The May 1915 eruptions of Lassen Peak, II: May 22 volcanic blast effects, sedimentology and stratigraphy of deposits, and characteristics of the blast cloud

The May 22, 1915 eruptions of Lassen Peak involved a volcanic blast and the emplacement of three geographically and temporally distinct lahar deposits. The volcanic blast occurred when a Vulcanian explosion at the summit unroofed a shallow magma source, generating an eruption cloud that rose to an estimated height of 9 km above sea level. The blast cloud was probably caused by the collapse of a small portion of the eruption column; absence of a flank vent associated with these eruptions argues against it originating as an explosion that has been directed by vent geometry or location. The volcanic blast devasted 7 km² of the northeast flank of the volcano, and emplaced a deposit of juvenile tephra and accidental lithic and mineral fragments. Decrease in blast deposit thickness and median grain size with increasing distance from the vent suggests that the blast cloud lost transport competence as it crossed the devastated area. Scanning electron microscope examination of pyroclasts from the blast deposit indicates that the blast cloud was a dry, turbulent suspension that emplaced a thin deposit which cooled rapidly after deposition. Lahar deposits were emplaced primarily in Lost Creek, with minor lahars flowing down gullies on the west, northwest and north flanks of the volcano. The initial lahar was apparently triggered early in the eruption when the blast cloud melted the residual snowpack as it moved down the northeast flank of the peak. The event that triggered the later lahars is enigmatic; the presence of approximately five times more juvenile dacite bombs on the surface of the later lahars suggests that they may have been triggered by a change in eruption style or dynamics.     

Rincón de la Vieja volcano, Guanacaste province, Costa Rica: geology of the southwestern flank and hazards implications

Acid rain and ongoing eruptive activity at Rincón de la Vieja volcano in northwestern Costa Rica have created a triangular, deeply eroded “dead zone” west-southwest of the Active Crater. The barren, steep-walled canyons in this area expose one of the best internal stratigraphic profiles of any active or dormant volcano in Costa Rica. Geologic mapping along the southwestern flank of the volcano reveals over 300 m of prehistoric volcanic stratigraphy, dominated by tephra deposits and two-pyroxene andesite lavas. Dense tropical forests and poor access preclude mapping elsewhere on the volcano. In the “dead zone” four stratigraphic groups are distinguished by their relative proportions of lava and tephra. In general, early volcanism was dominated by voluminous lava emissions, with explosive plinian eruptions becoming increasingly more dominant with time. Numerous phreatic eruptions have occurred in historic times, all emanating from the Active Crater. The stratigraphic sequence is capped by the Río Blanco tephra deposit, erupted at approximately 3500 yr B.P. Approximately 0.25 km³ (0.1 km³ DRE) of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating strong prevailing winds from the east and east-northeast at the time of the eruption. Grain-size studies of the deposit reveal that the eruption was subplinian, attaining an estimated column height of 16 km. A qualitative hazards assessment at Rincón de la Vieja indicates that future eruptions are likely to be explosive in style, with the zone of greatest hazard extending several kilometers north from the Active Crater.     

Sedimentological analysis of the tephra from the 12–15 August 1991 eruption of Hudson volcano

The tephra fallout from the 12–15 August 1991 explosive eruption of Hudson volcano (Cordillera de los Andes, 45°54 S-72°58 W; Chile) was dispersed on a narrow, elongated ESE sector of Patagonia, covering an area (on land) of more than 100 000 km². The elongated shape of the deposit, together with the relatively coarse mean and median values of the particles at a considerable distance from the vent, were the result of strong winds blowing to the southeast during the eruption. The thickness of the fall deposit decreases up to 250 km ESE from Hudson volcano, where it begins to thicken again. Secondary maxima are well developed at approximately 500 km from the vent. Secondary maxima, together with grainsize bimodality in individual layers and in the bulk deposit suggest that particle aggregation played an important role in tephra sedimentation. The fallout deposit is well stratified, with alternating fine-grained and coarsegrained layers, which is probably a result of strong eruptive pulses followed by relatively calm periods and/or changes in the eruptive style from plinian to phreatoplinian. The tephra is mostly composed of juvenile material: the coarse mode (mostly pumice) shifts to finer sizes with distance from the volcano; the fine mode (mostly glass shards) is always about 5/6 phi. Glass shards and pumice are mostly light gray to colorless. However, considerable amounts of dark, poorly vesiculated, blocky shards, suggest a hydromagmatic component in the eruption. A land-based tephra volume of 4.35 km³ was estimated, and a total volume of 7.6 km³ arose from an extrapolation, which took into account the probable volume sedimented in the sea. Bulk density ranges from 0.9 to 1.10 gr/cm³ (beyond 110 km from the vent). Rather uniform density values measured in crushed samples (2.45–2.50 gr/cm³ at all distances from the vent) reveal a relatively homogeneous composition. Mean and median sizes decrease rapidly up to 270 km from the vent; beyond that point they are more or less constant, whereas the maximum size (1 phi) shows a steady decrease up to 550 km. A concomitant improvement in sorting is observed. This is attributed to sorting due to wind transport combined with particle aggregation at different times and distances from the vent. The Hudson tephra fallout shares some strikingly similar features with the Mount St. Helens (18 May 1980) and Quizapu (1932) eruptions.     

