Most volcanoes on Earth host a volcano-hydrothermal system. Lying between the surface and magma reservoirs, they exert a dramatic influence on volcano dynamics. Understanding their behavior is however challenging because of the complex interplay between gas, liquid, and rocks. Volcanic gas can, for example, be completely scrubbed through interactions with groundwater whereas the kinetics of these reactions are controlled by thermodynamic conditions that are poorly constrained. While fluid circulation gives rise to a range of geophysical signals such as ground vibrations, self-potential, or variable resistivity observable at the surface, their complex dynamics complicate the isolation of pre-eruptive signals and the interpretation of the observed volcanic activity. Indeed, some volcanoes remain on alert for months or years without experiencing any eruption. Such situations severely affect the credibility of the agencies in charge of monitoring volcano activities.

In this contribution, we will focus on multi-disciplinary efforts to better characterise the time evolution of these complicated systems. Our ultimate goal is to deconvolve the contribution of dynamic processes occurring in such systems (temperature, gas saturation, alteration, precipitation) to possibly facilitate real-time monitoring efforts. Our examples come from different parts of the world, in both hemispheres.

How to cite: Caudron, C., Lecocq, T., Yates, A., Dempsey, D., Ardid, A., Hermans, T., Vanhooren, L., Fontaine, O., Bultreys, T., and Girona, T.: Towards monitoring volcanic hydrothermal alteration using geophysical approaches, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12564, https://doi.org/10.5194/egusphere-egu23-12564, 2023.

Volcanic Hydrothermal systems (VHS) are three-phase reservoirs between the magma chamber and the earth’s surface, they are present in most volcanoes on Earth but the dynamical behavior is currently poorly known. As fluid circulation gives rise to a range of geophysical and hydrothermal signals, it complicates the detection of pre-eruptive signals, hence certain volcanoes remain on alert for significant amounts of time without erupting. Moreover, VHS lie at the basis of phreatic or hydrothermal eruptions, which can cause significant casualties. Given these safety concerns it is thus paramount that we gain a better understanding of the dynamics of VHS.

Traditionally seismometers are used to monitor VHS but the high level of background noise complicates the application of standard processing techniques. Here we apply geo-electric methods for characterization and monitoring. Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) provide a relatively high resolution image of the subsurface electrical properties. The main dynamic processes occurring in VHS are temperature changes, variations in saturation and mineral precipitation, all of which influence the electrical signal making ERT/IP a suitable method to monitor this system. Similarly, the spontaneous potential signal (SP) is influenced by fluid flow and diffusion/conduction processes and should therefore bring complementary information to ERT/IP.  

In this project the Gunnuhver geothermal area in Iceland is monitored on a daily basis since October 2022. Prior to the monitoring campaign a field characterization was executed where 5 profiles were measured using ERT/IP and SP. Due to the proximity to the ocean, the groundwater in this area is saline which complicates the IP data acquisition, as saline environments have a low ability to store electrical charge. Hence, in the characterization phase we attempted to find the most suitable method for acquiring an IP signal in this high-temperature, saline setting. The static ERT profiles show a high spatial variability in the area, where the alteration zone, characterized by very low resistivity, is clearly distinguishable from the more resistive basalt. Adjacent to the geo-electrical methods the characterization and monitoring also include seismicity, fiber-grating, CO₂ measurements and shallow soil-moisture and -temperature measurements.

By developing a novel time-lapse inversion approach, where the auxiliary data helps to constrain the interpretation of the ERT/IP profiles, we are able to get new insights into the inner workings of Volcanic Hydrothermal Systems.

How to cite: Vanhooren, L., Fontaine, O., Caudron, C., Vrancken, E., Dekoninck, W., De Lathauwer, H., and Hermans, T.: Characterization and Monitoring of the Gunnuhver geothermal site using electrical methods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12873, https://doi.org/10.5194/egusphere-egu23-12873, 2023.

