion. The 2011 Cordon Caulle eruption, Chile, represents an ideals laboratory for the study of long-lasting eruptions thanks to the good deposit accessibility in medial to distal area. All layers analyzed are mostly characterized by bimodal grain-size distributions, with both the modes and the fraction of the coarse subpopulation decreasing rapidly with distance from vent and those of the fine subpopulation being mostly stable. Due to gradually changing wind direction, the two subpopulations characterizing the deposit of the first 2 days of the eruption are asymmetrically distributed with respect to the dispersal axis. The TGSD of the climactic phase is also bimodal, with the coarse subpopulation representing 90 wt% of the whole distribution. Polymodality of individual samples is related to size-selective sedimentation processes, while polymodality of the TGSD is mostly related to the complex internal texture (e.g., size and shape of vesicles) of the most abundant juvenile clasts. The most representative TGSD could be derived based on a combination of the Voronoi tessellation with a detailed analysis of the thinning trend of individual size categories. Finally, preferential breakage of coarse pumices on ground impact was inferred from the study of particle terminal velocity.
60 citations
TL;DR: In this article, textural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data.
Abstract: Textural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data. The data set defines a new gas-dominated type of “strombolian” eruption, named type 0. These events are characterized by high-velocity emission (150–250 m s−1) of a few relatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to be re-erupted. For the studied event, the explosion depth was more than 250 m deep, and the particles showed little residence time in the shallow system. Slug ascent velocities over the final 20–35 m of magma-filled conduit, and the low viscosity of the resident magma, are all consistent with simple bubble burst in a “clean” conduit. This conduit type and eruption style likely fit popular slug ascent and burst models used to explain “strombolian” eruptions. In contrast, the ballistic-dominated type of explosions (type 1) are associated with larger proportions of stagnant material in the shallow system magma mix. We argue that the additional volume of this stagnant material pushes the free surface upward. Because of the larger volume of material available for entrainment into a type 1 slug burst, which has to fragment through a thick cap of degassed material, type 1 events tend to be rich in particles. In contrast, the less spectacular, gas-rich (type 0) events have little material to entrain, thus being poor in lapilli and bombs.
56 citations
Eritrea Institute of Technology1, Imperial College London2, Institut de Physique du Globe de Paris3, Blaise Pascal University4, University of Bristol5, Michigan Technological University6, University of Alaska Fairbanks7, National Oceanography Centre, Southampton8, Addis Ababa University9, University of Leeds10
TL;DR: Analysis of ground deformation suggests the eruption was fed by a shallow, NW–SE-trending dike, which is consistent with field and satellite observations of vent distributions, and which was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor.
Abstract: We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of regional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region and of caldera systems in general, for the prodigious quantity of SO2 emitted into the atmosphere and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic significance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3,000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shallow, NW–SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response mitigated the human costs of the eruption.
55 citations
TL;DR: The volume, grain size, and depositional facies of material deposited outside an explosion crater, ejecta, are sensitive to the depth of the explosion, the explosion energy, and the presence or absence of a crater before the explosion.
Abstract: The volume, grain size, and depositional facies of material deposited outside an explosion crater, ejecta, are sensitive to the depth of the explosion, the explosion energy, and the presence or absence of a crater before the explosion. We detonate buried chemical explosives as an analog for discrete volcanic explosions in experiments to identify unique characteristics of proximal, medial, and distal ejecta facies and their distribution from a range of scaled depths in undisturbed and cratered ground. Ejecta are here discussed in terms of three facies: (1) proximal ejecta, which form a constructional landform around a crater; (2) medial ejecta, which form a continuous sheet deposit that thins much more gradually with distance; and (3) distal ejecta that are deposited as isolated clasts. The extent of proximal ejecta away from the crater, relative to crater size, is not sensitive to scaled depth, but the volume proportion of proximal ejecta to the total ejecta deposit is sensitive to the presence of a crater and scaled depth. Medial ejecta distribution and volume contributions are both sensitive to the presence of a crater and to scaled depth. Distal ejecta distance is dependent on scaled depth and the presence of a crater, while the volume proportion of distal ejecta is less sensitive to scaled depth or presence of a crater. Experimental facies and deposit structures inferred from observations of jet dynamics are used to suggest facies associations anticipated from eruptions dominated by explosions of different scaled depth configurations. We emphasize that significant differences in tephra ring deposits can result from the effects of scaled depth and preexisting craters on ejecta dynamics, and are not necessarily related to fundamental differences in explosion mechanisms or degree of magma fragmentation.