An explosive–intrusive subglacial rhyolite eruption at Dalakvísl,Torfajökull,Iceland

This paper describes unusual rhyolitic deposits at Dalakvísl, Torfajökull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km³), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.     

Strombolian and phreatomagmatic deposits of Ohakune craters, Ruapehu, New Zealand: A complex interaction between external water and rising basaltic magma

The Ohakune Craters form one of several parasitic centres surrounding Ruapehu volcano, at the southern end of the Taupo Volcanic Zone. An inner scoria cone and an outer, probably older, tuff ring are the principal structures in a nested cluster of four vents.The scoria cone consists of alternating lava flows and coarse, welded and unwelded, strombolian block and bomb beds. The strombolian beds consist of principally two discrete types of essential clast, vesicular bombs and dense angular blocks. Rare finer-grained beds are unusually block-rich. The tuff ring consists of alternating strombolian and phreatomagmatic units. Strombolian beds have similar grain size characteristics to scoria cone units, but contain more highly vesicular unoxidised bombs and few blocks. Phreatomagmatic deposits, which contain clasts with variable degrees of palagonitisation, consist of less well-sorted airfall deposits and very poorly sorted, crystal-rich pyroclastic surge deposits.Disruption by expanding magmatic gas bubbles was a major but relatively constant influence on both strombolian and phreatomagmatic eruptions at Ohakune. Instead, the nature of deposits was principally controlled by two other variables, vent geometry and the relative influence of external water during volcanism. During tuff-ring construction, magma is considered to have risen rapidly to the surface, and to have been ejected without sufficient residence time in the vent for non-explosive degassing. Availability of external water principally governed the eruption mechanism and hence the nature of the deposits. Essentials clasts of the scoria cone are, by comparison, dense, degassed and oxidised. It is suggested that a change in vent geometry, possibly the construction of the tuff ring itself, permitted lava ponding and degassing during scoria cone growth. During strombolian eruptions, magma remaining in the vent probably became depleted in gas, leading to the formation of an inert zone, or crust, above actively degassing magma. Subsequent explosions had therefore to disrupt both this passive crust and underlying, vesiculating magma “driving” the eruption. Cycles of strombolian eruption are thought to have stopped when the thickness of the inert crust precluded explosive eruption and only recommenced when some of this material was removed, either as a lava flow or during phreatomagmatic explosions when external water entered the vent. Such explosions probably formed the unusually fine-grained and block-rich beds in the strombolian sequence.The Ohakune deposits are an excellent example of the products of explosive eruption of fluid, gas-rich basic magma vesiculating under very near-surface conditions. A complex interplay of rate of magma rise, rate and depth of formation of gas bubbles, vent geometry, abundance of shallow external water, wind velocity and accumulation rate of ejecta determines the nature of deposits of such eruptions.     

