Three-dimensional inversion of audio-magnetotelluric data acquired from the crater area of Mt. Tokachidake, Japan

Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar.

Morphological Changes of Anak Krakatau after the 22 December 2018 Flank Collapsed

After the 22 December 2018 flank collapse, series of hydrothermal, phreatomagmatic, and effusive eruptions occurred and changed the morphology of Anak Krakatau. The ejected volcanic materials enlarge and increase the elevation of the west flank, which may indicate a reconstruction phase of the Anak Krakatau edifice. Here, we investigated the morphological changes of Anak Krakatau between 2019 and 2020 by using drone SfM photogrammetry, Sentinel and Pleiades satellite imageries, and fieldworks photograph data. The result shows volcaniclastic deposit due to the hydrothermal and/or phreatomagmatic eruptions that covered 0.08 km2 around an active crater lake at Anak Krakatau between February and January 2020. The large phreatomagmatic and effusive eruptions on 10 April 2020 produced tephra and lava flow deposits that significantly changed the morphology of Anak Krakatau. The deposit of tephra covered 0.815 km2 at the north – northwest flanks of Anak Krakatau, while the lava flow emplaced 0.2 km2 and elongated around 742 m from the pre-existing crater lake to the west shoreline of Anak Krakatau. The lava flow has a blocky surface and highly fractured that possibly formed due to compression – extension stresses during lava flow emplacement. The emplacement of the massive lava flow at the pre-existing crater lake may change the future eruption style at Anak Krakatau, which was previously dominated by hydrovolcanism activities, such as hydrothermal and phreatomagmatic events.

Metagenomic and Metabolic Analyses of Poly-Extreme Microbiome from an Active Crater Volcano Lake

Background: El Chichón volcano is one of the most active volcanoes in Mexico. Previous studies have described the poly-extreme conditions of the lake crater but its bacterial composition and the functional features of the complete microbiome have not been characterized yet. Methods: This study integrated two approaches to explain the microbiology diversity and abundance, one focused on the environmental genomic potential by metagenomics approach, and other culturomics of enrichment of bacteria and archaea. The microbial diversity of the anaerobic consortia cultivated in was carried out by metabarcoding analysis, the metabolic capacity by metabolomics fluxes of carbon and enzymologic techniques for the analysis of sulfate reduction in laboratory-grown prokaryotic cells. Results: This work provides new information on the taxonomic and functional diversity of the Archea representative phyla Crenarchaeota and Euryarchaeota as well as the phyla Thermotogales and Aquificae for Bacteria. Through the analysis of microbial consortia cultivation and the genetic information collected from the natural environment sampling, metabolic interactions were identified between the microorganisms that support the life of the microbiome under multi-extreme conditions. A close relationship is proposed between the cycles of carbon and sulfur in an active volcano. Conclusions: This research contributes to the understanding of microbial metabolism under extreme conditions and potential knowledge of "microbial dark matter" that can be applied in biotechnological processes and evolutionary studies.

CO2 and H2S Degassing at Fangaia Mud Pool, Solfatara, Campi Flegrei (Italy): Origin and Dynamics of the Pool Basin

The Fangaia mud pool provides a “window” into the hydrothermal system underlying the degassing Solfatara crater, which is the most active volcanic centre inside the restless Campi Flegrei caldera, Southern Italy. The present study aimed at unravelling the degassing dynamics of CO2 and H2S flushing through the pH 1.2 steam-heated Fangaia mud pool, an ideal field laboratory as a proxy of an active crater lake. Our results from MultiGAS measurements above Fangaia’s surface show that H2S scrubbing, demonstrated by high CO2/H2S ratios, was most efficient in the portions of the basin affected by diffusive degassing. Convective bubbling degassing instead was the most effective mechanism to release gas in quantitative terms, with lower CO2/H2S ratios, similar to the Solfatara crater fumaroles, the high-T end member of the hydrothermal system. Unsurprisingly, total estimated CO2 and H2S fluxes from the small Fangaia pool (~184 m2 in June 2017) were at least two orders of magnitude lower (CO2 flux < 64 t/d, H2S flux < 0.5 t/d) than the total CO2 flux of the Campi Flegrei caldera (up to 3000 t/d for CO2), too low to affect the gas budget for the caldera, and hence volcano monitoring routines. Given the role of the rising gas as “sediment stirrer”, the physical and chemical processes behind gas migration through a mud pool are arguably the creating processes giving origin to Fangaia. Follow-up studies of this so far unique campaign will help to better understand the fast dynamics of this peculiar degassing feature.