TL;DR: This paper used 40Ar/39Ar eruption ages for ice-marginal lava flows on Ruapehu, a glaciated andesite-dacite composite cone in the southern Taupo Volcanic Zone, New Zealand.
Abstract: Ice exerts a first-order control over the distribution and preservation of eruptive products on glaciated volcanoes. Defining the temporal and spatial distributions of ice-marginal lava flows provides valuable constraints on past glacial extents and is crucial for understanding the eruptive histories of such settings. Ice-marginal lava flows are well displayed on Ruapehu, a glaciated andesite-dacite composite cone in the southern Taupo Volcanic Zone, New Zealand. Flow morphology, fracture characteristics and 40Ar/39Ar geochronological data indicate that lavas erupted between ~51 and 15 ka interacted with large valley glaciers on Ruapehu. Ice-marginal lava flows exhibit grossly overthickened margins adjacent to glaciated valleys, are intercalated with glacial deposits, display fine-scale fracture networks indicative of chilling against ice, and are commonly ridge-capping due to their exclusion from valleys by glaciers. New and existing 40Ar/39Ar eruption ages for ice-marginal lava flows indicate that glaciers descending to 1300 m above sea level were present on Ruapehu between ~51–41 and ~27–15 ka. Younger lava flows located within valleys are characterised by blocky flow morphologies and fracture networks indicative of only localised and minor interaction with ice and/or snow, mainly in their upper reaches at elevations of ~2600–2400 m. An 40Ar/39Ar eruption age of 9 ± 3 ka (2σ error) determined for a valley-filling flow on the northern flank of Ruapehu indicates that glaciers had retreated to near-historical extents by the time of emplacement for this lava flow. The applicability of 40Ar/39Ar dating to ice-marginal flows on glaciated andesite-dacite composite volcanoes makes this technique an additional proxy for paleoclimate reconstructions.
TL;DR: In this article, a finite element numerical model of water migration in simplified dome geometries is combined with the rock alteration index (RAI) to predict zones of alteration and secondary mineral precipitation.
Abstract: Hydrothermal alteration is a recognized cause of volcanic instability and edifice collapse, including that of lava domes or dome complexes. Alteration by percolating fluids transforms primary minerals in dome lavas to weaker secondary products such as clay minerals; moreover, secondary mineral precipitation can affect the porosity and permeability of dome lithologies. The location and intensity of alteration in a dome depend heavily on fluid pathways and availability in conjunction with heat supply. Here we investigate postemplacement lava dome weakening by hydrothermal alteration using a finite element numerical model of water migration in simplified dome geometries. This is combined with the rock alteration index (RAI) to predict zones of alteration and secondary mineral precipitation. Our results show that alteration potential is highest at the interface between the hot core of a lava dome and its clastic talus carapace. The longest lived alteration potential fields occur in domes with persistent heat sources and permeabilities that allow sufficient infiltration of water for alteration processes, but not so much that domes cool quickly. This leads us to conclude that alteration-induced collapses are most likely to be shallow seated and originate in the talus or talus/core interface in domes which have a sustained supply of magmatic heat. Mineral precipitation at these zones of permeability contrast could create barriers to fluid flow, potentially causing gas pressurization which might promote deeper seated and larger volume collapses. This study contributes to our knowledge of how hydrothermal alteration can affect lava domes and provides constraints on potential sites for alteration-related collapses, which can be used to target hazard monitoring.
TL;DR: In this article, the authors present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland.
Abstract: The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet deflection into the subvertical ring fault. Our findings provide an alternative mechanical explanation for magma channeling along caldera ring faults, which is a process likely to be fundamental in controlling the location of post-caldera volcanism.
TL;DR: In this article, the authors used bathymetric and seismic reflection data to analyze the main volcanic, tectonic and mass-wasting features of Terceira offshore.