The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Volcanology of trachytic and associated basaltic pyroclastic deposits at Roccamonfina volcano, Roman Region, Italy

The 274 ka “Basalt-Trachytic Tuff of Tuoripunzoli” (TBTT) from Roccamonfina volcano (Roman Region, Italy) consists of a basaltic scoria lapilli fall (Unit A) overlain by a trachytic sequence formed by a surge (Unit B), repetitive pumice lapilli and ash-rich layers both of fallout origin (Unit C) and a pyroclastic flow deposit (Unit D). The TBTT is widespread (40 km²) in the northern sector of the volcano, but limited to a small area on the southern slopes of the main cone. Interpolation between the northern deposits and the latter one yields a minimum depositional area of 123 km², and an approximate bulk volume of 0.2-0.3 km³. Isopach and isopleth maps are consistent with a source vent within the main caldera of Roccamonfina.Unit A shows a fairly good sorting and a moderate grain size; glass fragments are cuspate and vesicular. Unit B is fine grained and poorly sorted; shards are blocky and nonvesicular. Pumice lapilli of Unit C are moderately sorted and moderately coarse grained. Glass shards are equant and vesicular. Lithic clasts are strongly comminuted to submillimetric sizes. By contrast, the ash-rich internal divisions are very fine grained and poorly sorted. They consist of a mixture of equant shards which are prevailingly blocky and poorly vesicular. Unit D is a massive, poorly sorted, moderately coarse-grained deposit. Glass fragments are nearly equant and slightly or nonvesicular.The TBTT is interpreted as due to eruption of a basaltic magma followed in rapid succession by one trachyte magma. Unit A formed by Subplinian fallout of a moderate, purely magmatic column. Interaction between a trachyte magma and water resulted in eruption of surge Unit B. A high-standing eruption column erupted alternating fallout pumice lapilli and fallout ashes. Pumice lapilli originated prevailingly from the inner part of the eruption column, whereas magma-water interaction on the external parts of the column resulted in ash fallout. The uppermost pyroclastic flow Unit D is interpreted as due to final collapse of the eruption column.     

Tarawera 1886, New Zealand — A basaltic plinian fissure eruption

New Zealand's biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km³) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min⁻¹. A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.     

Subaqueous eruption and emplacement of the Shinjima Pumice, Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan

The Shinjima Pumice is a fines-depleted pumice lapilli tuff emplaced several thousands years ago at about 100–140 m below sea level. This 40-m-thick deposit comprises many poorly defined flow units, which are 1–10 m thick, diffusely stratified and showing upward-coarsening of pumice clasts with a sharp to transitional base. Parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts, especially in the lower half of the flow units. Pumice clasts of block to coarse-lapilli size commonly have thermal-contraction cracks best developed on the surfaces, demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement. These features are suggestive of the direct origin of the Shinjima Pumice from subaqueous eruptions. A theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water-logged mass flows.     

Weather radar observations of the Hekla 2000 eruption cloud,Iceland

The Hekla eruption cloud on 26–27 February 2000 was the first volcanic cloud to be continuously and completely monitored advecting above Iceland, using the C-band weather radar near the Keflavík international airport. Real-time radar observations of the onset, advection, and waning of the eruption cloud were studied using time series of PPI (plan-position indicator) radar images, including VMI normal, Echotop, and Cappi level 2 displays. The reflectivity of the entire volcanic cloud ranges from 0 to >60 dBz. The eruption column above the vent is essentially characterised by VMI normal and Cappi level 2 values, >30 dBz, due to the dominant influence of lapilli and ash (tephra) on the overall reflected signal. The cloud generated by the column was advected downwind to the north-northeast. It is characterised by values between 0 and 30 dBz, and the persistence of these reflections likely result from continuing water condensation and freezing on ash particles. Echotop radar images of the eruption onset document a rapid ascent of the plume head with a mean velocity of ~30 to 50 m s^–1, before it reached an altitude of ~11–12 km. The evolution of the reflected cloud was studied from the area change in pixels of its highly reflected portions, >30 dBz, and tied to recorded volcanic tremor amplitudes. The synchronous initial variation of both radar and seismic signals documents the abrupt increase in tephra emission and magma discharge rate from 18:20 to 19:00 UTC on 26 February. From 19:00 the >45 dBz and 30–45 dBz portions of the reflected cloud decrease and disappear at about 7 and 10.5 h, respectively, after the eruption began, indicating the end of the decaying explosive phase. The advection and extent of the reflected eruption cloud were compared with eyewitness accounts of tephra fall onset and the measured mass of tephra deposited on the ground during the first 12 h. Differences in the deposit map and volcanic cloud radar map are due to the fact that the greater part of the deposit originates by fallout off the column margins and from the base of the cloud followed by advection of falling particle in lower level winds.Editorial responsibility: P. Mouginis-Mark     

