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A small phreatic eruption through Crater Lake producing vertically and radially directed dark surtseyan jets of Crater Lake water, gas and solid ejecta, and a whitish radial base surge with a diameter of c. 200 m. Note low angle of the jet falling to the left. Image captured in 1971 by P. Otway.
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At 20:26 (NZDT) on 25 September 2007 a moderate gas-driven eruption beneath the summit Crater Lake of Mt. Ruapehu, New Zealand generated a directed ballistic fallout apron and surtseyan jet that impacted an area of c. 2.5 km2 to the north of the vent. Two climbers were caught in the blast at a hut 600 m from the vent. Primary, ice-slurry lahars wer...
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... trend (major and trace element compositions) that spans a wide compositional range shown by eruptives between 1945 and 1996 (Gamble et al., 1999). The dense and vesicle-poor nature of the clasts suggests that they represent the degassed portions of magma from historic eruptions. Densities of the andesite lava and andesite breccia clast types were determined using the technique outlined by Carmichael (1984), and range from 1.7 g/cm 3 (cemented breccia) to 2.7 g/cm 3 (lava), with porosities of 1% (lava) to 41% (cemented breccia) (Table 1). The brecciated clasts tend to disintegrate on immersion in water, in part due to the breakdown of hydrothermal swelling clays. The lava clasts are relatively impermeable and there is little difference between wet and dry densities, while the andesite breccias and cemented lake sediments and/or vent debris are relatively permeable. The volume of ballistic ejecta produced by the 25 September 2007 event was calculated using dispersal data from aerial photographs and GIS, and clast size measurements (contoured point data) taken at various locations on the summit area (Fig. 6). The volume calculation is considered a minimum because a signi fi cant portion of the erupted material probably fell back into Crater Lake, was washed back in by run-off from the collapsing surtseyan jet, did not clear the lake rim, was carried off the summit by the eruption-triggered lahars, or was buried and masked by the jetted deposits. To calculate the volume of the ballistic material, we recorded clast sizes and types along the main dispersal axis and constructed an isopleth map (Fig 6). In order to convert non-uniformly distributed ballistics to isopachs a scaling factor, F , was determined, based on the percentage coverage of ballistic clasts and/or impact craters within a 100 m 2 quadrant at 3 locations on the summit plateau (Fig. 6). The value of F remains near-constant (0.015 to 0.013) despite increasing distance from the vent (i.e. 1.5 to 1.3% of each quadrant was occupied by ballistic clasts). The volume of the ballistic deposit was obtained by multiplying the isopleth by F = 0.013, giving a minimum estimated volume of 1.6 × 10 5 m 3 . Assuming a mean (measured) density of 2.36 g/cm 3 the erupted ballistic mass is thus c. 3.8 × 10 8 kg. In addition to the scattered ballistics emplaced across the summit plateau, an area of 2.5 km 2 was covered by a relatively thin, ash to lapilli-sized, and massively-bedded unit interpreted to be derived from a surtseyan-type event (Fig. 7). On the southern face of Dome Ridge, we excavated a pit into this deposit to determine whether there were changes in both grain size and componentry through the eruption. The deposit has a relatively uniform grain size distribution: the graphic mean ( M z ) ranges from 10 to 20 mm (Fig. 8). The surtseyan deposit consists of variably hydrothermally-altered andesite lava lithics, intensely altered rock fragments, weakly to strongly mineral-cemented lake sediments dominated by fi ne sand and mud, and recycled scoria and glass from previous historic eruptions characterised by alteration rims. Andesite lava lithics are similar in hand specimen to the discrete ballistics, comprising porphyritic andesitic lava fragments, with some lava clasts exhibiting coarsely porphyritic textures, highlighted by the range in plagioclase pheno- cryst abundance. A small proportion ( b 2 vol.