Abstract: Terceira Island, in the Azores Archipelago, lies at the intersection of four submarine volcanic ridges. New high-resolution bathymetric and seismic reflection data have been used to analyze the main volcanic, tectonic and mass-wasting features of the island offshore. Volcanic features such as linear volcanic centers, and pointy and flat-topped cones are mainly concentrated on the narrow western and north-western ridges, characterized by an overall rugged morphology. Fault scarps dominate mainly the broad eastern and south-eastern ridges, which are characterized by an overall smooth and terrace-like morphology. On the eastern ridge, faults form a series of horsts and grabens related to the onshore Lajes Graben. The strikes of the fault scarps, linear volcanic centers and alignment of volcanic cones on the ridges reveal two main structural trends, WNW–ESE and NNW–SSE, consistent with the main tectonic structures observed on the Azores Plateau. In contrast, a large variability of strike was observed in inter-ridge areas, reflecting the relative importance of regional and local stresses in producing these structures. Mass-wasting features are subordinate and mostly represented by hundred meter-wide scars that indent the edge of the insular shelf surrounding the island, apart from two large, deeper scars identified on the southern steep flank of the western ridge. Finally, the remarkable morpho-structural differences between the western and eastern ridges are discussed in the framework of the evolution of the Terceira volcanic edifice and hypothesized to reflect successive stages of ridge evolution.
TL;DR: In this paper, XRD, SEM, EPMA, Laser Raman, and synchrotron μ-cT were used to detect cristobalite in pyroclasts from the 2011-2012 eruption of Cordon Caulle (Chile).
Abstract: Cristobalite is a low-pressure high-temperature polymorph of SiO2 found in many volcanic rocks. Its volcanogenic formation has received attention because (1) pure particulate cristobalite can be toxic when inhaled, and its dispersal in volcanic ash is therefore a potential hazard; and (2) its nominal stability field is at temperatures higher than those of magmatic systems, making it an interesting example of metastable crystallization. We present analyses (by XRD, SEM, EPMA, Laser Raman, and synchrotron μ-cT) of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011–2012 eruption of Cordon Caulle (Chile). Cristobalite was not detected in pyroclasts, negating any concern for respiratory hazards, but it makes up 0–23 wt% of lava samples, occurring as prismatic vapour-deposited crystals in vesicles and/or as a groundmass phase in microcrystalline samples. Textures of lava collected near the vent, which best represent those generated in the conduit, indicate that pore isolation promotes vapour deposition of cristobalite. Mass balance shows that the SiO2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass. Textures of lava collected down-flow were modified during transport in the insulated interior of the flow, where protracted cooling, additional vesiculation events, and shearing overprint original textures. In the most slowly cooled and intensely sheared samples from the core of the flow, nearly all original pore space is lost, and vapour-deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix. Holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite. Vapour deposition and devitrification act concurrently but semi-independently. Both are promoted by slow cooling, and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies. Our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions, with insight afforded by the fundamentally different emplacement geometries of flows and domes.
TL;DR: The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2−ky recorded in polar ice cores.
Abstract: The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores. It is suspected to have impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40 km3 dense rock equivalent (DRE) of deposits, consisting of (i) 7–9 km3 DRE of pumiceous plinian fall products, (ii) 16 km3 DRE of pyroclastic density current deposits (PDC) and (iii) 8–9 km3 DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6 × 108 kg/s, a maximum plume height of 43 km and a dispersal index of 110,500 km2, the 1257 A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.
TL;DR: In this article, the authors used two TanDEM-X data pairs to generate digital elevation models (DEMs) prior to and immediately following the 2012-2013 eruption of Tolbachik Volcano, Kamchatka.
Abstract: The bistatic acquisition mode of the German TanDEM-X radar satellite mission provides a reliable source for measuring morphological changes associated with volcanic activity. We present the use of this system to measure key lava flow parameters including thickness, volume, runout, and flow extent by using two TanDEM-X data pairs to generate digital elevation models (DEMs) prior to and immediately following the 2012–2013 eruption of Tolbachik Volcano, Kamchatka. Morphometric parameters and areal distribution of the new lava flow field are determined using a cell-by-cell elevation difference between the two DEMs. A total flow volume of 0.53 ± 0.07 km3, a mean flow thickness of 14.5 m, and a modal thickness of 7.8 m are calculated. We use these calculated flow parameters as input to a volume-limited lava flow emplacement model. Model simulations are able to reproduce the SW portion of the 2012–2013 Tolbachik lava flow using a 75-m Shuttle Radar Topography Mission (SRTM) DEM and the 15-m TanDEM-X derived DEM, with goodness-of-fit measures of 56.3 and 59.6 %, respectively, based on the Jaccard similarity coefficient. The flow simulation done using SRTM data underestimates the observed 14.4 km flow runout by over 3 km, while the simulation with TanDEM-X data overestimates flow runout by about 1.5 km. Performance of the lava flow modeling algorithm is highly dependent on the modal lava thickness, highlighting the importance of using TanDEM-X DEMs to provide precise lava flow measurements in order to constrain input parameters for numerical modeling of lava flows.