Composite volcanoes can progressively weaken through hydrothermal alteration, which may lead to volcano collapse, forming far-reaching debris avalanches. Hydrothermal minerals can also contribute to flank instability as they play a critical role in moderating volcanic degassing by changing the porosity and permeability of the rock and thereby changing the local pore-pressure distribution. Therefore, a robust model and understanding of hydrothermal alteration within a volcanic edifice is important to improve hazard assessment efforts. This study investigates the type and extent of hydrothermal alteration on Mt Ruapehu, New Zealand, using a combination of mineralogical, hyperspectral imaging, and aero-magnetic studies.

Mt Ruapehu shows a diverse suite of surface weathering and hydrothermal alteration minerals, which are distributed heterogeneously on the surface. The surface weathering has abundant goethite, hematite and phyllosilicate mineral associations, while the hydrothermal alteration is characterised by phyllosilicates, Fe-oxides, pyrite, jarosite, alunite, gypsum anhydrite, and native sulphur minerals. Although surficial evidence of alteration on Mt Ruapehu is limited, aero-magnetic data and inversion modelling indicate deep-seated (≤500 m) alteration of demagnetized rocks. The decrease of magnetic susceptibility can be linked to the dissolution of (Ti-) magnetite phases, as well as the deposition of brecciated horizons between lava flows and intercalated glacial till and volcaniclastics. Surface outcrops mapped by airborne hyperspectral imaging combined with Scanning Electron Microscopy (SEM-EDS), Short wavelength Infrared (SWIR) Spectroscopy, X-Ray Diffraction and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) data of ground samples reveal a complex alteration history developed around older vent/crater systems of Mt. Ruapehu in last 250 ky. This study provides a simplified geological model to capture the hydrothermal processes on Mt Ruapehu, aiding future studies on delineating areas prone to mass movements.

How to cite: Kereszturi, G., Álvarez-Valero, A. M., D'Mello, N., Miller, C., and Coulthard Jr, D. A.: A multidisciplinary approach to constrain hydrothermal alteration history on Mt Ruapehu (New Zealand), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11501, https://doi.org/10.5194/egusphere-egu23-11501, 2023.

Fumarole fields and hydrothermal alteration are prominent signs of volcanic degassing at many volcanoes, and their monitoring is an essential part of the assessment of volcanic unrest. Yet, our knowledge about the detailed structure of fumarole fields and the spatiotemporal processes in their complexity is still poor, owing to limited accessibility. By using modern drone and sensor technologies we now are able to provide high-resolution data that allows us to analyze fumarole fields at cm-scales. From 2018 to 2022, we conducted repeated drone surveys at the fumaroles of La Fossa volcano on Vulcano Island (Italy). Drones equipped with a 20 MP camera and a radiometric thermal infrared sensor allowed the close-range acquisition of optical and thermal infrared images. By means of Structure from Motion (SfM) processing, the generation of high-resolution ortho- and infrared mosaic data was achieved. Applying Principal Component Analysis and image classification to the orthomosaic data, we detected and classified areas affected by degassing and hydrothermal alteration covering more than 60.000 sqm. By analyzing their spectral characteristics, we defined 4 surface types, of which type 1 and 2 are largely coincident with the thermally active surface. Type 3 is an altered low-temperature surface and type 4 is an unaltered surface. To evaluate these surface types, samples were analyzed in the lab for their mineralogical and geochemical composition by X-ray diffraction and fluorescence analysis, showing significant variability in composition. Further, we analyzed the spatial variability of the surface degassing activity using a portable multi-gas device. The combination of these methods allows us to constrain factors that are controlling the observed surface pattern of the degassing system, and to better understand the structural setup of the fumarole field and broader field of activity. We find that the actual high-temperature fumarole sites only account for <10% of the active surface. Besides, large thermally active areas, thermal aureoles for instance, display a rather diffuse activity. During the 2021 volcanic crisis, next to the high-temperature fumaroles, especially those diffuse features showed a response to the increased gas flux, emphasizing their structural importance. We summarize spatiotemporal variations during the crisis, and indicate possible widespread effects of the long-term gas-rock interaction like surface sealing, and forced lateral gas migration, affecting large parts of the fumarole field. The results suggest that detailed structural studies of fumarole fields by means of drone-based remote sensing in combination with in-situ measurements can contribute to a better understanding of degassing and alteration effects, with relevance for degassing sites elsewhere and during volcanic crises. 

