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Solfatara crater is located within the Campi Flegrei caldera to the west of Naples (Italy). It is one of the largest fumarolic manifestations known, and the rocks hosting the hydrothermal system are affected by intense hydrothermal alteration. Alteration can result in changes of degassing behavior, and in the formation of a cap rock thereby increas...
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The Boiling Lake - a major touristic attraction in the Morne Trois Pitons National Park, Dominica's (West Indies) World Heritage site - is located in an area of high hydrothermal activity in the southern part of the island. This spectacular lake is characterized by unstable behavior which has been interpreted to result from the presence of a water...
In the ninetieth, the discovery of hydrocarbons in volcanic or volcaniclastic rocks was not considered economically viable in the petroleum exploration because of the lack of reservoir quality. However, during last decades many examples from our country proved to be real targets like, Campo Océano, Octógono, Cupén Mahuida (Sruoga et al., 2004; Sruo...
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... In this study we did not characterize the effect of hydrothermal alteration on Vp/Vs as systematic sampling was not possible due to the lack of outcrop exposures for such tasks. However, mineral alteration plays a significant role in changing the porosity of rocks by precipitation and/or dissolution of minerals [Pola et al. 2014;Mayer et al. 2016;Kanakiya et al. 2017;Mordensky et al. 2018;Kanakiya et al. 2021] which in turn will increase or decrease the Vp and Vs in those rocks. ...
Measurements of ultrasonic elastic waves were carried out at atmospheric and in-situ pressure conditions on rock samples from Nevado del Ruiz Volcano (NRV). The study focuses on Vp/Vs as a function of Vp as indicators of mineralogy, porosity, fluid-types and anisotropy. We find that Vp/Vs can separate the effect of microcracks vs. vesicles as a function of fluid type and effective pressures. Little-to-no pressure dependence in Vp/Vs suggest that the rock porosity is mainly comprised by vesicles, while largely varying Vp/Vs vs Vp is associated to cracks. This effect occurs under both, dry and fluid saturation conditions. When cracks close, the Vp/Vs values are representative of the rock mineralogy. Finally, as foliation results in a wide range of Vp/Vs, which could lead to misinterpretations. For instance, on NRV foliated rocks, Vp/Vs at dry conditions and parallel to the foliation plane ranges at values which are commonly associated to fluid saturation.
... Rockfalls occurred frequently as magma continuously extruded in the southeastern part of the dome activity in the period between 2015 and 2017. The UAS data added constraints to the study of the interaction between hydrothermal fluids and dome rocks that may strongly alter intact rocks, changing their porosity and permeability ) by replacing the host rock minerals with secondary minerals and progressively reducing the strength of the dome rock (Pola et al. 2012;Wyering et al. 2014;Mayer et al. 2016). The location of hydrothermal activity was highly expressed at the structures of the Merapi dome, such as at the open NW-SE trending fissure and at the crescent-like structure described above. ...
... The activity of the Merapi dome was relatively calm and dominated by hydrothermal The UAS data added constraints to the study of the interaction between hydrothermal fluids and dome rocks that may strongly alter intact rocks, changing their porosity and permeability (Heap et al. 2019) by replacing the host rock minerals with secondary minerals and progressively reducing the strength of the dome rock (Pola et al. 2012;Wyering et al. 2014;Mayer et al. 2016). The location of hydrothermal activity was highly expressed at the structures of the Merapi dome, such as at the open NW-SE trending fissure and at the crescent-like structure described above. ...