TL;DR: The basaltic Al-Duaythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia as mentioned in this paper.
Abstract: The basaltic Al-Du’aythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia. Historical records indicate they may have erupted in AD 641. The four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion. Three cones display elongated tuff rings at the base, and two produced late-stage lava flows. The cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts. Spherical bombs and lapilli (cannonball bombs/lapilli), occasionally with country-rock fragments inside (both cored and loaded bombs/lapilli) are common within the tuff ring deposits. LiDAR data show a total volume of 1,664 × 10−6 km3 for the four cones (418 × 10−6 km3 DRE). Whole-rock chemical analyses indicate alkali-basalt compositions (SiO2 44.7–45.9 wt%), with little compositional variation and no relationship between chemistry and eruptive styles. Small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine. A magnetotelluric 2D cross-section shows that the cones are located adjacent to a buried sediment-filled alluvial channel along a NNW-SSE fault dipping to the east. The Al-Du’aythah eruption was related to the ascent of magma through this structure, with the first phase of the eruption triggered by the interaction of the magma with water from the northern Harrat Rahat aquifer that exists in the Al-Madinah basin. This initial water source was rapidly exhausted, while the eruption progressed roughly from north to south and from west to east, the latter motion probably along the fault-controlled feeding dyke. Our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the Al-Madinah basin, which, despite the hyperarid climate of the area, must be considered a potential future eruption hazard.
TL;DR: In this article, a review of Kawah Ijen's activity since 1770 is presented, with the purpose of reviewing the temporal evolution of its activity, including volcanic lake level and temperature measurements.
Abstract: Kawah Ijen is a composite volcano located at the easternmost part of Java island in Indonesia and hosts the largest natural acidic lake in the world. We have gathered all available historical reports on Kawah Ijen’s activity since 1770 with the purpose of reviewing the temporal evolution of its activity. Most of these observations and studies have been conducted from a geochemical perspective and in punctuated scientific campaigns. Starting in 1991, the seismic activity and a set of volcanic lake parameters began to be weekly available. We present a database of those measurements that, combined with historical reports, allow us to review each eruption/unrest that occurred during the last two centuries. As of 2010, the volcanic activity is monitored by a new multi-disciplinary network, including digital seismic stations, and lake level and temperature measurements. This detailed monitoring provides an opportunity for better classifying seismic events and forecasting volcanic unrest at Kawah Ijen, but only with the understanding of the characteristics of this volcanic system gained from the historical review presented here.
TL;DR: Tephra volume estimates are derived from isopach maps produced by modeling raw thickness data as cubic B-spline curves under tension, reflecting uncertainties in the volume estimation process.
Abstract: Characterization of explosive volcanic eruptive processes from interpretation of deposits is a key for assessing volcanic hazard and risk, particularly for infrequent large explosive eruptions and those whose deposits are transient in the geological record. While eruption size—determined by measurement and interpretation of tephra fall deposits—is of particular importance, uncertainties for such measurements and volume estimates are rarely presented. Here, tephra volume estimates are derived from isopach maps produced by modeling raw thickness data as cubic B-spline curves under tension. Isopachs are objectively determined in relation to original data and enable limitations in volume estimates from published maps to be investigated. The eruption volumes derived using spline isopachs differ from selected published estimates by 15–40 %, reflecting uncertainties in the volume estimation process. The formalized analysis enables identification of sources of uncertainty; eruptive volume uncertainties (>30 %) are much greater than thickness measurement uncertainties (~10 %). The number of measurements is a key factor in volume estimate uncertainty, regardless of method utilized for isopach production. Deposits processed using the cubic B-spline method are well described by 60 measurements distributed across each deposit; however, this figure is deposit and distribution dependent, increasing for geometrically complex deposits, such as those exhibiting bilobate dispersion.
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The online version of this article (doi:10.1007/s00445-015-0942-y) contains supplementary material, which is available to authorized users.
