Phreatic Explosions Research Articles

The Age of the Patom Crater: Dendrochronological and Biogeochemical Investigations

According to data of dendrochronological investigations of larch trees that had grown on the Patom crater (Irkutsk oblast, Bodaibinskii district) and near its base, the initial period of the formation of the crater can be dated back to the late 15th‒early 16th centuries. In 1841‒1842, the trees under investigation recorded in annual rings a catastrophic event which caused a disturbance to their root systems, damage to tree stems, and the formation of compression wood. It is obvious that the event is associated with the time of formation of the late stage ring swell on which sparse larch trees are 93‒101 years of age, and on the central mound emerging upon completion of the formation of the Patom crater the age of the oldest tree is 7 years. Thus dendrochronological investigations confirm a long-lasting and multievent formation of the Patoma crater. In some periods, the deposition cone was evolving with a different intensity, sometime showing an explosive character. X-ray fluorescent analysis revealed a double Sr enrichment of larch wood at the time of significant activity of crater formation (1852‒1859) when plutonic fluids from the igneous source were able to enter the upper horizons of the Earth’s crust at the time of magma intrusion into water-containing rocks. The fluids were enriched with CO2 and transported significant amounts of Sr, as a result of which the 87Sr/86Sr ratios in carbonate and terrigenous rocks assumed higher values in sandstone and schists on the early stage ring swell of the Patoma crater. The findings, coupled with geological data, suggest that the Patom crater emerged as a result of a phreatic explosion that occurred either at the time of magma intrusion into water-containing rocks or as a consequence of the fault and decompression of heated water-containing rocks.

Variability of ash deposits at Piton de la Fournaise (La Reunion Island): insights into fragmentation processes at basaltic shield volcanoes

It is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic shield volcanoes. At Piton de La Fournaise (La Réunion Island, France), eruptive activity occurs mostly within the non-populated Enclos Fouqué caldera and generally does not pose any risk to the population. However, historical observations, recent monitoring data, and field work on tephra deposits suggest that some eruptions have produced unusual and unexpected explosive phases. A comprehensive sampling of tephra from major historical and recent eruptions on this volcano allowed us to perform systematic componentry, grain size, and chemical and morphological analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions. This integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation efficiencies and multiple associated mechanisms. Primary ductile and partially brittle fragmentation of various intensity of juvenile magma emitted during Hawaiian fountaining or mild Strombolian explosions were identified as the most common fragmentation mechanisms, in particular during the 2014–2018 dominantly effusive eruptive sequence. In parallel, we distinguished more efficient short-lived fragmentation events related to (i) plug pressurization and brittle fragmentation enhanced by syn-eruptive crystallization, (ii) magma/water interactions, (iii) rare phreatic explosions, and (iv) secondary fragmentation producing fine ash during caldera collapse. We conclude that textural, geochemical, and morphological analyses make it possible to identify and characterize the variability in eruptive processes, with the grain size, and the componentry of the ash particles being probably the most important parameters to quantify both the efficiency and the nature of the fragmentation.

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, evidenced by multiparametric data

Abstract. Small steam-driven volcanic explosions are common at volcanoes worldwide but are rarely documented or monitored; therefore, these events still put residents and tourists at risk every year. Steam-driven explosions also occur frequently (once every 2–5 years on average) at Lascar volcano, Chile, where they are often spontaneous and lack any identifiable precursor activity. Here, for the first time at Lascar, we describe the processes culminating in such a sudden volcanic explosion that occurred on 30 October 2015, which was thoroughly monitored by cameras, a seismic network, and gas and temperature sensors. Prior to the eruption, we retrospectively identified unrest manifesting as a gradual increase in the number of long-period (LP) seismic events in 2014, indicating an enhanced level of activity at the volcano. Additionally, sulfur dioxide (SO2) flux and thermal anomalies were detected before the eruption. Then, our weather station reported a precipitation event, followed by an increase in steaming and a sudden volcanic explosion at Lascar. The multidisciplinary data exhibited short-term variations associated with the explosion, including (1) an abrupt eruption onset that was seismically identified in the 1–10 Hz frequency band, (2) the detection of a 1.7 km high white-gray eruption column in camera images, and (3) a pronounced spike in SO2 emission rates reaching 55 kg s−1 during the main pulse of the eruption as measured by a mini-differential optical absorption spectroscopy (DOAS) scanner. Continuous carbon dioxide (CO2) and temperature measurements conducted at a fumarole on the southern rim of the Lascar crater revealed a pronounced change in the trend of the relationship between the CO2 mixing ratio and the gas outlet temperature; we speculate that this change was associated with the prior precipitation event. An increased thermal anomaly inside the active crater as observed in Sentinel-2 images and drone overflights performed after the steam-driven explosion revealed the presence of a ∼50 m long fracture truncating the floor of the active crater, which coincides well with the location of the thermal anomaly. This study presents the chronology of events culminating in a steam-driven explosion but also demonstrates that phreatic explosions are difficult to predict, even if the volcano is thoroughly monitored; these findings emphasize why ascending to the summits of Lascar and similar volcanoes is hazardous, particularly after considerable precipitation.