Episodic growth and collapse of the lava dome of Merapi volcano is accompanied by significant hazards associated with material redeposition processes. Some of these hazards are preceded by over-steepening of the flanks of the dome, its destabilisation, fracturing and gravitational collapse, producing lethal pyroclastic density currents. With the emergence of unoccupied aircraft systems (UAS), these changes occurring high up at Merapi can now be monitored at unprecedented levels of detail. Here we summarise the use of UAS at Merapi to better understand the evolution of the lava dome following the 2010 eruption. Systematic UAS overflights and photogrammetric surveys were carried out in 2012, 2015, 2017, 2018 and 2019, allowing identification of the progression of major structures and a three-stage morphological evolution of the dome. We first highlight the significant morphological changes associated with steam-driven explosions that occurred in the period 2012–2014. A large open fissure formed and split the dome into two parts. In the years 2014–2018, hydrothermal activity dominated and progressively altered the dome rock. Lastly, in May–June 2018, a series of steam-driven explosions occurred and was followed by dome extrusion in August 2018, initially refilling the formerly open fissure. This work demonstrates the importance of reactivating pre-existing structures, and summarises the unique contribution realised by high resolution photogrammetric UAS surveys.
... Hydrothermal alteration observed at the Rotokawa DOC thermal area resulted from two main processes, i.e., leaching or precipitation, with opposite influences on porosity, permeability, and strength (Pola et al., 2012;Frolova et al., 2014;Mayer et al., 2016;Mordensky et al., 2019). Such changes in petrophysical and mechanical properties of rocks in shallow portions of geothermal areas can have important implications for ground deformation hazards (e.g., unstable grounds, sinkholes, and subsidence; Bromley et al., 2015;Frolova et al., 2020a). ...
Surface geothermal expressions such as mud pools, fumaroles, mineral deposits, collapse pits, and hydrothermal eruption craters vary in scale and type over space and time. The evolution in space and time of these surficial thermal features strongly relates to alteration processes caused by hydrothermal fluids. The changes in the physical and mechanical properties of top and subsoils that can control fluid flow, degassing patterns, and occurrence of geothermal hazards remain understudied. The thermal area located south of the Rotokawa geothermal field (New Zealand) includes a variety of major natural surface thermal features and widespread sulphur deposits precipitated by acid–sulphate fluids in steam-heated zones. Decades of sulphur mining induced the formation of new thermal features. Such a setting represents an exemplary case study for investigating old and recently formed thermal features, soil characteristics, types and patterns of superposed hydrothermal alteration, and degassing processes. We combined field and laboratory methods to define groups of thermal features, soil types, and associated alteration. Their spatial distribution indicates that fluid circulation, alteration intensity, and degassing are strongly influenced by the local and regional geological and structural settings, as well as by mining activity. We found that at the water table level and within excavated areas, acidic fluids led to the deposition of mud deposits, sinters, and stromatolites around warm springs and mud pools. In the vadose, steam-heated portions, fluids generally leach and degrade the Taupo Pumice, resulting in broad unstable grounds and collapse structures, while extensive sulphur-encrusted grounds and sulphur-rich soils formed in excavated sites. In this framework, the degassing and fluid circulation within the top and subsoils are strongly affected by the dominant soil layer type, and in turn by its granulometry, texture, and alteration state. Our study of top and subsoils yields precious insights into surface expression variability, fluid–rock interaction processes, and sulphur deposition patterns within steam-heated zones. Processes at such a scale may strongly influence the migration of thermal manifestations, gas outputs, and ground subsidence within geothermal environments. From a broader perspective, our results will help assess the evolution of geothermal activity and related hazards in similar areas worldwide.
... The presence of only silica (mainly cristobalite) as a secondary mineral in fresh to slightly altered lavas indicates alteration in a low-pH environment (pH<2), which upon subsequent neutralization (pH<4) due to dissolution would have added alunite to the mineral assemblage in moderately and highly altered lavas (Hemley et al., 1969;Stoffregen, 1987;Mayer et al., 2016;Hedenquist and Arribas, 2022). Given their surficial occurrence and secondary mineral assemblage, we interpret that the lavas have undergone acid-sulfate alteration at low temperatures (< 250 • C) characteristic of shallow environments (Zimbelman et al., 2005). ...