%) of the surtseyan deposit probably represents juvenile magma, which appears as fresh and unaltered moderately vesicular and glassy ash and fi ne lapilli. Particle morphol- ogies are indicative of melt quenching during fragmentation and the glass is dacitic in composition (Christenson et al., 2010-this issue). The surtseyan unit is variously distributed among individual ballistic blocks, indicating a complex emplacement chronology. The surtseyan deposit was distributed northwards along a similar azimuth to the ballistic apron, and its thickness appears to be locally controlled by summit topography, implying emplacement from a low-angle directed jet and a component of post-depositional fl ow and modi fi cation. This is seen clearly in the area of North Crater where the crater margins have contained the fl ow, causing minor, yet diffuse run-up (Fig. 7). Although the 25 September 2007 eruption occurred at night, dynamically similar eruptions have occurred during the 1970's, 1980's and 1990's during daylight and fi ne weather, and have been videoed and photographed by many eye-witnesses. These observations yield valuable insights into the nature of gas-driven, phreatic and phreatomagmatic blasts through Ruapehu's Crater Lake. Dark surtseyan jets (e.g. Kokelaar and Durant, 1983) of explosively displaced water droplets, also referred to as ‘ rooster-tails ’ or ‘ cockscomb jets ’ may have risen N 800 m above Crater Lake, while simultaneously ejected ballistic blocks fl ew further and faster due to greater inertia leaving white vapour trails in the atmosphere (Fig. 9). As the jets collapse and fall under the in fl uence of gravity, they impact the snow pack, leaving sharp-edged deposits that closely match their maximum projection range: a northward-directed jet from the 23 September 1995 eruption travelled c. 2 km in ∼ 20 s. Diffuse base surges develop from dilute parts of the collapsing jets and higher white steam column, but do not appear to produce signi fi cant deposits. Due to the different velocities and trajectories of the ballistics and surtseyan jets, they arrive at the ground surface in a variable and complex order: the last deposits to be emplaced are high- fl ying ballistics with launch angles N 45°. The volume of the September 2007 jetted deposit was determined by measuring ash thickness at various locations across and down the main northward-directed dispersal axis (Fig. 7). The variable topography of the summit plateau and syn- and post-depositional fl owage of the jetted deposit under the in fl uence of gravity resulted in a highly variable deposit thickness. At Dome Ridge, the jetted deposit is represented by a b 1 cm thick veneer, while in Central Crater it is ponded to a thickness of 10 – 12 cm. Based on a crude isopach map (Fig. 7), the minimum volume of the jetted deposit is estimated at c. 1 × 10 5 m 3 : giving an erupted mass of c. 2.4 × 10 8 kg assuming a mean density of 2.36 g/cm 3 . Some jetted and ballistic material was lost by run-off to form lahars (see below). Water ejected from Crater Lake in the directed surtseyan jet, together with condensate from the high eruption steam column, fell on the summit area of Mt. Ruapehu, running off and entraining the upper part of the seasonal snowpack to form a number of ice-slurry lahars (Lube et al., 2009). Similar multi-component volcanogenic mass- fl ows have occurred during previous eruptions of Ruapehu (Cronin et al., 1996a), and at other snow-clad volcanoes overseas (Pierson and Janda, 1994; Waitt et al., 1994; Major and Newhall, 1989). Two eruption-triggered lahars are recognised in the eastern Whangaehu catchment; with a third fl ow occurring c. 1.5 h after the initial explosion. The fi rst and largest lahar consisted of two near- simultaneous pulses (W1a and W1b) resulting from run-off of ejected Crater Lake water, ballistics, and entrained snow and ice from the head of the Whangaehu Glacier, the fl anks of Dome, and the southern half of Central Crater (Fig. 1). These fl ows left a 200 – 280 m wide grey deposit of ash-coated mm-diameter ice particles, fragments of ejected lake sediment, and ash, lapilli and blocks as a swath running down the centre of the 17 o Whangaehu Glacier (Figs. 1 and 2), before entering the North Branch of the Whangaehu Gorge (only minor overspill occurred into the South Branch). Evidence for erosion includes shallow gullies and potholes cut in the glacier surface. Pulses W1a and W1b travelled 7.3 km and 8.5 km respectively, giving H/L ratios of 0.162 and 0.168, similar to snow-slurry lahars at other volcanoes (Pierson et al., 1990; Pierson and Janda, 1994; Waitt et al., 1994; Cronin et al., 1996a). The combined area of W1 is 8.14 × 10 m , most of which represents the broad but thin swath on the Whangaehu Glacier; once con fi ned in the gorge the deposits are narrower but thicker. Total deposit volume is estimated at c. 3.5 – 7.0 × 10 5 m 3 . A second lahar (W2) was generated by syn-eruptive water displacement across the lake outlet coupled with a topographically- controlled base surge into the outlet