The rocks forming a volcanic edifice or dome are typically saturated or partially-saturated with water. However, most experiments aimed at better understanding the mechanical behaviour of volcanic ...rocks have been performed on dry samples, and therefore most large-scale models designed to explore volcano stability have used parameters representative for dry rock. Here, we present a combined laboratory and modelling study in which we (1) quantified the influence of water-saturation on the mechanical behaviour of variably altered dome rocks from La Soufrière de Guadeloupe (Eastern Caribbean) and (2) used these new data to investigate the influence of water on dome stability. Our laboratory data show that the ratio of wet to dry uniaxial compressive strength (UCS) and Young's modulus are ~0.30–0.95 and ~0.10–1.00, respectively. In other words, the dome rocks were all mechanically weaker when water-saturated. Further, the ratio of wet to dry UCS decreased with increasing alteration (the wt% of secondary minerals in the rocks). Micromechanical modelling suggests that the observed water-weakening is the result of a decrease in fracture toughness (K_IC) in the presence of water. The ratio of wet to dry K_IC also decreases with increasing alteration, explaining why water-weakening increased as a function of alteration. To explore the influence of water-saturation on lava dome stability, we numerically generated lava domes in Particle Flow Code using the experimental data corresponding to unaltered and altered rock under dry conditions. The strength of the dome-forming rocks was then reduced to values corresponding to wet conditions. Our modelling shows that, although the stability of the unaltered dome was not influenced by water-saturation, larger displacements were observed for the wet altered dome. Additional simulations in which we modelled a buried alteration zone within an otherwise unaltered dome showed that higher displacements were observed when the dome was water-saturated. We conclude that (1) the water-saturation reduces the UCS and Young's modulus of volcanic rock, (2) larger decreases in UCS in the presence of water are observed for altered rocks, and (3) the stability of a dome can be compromised by the presence of water if the dome is altered, or contains an altered zone. These conclusions highlight that the degree of alteration and water-saturation should be mapped and monitored at active volcanoes worldwide, and that large-scale models should use values for water-saturated rocks when appropriate.
The collapse of lava domes, inherently heterogeneous structures, represents a significant volcanic hazard. Numerical and analogue models designed to model dome instability and collapse have ...incorporated heterogeneity in the form of discrete zones with homogeneous properties. Based on an assessment of dome rock heterogeneity, we explore whether material property heterogeneity (“diffuse” heterogeneity) within these discrete zones can promote dome instability. X-ray computed tomography shows that dome samples are characterised by high microstructural heterogeneities; e.g. porosity varies from 0.07 to 0.20 over millimetric length scales. To explore how microstructural heterogeneity influences sample-scale strength, we performed numerical simulations using Rock Failure and Process Analysis. The mean mechanical properties of the numerical samples were constant, and we introduced heterogeneity by varying their distribution using a Weibull probability function. The models show that increasing heterogeneity can reduce sample-scale strength by more than a factor of 2. To explore the influence of dome-scale heterogeneity, we numerically generated lava domes in Particle Flow Code. The domes have the same bulk strength but are characterised by different degrees of heterogeneity by varying the distribution of cohesion using a Weibull probability function. The models show that a greater degree of heterogeneity induces higher dome-scale displacements and that, when there is also a discrete weakened zone, the addition of diffuse heterogeneity leads to more widely distributed deformation. Therefore, alongside discrete zones defined by different material properties, we find that the diffuse heterogeneity inherent to a dome is sufficient to compromise dome stability and should be incorporated in future modelling endeavours.
The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores. It is suspected to have ...impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40 km
3
dense rock equivalent (DRE) of deposits, consisting of (i) 7–9 km
3
DRE of pumiceous plinian fall products, (ii) 16 km
3
DRE of pyroclastic density current deposits (PDC) and (iii) 8–9 km
3
DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6 × 10
8
kg/s, a maximum plume height of 43 km and a dispersal index of 110,500 km
2
, the 1257 A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.
Volcanic gas dispersal can be a serious threat to people living near active volcanoes since it can have short- and long-term effects on human health, and severely damage crops and agricultural land. ...In recent decades, reliable computational models have significantly advanced, and now they may represent a valuable tool to make quantitative and testable predictions, supporting gas dispersal forecasting and hazard assessments for public safety. Before applying a specific modelling tool into hazard quantification, its calibration and its sensitivity to initial and boundary conditions should be carefully tested against available data, in order to produce unbiased hazard quantifications. In this study, we provided a number of prototypical tests aimed to validate the modelling of gas dispersal from a hazard perspective. The tests were carried out at La Soufrière de Guadeloupe volcano, one of the most active gas emitters in the Lesser Antilles.