Hydrothermal alteration is generally associated with the weakening of volcanic rocks. Here we evaluate the possible role of hydrothermal alteration in lava dome collapses at Mt. Taranaki by evaluating the petrophysical (mineralogy, fluid pathways, crystallinity) and elastic properties (stiffness) of variably altered lavas from its summit dome area and block and ash flow deposits. Our results show that acid-sulfate alteration changes the mineralogy of the lavas by dissolving primary feldspars, pyroxenes, amphiboles, Fe-Ti oxides, and volcanic glass and precipitating secondary alunite and silica. These changes alter the fluid pathways and crystallinity of the lavas. However, despite these alteration-related petrophysical changes, we find that altered lavas are stiffer, and inferentially stronger, than fresh lavas of similar porosity. We attribute this at least partially to the precipitation of relatively strong secondary minerals like alunite and silica instead of weaker minerals like clays. We discuss the implications of these findings for dome stability at Mt. Taranaki. We suggest that the role of hydrothermal alteration in weakening volcanic rocks is not merely dependent on the alteration intensity but also on the type of alteration. Altered lavas, without extensive dissolution and with precipitation of strong secondary minerals, are unlikely to weaken the dome.
... The influence of hydrothermal alteration on rock properties of volcanic rocks has been investigated in the past to assess not only the longevity of geothermal reservoirs ) but also regarding slope instability (Sánchez-Núñez et al. 2021), heat flux, volcanic activity, and the possible impact on phreatic eruptions (Mayer et al. 2016;Heap et al. 2019Heap et al. , 2022. Previous studies aimed to identify general trends in altered rocks, e.g., increased porosity and permeability along with reduced rock strength (Pola et al. 2014;Wyering et al. 2014), reduced permeability due to mineral precipitation (Mordensky et al. 2018), reduced porosity and permeability due to silicification (Dobson et al. 2003), or reduced porosity and increased thermal conductivity associated with an increased degree of hydrothermal alteration (Mielke et al. 2015), respectively. ...
Abstract Hydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities ( 0.91 10–6 m2 s−1), but a significant loss of magnetic susceptibility (10–3–10–6 SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings.
... Chiodini 2010), and mud pools and thermal springs with intense gas bubbling occurring over highly hydrothermally altered terrains (De Gennaro et al. 1980, Piochi et al. 2015. In particular, the soils surrounding the pools, dominated by amorphous silica and alunite with minor amounts of pyrite (De Gennaro et al. 1980, Pinto et al. 2007, Mayer et al. 2016, Piochi et al. 2019, are the result of the alteration of trachytic rocks through the interaction with hot acidic fluids. On the other hand, the water in the pools is a mixture of meteoric waters and magmatic-hydrothermal condensates characterized by extreme temperatures (up to 95 • C) and pH conditions (down to 1 and below) (e.g. ...
... Differences between sediments were also observed in terms of organic matter, with a higher content of AFDW, total N and C in sites P9, P10 and P11 (Fig. 2). Overall, sediment samples displayed relatively high concentrations of Si and K. Accordingly, potassium alunite [KAl 3 (OH) 6 (SO 4 ) 2 ] was observed to be widely distributed at Pisciarelli (Mayer et al. 2016, Piochi et al. 2019, whilst amorphous silica (opal) was recognized as the prevailing mineral in the muddy bed of the Pisciarelli thermal waters (Pinto et al. 2007). ...
