Dynamical analysis of volcanic explosion

Visible phenomena accompanied by volcanic explosions at Sakurajima Volcano in Kyushu, Japan, were recorded by means of a TV camera and still cameras to make clear the process of explosive eruption of a Vulcanian type by image analysis and to enable a discussion of the process of explosive eruption. The most interesting phenomenon observed by the TV camera was visible shock waves passing through the atmosphere above the crater. The instant disappearance of thin clouds and the condensation of dense clouds were induced by the passage of shock waves. Explosion-quakes, which occurred at a depth of 1–2 km beneath the active crater, clearly preceded the explosion at the crater bottom. The atmospheric shock waves were generated in the crater 1.1–1.5 seconds later than the occurrence of the explosion-quake and propagated with the velocity of Mach 1.3–1.5 in a height range from 300 m to 600 m above the crater. Eruption clouds expanded subspherically for several seconds after the ejection and then the eruption column developed upwards at a certain velocity. The maximum ejection velocity of volcanic blocks, which was obtained from the analysis of photo-trajectories, was 112–157 m/sec. The internal pressure which ejected the volcanic blocks was estimated to be 138–271 bars in the case of the explosive eruptions analyzed. The results of analysis suggest that a high-pressure gas chamber was formed just beneath the crater bottom before the explosive eruption and that pressure waves caused by the explosion-quake acted as the trigger for the explosive eruption.     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO₂, with 7–11 wt.% CO₂, high alkalies (Na₂O 15–19%, K₂O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite nephelinite (CWN) of OL by enrichment of CO₂ and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.     

Geochemical monitoring of the 2002–2003 eruption at Stromboli volcano (Italy): precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters

Significant changes in the helium and carbon isotopic composition of shallow thermal waters vs. gas and a crater fumarolic gas have been recorded at Stromboli prior and during the 2002–2003 eruption. The³He/⁴He ratios corrected for air contamination (Rc/Ra), and δ¹³C of fumarolic gases gradually increased from May to November 2002 before the eruption onset. These variations imply early degassing of a gas-rich magma at depth that likely fed both the intense Strombolian activity and small lava overflows recorded during that period. The lava effusion of late December 2002 was shortly preceded by a marked Rc/Ra decrease both in water and fumarolic gases. Comparison of He/CO₂ and CH₄/CO₂ ratios in dissolved gas and with values rules out the Rc/Ra decrease due to an increasing input of radiogenic⁴He. The Rc/Ra decrease is attributed to the He isotope fractionation during rapid magma ascent and degassing. A new uprising of ³He-rich magma probably occurred in January to February 2003, when Rc/Ra ratios displayed the highest values in dissolved gases ever measured before (4.56 Rc/Ra). The increase in He/CO₂ and CH₄/CO₂ ratios and decrease in δ¹³C of dissolved CO₂ was recorded after the 5 April 2003 explosive paroxysm, likely caused by enhanced gas-water interaction inducing CO₂ dissolution. No anomalous Rc/Ra values were recorded in the same period, when usual Strombolian activity gradually resumed.Editorial responsibility: H Shinohara     

Les éruptions de Mars–Avril 1982 du Volcan Chichonal

El Chichonal Volcano, within the Sierra of Chiapas (Mexico), became active on March 28, 1982. The summit dome was blasted away, leaving a 1000 m wide and 290 m deep crater in its place. Plinian eruption of March 28–29 produced air fall deposits. More vigorous activity on April 3–4 produced air falls, surges and pyroclastic flows. Villages within 6 km radius were entirely destroyed. The explosive eruptions of Chichonal produced large amounts of K-rich andesitic ash and pumice. The juvenile magma shows little compositional variability. Much of eruptive products is highly porphyritic pumice containing andesine, hornblende, clinopyroxene, biotite and anhydrite phenocrysts.     

The Miocene Costa Giardini diatreme,Iblean Mountains,southern Italy: model for maar-diatreme formation on a submerged carbonate platform

In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper, late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity. We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions.     