Characterization of tephra deposits with limited exposure: the example of the two largest explosive eruptions at Nisyros volcano (Greece)

Explosive eruptions associated with tephra deposits that are only exposed in proximal areas are difficult to characterize. In fact, the determination of physical parameters such as column height, mass eruption rate, erupted volume, and eruption duration is mainly based on empirical models and is therefore very sensitive to the quality of the field data collected. We have applied and compared different modeling approaches for the characterization of the two main tephra deposits, the Lower Pumice (LP) and Upper Pumice (UP) of Nisyros volcano, Greece, which are exposed only within 5 km of the probable vent. Isopach and isopleth maps were compiled for two possible vent locations (on the north and on the south rim of the caldera), and different models were applied to calculate the column height, the erupted volume, and the mass eruption rate. We found a column height of about 15 km above sea level and a mass eruption rate of about 2 × 10⁷ kg/s for both eruptions regardless of the vent location considered. In contrast, the associated wind velocity for both UP and LP varied between 0 and 20 m/s for the north and south vent, respectively. The derived erupted volume for the south vent (considered as the best vent location) ranges between 2 and 27 × 10⁸ m³ for the LP and between 1 and 5 × 10⁸ m³ for the UP based on the application of four different methods (integration of exponential fit based on one isopach line, integration of exponential and power-law fit based on two isopach lines, and an inversion technique combined with an advection–diffusion model). The eruption that produced the UP could be classified as subplinian. Discrepancies associated with different vent locations are smaller than the discrepancies associated with the use of different models for the determination of erupted mass, plume height, and mass eruption rate. Proximal outcrops are predominantly coarse grained with ≥90 wt% of the clasts ranging between −6ϕ and 0ϕ. The associated total grainsize distribution is considered to result from a combination of turbulent fallout from both the plume margins and the umbrella region, and as a result, it is fines-depleted. Given that primary deposit thickness observed on Nisyros for both LP and UP is between 1 and 8 m, if an event of similar scale were to happen again, it would have a significant impact on the entire island with major damage to infrastructure, agriculture, and tourism. Neighboring islands and the continent could also be significantly affected.     

Volcanic earthquakes, and their relationship to eruptions at Ruapehu and Ngauruhoe volcanoes

Crustal earthquakes near Ruapehu and Ngauruhoe fall into two classes, each of which can be subdivided. On the one hand, there are high-frequency events ( 3 Hz) with sharp, well-defined phases, mainly concentrated beneath Ruapehu Crater Lake. Low-frequency events (< 2 Hz), on the other hand, are common at shallower depths under both volcanoes. These are usually emergent multiple events, and are often closely associated with eruptions.The low-frequency events resemble Minakami's B-type and explosion earthquakes, but sometimes occur where no vent exists and rather deeper than his formal definition (< 1 km) permits. More importantly, they lack reliable criteria (wave-form or magnitude differences) to distinguish between his two groups. Whether or not they accompany an eruption (Minakami's definition of explosion earthquake) appears to depend on whether the volcanoes are in a “closed-” or “open-vent” condition. The high-frequency earthquakes are similar in wave-form to Minakami's A-type. However, many at Ruapehu (here designated “roof-rock” earthquakes) originate at shallower depths than the B-type earthquakes, which is contrary to Minakami's definition.Difficulty in applying Minakami's classification rigorously, and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path, rather than to their suppression or absence at the source, has led to reclassification of earthquakes near the volcanoes into two broad groups, tectonic and volcanic. The former includes all high-frequency earthquakes, and those discrete events in which dominant low frequencies are due to path effects. The latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism. Dominant low frequencies are ascribed to occurrence in heat-weakened material, and high frequencies to instantaneous source mechanisms operating in competent rock. The term volcano-tectonic describes tectonic earthquakes within some arbitrary distance of a volcano.At Ngauruhoe and Ruapehu, volcanic earthquakes accompany explosive, vent-clearing eruptions. Subsequent “open-vent” degassing and ash emission, however, although often powerful and prolonged, usually occurs without earthquakes. Such activity is, however, frequently accompanied by volcanic tremor. At Ruapehu, under “closed-vent” conditions, when lake temperature is low, low-frequency earthquakes up to magnitude M_L = 3.4 have occurred without any eruption.Five types of phreatic eruptions are identified at Ruapehu, each having a distinctive seismic pattern. The three most explosive types appear to be generated by a chain reaction process, and all involve flashing of water to steam; the first by failure of the roof, with little precursory seismicity, after a “closed-vent” period, during which lake temperature decreases; the second, after prolonged heating of the lake and much preliminary volcanic tremor, interpreted as due to rising magma; and the third, under “open-vent” conditions in the wake of one of the two preceding types. A fourth probably occurs in wet sediments near the base of the lake, as a result of upward migration of hot gas, and a fifth, aseismic, or accompanied by very weak volcanic tremor, is associated with convective overturn within Crater Lake.     