Although terrestrial hydrothermal systems are considered among the most fascinating environments, how their unique and extreme conditions can affect microorganisms selection and the role in biogeochemical cycles has not yet been well elucidated. A combined geochemical and microbiological exploration in waters and sediments from ten sampling points along a sharp temperature gradient (15–90 °C) within an extremely acidic hydrothermal system (Pisciarelli Spring, Campi Flegrei area, southern Italy) displayed how hydrothermal fluids influence the microbial dynamics. This area was characterized by high levels of reduced gaseous species (e.g. H2S, H2, CH4, CO), and very low pH values (<2.3). Thermodynamic calculations revealed a high microbial catabolic potential in oxidation/reduction reactions of N-, S-, and Fe-bearing species. Overall, an increase of the archaeal/bacterial abundance ratio was observed by decreasing temperature and pH values. In particular, Archaea and Bacteria were present in almost equal cell abundance (up to 1.1 × 109 and 9.3 × 108 cell/g, respectively) in the <70 °C sampling points (average pH = 2.09); on the contrary, highest temperature waters (85–90 °C; average pH = 2.26) were characterized by low abundance of archaeal cells. The high-throughput sequencing of 16S rRNA gene indicated strong differences in archaeal and bacterial communities’ composition along temperature gradient. However, the microbiome in this extreme environment was mainly constituted by chemoautotrophic microorganisms that were likely involved in N-, S-, and Fe-bearing species transformations (e.g. Acidianus infernus, Ferroplasma acidarmanus, Acidithiobacillus, Sulfobacillus, Thaumarchaeota), in agreement with thermodynamic calculations.
... Currently, CF hosts an intense hydrothermal activity and an acid sulfate alteration environment that can be observed on pyroclastic deposits (e.g., Agnano Monte Spina, Astroni, and Solfatara tephra) and lava domes (Monte Olibano, and Solfatara cryptodome) constituting the Solfatara vent structure [1,2,6,37,38]. In particular, hot fumaroles, thermal springs, mud pools, and diffuse degassing both occur inside the crater as well as on the eastern flanks such as at Pisciarelli ( [3,9,37,[39][40][41] (and references therein [2,3,9,37,39,[41][42][43])). ...
This paper explored the relationship between acidic sulfate alteration, geostructural frameworks, and geomorphological changes that can be observed in active volcanic hydrothermal systems. The target area was Pisciarelli in the Campi Flegrei volcano, where diffuse acidic sulfate alteration and hydrothermal dynamics have been growing since 2012, causing a progressive deterioration of landscapes. Terrestrial Laser Scanner (TLS), photogrammetry of proximity survey, geological field work, mineralogical and geochemical analysis with Optical Microscopy (OM), electron microscopy, and energy dispersive micro-analysis (BSEM-EDS) and X-ray Powder Diffraction (XRPD) to characterize (and monitor) altered rock outcrops were repeatedly carried out in the area. We present the multi-temporal acquisition and analysis referring to Terrestrial Laser Scanning (TLS) datasets (2014 survey) with 3D-point clouds obtained from the Structure for Motion (SfM) photogrammetry (2021 survey) with a high-resolution digital camera aimed at evaluating volumetric changes on the mostly damaged and altered fault scarp. For each survey, we obtained a vertical Digital Elevation Model (DEM) and a true color RGB orthomosaic that provided the setting of the area at the different times and its evolution through their comparison. Changing sites were examined in the field and characterized for mineralogical and geochemical purposes. The investigated slope lost up to about 4 m3 of deposits between 2014 and 2021, mostly related to hydrothermal alteration induced by gas emissions and meteoric infiltration. Our methodological approach appears promising to evaluate evolution and rock-fall susceptibility of solfataric terrains subjected to hydrothermal dynamics.
... This contact probably causes a geochemical alteration in the shallow unconsolidated tephra (Figure 2). According to Mayer et al. (2016), the interaction shown in Figure 9 with a resolution of meters may be caused by the presence of the Alunite in the formation due to the hydrothermal alteration between GDF and the LDF. Therefore, the alunite represents the cap-rock where the fluids, from the deeper hydrothermal plume through the faults and the chaotic zones identified by the seismic, are trapped. ...