The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions

 Lascar Volcano (5592 m; 23°22'S, 67°44'W) entered a new period of vigorous activity in 1984, culminating in a major explosive eruption in April 1993. Activity since 1984 has been characterised by cyclic behaviour with recognition of four cycles up to the end of 1993. In each cycle a lava dome is extruded in the active crater, accompanied by vigorous degassing through high-temperature, high-velocity fumaroles distributed on and around the dome. The fumaroles are the source of a sustained steam plume above the volcano. The dome then subsides back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by violent explosive activity. Subsidence of both the dome and the crater floor is accommodated by movement on concentric, cylindrical or inward-dipping conical fractures. The observations are consistent with a model in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, triggering a large explosive eruption. Factors that can lead to a decrease in gas loss include a decrease in magma permeability by foam collapse, reduction in permeability due to precipitation of hydrothermal minerals in the pores and fractures within the dome and in country rock surrounding the conduit, and closure of open fractures during subsidence of the dome and crater floor. Dome subsidence may be a consequence of reduction in magma porosity (foam collapse) as degassing occurs and pressurisation develops as the permeability of the dome and conduit system decreases. Superimposed upon this activity are small explosive events of shallow origin. These we interpret as subsidence events on the concentric fractures leading to short-term pressure increases just below the crater floor. Received: 12 December 1996 / Accepted: 6 May 1997     

Anomalous Emissions of SO2 During the Recent Eruption of Santa Ana Volcano, El Salvador, Central America

Santa Ana volcano in western El Salvador, Central America, had a phreatic eruption at 8:05 am (local time) on October 1, 2005, 101 years after its last eruption. However, during the last one hundred years this volcano has presented periods of quiet degassing with fumarolic activity and an acidic lake within its crater. This paper presents results of frequent measurements of SO₂ degassing using the MiniDOAS (Differential Optical Absorption Spectroscopy) system and a comparison with the volcanic seismicity prior to the eruption, during, and after the eruption. Vehicle measurements of SO₂ flux were taken every hour during the first nine days of the eruption and daily after that. The period of time reported here is from August to December, 2005. Three periods of degassing are distinguished: pre-eruptive, eruptive, and post-eruptive periods. The intense activity at Santa Ana volcano started in July 2005. During the pre-eruptive period up to 4306 and 5154 ton/day of SO₂ flux were recorded on October 24 and September 9, 2005, respectively. These values were of the same order of magnitude as the recorded values just after the October 1^st eruption (2925 ton/day at 10:01 am). Hourly measurements of SO₂ flux taken during the first nine days after the main eruptive event indicate that explosions are preceded by an increase in SO₂ flux and that this parameter reaches a peak after the explosion took place. This behavior suggests that increasing accumulation of exsolved magmatic gases occurs within the magmatic chamber before the explosions, increasing the pressure until the point of explosion. A correlation between SO₂ fluxes and RSAM (Real Time Seismic Amplitude Measurements) is observed during the complete sampling period. Periodic fluctuations in the SO₂ and RSAM values during the entire study period are observed. One possible mechanism explaining these fluctuations it that convective circulation within the magmatic chamber can bring fresh magma periodically to shallow levels, allowing increasing degasification and then decreasing degasification as the batch of magma lowers its gas content, becomes denser, and sinks to give space to a new magma pulse. These results illustrate that the measurements of SO₂ flux can provide important warning signals for incoming explosive activity in active volcanoes.     

Mount Vesuvius: 2000 years of volcanological observations

Mount Vesuvius had eruptions ranging between VEI 5+ to 0–1 during the last 2000 years. Infrequent explosive eruptions are recorded during the period 79 AD to 1631. Since the violent explosive eruption of 1631, the volcano has been in persistent activity, rebuilding the morphology that it had before that eruption. A succession of explosive and effusive eruptions occurred until 1944, with a predominance of short and violent episodes until 1872 and longer effusive eruptions since that date. Two factors mainly controlled the character of volcanic activity during this period: (1) the strength of the cone, which allowed, in the earlier period, an easy fracturing, rapid drainage, and pressure release of the magma column; (2) the interaction between magma and water, which enhanced the explosivity of several eruptions.The volcano appears to have reached a stage of quiescence because it finally attained a shape of equilibrium in which the height of the mountain is sufficient to counterbalance the buoyancy of the magma.     

