Characterisation of the 1997 Vulcanian explosions of Soufrière Hills Volcano,Montserrat, by video analysis

The activity of Convention at Montserrat Soufrière Hills Volcano, Montserrat, during the period 1995–1999 included numerous violent explosions. Two major cycles of Vulcanian explosions occurred in 1997: a first of 13 explosions between 4 and 12 August and a second of 75 between 22 September and 21 October. The explosions were short-lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows, and buoyant plumes ascended 3–15 km into the atmosphere. Each explosion discharged on average 3×10⁵ m³ (dense-rock equivalent, DRE) of magma, draining the conduit to depths of 1–2 km. The paper focuses on the first few seconds of three explosions of the 75 that occurred in September/October 1997: 6 October 1997 at 17:50, 7 October 1997 at 16:02 and 9 October 1997 at 12:32. Physical parameters such as exit velocities, magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum-dominated finger jets visible on videos during the initial stages of each explosion. The model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent, and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters. Each explosion reveals a progressive increase in eruptive intensity with time, jet exit velocities increasing from 40 m s^–1 at the beginning of the explosion up to 140 m s^–1 after a few seconds. Modelling suggests that the first magma to exit was largely degassed, whereas that discharged after a few seconds contained up to 2 wt% water. Magma overpressures up to ~10 MPa are estimated to have existed in the conduit immediately prior to each explosion. Progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre-eruptive gradients in water content and magma pressure in the upper reaches (probably 100–500 m) of the conduit. Fountain collapse occurred during the first 10–20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant. Such high eruptive densities were due to the presence of partially degassed magma in the conduit.Editorial responsibility: A. Woods     

Petrologic constraints on the decompression history of magma prior to Vulcanian explosions at the Soufrière Hills volcano,Montserrat

A series of 88 Vulcanian explosions occurred at the Soufrière Hills volcano, Montserrat, between August and October, 1997. Conduit conditions conducive to creating these and other Vulcanian explosions were explored via analysis of eruptive products and one-dimensional numerical modeling of magma ascent through a cylindrical conduit. The number densities and textures of plagioclase microlites were documented for twenty-three samples from the events. The natural samples all show very high number densities of microlites, and > 50% by number of microlites have areas < 20 μm². Pre-explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions. Our comparisons suggest quench pressures < 30 MPa (origin depths < 2 km) and multiple rapid decompressions of > 13.75 MPa each during ascent from chamber to surface. Values are consistent with field studies of the same events and statistical analysis of explosion time-series data. The microlite volume number density trend with depth reveals an apparent transition from growth-dominated crystallization to nucleation-dominated crystallization at pressures of ∼ 7 MPa and lower. A concurrent sharp increase in bulk density marks the onset of significant open-system degassing, apparently due to a large increase in system permeability above ∼ 70% vesicularity. This open-system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to Vulcanian explosions. The corresponding inferred depth of overpressure at 250–700 m, near the base of the dense plug, is consistent with depth to center of pressure estimated from deformation measurements. Here we also illustrate that one-dimensional models representing ascent of a degassing, crystal-rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis. The comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open-system degassing through conduit walls.     

Changes in eruptive style during the A.D. 1538 Monte Nuovo eruption (Phlegrean Fields, Italy): the role of syn-eruptive crystallization

The Monte Nuovo eruption is the most recent event that occurred at Phlegrean Fields (Italy) and lasted from 29 September to 6 October 1538. It was characterized by 2 days of quasi-sustained phreatomagmatic activity generating pumice-bearing pyroclastic density currents and forming a 130-m-high tuff cone (Lower Member deposits). The activity resumed after a pause of 2 days with two discrete Vulcanian explosions that emplaced radially distributed, scoria-bearing pyroclastic flows (Upper Member deposits). The juvenile products of Lower and Upper Members are, respectively, phenocryst-poor, light-coloured pumice and dark scoria fragments with K-phonolitic bulk compositions, identical in terms of both major and trace elements. Groundmass is formed by variable proportions of K-feldspar and glass, along with minor sodalite and Fe-Ti oxide present in the most crystallized samples. Investigations of groundmass compositions and textures were performed to assess the mechanisms of magma ascent, degassing and fragmentation along the conduit and implications for the eruptive dynamics. In pumice of the Lower Member groundmass crystal content increases from 13 to 28 vol% from the base to the top of the sequence. Products of the Upper Member consist of clasts with a groundmass crystal content between 30 and 40 vol% and of totally crystallized fragments. Crystal size distributions of groundmass feldspars shift from a single population at the base of the Lower Member to a double population in the remaining part of the sequence. The average size of both populations regularly increases from the Lower to the Upper Member. Crystal number density increases by two orders of magnitude from the Lower to the Upper Member, suggesting that nucleation dominated during the second phase of the eruption. The overall morphological, compositional and textural data suggest that the juvenile components of the Monte Nuovo eruption are likely to record variations of the magma properties within the conduit. The different textures of pumice clasts from the Lower Member possibly reflect horizontal gradients of the physical properties (P, T) of the ascending magma column, while scoriae from the second phase are thought to result from the disruption of a slowly rising plug crystallizing in response to degassing. In particular, crystal size distribution data point to syn-eruptive degassing-induced crystallization as responsible for the transition in eruptive style from the first to the second phase of the eruption. This mechanism not only has been proved to profoundly affect the dynamics of dome-forming calc-alkaline eruptions, but may also have a strong influence in driving the eruption dynamics of alkaline magmas of intermediate to evolved compositions.Editorial responsibility: J. Donnelly-Nolan     

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano,Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4–0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H₂O contents of >∼0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H₂O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H₂O content of 3.1 wt% establishes the maximum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H₂O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO₂ (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open-system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressurization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.     