gorge. It fl owed across the toe of a large landslide c. 500 m downstream where it picked up additional lithic material that gave the deposit a distinctive brown colour. W2 fi lled the outlet gorge to a width of c. 60 – 80 m and a depth of 6 – 8 m, giving it a cross-section area similar to the much wider but shallower W1 pulses on the Whangaehu Glacier, and travelled c. 10.7 km from source (H/L ratio = 0.122) to cover an area of 330,000 m 2 with a volume estimated at c. 2.5 – 5.0 × 10 5 m 3 . At the RTMT lahar monitoring site, 7.1 km downstream from Crater Lake, lahar pulses W1a and W1b formed a single grey depositional unit that buried the 80 m-wide valley fl oor beneath 1 – 3 m of frozen ice slurry containing clay to boulder-sized lithic material (Fig. 10). This was partially overlain by the grey-brown deposits of W2. The matrix of both consisted of mm- sized sub-rounded particles of clear ice, representing modi fi ed snow and fi rn, coated with a fi lm of silt and clay. Both units showed coarse- tail normal grading: coarse clasts included ice fragments up to 15 cm in diameter and lithic boulders up to 70 cm across. Lithic clasts were dominated by hydrothermally-altered andesite lava (36 – 50 wt.%) and scoria (18 – 20 wt.%), often veined or crusted with gypsum and alunite, and pieces of pale, laminated lake- fl oor sediment (34 – 43 wt.%). Black mm-sized globules of sulphur were abundant in the matrix. The deposits of W2 contained a slightly higher proportion ...
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The ballistic ejection of blocks during explosive eruptions constitutes a major hazard near active volcanoes. Fields of ballistic clasts can provide important clues towards quantifying the energy, dynamics and directionality of explosive events, but detailed datasets are rare. During the 6 August 2012 hydrothermal eruption of Upper Te Maari (Tongar...
Citations
... ash dispersal and fallout) impacts ranging from the proximal vent area to a few to hundreds of kilometres (e.g. Baxter et al. 2017;Biass et al. 2014Biass et al. , 2017Deligne et al. 2022;Hayes et al. 2019;Jenkins et al. 2017;Kaneko et al. 2016;Kilgour et al. 2010;Neal et al. 2018;Spence et al. 1996). Hazards causatively connected to the eruption include interactions between two volcanic processes (e.g. ...
The simultaneous or sequential occurrence of several hazards—be they of natural or anthropogenic sources—can interact to produce unexpected compound hazards and impacts. Since success in responding to volcanic crises is often conditional on accurate identification of spatiotemporal patterns of hazard prior to an eruption, ignoring these interactions can lead to a misrepresentation or misinterpretation of the risk and, during emergencies, ineffective management priorities. The 2021 eruption of Tajogaite volcano on the island of La Palma, Canary Islands (Spain), was an 86 day-long hybrid explosive-effusive eruption that demonstrated the challenges of managing volcanic crises associated with the simultaneous emission of lava, tephra and volcanic gases. Here, we present the result of a small-scale impact assessment conducted during three-field deployments to investigate how tephra fallout and lava flow inundation interacted to cause compound physical impact on buildings. The study area was a neighbourhood of 30 buildings exposed to tephra fallout during the entire eruption and by a late-stage, short-lived lava flow. Observations highlight, on one hand, the influence of clean-up operations and rainfall on the impact of tephra fallout and, on the other hand, the importance of the dynamics of lava flow emplacement in controlling impact mechanisms. Overall, results provide an evidence-based insight into impact sequences when two primary hazards are produced simultaneously and demonstrate the importance of considering this aspect when implementing risk mitigation strategies for future long-lasting, hybrid explosive-effusive eruptions in urban environments.
Supplementary Information
The online version contains supplementary material available at 10.1007/s00445-023-01700-w.
... As recently demonstrated by the deadly events at Mount Ruapehu (New Zealand, 2007Kilgour et al, 2010, Ontake volcano (Japan, 2014, Maeno et al, 2016 or Whakaari (White Island, New Zealand, 2019, Dempsey et al, 2020, even purely steam-driven/hydrothermal eruptions can be very hazardous given their sudden onset. Therefore, monitoring the spatial distribution and temporal evolution of hydrothermal signals at volcanoes with long-lived hydrothermal activity is critical to detect and interpret precursory signals of explosive activity. ...