Air-Fall Ash from the Main Crater of Asama Volcano on August 7, 2019, and its Water-Soluble Components

We collected volcanic ash immediately following the eruption of Mt. Asama on August 7, 2019, observed the characteristics of ash particles, and analyzed the water-soluble components. The volcanic ash consisted mostly of altered fragments, and no clear evidence of essential materials was found. The volcanic ash contained large amounts of water-soluble components, Cl and SO4 at concentrations of 8,710 mg/kg and 49,100 mg/kg, respectively. These results indicate that this eruption was caused by the phreatic explosion and that part of the volcanic edifice of Mt. Asama was fractured and emitted.

Complex crater fields formed by steam-driven eruptions: Lake Okaro, New Zealand

Abstract Steam-driven eruptions are caused by explosive vaporization of water within the pores and cracks of a host rock, mainly within geothermal or volcanic terrains. Ground or surface water can be heated and pressurized rapidly from below (phreatic explosions), or already hot and pressurized fluids in hydrothermal systems may decompress when host rocks or seals fail (hydrothermal eruptions). Deposit characteristics and crater morphology can be used in combination with knowledge of host-rock lithology to reconstruct the locus, dynamics, and possible triggers of these events. We investigated a complex field of >30 craters formed over three separate episodes of steam-driven eruptions at Lake Okaro within the Taupo volcanic zone, New Zealand. Fresh unaltered rock excavated from initially >70 m depths in the base of phase I breccia deposits showed that eruptions were deep, “bottom-up” explosions formed in the absence of a preexisting hydrothermal system. These phreatic explosions were likely triggered by sudden rise of magmatic fluids/gas to heat groundwater within an ignimbrite 70 m below the surface. Excavation of a linear set of craters and associated fracture development, along with continued heat input, caused posteruptive establishment of a large hydrothermal system within shallow, weakly compacted, and unconsolidated deposits, including the phase I breccia. After enough time for extensive hydrothermal alteration, erosion, and external sediment influx into the area, phase II occurred, possibly triggered by an earthquake or hydrological disruption to a geothermal system. Phase II produced a second network of craters into weakly compacted, altered, and pumice-rich tuff, as well as within deposits from phase I. Phase II breccias display vertical variation in lithology that reflects top-down excavation from shallow levels (10–20 m) to >70 m. After another hiatus, lake levels rose. Phase III hydrothermal explosions were later triggered by a sudden lake-level drop, excavating into deposits from previous eruptions. This case shows that once a hydrothermal system is established, repeated highly hazardous hydrothermal eruptions may follow that are as large as initial phreatic events.

Modern Mineral Formation in the Thermal Lake Fumarolnoe (Uzon Caldera, Kamchatka) as a Key to Paleoreconstruction

Paleoreconstruction of events involved in the bottom sedimentation has been carried out on the basis of mineral composition of the section of the minor lake IV (Lake Fumarolnoe, Kamchatka) bottom sediments. The lower sediments are composed mainly of smectite in association with gypsum and jarosite; they are overlain by smectite–kaolinite with a large amount of framboidal pyrite and diatom shells. There is also a layer of sinter opal–pyrite units and calcite. It is concluded that initially the minor lake IV was a mud pit with sulfide-bearing solutions. Then, after the lowering of the water level and the partial exposure of the bottom sediments on the surface, the latter were oxidized. The subsequent phreatic explosion provided the covering of oxidized sediments with the reduced substance from the lower horizons of the pit. Further setting of sediment accumulation corresponded to a source with cyanobacterial community developing around the gryphon, and then it changed to that of a shallow-water lake similar to a contemporary one. Sediments also include two layers of pyroclastic material originated from eruptions of neighboring volcanoes.