Dynamics of explosive volcanism at Unzen volcano: an experimental contribution

Knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behaviour of silicic volcanoes. Here we have measured the fragmentation speed and the fragmentation threshold of five dacitic samples (6.7–53.5 vol% open porosity) from Unzen volcano, Kyushu, Japan. The measurements were carried out using a shock-tube-based fragmentation apparatus modified after Alidibirov and Dingwell (1996a,b). The results of the experimental work confirm the dominant influence of porosity on fragmentation dynamics. The velocity of the fragmentation front increases and the value of the fragmentation threshold decreases with increasing porosity. Further, we observe that the fragmentation speed is strongly influenced by the initial pressure difference and the texture of the dacite. At an initial pressure difference of 30 MPa, the fragmentation speed varies from 34 m/s for the least porous sample to 100 m/s for the most porous sample. These results are evaluated by applying them to the 1990–1995 eruptive activity of Unzen volcano. Emplacements of layered lava dome lobes, Merapi-type pyroclastic flows and minor explosive events dominated this eruption. The influence of the fragmentation dynamics on dome collapse and Vulcanian events is discussed.     

Explosive activity and generation mechanisms of pyroclastic flows at Arenal volcano, Costa Rica between 1987 and 2001

Explosive activity at Arenal and associated tephra fall that has occurred over the 14-year period from 1987–2001 is described. Explosions have been notably variable in both frequency and size. A marked decrease in both frequency and quantity of tephra fallout occurred in early 1998 until the end of 2001. Grainsize distributions of cumulative tephra samples collected once a month are typically bimodal. Aggregation causing premature fallout of fine ash and possibly fallout from ash plumes produced by pyroclastic flows are considered responsible for the bimodality of fallout. Scanning electron microscopy of the glass component of tephra from single explosions show predominantly blocky and blocky/fluidal clast types, interpreted as being the product of vulcanian type explosions. Fragmentation of a mainly rigid, degassed magma body, and a minor molten component is inferred for these explosions. Pyroclastic flows were produced either associated with the larger explosions by a mechanism of column collapse (1987–1990), or unrelated to explosions by partial collapse of the crater wall (1993, 1998, 2000, 2001). Pyroclastic flow activity has migrated from west to north during the period reported. Pyroclastic flow deposits are variable in the quantity of juvenile material and any associated surge component. Large juvenile blocks were partially molten on emplacement and many have a typical cauliform texture. Blocks with both juvenile and lithic textures indicate that at the summit magma was in intimate contact with the pre-existing edifice, rather than as a simple open crater or lava pool. Crater wall collapse may have been promoted by the reduction in explosive activity, which has increased the lava accumulation at the summit and in turn increased instability of the summit region. Thus although explosive activity has waned, if the lava output is maintained, the hazard of pyroclastic flows is likely to continue.Editorial responsibility: R. Cioni     

Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption,Guatemala

The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (∼0.02 km³ DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal area (7–80 km from the volcano) have been size analyzed down to 8 φ (4 μm). Tephra along the dispersal axis were all well-sorted (σ _φ = 0.25–1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Md_φ < 3φ or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Md_φ = 0.58φ or 0.7 mm σ _φ = 1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained, thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent geologic record. The fraction of finest particles (Md_φ = 4–8φ or 4–63 μm) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around the margin of the deposit. A previously undocumented bimodal–unimodal–bimodal change in grain size distribution across the dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher subplinian column and a lower “co-pf” cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds, especially across the tropopause. The distribution of fine particles (<8 μm diameter) in the tephra deposit is asymmetrical, mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance.     

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy,and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K₂O/Na₂O, K₂O/TiO₂ and intermediate CaO/Al₂O₃ that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K₂O/Na₂O, K₂O/TiO₂ and CaO/Al₂O₃ that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K₂O/TiO₂ similar to those from VF, and intermediate CaO/Al₂O₃ as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO₂–eclogites) whereas the less important EM component to recycled marine or continental sediments.     

Petrology and geochemistry of lava and ash erupted from Volcán Colima, Mexico, during 1998–2005

Lava (n = 8) and bulk ash samples (n = 6) erupted between July 1999 and June 2005 were investigated to extend time-series compositional and textural studies of the products erupted from Volcán Colima since 1869. In particular, we seek to evaluate the possibility that the current activity will culminate in major explosive Plinian-style event similar to that in 1913. Lava samples continue to show relatively heterogeneous whole-rock compositions with some significant mafic spikes (1999, 2001) as have prevailed since 1976. Groundmass SiO₂ contents continue trends to lower levels that have prevailed since 1961, in the direction of the still lower groundmass SiO₂ contents found in 1913 scoriae. Importantly, ash samples from investigated Vulcanian-style explosive eruptions in 2005 are devoid of particles with micro-vesiculated groundmass textures; such textures characterized the 1913 scoriae, signifying expansion of in-situ magmatic gas as the propellant of the 1913 eruption. All magmas erupted since 1913 appear to have arrived in the upper volcanic conduit system in a degassed state. The small to moderate Vulcanian-style explosive eruptions, which have been common since 1999 (> 16,000 events), have blasted ash clouds as high as 11 km a.s.l. and sent pyroclastic flows out to distances of 5 km. These eruptions do not appear to be powered by expansion of in-situ magmatic gas. New small lava domes have been observed in the crater prior to many explosive eruptions. These plugs of degassed lava may temporarily seal the conduit and allow the build-up of magmatic gases streaming upward from below ahead of rising and degassing magma. In this interpretation, when gas pressure exceeds the strength of the plug seal in the upper conduit, an explosive Vulcanian-style eruption occurs. Alternatively these explosive eruptions may represent interactions of hot rock and groundwater (phreato-magmatic).     

Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano,Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m³ from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.