Quantifying subsurface fluid flows and related heat and gas fluxes can provide essential clues for interpreting the evolution of volcanic unrest in volcanoes with active hydrothermal systems. To better constrain the distribution of current hydrothermal activity, we mapped diffuse soil CO \(_2\) degassing, ground temperature and self-potential covering the summit of La Soufrière de Guadeloupe during 2022-23. From these mappings, we identify areas of fluid recharge and the zones and extent of major ascending hydrothermal flows. We provide a first estimate for ground CO$_2$ flux of \SI{3.76+-0.52}{\tonne\per\day} (\SI{0.044+-0.006}{\kg\per\s}), representing about half the CO$_2$ emissions from the summit fumaroles. We find an extensive area of ground heating of at least \SI{15175+-4200}{\m\squared} in area and a total ground heat flux of \SI{2.29+-0.88}{\MW} to \SI{2.79+-0.98}{\MW}, dominated by a convective flux of \SI{2.00+-0.86}{\MW}. These observations indicate that conduction is not always the primary mode of heat transport in hydrothermal volcanoes, especially in highly-altered settings. The prominent summit fractures exert significant control over hydrothermal fluid circulation and delimit a main active zone in the NE sector. The observed shift in subsurface fluid circulation towards this sector may be attributed to a changing ground permeability and may also be related to observed fault widening and the gravitational sliding of the dome's SW flank. Our results indicate that the state of sealing of the dome may be inferred from the mapping of hydrothermal fluid fluxes and that such mappings may help evaluate potential hazards associated with fluid pressurisation.
... All blocks are considered surface samples excluding RH50; RH50 is a ballistic block from the 1995-1996 eruption, composed of moderately to highly altered andesites with pores and vesicles rarely filled with elemental sulfur and associated anhydrite, pyrite, and natroalunite (Christenson et al. 2010). During this eruption, the introduction of gas/fluid at depth promoted catastrophic failure of a partial mineralogic seal developed at the top of the hydrothermal systems in the northward-inclined vent beneath Crater Lake (Kilgour et al. 2010). Estimates of penetration depth into the vent complex indicate depths between 275 and 410 m, and the formerly molten elemental sulfur indicates a minimum pre-eruption temperature in the source area of 119 °C (Christenson et al. 2010). ...
... This sample was subjected to intense alteration and precipitation of clay infilling minerals, particularly in the outer 6 cm, resulting in lower porosity-to-UCS ratios than is typical for lava with low primary porosity (Fig. 4). Sample RH50, a block from the 1995-1996 eruption, represents lava from the upper conduit area within the current vent-hosted hydrothermal system (Kilgour et al. 2010). RH50 has a high percentage of phyllosilicate clay minerals, resulting in anomalously low porosity, low strength, and low magnetic susceptibility (Fig. 4, Fig. 6c, f, i). ...
The geomechanical characterization of volcanic material has important implications for geothermal and mineral exploration, engineering design, geophysical signals of volcano unrest, and models of instability and mass flows. Chemical weathering and hydrothermal systems can alter the host rock, leading to changes in mechanical behavior and failure mode. Here, we compare the physical and mechanical properties of lava, autoclastic breccia, and pyroclastic (scoria) rocks from Mount Ruapehu volcano (Ruapehu) in New Zealand to mineralogical composition determined via infrared spectroscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). We use correlation matrices, principal component analysis, and parametric analysis to determine which parameters can be used to predict physical and mechanical properties and form the basis for transfer functions. Laboratory-based spectroscopy shows that the samples contain absorption features indicative of Al- and Mg-rich hydrous phyllosilicates (e.g., kaolinite, halloysite, montmorillonite), Fe- oxides (e.g., goethite), and sulfates attributed to surface weathering, supergene, and steam-heated alteration. We find that porosity and primary lithology are the predominant control on physical and mechanical properties, followed by the pervasiveness of weathering/alteration, and then mineralogical composition. Several properties, such as porosity, uniaxial compressive strength, P-wave seismic velocity, density, and Young’s modulus, show strong correlations with other properties, indicating the potential for transfer functions between these properties. Hydrothermally altered rocks near the vent complex (up to ~ 400 m depth beneath the crater lake) with high-intensity hydrothermal alteration do not follow typical physical and mechanical property trends due to high clay content, low permeability, and low strength. The presence of these rocks within the edifice at Ruapehu implies local barriers to fluid flow and subsequent pore pressure variations. Additionally, they may have less than half the strength than would be dictated by typical porosity-strength trends for surface rocks, increasing the likelihood of structural failure. Trends in the pervasiveness of weathering with physical and mechanical properties, along with shifts in the position of spectral absorption peaks as hydrothermal/weathering alteration increases, suggest that it may be possible to extrapolate properties from imaging spectroscopy.