Public response to news reports on the Mount Kusatsu-Shirane volcanic eruption: A content analysis of online reader comments

This paper reports on the real-time public responses to the eruption disaster on Mt. Kusatsu-Shirane in January 2018. This volcanic disaster attracted significant media coverage, with many people again acknowledging the risk of volcanic hazards from the many news reports. These days, as it has become increasingly common for people to express their opinions using online sites, online reader comments can be important sources of public opinion. This paper used a content analysis approach to analyze the comments posted in online news reports on the volcanic disaster to determine the public's perspective of the risks associated with volcanic disasters and volcanology. Applying an inductive category-extraction strategy, the comments were sorted into six categories of comments (1) that made reference to volcanic risk perception, (2) on volcanology or volcanologists, (3) that had fatalistic perceptions of natural disasters as blameless misfortunes, (4) on eruption predictability, (5) on disaster risk reduction measures implemented by central/local governments and disaster management agencies, and (6) explaining the phreatic explosion mechanism. It was found that many comments tended to have a negative tone, and a prominent criticism was that modern science could not detect volcanic activities reliably. However, some commenters suggested that experts with a sophisticated knowledge of volcanology should be available to support local communities in a similar way to the “home doctor”. Volcanologists need to disseminate information, such as the current state of geological and earth science education systems, volcanological knowledge, and application of the lessons learned from volcanic disasters.

Late Pleistocene–Holocene Volcaniclastic Ejecta Along the Southern Apron of the Esan Volcanic Complex, Japan

The Esan volcanic complex (EVC), northern Japan, is an active volcanic complex. The EVC represents a potential threat owing to its close proximity to inhabited areas, and abundant phreatic deposits occur around its volcanic aprons. Eleven samples of black paleosol and charcoal were collected from trenches dug at two sites (A and B) for the purpose of 14C dating. Sites A and B are located 2.6 km and 2.2 km from the source crater, respectively, and the deposits at these sites represent a proximal facies. At least 11 (at site A) and 12 (at site B) volcaniclastic units are identified. A newly identified Holocene phreatic unit (Es-0) is dated at ca 11 ka and is older than the EsMP unit that is a Holocene lava-dome building episode at ca 9 ka. Holocene phreatic episodes Es-1 and Es-3 can be divided into three and five subunits, respectively, and include a pyroclastic surge deposit that would have posed a threat to current local communities. The sequence of Holocene eruptions includes at least 11 tephra events over 11,000 years. The material ejected during phreatic explosions is characterised by highly heterogeneous grain size. Therefore, grain size analysis of erupted phreatic material might not effectively discriminate the processes of deposition, in contrast to such analysis of magmatic ejecta. The topography of the EVC largely constrains the area of deposition of phreatic ejecta.

Mechanisms of Unrest and Eruption at Persistently Restless Volcanoes: Insights From the 2015 Eruption of Telica Volcano, Nicaragua

AbstractMany of Earth's volcanoes experience well‐defined states of “quiescence” and “unrest,” with unrest occasionally culminating in eruption. Some volcanoes, however, experience an unusually protracted (i.e., decades‐long) period of noneruptive unrest and are thus categorized as “persistently restless volcanoes” (PRVs). The processes that drive persistently restless volcanism are poorly understood, as our knowledge of PRVs is currently based on a small number of case studies. Here we examine multidisciplinary observations of the 2015 eruptive episode at Telica Volcano, Nicaragua, in the context of its long‐term behavior. We suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir. Based on previous geodetic‐seismic studies of Telica (Geirsson et al., 2014, https://doi.org/10.1016/j.jvolgeores.2013.11.009; Rodgers et al., 2013, https://doi.org/10.1016/j.jvolgeores.2013.08.010 and 2015, https://doi.org/10.1016/j.jvolgeores.2014.11.012) and on multiparameter observations at Telica over a 7‐year period, we propose that three distinct states of unrest occur at Telica over decadal timescales: a stable open state involving steady conduit convection and two distinct “unstable” states that may lead to eruptions. In the “weak sealing” state, phreatic explosions result from steady conduit convection underlying a weak seal. In the “destabilized” state, destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong/impulsive phreatomagmatic explosions. Our observations and interpretations suggest that continuous seismic, ground‐based deformation, gas emission, and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential, energy, and duration at Telica and similar PRVs worldwide.