The imaging of volcanic systems is a challenging topic that attracts the scientific community’s attention. The characterization of structures and rock properties by means of seismic active methods is becoming fundamental for providing ultra-high-resolution images of the structures of interest. The Solfatara Volcano is a quiescent volcano in the Campi Flegrei resurgent nested caldera that is continuously under investigation and monitoring for its shallow activity, such as fumaroles. The purpose of this work is to characterize the fluid accumulation zone in the first 150 m depth in the middle of the crater, using several post-stack seismic attributes and Amplitude Versus Offset (AVO) analysis to characterize the contact between the CO2 and condensed water in the shallower accumulation zone. The two 400 m-long profiles to which we refer in this work have been acquired during the active Repeated InduCed Earthquakes and Noise experiment. The profiles were deployed along with the NNE-SSW and WNW-ESE directions across the whole surface of the crater including the main surface anomalies of the fumaroles, in the eastern area, and the mud-pool of Fangaia, located in the western area. The seismic pre-processing, pre-stack processing, and post-stack analysis previously applied on the NNE-SSW profile are here performed for the first time on the WNW-ESE profile, while partial-stack AVO analysis is performed for both profiles. The post-stack attributes including time gain, envelope, energy, and root mean square have been computed and extracted for determining the maximum and minimum values of amplitude zones on the migrated post-stack seismic profiles. Such anomalies are provided by complex and geometrical attributes embedding information on faults and chaotic zones. The AVO technique has also been used as a direct gas indicator to enhance fluid discrimination and identification. Finally, the analysis of the profile, seismic attributes, and near-surface structural interpretation related to the Solfatara Volcano has been incorporated into the proposed analysis. The multi-2D image depicts fluids trapped in the Solfatara Volcano at depths ranging from 10 to 50 m below the crater’s surface, as well as their migration paths up to 150 m deep: this evidenced contact between the fluids has been probably due to the solfataric alteration of the minerals, caused by the arising plume and the abovecondensed water which decreases the permeability of the rocks and forms an argillic phase working as cap-rock and trapping the gases. The application of the AVO analysis, coupled with the seismic attribute’s investigation, provides a very detailed multi-2D image of the shallower Solfatara Volcano, which outperforms in terms of accuracy the ones obtained with different tools in previous works, and that evidences the presence and the position of the liquid and the gases in the north-east area of the Solfatara Volcano.
... Both parameters can be influenced by alteration, fracturing and compaction processes (Fig. 3a). Mineral dissolution can increase permeability and reduce rock strength, whereas mineral precipitation has the opposite effect, favouring brittle behaviour (Pola et al. 2012;Mayer et al. 2016;Mordensky et al. 2019). Fractures tend to enhance permeability, favouring fluid circulation and eventual dissolution/precipitation, but might weaken host rocks, whereas compaction tends to close pores and fractures reducing permeability and increasing strength (Heap et al. 2015;Heap and Violay 2021). ...
... Thiéry and Mercury 2009; Montanaro et al. 2016a); 2. The state of the fluid (gas, liquid, gas + liquid) and its volume(Mastin 1995;Ohba et al. 2007;Thiéry and Mercury 2009;Toramaru and Maeda 2013; Montanaro et al. 2016c, a;Fullard and Lynch 2012); 3. The geometry and properties (connected porosity, permeability and strength) of the aquifer host rock(Haug et al. 2013;Galland et al. 2014;Mayer et al. 2016;Kennedy et al. 2020; Montanaro et al. 2021a, b;Fullard and Lynch 2012). ...
Over the last decade, field investigations, laboratory experiments, geophysical exploration and petrological, geochemical and numerical modelling have provided insight into the mechanisms of phreatic and hydrothermal eruptions. These eruptions are driven by sudden flashing of ground-or hydrothermal water to steam and are strongly influenced by the interaction of host rock and hydrothermal system. Aquifers hosted in volcanic edifices, calderas and rift environments can be primed for instability by alteration processes affecting rock permeability and/or strength, while magmatic fluid injection(s), earthquakes or other subtle triggers can promote explosive failure. Gas emission, ground deformation and seismicity may provide short-to medium-term forerunner signals of these eruptions, yet a definition of universal precursors remains a key challenge. Looking forward in the next 10 years, improved warning and hazard assessment will require integration of field and experimental data with models combining case studies, as well as development of new monitoring methods integrated by machine learning approaches.