... Ontake, and Meakan-dake in Japan; and Rincón de la Vieja in Costa Rica Procter et al., 2014;Battaglia et al., 2019;Kilgour et al., 2019;Jolly et al., 2020;Matsunaga et al., 2020;Miyagi et al., 2020;Caudron et al., 2021;Yamada et al., 2021). In these cases, the pre-conditioning (i.e., res-ervoir pressurization) and eruption triggering are driven by shallow magma-related fluids (Kilgour et al., 2010;Jolly et al., 2014;Christenson et al., 2017;Rosi et al., 2018;Himematsu et al., 2020;Narita et al., 2020;Yamada et al., 2021). Far from active volcanoes, similar violent hydrothermal eruptions also occur at geothermal fields, for example, at Yellowstone's Lower and Upper Basins in the USA; Waiotapu in New Zealand; Hakone and Ebinokogen Ioyama in Japan; Domuyo's El Humazo in Argentina; the Valley of Desolation in Dominica; and Krafla in Iceland (Hedenquist and Henley, 1985;Morgan et al., 2009;Mayer et al., 2017;Kobayashi et al., 2018;Fowler et al., 2019;D'Elia et al., 2020;Eggertsson et al., 2020;Tajima et al., 2020). ...
Hydrothermal eruptions are the most violent and hazardous phenomena within geothermal fields. The largest of these may produce kilometer-sized craters and breccia deposits that are tens of meters thick. The geological and hydrothermal priming that leads to these types of eruptions is poorly understood. To understand large hydrothermal eruptions, we investigated a series of prehistoric events at the Rotokawa geothermal field in New Zealand. By revising the stratigraphy and distribution of hydrothermal breccia deposits and correlating these with componentry, crater morphology, and subsurface geological structure, we estimated the frequency, priming processes, triggers, and dynamics of multiple eruptions.
Seventeen large hydrothermal eruptions occurred centuries to millennia apart in the period from ca. 22 cal ka B.P. to ca. 3.4 cal ka B.P. Of six hydrothermal eruptions since ca. 7 ka, four produced oval-shaped craters up to 2 km in diameter, creating a broad, shallow depression within the geothermal field. The two youngest eruptions occurred northeast of earlier eruption centers and have narrower and elongated vents. We infer that in the central depression, newly formed craters rimmed by breccia deposits and high-relief country rock hosted temporary lakes tens of meters deep. Crater-lake breakout(s) and/or seismic events caused sudden pressure reduction above the hydrothermal aquifer, triggering hydrothermal eruptions. Northeast of the basin, hydrothermal alteration produced caprocks above intensively fractured areas. In this case, earthquakes are the most likely trigger for cap-rupture and eruption. All eruptions excavated shallow and large craters mostly within partially altered Oruanui Formation and pre-fragmented breccias. The size and localization of the eruptions was likely due to a combination of (1) availability of undisturbed porous ignimbrite hosting large thermal aquifers, (2) efficient crater excavation within or alongside pre-fragmented breccia, and (3) the location of fracture and fault zones that channeled deep fluid upflow, favoring priming processes.
This study highlights how an interplay of tectonic, magmatic, and hydrologic processes is responsible for the timing, dynamics, and ultimate size of hydrothermal eruptions in geothermal fields. Some events may be very large and destructive depending on the right priming and geological conditions.
... We aim to establish field conditions that emulate those captured by a set of acoustic sensors in the near-field (where the source dimension is large compared to the observation array) volcanic environment while controlling important features of an eruption, including the event size, source response and discharge direction, such that waveform features and spectral characteristics can be monitored. The eruption conditions emulated here are mixed liquid-gas jets (e.g., Zhang et al. 2016) that are akin to eruptions through volcanic lakes (e.g., Kilgour et al. 2010;Jolly et al. 2010;Fee et al. 2020;Lyons et al. 2019), and may be dynamically different from ash-gas jets from dry bed volcanic systems. Regardless, by controlling the eruption parameters, we aim to determine which characteristic features may be identified for real volcanic eruptions, to improve eruption source characterization and inform volcano monitoring decisions. ...