Explosive summit collapse of Kīlauea Volcano in 1924 preceded by a decade of crustal contamination and anomalous Pb isotope ratios

Abstract A geochemical time-series analysis of lavas from frequently active basaltic volcanoes has the potential to reveal the enigmatic mantle controls on volcanic behavior and hazards. In May 1924, the century-long lava lake within Halemaʻumaʻu pit crater at the summit of Kīlauea Volcano drained and the floor of Halemaʻumaʻu collapsed, triggering ∼3 weeks of phreatic explosions due to the interaction of groundwater with hot rock. For the next three decades, eruptions at Kīlauea were sporadic (the longest hiatus was from 1934 to 1952), small in volume, and short (typically 10 km). The Pb isotopic heterogeneity of the samples on short length (hand specimen to lava flow) and time (∼1–4 yr) scales can be explained by inefficient mixing as small batches of contaminated magma were delivered to the remnants of Kīlauea’s summit magma storage reservoir. Our results confirm that the Pb isotope ratios of basalts from ocean-island volcanoes may be significantly modified by assimilation of materials from the underlying oceanic crust. In particular, the 207Pb/204Pb ratio may be a sensitive tracer of such crustal contamination at Hawaiian shield volcanoes. Mauna Loa lavas display a factor of ∼5 more scatter towards higher 207Pb/204Pb at a given 206Pb/204Pb ratio than most Kīlauea lavas (excluding the samples from 1912 to 1954). This might be caused by more pervasive crustal contamination at Mauna Loa due to its lower magma supply rate over the last ∼4 kyr.

Tephra fallout hazard assessment at Tacaná volcano (Mexico)

Tacaná volcano is one of the dormant volcanoes in Mexico. Its activity is demonstrated by recent phreatic explosions occurred in 1949 and 1986. At least four Plinian to sub Plinian eruptions have been recorded in the eruptive history of this volcano, which produced pumice and ash deposits that blanketed a vast area straddling the Mexican and Guatemalan territory. Currently, there is no quantitative tephra hazard study of the volcano fallout deposits nor of the dispersion of ash in the atmosphere.In this work, we present the first probabilistic tephra hazard assessment for the Tacaná volcano. Probabilistic hazard maps were computed both, for the tephra deposit, and for critical thresholds of airborne ash concentration at different flight levels. The FALL3D numerical model was used to perform hazard maps for a Plinian scenario defined on the basis of the Sibinal Pumice eruption occurred ∼23.5 kyrs ago, which is the most studied deposit of this type in the volcano's surroundings, even when just few of their most distal outcrops are preserved. Results of this work show that at ground level, the disturbance for this type of hazard would affect mainly Guatemalan territory and the state of Chiapas in Mexico. It could cause serious repercussions on the economy of the region, which is characterized mainly by cocoa, coffee and banana plantations. The ash dispersion into the atmosphere could disturb the airspace and major airports in Mexican territory and neighboring countries. In fact, probability ash dispersal maps show that the level of affectation to the air traffic would involve the possible closure of at least 23 airports in Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua. Another collateral damage would be the remobilization of ash by lahars considering the near 4000 mm of rain in average poured yearly in the Soconusco region that would increase the sediment load of the Coatán and Suchiate rivers, and then could produce a severe damage to the lowlands, which in the case of the Mexican territory, is the most populated one.