TL;DR: In this article, a building-damage scale has been developed that categorises likely lahar damage levels and, through theoretical calculations of expected building resistance to impact, approximate ranges of impact pressures.
Abstract: Lahar damage to buildings can include burial by sediment and/or failure of walls, infiltration into the building and subsequent damage to contents. The extent to which a building is damaged will be dictated by the dynamic characteristics of the lahar, i.e. the velocity, depth, sediment concentration and grain size, as well as the structural characteristics and setting of the building in question. The focus of this paper is on quantifying how buildings may respond to impact by lahar. We consider the potential for lahar damage to buildings on Merapi volcano, Indonesia, as a result of the voluminous deposits produced during the large (VEI 4) eruption in 2010. A building-damage scale has been developed that categorises likely lahar damage levels and, through theoretical calculations of expected building resistance to impact, approximate ranges of impact pressures. We found that most weak masonry buildings on Merapi would be destroyed by dilute lahars with relatively low velocities (ca. 3 m/s) and pressures (ca. 5 kPa); however, the majority of stronger rubble stone buildings may be expected to withstand higher velocities (to 6 m/s) and pressures (to 20 kPa). We applied this preliminary damage scale to a large lahar in the Putih River on 9 January 2011, which inundated and caused extensive building damage in the village of Gempol, 16 km southwest of Merapi. The scale was applied remotely through the use of public satellite images and through field studies to categorise damage and estimate impact pressures and velocities within the village. Results were compared with those calculated independently from Manning’s calculations for flow velocity and depth within Gempol village using an estimate of flow velocity at one upstream site as input. The results of this calculation showed reasonable agreement with an average channel velocity derived from travel time observations. The calculated distribution of flow velocities across the area of damaged buildings was consistent with building damage as classified by the new damage scale. The complementary results, even given the basic nature of the tools and data, suggest that the damage scale provides a valid representation of the failure mode that is consistent with estimates of the flow conditions. The use of open-source simplified tools and data in producing these consistent findings is very promising.
TL;DR: In this article, the authors use the elliptically constrained vents in the Auckland Volcanic Field to examine how spatial intensity models can be tested to assess whether they are consistent with such features.
Abstract: The idea of a volcanic field ‘boundary’ is prevalent in the literature, but ill-defined at best. We use the elliptically constrained vents in the Auckland Volcanic Field to examine how spatial intensity models can be tested to assess whether they are consistent with such features. A means of modifying the anisotropic Gaussian kernel density estimate to reflect the existence of a ‘hard’ boundary is then suggested, and the result shown to reproduce the observed elliptical distribution. A new idea, that of a spatio-volumetric model, is introduced as being more relevant to hazard in a monogenetic volcanic field than the spatiotemporal hazard model due to the low temporal rates in volcanic fields. Significant dependencies between the locations and erupted volumes of the observed centres are deduced, and expressed in the form of a spatially-varying probability density. In the future, larger volumes are to be expected in the ‘gaps’ between existing centres, with the location of the greatest forecast volume lying in the shipping channel between Rangitoto and Castor Bay. The results argue for tectonic control over location and magmatic control over erupted volume. The spatio-volumetric model is consistent with the hypothesis of a flat elliptical area in the mantle where tensional stresses, related to the local tectonics and geology, allow decompressional melting.
TL;DR: In this article, the eruptive history of Ciomadul volcano in the Carpathian-Pannonian region was reconstructed based on a new set of K-Ar age data and detailed field observations.