Dynamic pile response using two pile-driving techniques

Because of disadvantages caused by pile top driving, a new pile driving technique by which the hammer is inside the pile has been developed by contractors. The so called “down-the hole” piling system is used to drive a tubular pile in an experimental set-up in the laboratory. This new technique is tested and compared with pile top driving using similar hammer energy. A reduction of noise and the opportunity to save steel are confirmed during a field test program. Also, a reduction in driving time and a higher bearing capacity have been observed. Some similar conclusions are arrived at in the laboratory study, specially the low level of stresses in the pile and the shaft friction.     

The 5,660?yBP Boquerón explosive eruption, Teide–Pico Viejo complex, Tenerife

Quantitative hazard assessments of active volcanoes require an accurate knowledge of the past eruptive activity in terms of eruption dynamics and the stratified products of eruption. Teide–Pico Viejo (TPV) is one of the largest volcanic complexes in Europe, but the associated eruptive history has only been constrained based on very general stratigraphic and geochronological data. In particular, recent studies have shown that explosive activity has been significantly more frequently common than previously thought. Our study contributes to characterization of explosive activity of TPV by describing for the first time the subplinian eruption of El Boquerón (5,660?yBP), a satellite dome located on the northern slope of the Pico Viejo stratovolcano. Stratigraphic data suggest complex shifting from effusive phases with lava flows to highly explosive phase that generated a relatively thick and widespread pumice fallout deposit. This explosive phase is classified as a subplinian eruption of VEI 3 that lasted for about 9–15?h and produced a plume with a height of up to 9?km above sea level (i.e. 7?km above the vent; MER of 6.9–8.2?×?10⁵?kg/s). The tephra deposit (minimum bulk volume of 4–6?×?10⁷?m³) was dispersed to the NE by up to 10?m/s winds. A similar eruption today would significantly impact the economy of Tenerife (e.g. tourism and aviation), with major consequences mainly for the communities around the Icod Valley, and to a minor extent, the Orotava Valley. This vulnerability shows that a better knowledge of the past explosive history of TPV and an accurate estimate of future potentials to generate violent eruptions is required in order to quantify and mitigate the associated volcanic risk.     

Reconstruction of a kimberlite eruption,using an integrated volcanological,geochemical and numerical approach: A case study of the Fox Kimberlite,NWT, Canada

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. > 40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al₂O₃ and Na₂O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~ 30% loss of fines whereas the column collapse deposit exhibits ~ 50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.     

Sulfur as a binding agent of aggregates in explosive eruptions

The 2013 eruption of Pavlof Volcano, Alaska began on 13 May and ended 49 days later on 1 July. The eruption was characterized by persistent lava fountaining from a vent just north of the summit, intermittent strombolian explosions, and ash, gas, and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano. During the first several days of the eruption, accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano. Continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano, about 3–4 km beyond the vent. The generation of fountain-fed lava flows was a dominant process during the 2013 eruption; however, episodic collapse of spatter accumulations and formation of hot spatter-rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars. The lahars and ash plumes generated during the eruption did not pose any serious hazards for the area. However, numerous local airline flights were cancelled or rerouted, and trace amounts of ash fall occurred at all of the local communities surrounding the volcano, including Cold Bay, Nelson Lagoon, Sand Point, and King Cove.     