Numerical simulation of block-and-ash flows for different eruptive scenarios of the Tacaná Volcanic Complex, México-Guatemala

The Tacaná Volcanic Complex (TVC), shared by México and Guatemala, is considered an active volcano that resumed phreatic explosions in 1949 and 1986. The TVC has recorded at least eight eruptions emplacing block-and-ash flows (BAFs) around the volcanic edifice during the past 42,000 years. The largest event generated the Mixcun deposit some 1950 yr B.P. from San Antonio Volcano, the youngest structure of the complex. Based on the eight mapped BAF deposits of the complex, we perform numerical simulations with the TITAN2d code to recreate the distribution and extent of these deposits. The results were used to prepare the zonation of the TVC based on the surface bed friction angles, which are related to the different morphologies of the volcano. From this zonation and the features of the past BAF deposits, we define three new eruptive impact scenarios (low, medium and high), to perform a hazard assessment of this phenomenon but considering the present morphology of the complex of the four eruptive centers. Next, we ranged the input parameters for each case scenario to analyze the dispersal, thickness and maximum runout of simulated flows. From the numerical simulations, we found that the most influential parameter in the model outputs is the basal friction angle, which varied from 23° to 12°. Then, we merged the eruptive scenarios simulated to construct new hazard maps for the different mass volumes simulated for each scenario. These maps clearly evidenced that the Mexican territory would be seriously affected in case of a renewed explosive activity of the TVC. In fact, even the partial collapse of any of the TVC domes would produce BAFs capable to flow over ~70 communities affecting ca. 20,000 inhabitants, as well as promote the generation of syn-and-post eruptive lahars if they flow into the main rivers that drain the volcano. Therefore, these maps are a great advance to evaluate the destructive potential of the TVC and must serve as the basis for the construction of a new risk map by the local authorities to update the operative and evacuation plans of the region. These actions would help to the >350,000 inhabitants living in a 35 km radius from the volcano's summit to be aware of the perils posed by the volcano, and how to act and evacuate in case of a volcanic crisis.

Современное минералообразование в термальном озере Фумарольное (кальдера Узон, Камчатка) — ключ к палеореконструкции

Paleoreconstruction of events involved in the bottom sedimentation has been carried out on the base of mineral composition of a section of bottom sediments in the Lake IV (the Lake Fumarolnoe). The lower levels are composed there mainly of smectite in association with gypsum and jarosite; they are overlapped by smectite-kaolinite depositions with the large amount of diatom algae shells and the fraboidal pyrite. There is also a layer of sinter opal-pyrite structures and calcite. It is assumed that initially the lake IV was a mud pit with sulfide-bearing solutions, then, after the lowering of water level and the partial exposure of bottom sediments of the surface, the substance has been oxidized. The subsequent phreatic explosion has provided the covering of oxidized substance by the matter of lower layers of the pit. The further situation corresponded to the source, with the cyano-bacterial community developing around the griffin, and it was replaced by the site of a shallow-water lake similar to the contemporary one. The studied sedimentary formation includes two layers of pyroclastic material originated from neighbouring eruptions.

Eruptive Fluid Fracture Formations of the Nakyn Diamondiferous Field of Yakutia

In the Nakyn diamondiferous field of the West Yakutia province various fluid fracture formations, including kimberlites, eruption breccias of basites, fluid fracture carbonate breccias, and accompanying fluidizite streaks were identified. The characteristic petrographic features and patterns of distribution of these formations in the rocks of the Lower Paleozoic sedimentary cover are presented. The predominantly tectonic control of the fluid fracture formations and their bi- and confocal distribution was revealed. It is suggested that the formations resulted from phreatic explosions of different ages directed along faults.

Eruptive fluid fracture formations of Nakyn diamondiferous field of Yakutia

In Nakyn diamondiferous field of West-Yakutian province various fluid fracture rocks determined, including kimberlites, eruptive breccias of basites, fluid fracture carbonate breccias and accompanying fluidizite streaks. Shown their characteristic petrographic features and regularity of distribution in the rocks of the lower Paleozoic sedimentary cover. Revealed a predominant tectonic control of fluid fracture formations with bi — and confocal distribution. Suggested the formation of these fluid fracture rocks as a result of different ages directed at faults phreatic explosions.

The 2010 seismovolcanic crisis at Taal Volcano (Philippines)