Abstract: Based on a new set of K–Ar age data and detailed field observations, the eruptive history of the youngest volcano in the whole Carpathian-Pannonian region was reconstructed. Ciomadul volcano is a dacitic dome complex located at the southeastern end of the Calimani-Gurghiu-Harghita Neogene volcanic range in the East Carpathians. It consists of a central group of extrusive domes (the Ciomadul Mare and Haramul Mare dome clusters and the Koves Ponk dome) surrounded by a number of isolated peripheral domes, some of them strongly eroded (Balvanyos, Puturosul), and others topographically well preserved (Haramul Mic, Dealul Mare). One of the domes (Dealul Cetaţii) still preserves part of its original breccia envelope. A large number of bread-crust bombs found mostly along the southern slopes of the volcano suggest that the dome-building activity at Ciomadul was punctuated by short Vulcanian-type explosive events. Two late-stage explosive events that ended the volcanic activity of Ciomadul left behind two topographically well-preserved craters disrupting the central group of domes: the larger-diameter, shallower, and older Mohos phreatomagmatic crater and the smaller, deeper and younger Sf. Ana (sub)Plinian crater. Phreatomagmatic products of the Mohos center, including accretionary lapilli-bearing base-surge deposits and poorly sorted airfall deposits with impact sags, are known close to the eastern crater rim. A key section studied in detail south of Baile Tusnad shows the temporal succession of eruptive episodes related to the Sf. Ana (sub)Plinian event, as well as relationships with the older dome-building stages. The age of this last eruptive event is loosely constrained by radiocarbon dating of charcoal pieces and paleosoil organic matter at ca. 27–35 ka. The age of the Mohos eruption is not constrained, but we suggest that it is closely related to the Sf. Ana eruption. The whole volcanic history of Ciomadul spans over ca. 1 Myr, starting with the building up of peripheral domes and then concentrating in its central part. Ciomadul appears as a small-volume (ca. 8.74 km3) and very low-frequency and low-output rate volcano (ca. 9 km3/Myr) at the terminus of a gradually diminishing and extinguishing volcanic range. A number of geodynamically active features strongly suggest that the magma plumbing system beneath Ciomadul is not completely frozen, so future activity cannot be ruled out.
TL;DR: In this paper, the authors propose a mechanism for conduit widening that precedes flow localization, and present geologic evidence from an Oligocene-aged dike-plug system at Ship Rock, New Mexico.
Abstract: Basaltic volcanic eruptions typically initiate as fissures fed by dikes. In some cases, eruptions become localized through discrete vents, until the flow is sustained through a single vent underlain by a volcanic plug, at which point, the eruption is capable of delivering much greater volumes of magma to the surface. Existing theories to explain flow localization focus on the relationship between non-uniform dike thickness and variations in magma flow rates. Here, we propose a mechanism for conduit widening that precedes flow localization, and we present geologic evidence from an Oligocene-aged dike-plug system at Ship Rock, New Mexico. We use techniques of structure from motion to map 621 joints in the sedimentary rock adjacent to the intrusions and find that these joints are part of a systematic set oriented perpendicular to the dikes and localized to within a meter of the dike contacts. This joint set, along with a set of dike-parallel joints and the bedding planes, divides the strata into rectangular blocks that subsequently are entrained in the flowing magma to widen the dikes. Using field and lab data and a stress analysis, we show that the dike-perpendicular joints are intimately associated with the emplacement of magma and thermal pressurization of pore fluids in the host rock. This fracturing process is a precursor to entrainment of the host rock and sustained flow through discrete vents.
TL;DR: In this paper, the authors investigated the Late Holocene tephrostratigraphic record of Agung volcano using stratigraphic logging, and geochemical and geochronological analyses.
Abstract: The 1963 AD eruption of Agung volcano was one of the most significant twentieth century eruptions in Indonesia, both in terms of its explosivity (volcanic explosivity index (VEI) of 4+) and its short-term climatic impact as a result of around 6.5 Mt SO2 emitted during the eruption. Because Agung has a significant potential to generate more sulphur-rich explosive eruptions in the future and in the wake of reported geophysical unrest between 2007 and 2011, we investigated the Late Holocene tephrostratigraphic record of this volcano using stratigraphic logging, and geochemical and geochronological analyses. We show that Agung has an average eruptive frequency of one VEI ≥2–3 eruptions per century. The Late Holocene eruptive record is dominated by basaltic andesitic eruptions generating tephra fall and pyroclastic density currents. About 25 % of eruptions are of similar or larger magnitude than the 1963 AD event, and this includes the previous eruption of 1843 AD (estimated VEI 5, contrary to previous estimations of VEI 2). The latter represents one of the chemically most evolved products (andesite) erupted at Agung. In the Late Holocene, periods of more intense explosive activity alternated with periods of background eruptive rates similar to those at other subduction zone volcanoes. All eruptive products at Agung show a texturally complex mineral assemblage, dominated by plagioclase, clinopyroxene, orthopyroxene and olivine, suggesting recurring open-system processes of magmatic differentiation. We propose that erupted magmas are the result of repeated intrusions of basaltic magmas into basaltic andesitic to andesitic reservoirs producing a hybrid of bulk basaltic andesitic composition with limited compositional variations.