La Soufrière de Guadeloupe has shown quasi-permanent degassing of a low-temperature hydrothermal nature since its last magmatic eruption in 1530 CE, when the current dome was emplaced. We focused on the distribution of CO2 and H2S discharged from the three main present-day fumarolic sources at the summit, using the measurements of continuous gas concentrations collected in the period March–April 2017. We developed a new probabilistic implementation of the Eulerian code DISGAS-2.0 for passive gas dispersion coupled with the mass-consistent Diagnostic Wind Model, using local wind measurements and atmospheric stability information from a local meteorological station and ERA5 reanalysis data. We found that model outputs were not significantly affected by the type of wind data but rather upon the relative positions of fumaroles and measurement stations. Our results reproduced the statistical variability in daily averages of observed data over the investigated period within acceptable ranges, indicating the potential usefulness of DISGAS-2.0 as a tool for reproducing the observed fumarolic degassing and for quantifying gas hazard at La Soufrière. The adopted testing procedure allows for an aware application of simulation tools for quantifying the hazard, and thus we think that this kind of testing should actually be the first logical step to be taken when applying a simulator to assess (gas) hazard in any other volcanic contexts.
•A new probabilistic implementation of DISGAS-2.0 is provided.•Prototypical tests validated the modelling of gas dispersal from hazard perspective.•Outputs not affected by wind data but upon the positions of sources and stations.
Volcano slope stability analysis is a critical component of volcanic hazard assessments and monitoring. However, traditional methods for assessing rock strength require physical samples of rock which ...may be difficult to obtain or characterize in bulk. Here, visible to shortwave infrared (350–2500 nm; VNIR–SWIR) reflected light spectroscopy on laboratory-tested rock samples from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria) and La Soufrière de Guadeloupe (Eastern Caribbean) volcanoes was used to design a novel rapid chemometric-based method to estimate uniaxial compressive strength (UCS) and porosity. Our Partial Least Squares Regression models return moderate accuracies for both UCS and porosity, with R2 of 0.43–0.49 and Mean Absolute Percentage Error (MAPE) of 0.2–0.4. When laboratory-measured porosity is included with spectral data, UCS prediction reaches an R2 of 0.82 and MAPE of 0.11. Our models highlight that the observed changes in the UCS are coupled with subtle mineralogical changes due to hydrothermal alteration at wavelengths of 360–438, 532–597, 1405–1455, 2179–2272, 2332–2386, and 2460–2490 nm. These mineralogical changes include mineral replacement, precipitation hydrothermal alteration processes which impact the strength of volcanic rocks, such as mineral replacement, precipitation, and/or silicification. Our approach highlights that spectroscopy can provide a first order assessment of rock strength and/or porosity or be used to complement laboratory porosity-based predictive models. VNIR-SWIR spectroscopy therefore provides an accurate non-destructive way of assessing rock strength and alteration mineralogy, even from remote sensing platforms.
•VNIR-SWIR reflectance spectroscopy can predict rock mechanical properties (e.g., UCS with R2 of 0.43-0.82).•Spectroscopy-based UCS predictions are most accurate when combined with measured porosity.•Mineralogical changes and metal cation substitution exert a strong control on UCS and porosity.•Spectroscopy is an appropriate new tool for assessing in-situ volcanic rock masses.
Volatiles exert a critical control on volcanic eruption style and in turn impact the near source environment and global climate. La Soufrière de Guadeloupe in the Lesser Antilles has been ...experiencing volcanic unrest since 1992, increasing to a peak in 2018. The lack of data available on volatiles from past eruptions, and the well-developed hydrothermal system makes understanding deep-released volatile behaviour challenging. In this study, we analyse new melt inclusions and shed light on the volatile lifecycle and impacts at La Soufrière de Guadeloupe. We focus on four eruptions: 1657 CE (Vulcanian), 1010 CE (Plinian), 341 CE (Strombolian) and 5680 BCE (Plinian), and compare to the well-studied 1530 CE (Sub-Plinian) eruption. The maximum volatile content of these eruption melt inclusions are: 4.42 wt% H
2
O, 1700 CO
2
ppm, 780 ppm S, 0.36 wt% Cl and 680 ppm F. We observe a decrease in S content over time indicating the whole system is evolving by early separation of FeS, resulting in a lower S content in younger magma. Using the CHOSETTO v1 model, we modelled degassing paths related to decompression at low pressures, suggesting the majority of S degassing has occurred during magma ascent. We also calculate the SO
2
emissions using the petrologic method, and while the 1657 CE, 1530 CE and 341 CE eruptions have negligible emissions (0.0001–0.001 Mt of SO
2
), the 1010 CE and 5680 BCE eruptions (0.2 Mt and 0.3 Mt of SO
2
, respectively) are greater. Using the SO
2
emissions and plume height, we calculated the climate forcing associated with each event. The 1010 CE and 5680 BCE Plinian eruptions produced a peak global mean stratospheric aerosol optical depth (SAOD) of 0.0055 and 0.0062, respectively. This suggests, that even the largest eruptions of La Soufrière de Guadeloupe did not exert a significant climate forcing individually, but are important contributors to the volcanic stratospheric sulfate aerosol background resulting from relatively moderate but frequent explosive eruptions. Overall, this study provides new insights into degassing processes and climate forcing not only at La Soufrière de Guadeloupe, but also for other basaltic-andesitic, magmatic-hydrothermal systems. These new constraints are vital particularly if the volcano is currently in a state of unrest and will contribute to improving monitoring crisis management and long-term planning.