Typical eruptions of Taal Volcano in the Philippines are frequent and violently explosive. The last devastating phreatomagmatic eruption from 1965 to 1977 resulted in the death of about 200 persons. For this and other earlier events, no warnings of impending eruptions were issued. Since this time, volcanic crises involving episodes of seismicity, deformation, gas release, temperature change, etc., have occurred frequently. Each of these did not, but possibly could have, evolved into an explosive eruption. One such event occurred between April 2010 and March 2011. This paper reports various types of geophysical data (electric/magnetic field, ground temperature, seismicity and ground deformation) taken during this period of activity together with the current state of geophysical knowledge about the volcano in order to throw light on activity evolution. While the 2010 seismovolcanic crisis was preceded by unusual changes in electric field, ground tilting and uplift, the major signals from all data were concomitant with the evolution of the crisis. On April 20, 2010, felt earthquakes first started to occur. The numbers increased sharply after April 29 with repeated pulses of seismicity that ended finally in March 2011. Overall, we can identify three periods of seismic activity, from April 20 to August 4–8, from August 4–8 to November 18, and from November 18 to March 28, 2011. Although only a single tiltmeter was in operation, it recorded three phases of deformation that correspond generally with the pulses in seismic activity. Phase 1 appears to correspond to inflation at a depth of about 5 km just to the NW of Main Crater Lake from April 20 to June 11, 2010. This resulted in an eastward tilting of the northern flank of the volcano. This apparently triggered rapid but shallower inflation (Phase 2) in the hydrothermal region of the volcano under the northern edge of Main Crater Lake from June 11 to July 13 just to the SE of the initial deeper source as indicated by northward tilting of the volcano’s northern flank. Progressive deflation began after July 13. By March 28, 2011, the deformation state recovered to the pre-crisis level and felt seismic activity ceased. Apparent related changes in seismicity, tilt, magnetic and electric fields occur during smaller NS inflations in the Phase 3 deflation—Phase 3a (July 13 to August 4–8), Phase 3b (August 4–8 to November 18) and Phase 3c (November 18 to March 28, 2011). These likely result from interrelated stress change, fluid/gas migration and thermal activity at depth and in the hydrothermal system. The rapidity of changes in the state of the volcano over a matter of hours during this entire process indicates rapid communication occurs between processes at depth and in the hydrothermal system and this could easily result in the dramatic evolution to violent phreatic explosions with little warning.

Stability assessment of the Crater Lake/Te Wai-ā-moe overflow channel at Mt. Ruapehu (New Zealand), and implications for volcanic lake break-out triggers

The presence of lakes on active volcanoes gives rise to several unique volcano-hydrologic hazards, including those directly related to volcanic activity (e.g. phreatic explosions) and non-volcanic phenomena (e.g. lake outbursts). Mt. Ruapehu volcano hosts the 107 m3 Crater Lake/Te Wai-ā-moe in the active summit crater, making it prone to lahars due to the abundance of water stored at high elevation. Apart from eruptive periods when water is commonly ejected from the lake, large lahar events can occur when portions of the crater rim fail during quiescent periods, leading to partial lake drainage; the latter occurred recently in 1953 and 2007. Here, we investigate the potential of future lahars due to rim failure at the current overflow channel considering several triggering scenarios during non-eruptive periods. The geomechanical properties of a matrix-dominated breccia unit composing part of the overflow channel were analyzed using physical and mechanical laboratory methods. These properties, combined with previous geomechanical studies of lava units in the overflow channel, were used in finite element and limit equilibrium method-based groundwater and stability models. Groundwater models indicate that a low-permeability barrier inside the crater rim is preventing water from seeping through the unconsolidated and potentially permeable breccia unit of the overflow channel. The removal of this barrier, or weathering of a resistive lava cap, could cause water to seep through the overflow channel, elevating the potential of collapse. Slope stability models indicate that under certain conditions, the overflow channel is below a conservative threshold of stability and can be destabilized with continued mechanical or chemical weathering that reduces strength properties. This is also true during high magnitude tectonic or volcanic earthquakes, where large ground accelerations could result in several centimeters of displacement at the overflow channel, corresponding to high hazard categories of stability. In the worst-case scenario modeled here, full collapse of the overflow channel could result in the release of c. 2.4 million m3 of lake water. This could drive a lahar substantially larger than well-documented historic events, including the more common lahars produced during eruptions. The identification of key factors affecting stability can be used to inform future monitoring and the risk of these high impact events.