How to cite: Müller, D., Walter, T. R., Troll, V., Stammeier, J. A., Karlsson, A., De Paolo, E., and Pisciotta, A. F.: Drone and field-based monitoring of the degassing and alteration structures of the La Fossa fumarole field (Vulcano Island), before and during the 2021 volcanic crisis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12942, https://doi.org/10.5194/egusphere-egu23-12942, 2023.

The silicic flows and domes can impose mechanical and thermal stress on the underlying substrate causing mineralization and lithification of granular bodies. In addition, the released water from the permeable substrate as dominant volatile species can contribute to the glass hydration of the flow. This fluid-lava interaction can be directly studied in ancient successions with exposed contacts. The Lebuj flow (Tokaj Mountains, Hungary) developed in a Miocene caldera setting, where the erosion revealed its basal zone including lava-substrate interaction textures. The main textural units comprise (1) a rhyolitic lava flow (F₁: perlitic glass with obsidian marekanite, F₂: microcrystalline-glass transition and F₃: a basal breccia layer) and (2) the underlying mixed substrate unit (S₁: massive rhyolite and breccia S₂: enclosed partially sintered rhyolite tuff). The thin section textural analyses were completed by BSE imaging, Raman mapping (SiO₂ polymorphs) and FTIR spot measurement (perlite H₂O, clays). Glass transition temperature (T_g) was estimated using the chemical based GRD model.

The flow margin contacted with underlying volcanoclastic deposits along a steeply inclined (50-75°) plane with subordinate fragmentation. The substrate suffered re-heating by the flow where porosity loss and welding (solid-state sintering) occurred. The silica polymorphs are observed growing into open pore spaces and fractures and interpreted as precipitates from vapor phase fluids passing through the permeable lithologies. The smectite group minerals typically record acidic type alteration, where the water-rock interaction commonly produces glass replacement minerals. The FTIR-identified clays (mixed layer kaolinite/montmorillonite or beidellite) indicate low-to medium alteration degree (estimated temperature between 50-100 °C).

The lithophysae, spherulites and microcrystalline bands in the flow unit are textural evidence for prolonged groundmass crystallization above T_g. The relict obsidian grains in the glass are proofs of an incomplete hydration process. The FTIR and BSE investigations demonstrate the presence of sharp transitions from the hydrated ~3 wt.% perlitic rims to non-hydrated obsidian cores.

Textural and mineralogical evidence suggest that the following series of events occurred as the consequences of the lava-substrate interaction: a) a viscous rhyolite flow advanced on an irregular topography; b) shear and brittle fracturing occurred at the contact; c) groundmass crystallization (above T_g, ~ 690-715 °C) and hydration (below T_g) acted in the flow; d) low temperature mineralization and variable scale sintering occurred in the substrate (below T_g). According to the fluid exchange model beneath silicic lava domes (Ball et al. 2015), the water – rock interaction resulted in weak hydrothermal alteration of the substrate and water flux to the quenched glass (flow). As an interaction of the two processes, the increased sintering and mineralization reduced the porosity of the substrate which probably restricted further water uptake for hydration. Thus the obsidian results from a ‘quenched’ hydration front (Bindeman and Lowenstern 2016).

Ball J.L., Stauffer P.H., Calder E.S., Valentine G.A. (2015). Bull Volcanol 77:1–16.

Bindeman I.N., Lowenstern J.B.  (2016).  Contrib to Mineral Petrol 171:89

Aknowledgements

This research has been funded by the Hungarian–Italian MTA-CNR bilateral research project 2019–2022. The research was also supported by Development and Innovation Office–NKFIH No. 131869 OTKA project. RL was supported by the Bolyai János Research Fellowship.