The role of syn-eruptive vesiculation on explosive basaltic activity at Mt. Etna,Italy

We investigated the dynamics of explosive activity at Mt. Etna between 31 August and 15 December 2006 by combining vesicle studies in the erupted products with measurements of the gas composition at the active, summit crater. The analysed scoria clasts present large, connected vesicles with complex shapes and smaller, isolated, spherical vesicles, the content of which increases in scoriae from the most explosive events. Gas geochemistry reports CO₂/SO₂ and SO₂/HCl ratios supporting a deep-derived gas phase for fire-fountain activity. By integrating results from scoria vesiculation and gas analysis we find that the highest energy episodes of Mt. Etna activity in 2006 were driven by a previously accumulated CO₂-rich gas phase but we highlight the lesser role of syn-eruptive vesicle nucleation driven by water exsolution during ascent. We conclude that syn-eruptive vesiculation is a common process in Etnean magmas that may promote a deeper conduit magma fragmentation and increase ash formation.     

Evolution and hydrological conditions of a maar volcano (Atexcac crater,Eastern Mexico)

The Atexcac maar is located in the central part of the Serdán–Oriental lacustrine/playa basin in the eastern Mexican Volcanic Belt. It is part of a dispersed and isolated monogenetic field consisting of maar volcanoes, basaltic cinder cones and rhyolitic domes. Atexac is a maar volcano excavated into pyroclastic deposits, basaltic lava flows and the flanks of a cinder cone cluster, which itself was built on a topographic high consisting of limestone. It has an ENE-trending elliptical shape with beds, mostly unconsolidated deposits that dip outward at 16–22°. The Atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water, temporal migration of the locus of the explosion, and periodic injection of new magma were important controls on the evolution of the maar crater. Variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions, producing different eruptive conditions with fluctuating water–magma interactions. Deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli. This could suggest differential degrees of fragmentation due to inherited (previously acquired) fragmentation and/or relative distance to the locus of explosions. Initial short-lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma. Drier explosions progressed downward and/or laterally northward, sampling subsurface rock types, particularly intrusive, limestone and andesitic zones as well as localized altered zones (N-NE), caused by repetitive injection of basaltic magma. A final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation. We infer the aquifer was formed by fractured rocks, predominantly andesitic lava flows and limestone rocks. Andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area. These local hydrogeological conditions contrast with those at nearby maar volcanoes, where the water for the magma/water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer, producing tuff rings with a much lower profile than Atexcac.     

Small inflation sources producing seismic and infrasonic pulses during the 2000 eruptions of Miyake-jima, Japan

During the 2000 activity of Miyake-jima volcano, Japan, we detected long period seismic signals with initial pulse widths of 1-2 s, accompanied by infrasonic pulses with almost the same pulse widths. The seismic signals were observed from 13 July 2000, a day before the second summit eruption. The occurrences of the seismic signals were intermittent with a gradual increase in their magnitudes and numbers building toward a significant explosive eruption on 18 August. After the eruption, the seismic and infrasonic events ceased. The results of a waveform inversion show that the initial motions were excited by an isotropic inflation source beneath the south edge of the caldera at a depth of 1.4 km. On the other hand, the sources of the infrasonic pulses were located in the summit caldera area. The times at which the infrasonic pulses were emitted at the surface were delayed by about 3 s from the origin times of the seismic events. It is suggested that small isotropic inflations excited seismic waves in the crust and simultaneously caused acoustic waves that traveled in the conduit and produced infrasonic pulses at the crater bottom. Considering the observed time differences and gas temperatures emitted from the vent, the conduit should have been filled with vapor mixed with SO₂ gas and volcanic ash. The change of the time differences between the seismic and infrasonic signals suggests that the seismic source became shallower within half a day before the August 18 explosive eruption. We interpret the source process as a fragmentation process of magma in which gas bubbles burst and quickly released part of the pressure that had been sustained by the tensional strength of magma.     