TL;DR: In this paper, the authors used high-spatial-resolution (HSR) imagery from GeoEye, Pleiades, IKONOS, and SPOT5 satellites to assess the extent and effects of the pyroclastic density currents (PDCs) and subsequent lahars.
Abstract: The 26 October–23 November 2010 eruption is Merapi’s largest event (VEI 4) over the past 140 years. We used high-spatial-resolution (HSR) imagery from GeoEye, Pleiades, IKONOS, and SPOT5 satellites to assess the extent and effects of the pyroclastic density currents (PDCs) and subsequent lahars. We have tracked the geomorphic and structural (fracturing) changes of Merapi’s summit crater and dome between 2008 and 2012. The 4 September 2011 GeoEye image shows that due to the explosive eruption, the summit area lost about 10 × 106 m3. The eruption enlarged the SSE-trending Gendol breach to be 1.3 × 0.3 × 0.2 km. The 2010 tephra and PDC deposits covered about 26 km2 in the two catchments of Gendol and Opak Rivers on Merapi’s south flank, i.e., 60–75 % of the total PDC deposit area, with a total bulk volume of 45 × 106 m3. The tephra-fall deposit covered an area of about 1300 km2 with a range in volume of 18–21 × 106 m3. Supervised and object-oriented classification on HSR imagery enables us to map in detail the PDC deposits across the Gendol-Opak catchment. We delineated 16 spectrally and/or texturally distinct units of PDC deposits and compared them with previously published results. They encompass high-energy surge deposits within ca. 8 km of the summit, valley-confined PDC deposits channeled as far as 16.5 km in the Gendol River, and widespread overbank PDC with ash-cloud surge deposits on valley margins. Additional high-resolution data are provided to map and analyze flooded areas due to lahar activity in 2011–2012 on the south and west flanks. Subsequent overbank lahars impacted selective small areas in the populated ring plain, devastating villages along the Putih River on the southeast flank and the Gendol River. We have analyzed the morphometric parameters (channel cross-sectional capacity, longitudinal rate of channel confinement, and channel sinuosity) of the Gendol-Opak River channels that govern overbank and avulsion of lahars in the ring plain. The paper demonstrates the potential of HSR satellite imagery to outline tephra, PDC, and lahar deposits; map the geomorphic and structural evolution of the summit area of persistently active composite cones; and thus improve hazard assessment for Merapi, a volcano whose summit, slopes, and drainages have changed more since October 2010 than at any other time since 1931.
TL;DR: In this article, a high-resolution tephra fall map from the basaltic AD1256 Harrat Al-Madinah fissure eruption in Saudi Arabia was used to identify the most likely combination of multiple fall lobes.
Abstract: Complex eruption episodes commonly produce several phases of tephra fall and/or concurrent falls from multiple vents. Phases of eruption are challenging to recon- struct from the geological record, especially where there is a lack of distinct physical or chemical variations during an eruption episode. A statistical method is proposed for identi- fying the most likely combination of multiple fall lobes for composite tephra deposits, using a new high-resolution tephra fall map from the basaltic AD1256 Harrat Al-Madinah fissure eruption in Saudi Arabia. This dominantly effusive eruption episode lasted 52 days periodically producing tephra from several vents along the fissure. Most tephra was produced from high Hawaiian fountains and dispersed under differing wind conditions. The widest-dispersed tephra occurred under phases of the highest fountains, at least 500 m high and probably closer to 1000 m. These high fountains produced pyroclasts with a broad range of vesicularity. Similar total versus lobe-specific grain size determinations showed little systematic variation of maximum fountain-height phases. Individual tephra lobe properties (vesicle form, density, parti- cle shape and particle-size distribution) in different sectors around the volcano varied only subtly. From the statistical distribution of spot fall-thickness measurements, a semi- empirical tephra fallout model, modified to account for weathering, wind remobilisation and settling, was fitted using maximum likelihood estimation. A range of likely eruption- event scenarios were evaluated, concluding that the AD1256 eruption most likely comprised three separate fall-producing eruptions from its northern vent under differing wind condi- tions. The first of these occurred concurrently with high- fountaining events from two other major vents southward along the fissure, producing overlapping fall lobes. Applying this method to other similar compound tephra deposits will help elucidate more realistic eruption scenarios and event reconstructions from the geological record.