Laterally directed explosive eruptions are responsible for multiple fatalities over the past decade and are an increasingly important volcanology problem. To understand the energy dynamics for these events, we collected field-scale explosion data from nine acoustic sensors surrounding a tiltable cannon as part of an exploratory experimental design. For each cannon discharge, the blast direction was varied systematically at 0°, 12°, and 24° from vertical, capturing acoustic wavefield directivity related to the tilt angle. While each event was similar in energy discharge potential, the resulting acoustic signal features were variable event-to-event, producing non-repetitious waveforms and spectra. Systematic features were observed in a subset of individual events for vertical and lateral discharges. For vertical discharges, the acoustic energy had a uniform radiation pattern. The lateral discharges showed an asymmetric radiation pattern with higher frequencies in the direction of the blast and depletion of those frequencies behind the cannon. Results suggest that, in natural volcanic systems, near-field blast directionality may be elucidated from acoustic sensors in absence of visual data, with implications for volcano monitoring and hazard assessment.
Graphical Abstract
... La dinámica del transporte balística ha sido analizando usando observaciones de campo de los depósitos (Biass et al., 2016;Fitzgerald et al., 2014;Kilgour et al., 2010), por medio de video (Chouet, Hamisevicz y McGetchin, 1974;Taddeucci et al., 2012;Taddeucci, Alatorre-Ibarguengoitia, Palladino, Scarlato y Camaldo, 2015), por experimentos de laboratorio (Alatorre-Ibargüengotia, Morales-Iglesias, Ramos-Hernandez, Jon-Selvas y Jimenez-Aguilar, 2016; Graettinger, Valentine; Sonder, Ross y White, 2015), por simulación numérica (De' Michieli Vitturi, Neri, Ongaro, Savio y Boschi, 2010;Biass et al., 2016). ...
Con la ayuda del software BALÍSTICA, desarrollado por el autor, fue posible analizar la posible trayectoria de un bloque balístico eyectado el 29 octubre del 2014, que impactó el puesto de guardaparques del volcán Turrialba, causando destrozos que pudieron ser mortales. En el análisis se pudo incorporar el concepto de que, en las primeras decenas de metros de recorrido, el fragmento fue arrastrado por la nube de gas y ceniza por lo que se mueve como un proyectil ideal. El resto del recorrido se supuso fue hecho en un régimen Newtoniano con coeficiente de arrastre Cd constante. Ambas etapas estuvieron afectadas por la asimetría del cráter. Bajo estas condiciones se calculó 125 ms-1 y 65° para la velocidad y ángulo de salida, de 83,3 ms-1 y 74,6° para la velocidad y ángulo de llegada, con un tiempo de vuelo 20,3 s (utilizando un Cd = 0,42 asociado a una semiesfera). Al final se incluyen unas recomendaciones generales sobre cómo construir los próximos refugios de manera que se reduzca el riego tanto de los guardaparques como de los visitantes.
... Kereszturi et al. (2021) notes that the large extent of altered material beneath Ruapehu's summit may potentially explain the lack of surficial hydrothermal activity at Ruapehu's summit plateau and indicate the presence of shallow hydrothermal seals (Christenson and Wood, 1993;Christenson, 2000;Miller et al., 2020). Ruapehu's 25 September 2007 eruption is the likely result of over pressurization that led to a failure of a hydrothermal seal within this alteration environment (Kilgour et al., 2010). ...
Volcanic edifices are composed of an assortment of materials with varying mechanical behaviours. The range in mechanical behaviours of these rock materials presents a critical unknown element to volcanic stability assessments. In this study, we explore the deformation behaviour (e.g. uniaxial, triaxial, and tensile deformation behaviour) of volcanic materials from seven geotechnical units associated with a small, shallow intrusion at Pinnacle Ridge, Ruapehu, New Zealand. First, we provide a complete characterisation of the rock masses using field (e.g. rock mass characterisation) and lab (compressive and tensile strength analyses) data for each geotechnical unit. The new data show a range of tensile and triaxial compressive strengths dependent largely on porosity, which is further modified by hydrothermal alteration. More altered rocks are generally weaker (with the exception of the brecciated lava margins) under triaxial conditions. Altered rocks generally transition from brittle to ductile behaviour at lower confining pressure, although this depends on the porosity.