Internal structure and building of basaltic shield volcanoes: the example of the Piton de La Fournaise terminal cone (La Réunion)

In April 2007, a caldera collapsed at the Dolomieu summit crater of Piton de La Fournaise (La Réunion Island, Indian Ocean) revealing new outcrops up to 340?m high along the crater walls. The lithostratigraphic interpretation of these new exposures allows us to investigate the most recent building history of a basaltic shield volcano. We present the history of the Piton de La Fournaise terminal cone, from the building of a juvenile cone during which periods of explosive activity dominated, to the most recent effusive period. The changes in eruptive dynamics are the cause of successive summit crater/pit–crater collapses. In April 2007, such an event occurred during rapid emptying of the shallow plumbing system feeding a large effusive lateral eruption. During the most recent effusive period, an eastward migration of the eruptive crater was observed and was linked to the successive destructions of the shallow magma reservoir during each collapse. The resulting changes in the local stress field favor the formation of a new reservoir and thus the migration of activity. Internal structures reveal that the building of the upper part of the terminal cone was predominantly by exogenous growth and that the hydrothermal system is confined at a depth >?350?m. These observations on Piton de La Fournaise provide new insights into construction of the summits of other basaltic shield volcanoes.     

Summit eruptions of Klyuchevskoi Volcano,Kamchatka in the early 21st century, 2003–2013

This paper is concerned with eruptions, seismicity, and deformation on Klyuchevskoi Volcano during the summit eruptions of 2012–2013, with the condition of the central crater during the eruptions, and with the effect that is exerted by the height of the lava in the crater on the start of the eruptions. The recurrence of eruptions in the North Volcanic Cluster (NVC), Kamchatka showed that all the four volcanoes in the cluster (Klyuchevskoi, Tolbachik, Shiveluch, and Bezymyannyi) become active during definite phases that were identified in the 18.6-year lunar cycle. This relationship of the NVC eruptions to the active phases in the 18.6-year lunar cycle, as well as the relationship to the 11-year solar activity, showed that eruptions can be predicted, yielding long-term estimates of activity for the NVC volcanoes. The short-term prediction of volcanic eruptions requires knowledge of seismicity and deformation that occur during the precursory period and during the occurrence of eruptions. Seismic activity during the summit eruptions of 2003–2013 took place in the depth range 20–25 km during repose periods of the volcano and at depths of 0–5 km in the volcanic edifice during the eruption. One notes an almost complete absence of any earthquakes at great depths during the summit eruptions. Volcanic tremor (VT) was recorded from the time that the eruptions began and continued to occur until the end. Geodetic measurements showed that the center of the magma pressure beneath the volcano during the parasitic and summit eruptions of 1979–1989 moved in the 4–17 km depth range, while during the summit eruptions of 2003–2013 the center moved in the 15–20 km range. These changes in the depth of the center of magma pressure may have been related to evacuation from shallow magma chambers.     

A CO2-rich gas trigger of explosive paroxysms at Stromboli basaltic volcano,Italy

In addition to rhythmic slug-driven Strombolian activity, Stromboli volcano occasionally produces discrete explosive paroxysms (2 per year on average for the most frequent ones) that constitute a major hazard and whose origin remains poorly elucidated. Partial extrusion of the volatile-rich feeding basalt as aphyric pumice during these events has led to consider their triggering by the fast ascent of primitive magma blobs from possibly great depth. Here I examine and discuss the alternative hypothesis that most of the paroxysms could be triggered and driven by the fast upraise of CO₂-rich gas pockets generated by bubble foam growth and collapse in the sub-volcano plumbing system. Data for the SO₂ and CO₂ crater plume emissions are used to show that Stromboli's feeding magma may originally contain as much as 2 wt.% of carbon dioxide and early coexists with an abundant CO₂-rich gas phase with high CO₂/SO₂ molar ratio (≥ 60 at 10 km depth below the vents, compared to ～ 7 in time-averaged crater emissions). Pressure-related modelling indicates that the time-averaged crater gas composition and output are well accounted for by closed system decompression of the basalt–gas mixture until the volcano–crust interface (～ 3 km depth), followed by open degassing and crystallization in the volcano conduits. However, both the low viscosity and high vesicularity of the basaltic magma permit bubble segregation and bubble foam growth at deep sill-like feeder discontinuities and at shallower physical boundaries (such as the volcano–crust interface) where the gas-rich aphyric basalt interacts with the unerupted crystal-rich and viscous magma drained back from the volcano conduits. Gas pressure build-up and bubble foam collapse at these boundaries will intermittently trigger the sudden upraise of CO₂-rich gas blobs that constitute the main driving force of the paroxysms. Deeper-sourced gas blobs, driving the most powerful explosions, will be the richest in CO₂ and have highest CO₂/SO₂ ratios. This mechanism is shown to account well for the dynamic, seismic and petrologic features of Stromboli's paroxysms and, hence, to provide a potential alternative interpretation for their genesis and their forecasting. Enhanced bubble foam leakage prior to a paroxysm, or foam emptying in several steps, should lead indeed to precursory upstream of CO₂-rich gas and increasing CO₂/SO₂ ratio in crater plume emissions. The recent detection of such signals prior to two explosions in December 2006 and March 2007 strongly supports this expectation and the model proposed in this study.     