The October 1902 plinian eruption of Santa Maria volcano, Guatemala

The October, 1902, eruption of Santa Maria Volcano, Guatemala, was one of the largest this century. It was preceded by a great earthquake on April 19 centered at the volcano, as well as numerous other major earthquakes. The 18–20 hour-long plinian eruption on October 25 produced a column at least 28 km high, reaching well into the stratosphere.The airfall pumice deposit covered more than 1.2 million km² with a trace of ash and was only two meters thick at the vent. White dacitic pumice, dark gray scoriaceous basalt (with physically and chemically mixed intermediate pumice) and loose crystals of plagioclase, hornblende, hypersthene, biotite and magnetite make up the juvenile components of the deposit. Lithic fragments are of volcanic, plutonic, and metamorphic origin. The plinian deposit is a fine-grained, crystal-rich, single pumice fall unit and shows inverse grading. Mapping of the deposit gives a volume of 8.3 km³ within the one mm isopach. Crystal concentration studies show that the true volume erupted was at least 20 km³ (equivalent to 8.5 km³ of dense dacite) and that 90% of the ejecta was less than 2 mm in diameter.The plinian volume eruption rate averaged 1.2 × 10⁵ m³s⁻¹ and the average gas muzzle velocity of the column exceeded 270 ms⁻¹. A total of 8.3 × 10¹⁸ J of energy were released by the eruption. A knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms.This eruption was the major stratospheric aerosol injection in the 1902–1903 period. However, mid- to low- latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease. This may be explained by the sulfur-poor nature of dacite magmas, suggesting that volatile composition, rather than mass of volatiles, is the controlling parameter in climatic response to explosive eruptions.     

Flow directions inferred from imbrication in the Handa pyroclastic flow deposit in Japan

Flow direction patterns have been determined by imbrication measurements of pumice and lithic fragments of the Handa pyroclastic flow deposit, in order to estimate the source vent location and to analyze the flow behavior. The pyroclastic flow deposit studied is dacitic in composition, 2 km² in volume, and >32,300 Y.B.P. in age. Flow directions from 52 outcrops indicate a source vent located within the area of recent lava domes of Kuju Volcano. The distribution of the pyroclastic flow deposit and the flow direction patterns determined by imbrication suggest that the pyroclastic flow accurately followed the topographic relief at the time of eruption. The presence of imbrication indicates the change of flow-regime from turbulent condition to laminar condition according to the distance from the source vent. Imbrication is visible within the lower-half reaches of the pyroclastic flow distribution, where the pyroclastic flow had developed the laminar flow characteristics of a dense gravity current.     

The physical volcanology of the 1600 eruption of Huaynaputina, southern Peru

Volcán Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km³ of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19-20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km³). Throughout medial-distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km³) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km³). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km³). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe-Ti oxides-apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO₂), lower crystal contents (17-20%), lower density (1.0-1.3 g/cm³), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO₂) compositions, higher crystal contents (25-35%), higher densities (up to 2.2 g/cm³), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area^1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24-32 km and a total column height of 34-46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4᎒⁸ kg/s and the volumetric discharge rate is ~3.6᎒⁵ m³/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km³ total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW-SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano.     

A model for the numerical simulation of tephra fall deposits

A simple semianalytical model to simulate ash dispersion and deposition produced by sustained Plinian and sub-Plinian eruption columns based on the 2D advection–dispersion equation was applied. The eruption column acts as a vertical line source with a given mass distribution and neglects the complex dynamics within the eruption column. Thus, the use of the model is limited to areas far from the vent where the dynamics of the eruption column play a minor role. Vertical wind and diffusion components are considered negligible with respect to the horizontal ones. The dispersion and deposition of particles in the model is only governed by gravitational settling, horizontal eddy diffusion, and wind advection. The model accounts for different types and size classes of a user-defined number of particle classes and changing settling velocity with altitude. In as much as wind profiles are considered constant on the entire domain, the model validity is limited to medium-range distances (about 30–200 km away from the source).The model was used to reconstruct the tephra fall deposit from the documented Plinian eruption of Mt. Vesuvius, Italy, in 79 A.D. In this case, the model was able to broadly reproduce the characteristic medium-range tephra deposit. The results support the validity of the model, which has the advantage of being simple and fast to compute. It has the potential to serve as a simple tool for predicting the distribution of ash fall of hypothetical or real eruptions of a given magnitude and a given wind profile. Using a statistical set of frequent wind profiles, it also was used to construct air fall hazard maps of the most likely affected areas around active volcanoes where a large eruption is expected to occur.