Using the stratigraphy and geomorphology of Pinnacle Ridge in conjunction with the rock property data we collect, we then use finite element modelling to depict the deformation behaviour of these units under different pore pressure and seismic conditions. The modelling shows that a relatively minor increase in pore pressure (e.g. < 3 MPa above hydrostatic conditions) is sufficient to change the slope failure type from deep-seated rotational sliding to extensional rupture. The presence of a low-permeability, low-rock mass strength material, even if very localised like a hydrothermal vein, can reduce slope stability by locally increasing pore pressure and providing a weakness plane that facilitates failure. The modelling also shows that a relatively minor increase in pore pressure (e.g. < 6 MPa above hydrostatic conditions) in a shallow hydrothermal system is sufficient to cause localised failure in the material, especially in the hydrothermal vein material. Additionally, the low-permeability hydrothermal vein material can cause pore pressure compartmentalisation, decreasing the strength factor in all materials in the compartment. While Pinnacle Ridge and its associated hydrothermal veins are >1 km from the currently active hydrothermal system, similar veins may be present near active parts of the hydrothermal system at Ruapehu and are an important consideration for future stability models at Ruapehu and other volcanoes.
... The breaking of relatively hard, low porosity spatter and lava likely led to an initial bottom-up explosion, eventually breaking to the surface with low energy and emplacing a small breccia deposit (lower bed; Fig. 16). The breaching of levels capping a pressurized hydrothermal system, such as the scoria cone, then led to a top-down rarefaction wave followed by expansion of flashing fluids that fragmented and ejected clasts (Germanovich and Lowell 1995;Kilgour et al. 2010;Gallagher et al. 2020;Montanaro et al. 2020). This eruption phase produced a wider ejection of larger particles and a thicker breccia dominated by hyaloclastite and lavas from shallow-intermediate depths (middle bed). ...
Krafla central volcano in Iceland has experienced numerous basaltic fissure eruptions through its history, the most recent examples being the Mývatn (1724‒1729) and Krafla Fires (1975–1984). The Mývatn Fires opened with a steam-driven eruption that produced the Víti crater. A magmatic intrusion has been inferred as the trigger perturbing the geothermal field hosting Víti, but the cause(s) of the explosive response remain uncertain.
Here, we present a detailed stratigraphic reconstruction of the breccia erupted from Víti crater, characterize the lithologies involved in the explosions, reconstruct the pre-eruptive setting, fingerprint the eruption trigger and source depth, and reveal the eruption mechanisms. Our results suggest that the Víti eruption can be classified as a magmatic-hydrothermal type and that it was a complex event with three eruption phases. The injection of rhyolite below a pre-existing convecting hydrothermal system likely triggered the Víti eruption. Heating and pressurization of shallow geothermal fluid initiated disruption of a scoria cone “cap” via an initial series of small explosions involving a pre-existing altered weak zone, with ejection of fragments from at least 60 m depth. This event was superseded by larger, broader, and dominantly shallow explosions (~200 m depth) driven by decompression of hydrothermal fluids within highly porous, poorly compacted tuffaceous hyaloclastite. This second phase was triggered when pressurized fluids broke through the scoria cone complex “cap”. At the same time, deep-rooted explosions (~1 km depth) began to feed the eruption with large inputs of fragmented juvenile and host rock from a deeper zone. Shallow explosions enlarging the crater dominated the final phase.
Our results indicate that at Krafla, as in similar geological contexts, shallow and thin hyaloclastite sequences hosting hot geothermal fluids and capped by low-permeability lithologies (e.g., altered scoria cone complex and/or massive, thick lava flow sequence) are susceptible to explosive failure in the case of shallow magmatic intrusion(s).