How to cite: Szepesi, J., Vona, A., Kovács, I. J., Fintor, K., Molnár, K., Scarani, A., Giordano, G., and Lukács, R.: Lava – substrate interaction: constraints on perlitic hydration and low temperature mineralization, Lebuj rhyolitic flow, Tokaj Mountains, Carpathian-Pannonian region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12320, https://doi.org/10.5194/egusphere-egu23-12320, 2023.

The growing structure of active volcanoes due to material addition can lead to oversteepening and overloading (McGuire, 2003). This situation can be worsened by seismic activity as most volcanoes are in seismically active areas. Moreover, the materials forming volcanic edifices are subjected to extreme conditions in terms of temperature, pore pressure (consequence of several combined factors) and chemically aggressive fluids (very low pH for example) which can all destabilize volcanoes’ flanks. Among these factors, hydrothermal activity is of particular interest as it enhances rock dissolution (and thus, increases rock porosity), promotes high pore pressures and leads to the creation of mechanically weaker materials (like clay-rich rocks) and promote the instability (Rattez and Veveakis, 2020). However, the effects that these processes have on volcano stability have been barely quantified (Heap and Violay, 2021).

To better understand the influence of different types of hydrothermal alteration on the hydraulic and mechanical properties of volcanic rocks, permeameter and triaxial experiments have been performed on samples retrieved by Detienne et al. (2016) from the Tutupaca volcano (17° 01′ S, 70° 21′ W). This volcano is a dacitic dome complex located at the southern end of the Peruvian arc. The study focuses on a remarkably well-preserved debris avalanche deposit emplaced to the northeast of the volcano. The debris avalanche is sourced to Eastern Tutupaca; it left a horseshoe-shaped crater open to the northeast and was accompanied by a pyroclastic flow (volume: 6.5-7.5 x 10⁷ m³) (Samaniego et al., 2015). The mineralogy and the microstructure of the samples have been investigated using X-ray diffraction and micro-computed tomography respectively. Preliminary results exhibit a high variability of mineralogy, microstructures, and mechanical properties. It appears that the alteration degree may have more influence on the mechanical behavior of volcanic rocks than the porosity. This dataset could be further used in numerical models of flank collapses to better constrain the role of hydrothermal alteration on the nucleation of those events. 

References

Detienne M. (2016) “Unravelling the role of hydrothermal alteration in volcanic flank and sector collapses using combined mineralogical, experimental, and numerical modelling studies”. PhD thesis, UCLouvain.

Heap M. and Violay M. (2021) “The mechanical behaviour and failure modes of volcanic rocks: a review”, Bulletin of Volcanology, 83:33. 

Lipman PW, Mullineaux DR (eds) (1981) “The 1980 eruptions of Mount St Helens, Washington.” U.S. Geological Survey, Professional Paper 1250, 844 pp.

McGuire WJ (2003) Volcano instability and lateral collapse. Revista 1:33–45.

Rattez H, Veveakis M (2020). “Weak phases production and heat generation control fault friction during seismic slip”, Nature Communications, doi: 10.31223/osf.io/xupr8

Samaniego, P., Valderrama, P., Mariño, J., De Vries, B. V. W., Roche, O., Manrique, N., Chédeville, C., Liorzou, C., Fidel, L. & Malnati, J. (2015). “The historical (218±14 aBP) explosive eruption of Tutupaca volcano (Southern Peru) ». Bulletin of Volcanology, 77, 1-18.

How to cite: Niclaes, J., Detienne, M., Heap, M., Delmelle, P., and Rattez, H.: Experimental insight into the role of hydrothermal alteration on the mechanical and microstructural properties of the volcanic rocks: The case of Tutupaca, Peru, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7059, https://doi.org/10.5194/egusphere-egu23-7059, 2023.

The global dominance of basaltic rock in the crust plays a dominant role in the global elemental geochemical cycle. Smectite clay is a ubiquitous alteration product of basalt. Despite having a complex silicate structure, a phyllosilicate dominance of smectite raises a question on the kinetics aspect of its formation and underlying nucleation and growth mechanism during basalt alteration. In the experimental formation of smectite, Zhang et el., 2019 have shown that coherent scattering domain size (CSDS) remains constant for starting few days during the growth of the sheet, and then stacking of fundamental particles takes place, resulting in the growth along C*. A later study by Kuliviesz et al., 2018 has shown variation in the layer charge with clay size. This implies chemical changes in the clay with its growth, which complicates the problem. The current study aims to investigate the mechanism and pathway of smectite crystallization. We have studied the role of magnesium in the growth of smectite clay and its mechanism.