Background
Scientists monitoring active volcanoes are increasingly required to provide decision support to civil authorities during periods of unrest. As the extent and resolution of monitoring ...improves, the process of jointly interpreting multiple strands of indirect evidence becomes increasingly complex. Similarities with uncertainties in medical diagnosis suggest a formal evidence-based approach, whereby monitoring data are analysed synoptically to provide probabilistic hazard forecasts. A statistical tool to formalize such inferences is the Bayesian Belief Network (BBN). By explicitly representing conditional dependencies between the volcanological model and observations, BBNs use probability theory to treat uncertainties in a rational and auditable manner, as warranted by the strength of the scientific evidence. A retrospective analysis is given for the 1976 Guadeloupe crisis, using a BBN to provide inferential assessment of the state of the evolving magmatic system and probability of incipient eruption. Conditional dependencies are characterized quantitatively by structured expert elicitation.
Results
Analysis of the available monitoring data suggests that at the height of the crisis the probability of magmatic intrusion was high, in accordance with scientific thinking at the time. The corresponding probability of magmatic eruption was elevated in July and August 1976 and signs of precursory activity were justifiably cause for concern. However, collective uncertainty about the future course of the crisis was also substantial. Of all the possible scenarios, the most likely outcome evinced by interpretation of observations on 31 August 1976 was 'no eruption’ (mean probability 0.5); the chance of a magmatic eruption/blast had an estimated mean probability of ~0.4. There was therefore no evidential basis for asserting one scenario to be significantly more likely than another.
Conclusions
Our analysis adds objective probabilistic expression to the volcanological narrative at the time of the 1976 crisis, and demonstrates that a formal evidential case could have supported the authorities' concerns about public safety and decision to evacuate. Revisiting the episode highlights many challenges for modern, contemporary decision making under conditions of considerable uncertainty, and suggests the BBN is a suitable framework for marshalling multiple, uncertain observations, model results and interpretations. The formulation presented here can be developed as a tool for ongoing use in the volcano observatory.
•Successful mass flow simulations with up to two rheological parameters.•Extensive use of field data for model calibration and scenario definition.•Mapping of areas exposed to high discharge debris ...flow, for hazard assessment.
High discharge debris flows in mountainous and volcanic areas are major threats to populations and infrastructures. Modeling such events is challenging because the associated processes are complex, and because we often lack data to constrain rheological parameters. In this work, we show how extensive field data can help model a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow, with a single one-phase thin-layer numerical code, SHALTOP, and up to two rheological parameters. With the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analyses to define realistic simulation scenarios and determine the main characteristics of documented events for model calibration. Then, we model a possible 1.9×106 m3 rock avalanche. The resulting deposits are remobilized instantaneously as a high discharge debris flow. We show that, for a given unstable volume, successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus the initial conditions of the subsequent debris flow simulation. The location of the debris flow initiation has also little influence on simulation results. However, progressive remobilization of materials slows down the debris flow and limits overflows, in comparison to an instantaneous release. Nevertheless, high discharge debris flows are well reproduced with an instantaneous initiation. Besides, the range of travel times measured for other significant debris flows in the Prêcheur river is consistent with our simulation results.
The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and ...intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.
•Hydrothermal alteration changes the thermal properties of volcanic rocks.•Alteration can increase or decrease thermal properties, depending on alteration type.•Alteration can change the conductive heat flow density at a volcano.•Alteration should be considered when interpreting volcano heat flux data.
The seismic crisis that began in May, 2018 off the coast of Mayotte announced the onset of a volcanic eruption that started two months later 50 km southeast of the island. This seismicity has since ...been taken as an indicator of the volcanic and tectonic activity in the area. In response to this activity, a network of stations was deployed on Mayotte over the past three years. We used the machine learning-based method PhaseNet to re-analyze the seismicity recorded on land since March 2019. We detect 50,512 events compared to around 6508 manually picked events between March 2019 and March 2021. We locate them with NonLinLoc and a locally developed 1-D velocity model. While eruptions are often monitored through the analysis of Volcano-Tectonic (VT) seismicity (2–40 Hz), we focus on the lower frequency, Long Period (LP) earthquakes (0.5–5 Hz), which are thought to be more directly related to fluid movement at depth. In Mayotte, the VT events are spread between two clusters, whereas the LP events are all located in a single cluster in the bigger proximal VT cluster, at depths ranging from 25 to 40 km. Moreover, while the VT earthquakes of the proximal cluster occur continuously with no apparent pattern, LP events occur in swarms that last for tens of minutes. We show that during the swarms, LP events generally migrate downward at a speed of 5 m/s. While these events do not appear directly linked to upward fluid migration, their waveform signature could result from propagation through a fluid-rich medium. They occur at a different location than VT earthquakes, also suggesting a different origin which could be linked to the Very Long Period events (VLP) observed above the LP earthquakes in Mayotte.
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