Explosive magma-water interactions: Thermodynamics,explosion mechanisms,and field studies

Physical analysis of explosive, magma-water interaction is complicated by several important controls: (1) the initial geometry and location of the contact between magma and water; (2) the process by which thermal energy is transferred from the magma to the water; (3) the degree to and manner by which the magma and water become intermingled prior to eruption; (4) the thermodynamic equation of state for mixtures of magma fragments and water; (5) the dynamic metastability of superheated water; and (6) the propagation of shock waves through the system. All of these controls can be analyzed while addressing aspects of tephra emplacement from the eruptive column by fallout, surge, and flow processes. An ideal thermodynamic treatment, in which the magma and external water are allowed to come to thermal equilibrium before explosive expansion, shows that the maximum system pressure and entropy are determined by the mass ratio of water and magma interacting. Explosive (thermodynamic) efficiency, measured by the ratio of maximum work potential to thermal energy of the magma, depends upon heat transfer from the pyroclasts to the vapor during the expansion stage. The adiabatic case, in which steam immediately separates from the tephra during ejection, produces lower efficiencies than does the isothermal case, in which heat is continually transferred from tephra to steam as it expands. Mechanisms by which thermal equilibrium between water and magma can be obtained require intimate mixing of the two. Interface instabilities of the Landau and Taylor type have been documented by experiments to cause fine-scale mixing prior to vapor explosion. In these cases, water is heated rapidly to a metastable state of superheat where vapor explosion occurs by spontaneous nucleation when a temperature limit is exceeded. Mixing may also be promoted by shock wave propagation. If the shock is of sufficient strength to break the magma into small pieces, thermal equilibrium and vapor production in its wake may drive the shock as a thermal detonation. Because these mechanisms of magma fragmentation allow calculation of grain size, vapor temperature and pressure, and pressure rise times, detailed emplacement models can be developed by critical field and laboratory analysis of the resulting tephra deposits. Deposits left by dense flows of tephra and wet steam as opposed to those left by dilute flows of dry steam and tephra show contrasts in median grain size, dispersal area, grain shape, grain surface chemistry, and bed form.     

Compositionally diverse magmas erupted close together in space and time within a Karoo flood basalt crater complex

Geochemical data and mapping from a Karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism. Flood basalt eruptions at Sterkspruit, South Africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km² filled by 9–18 km³ of volcaniclastic debris. Rising magma flux rates combined with reduced access of magma to external water led to effusion of thick Karoo flood basalts, burying the crater-complex beneath the >1.5 km-thick Lesotho lava pile. Geochemical data is consistent with flood basalt effusion from local dikes, and some lava flows likely shared or re-occupied vent sites active during explosive eruptions at Sterkspruit. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the Sterkspruit crater complex. Stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater-complex. Two magma types recognized there have not been recognized in the Karoo province before. The variable composition of flood basalts at Sterkspruit argues that magma batches in flood basalt fields may be small (0.5–1 km³) and not simply related to one another. This implies in turn that heterogeneities in the magma source region may be close to each other in time and space, and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields.     

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.     

Origins and energetics of maar volcanoes: examples from the ultrapotassic Sabatini Volcanic District (Roman Province,Central Italy)

Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water. Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions, eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from either monogenetic or polygenetic maars ranged 0.004–0.07 km³ during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 10¹⁵–4 × 10¹⁷ J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation.     

Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand

The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.