... Such proximity to the explosions exposes visitors and guides to multiple volcanic hazards, with Volcanic Ballistic Projectiles (VBPs) globally the most common cause of volcanic fatalities for tourists [Brown et al. 2017]. VBPs are fragments of molten lava or solid rock and can range from a few centimetres to tens of metres in diameter [Andronico et al. 2015;Bower and Woods 1996;Gurioli et al. 2013;Nairn and Self 1978;Taddeucci et al. 2017;Tsunematsu et al. 2016], can travel tens to hundreds of metres per second [Fudali and Melson 1971;Pioli et al. 2008;Taddeucci et al. 2012;Yamagishi and Feebrey 1994] and Corresponding author: bec.fitzgerald4@gmail.com up to~10 km from vent, although they are usually limited to within 5 km [Alatorre-Ibargüengoitia et al. 2012;Blong 1984;Gurioli et al. 2013;Kilgour et al. 2010;Minakami 1942]. Their often high impact and thermal energies make them a potentially lethal hazard [Alatorre-Ibargüengoitia et al. 2012;Baxter and Gresham 1997;Blong 1984;Tsunematsu et al. 2016;Wardman et al. 2012]. ...
... We see a change in spatial density from the south to the east, with higher densities in the S-SSE and ESE-E than the SE. Four main factors might influence where VBPs cluster or appear to cluster: (1) topography [Kilgour et al. 2019], (2) preservation of the deposit (3) VBP fragmentation and (4) eruption directionality [Gurioli et al. 2013;Kilgour et al. 2010]. A topographic high or steep sided crater wall may block VBP deposition as VBPs may not be able to reach heights capable of landing on the other side of the topography or angled VBPs may get blocked by steep sided crater walls. ...
... Directed eruptions can produce asymmetric VBP fields Gurioli et al. 2013;Houghton et al. 2011;Kilgour et al. 2010]. From the observational logs taken over the two-month period ( Figure 7C), more VBPs were reported landing to the south of the vents in South Crater (i.e. both on the southern crater wall and outside the crater rim to the south). ...
Volcanic ballistics are the main hazard to life and infrastructure from Strombolian eruptions, which are a tourist drawcard, exposing people to this hazard. Most research to date has been to understand this style of eruption and how ballistics form and travel. However, little focus has been placed on how ballistics are distributed within ballistic fields or the inclusion of this data into hazard and risk assessments. In this study we used a UAV to image the ballistic field, and cameras to record eruptions at Yasur Volcano, Vanuatu from 28 July – 2 August and 17 – 19 October 2016. We present the mapped distributions from the two trips, how the field changes with distance and direction from the vent, and how eruption dynamics influence these changes. Our evidence for directionality results in considerable variation in summit ballistic hazard and is an important consideration for ballistic hazard and risk assessments.
... We infer this material to be a region of altered rock most likely reflecting the present day and previous hydrothermal systems. Andesite ballistic blocks from beneath Crater Lake, ejected in the 2007 eruption (Kilgour et al., 2010), are moderately to highly altered and have measured susceptibility of <0.002 SI, consistent with the low apparent susceptibility region recovered by our model. We tentatively interpret this to be the upper portion of the plumbing system ("heat pipe") (Christenson & Wood, 1993;Hurst et al., 1991) that supplies heat, gas, liquid, and occasionally magma to Crater Lake. ...
Combining 3‐D inversion of high‐resolution aeromagnetic data with airborne hyperspectral imaging creates a new method to map buried structure and hydrothermal alteration, applied to Mt. Ruapehu volcano, New Zealand. Hyperspectral imaging is sensitive to surface mineralogy including alteration minerals, while magnetic vector inversion reveals the volumetric distribution of magnetic susceptibility from which we interpret buried geology. Probability assessment from multiple model regularizations provides an important model uncertainty estimate. At Ruapehu, hyperspectral imaging highlights two main regions of surface alteration: the Pinnacle Ridge and the southeast flanks. The magnetic model of Pinnacle Ridge shows that alteration seen at surface continues to depth, but strongly magnetic, unaltered dikes form the core of the ridge. On the southeast flanks, the magnetic model also shows alteration imaged on the surface continues to depth; however, a previously unknown, magnetized sill intrudes part of the flank. Several smaller demagnetized regions are modeled, unlike at neighboring Mt. Tongariro where the hydrothermal system created a large demagnetized core. We propose that these differences relate to spatially focused (Ruapehu) vs distributed (Tongariro) eruption vents, the degree of faulting of the edifice and its glaciation history. Lava‐ice interaction produces fine‐grained lavas with measured magnetic susceptibilities similar to some moderately altered lavas, illustrating that care must be taken in the interpretation of magnetic data in the absence of geological information. The combination of hyperspectral imaging and aeromagnetic data inversion distinguishes shallow surface weathering from deeper‐seated hydrothermally altered rock masses, with implications for the magnitude and probability of collapse events.