An Overview of the Dynamics of the Volcanic Paroxysmal Explosive Activity, and Related Seismicity, at Andesitic and Dacitic Volcanoes (1960–2010)

Understanding volcanic paroxysmal explosive activity requires the knowledge of many associated processes. An overview of the dynamics of paroxysmal explosive eruptions (PEEs) at andesitic and dacitic volcanoes occurring between 1960 and 2010 is presented here. This overview is based mainly on a description of the pre-eruptive and eruptive events, as well as on the related seismic measurements. The selected eruptions are grouped according to their Volcanic Explosivity Index (VEI). A first group includes three eruptions of VEI 5-6 (Mount St. Helens, 1980; El Chichon, 1982, and Pinatubo, 1991) and a second group includes three eruptions of VEI 3 (Usu volcano, 1977; Soufriere Hills Volcano (SHV), 1996, and Volcan de Colima, 2005). The PEEs of the first group have similarity in their developments that allows to propose a 5-stage scheme of their dynamics process. Between these stages are: long (more than 120 years) period of quiescence (stage 1), preliminary volcano-tectonic (VT) earthquake swarm (stage 2), period of phreatic explosions (stage 3) and then, PEE appearance (stage 4). It was shown also that the PEEs of this group during their Plinian stage “triggered” the earthquake sequences beneath the volcanic structures with the maximum magnitude of earthquakes proportional to the volume of ejecta of PEEs (stage 5). Three discussed PEEs of the second group with lower VEI developed in more individual styles, not keeping within any general scheme. Among these, one PEE (SHV) may be considered as partly following in development to the PEEs of the first group, having stages 1, 3 and 4. The PEEs of Usu volcano and of Volcan de Colima had no preliminary long-term stages of quiescence. The PEE at Usu volcano came just at the end of the preceding short swarm of VT earthquakes. At Volcan de Colima, no preceding swarm of VT occurred. This absence of any regularity in development of lower VEI eruptions may refer, among other reasons, to different conditions of opening of the magmatic conduit during these eruptions.

Fluid Vents, Flank Instability, and Seafloor Processes along the Submarine Slopes of the Somma-Vesuvius Volcano, Eastern Tyrrhenian Margin

We report the geomorphological features of the continental shelf of the Gulf of Naples along the submarine slopes of the Somma-Vesuvius volcanic complex. This area is characterized by seafloor morphologies that are related to mantle degassing. Significant phenomena associated with this process occur. Doming of the seafloor has been detected in the area of Banco della Montagna, whereas a hole-like morphology has formed at Bocca dei Pescatori, likely as a result of a phreatic explosion. Outcropping or partially submerged volcanic bodies are also present as well as two main debris avalanche deposits arising from the main Somma-Vesuvius edifice. A large area characterized by an overall concave external profile and a global sediment wave morphology covers most of the southwestern area of the volcano.

Phreatic explosions during basaltic fissure eruptions: Kings Bowl lava field, Snake River Plain, USA

Physical and compositional measurements are made at the ~7km-long (~2200years B.P.) Kings Bowl basaltic fissure system and surrounding lava field in order to further understand the interaction of fissure-fed lavas with phreatic explosive events. These assessments are intended to elucidate the cause and potential for hazards associated with phreatic phases that occur during basaltic fissure eruptions. In the present paper we focus on a general understanding of the geological history of the site. We utilize geospatial analysis of lava surfaces, lithologic and geochemical signatures of lava flows and explosively ejected blocks, and surveys via ground observation and remote sensing.Lithologic and geochemical signatures readily distinguish between Kings Bowl and underlying pre-Kings Bowl lava flows, both of which comprise phreatic ejecta from the Kings Bowl fissure. These basalt types, as well as neighboring lava flows from the contemporaneous Wapi lava field and the older Inferno Chasm vent and outflow channel, fall compositionally within the framework of eastern Snake River Plain olivine tholeiites. Total volume of lava in the Kings Bowl field is estimated to be ~0.0125km3, compared to a previous estimate of 0.005km3. The main (central) lava lake lost a total of ~0.0018km3 of magma by either drain-back into the fissure system or breakout flows from breached levees. Phreatic explosions along the Kings Bowl fissure system occurred after magma supply was cut off, leading to fissure evacuation, and were triggered by magma withdrawal. The fissure system produced multiple phreatic explosions and the main pit is accompanied by others that occur as subordinate pits and linear blast corridors along the fissure. The drop in magma supply and the concomitant influx of groundwater were necessary processes that led to the formation of Kings Bowl and other pits along the fissure. A conceptual model is presented that has relevance to the broader range of low-volume, monogenetic basaltic fissure eruptions on Earth, the Moon and other planetary bodies.