Experimental alteration of basaltic glass has been performed at 150 degree Celsius in two different fluid compositions. One is containing 0.3M MgCl₂ and the other pure water. Products have been investigated under XRD, FTIR, TEM, and SEM; solution has been investigated under ICP AES.

We found that both the experimental setup resulted globular flower-like feature along with a honeycomb structure on the glass surface. There is no XRD-detectable crystalline product, but electron microscopy has confirmed smectite, the globular flower-like structure (similar to Fiore et al. 2001). There is a discernible difference in the size of globules. Mg bearing condition has leather size flakes and globule than without Mg. Magnesium bearing experiment has shown higher Si concentration in the solution than without Mg, similar to Al's behaviour.

Smectite formed in the Mg bearing experiment has twice the amount of Mg in clay structure than smectite formed in an experiment without Mg. Based on 11 oxygen, the chemical composition of smectite has been used to calculate the abundance of cation pairs in the octahedral sheet if there is a random distribution of all cations. At the same time, observation based on FTIR spectra has shown a dominance of Mg-Mg pairs. This has been observed in both experiments. Our study confirms a preferential ordering of Mg-Mg cation pairs in the octahedral sheet and better growth in the presence of Mg.

Further TEM investigation is being performed to observe the fundamental particle size and chemical relation.

How to cite: Saini, V., Sriwastava, P., and Mathew, G.: Experimental Study of Basalt Alteration At 150 °C: An Approach To Understand Smectite Nucleation And Crystallization Pathways, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14028, https://doi.org/10.5194/egusphere-egu23-14028, 2023.

Telica volcano is a persistently restless volcano with activity expressed as long-lived high-temperature fumaroles and magmatic degassing, high rates of background seismicity and frequent (sub-decadal) explosive, phreatic to phreatomagmatic eruptive activity. We are studying this system through the combination of geophysical, geochemical, and geologic observations and analyses. To date our observations and analyses indicate: 1) long-lived fumaroles and subsequent hydrothermal alteration of the crater walls lead to regions of preferential crater wall collapse into the active the crater; and 2) hydrothermal alteration and mineralization within the shallow hydrothermal system leads to sealing of the conduit, a build-up of pressure and explosions. The partial to complete sealing of the top of the volcanic conduit is thought to occur through the deposition of salts and silicate minerals from hydrothermal fluids, moving the volcano from an open to a closed system. The formation of a seal is observed indirectly by the eruption of hydrothermal mineral species and altered rock, decreases in measured gas flux, thermal anomalies, and LF seismicity that indicates a decrease in gas flow, and deformation of the volcanic edifice due to the increase in pressure from gas accumulation. Additionally, the eruption of a small volume lava dome in 2017 indicates the presence of highly viscous basaltic andesite magma in the conduit, which may also contribute to sealing of the conduit and the observed deformation. Here, we model cGPS displacement time series based on our conceptual model for the transition from an open to a closed system, utilizing our disparate, but complimentary data sets, including gas flux, terrestrial and remotely sensed thermal, web camera and cGPS displacement time series. Our numerical model incorporates changes in permeability of the conduit due to mineralization and the subsequent accumulation of gas beneath the seal. The increase of pressure due to sealing of the system and rising magma eventually leads to failure of the seal and phreatic explosions. Improving our knowledge of this transition from an open to closed system is important for understanding Telica’s eruptive processes and hazards, as well as gaining a better understanding of how this transition could manifest at other volcanic systems.

How to cite: LaFemina, P., Saucier, E., Feineman, M., and Saballos, A.: Persistent Activity and Crater Formation at Telica Volcano Driven by the Shallow Hydrothermal System, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9333, https://doi.org/10.5194/egusphere-egu23-9333, 2023.