Ultra-acid volcanic waters: origin and response on volcanic activity. A review

&lt;p&gt;Some active volcanoes host thermal springs with ultra- (1&lt;pH&lt;2) and even hyper- (pH &lt; 1) acidic waters with composition corresponding to a mixture of HCl and H2SO4 acids and with cations where Al and Fe are often the major components. Such springs sometimes are known as inferred drainages from active crater lakes (e.g., Rios Agrio at Poas and Copahue volcanoes). However, there are a number of acidic volcano-hydrothermal systems of Cl-SO4 composition at volcanoes without crater lakes. &amp;#160;At least ten groups of manifestation of this type are known for Kuril Islands. Several groups of acid volcanic springs including the famous Tamagawa springs are described in Japan.&amp;#160; Most of the acid Cl-SO4 volcano-hydrothermal systems are characteristic for island volcanoes, probably due to specific hydrological conditions of small volcanic islands. Maybe most known are coastal acid springs at Satsuma Iwojima volcano, Ryukyu arc, Japan. The accepted idea about the origin of such systems is scrubbing (dissolution) of magmatic HCl, HF and SO2 by groundwaters above magmatic conduits.&amp;#160; If so, the composition of acid springs must reflect the state of activity of a volcano. This review describes case histories that are known from the literature and from authors&amp;#8217; studies. Most of the volcanoes hosting acid systems show frequent phreatic activity. We show that &amp;#160;in contrast to crater lakes (Poas, Ruapehu, Copahue, White Island), acid springs on slopes of active volcanoes generally do not response on the preparing or ongoing volcanic eruptions. The aquifers and flow paths of the acid waters in volcanic edifices can be not associated with active conduits but with other degassing magmatic bodies and/or with deeper aquifers. One of the examples of such a complicated system is Ebeko volcano with Yuryevskye springs in Kuril Islands. These springs have a hydrochemical record since 1950s, and during this period Ebeko volcano had at least 10 strong phreatic eruptions.&lt;/p&gt;

Silent VLPs At Stromboli: Implications For Slug Vs. Plug

&lt;p&gt;Very long period (VLP) seismic signals observed in volcanic environments are thought to be produced by magma and gas flow through conduits. Stromboli Volcano, Italy, typically produces hundreds of VLPs per day. These have been generally attributed to the flow of gas slugs through the shallow plumbing system and thus linked to the mechanism thought to drive Strombolian explosions. During a 6-day-long seismo-acoustic campaign in May 2018 (a period characterized by relatively low activity) we recorded 1900+ seismic events, the majority of which have significant energy in the VLP (2-100 s) band. We used a coincident STA/LTA trigger to identify seismic events in continuous waveform data and then used the PeakMatch algorithm (Rodgers et al., 2015) to identify seismic multiplets, with a focus on VLPs. To identify explosions, we applied the same coincident trigger to infrasound data, and manually identified gas jetting events using spectrograms and high-pass-filtered (20 Hz) waveforms.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We identified ~250 explosions and ~600 jetting events. Seismic multiplet analysis identified two main families of repeating events. Family 1 (F1) has over 500 events and Family 2 (F2) has over 150 events based on a 0.7 correlation threshold. We find that F1 VLPs coincide in time with ~6% of explosions and ~0.8% of jetting events and F2 VLPs coincide in time with ~28% of explosions and ~2.7% of jetting events (we term these &amp;#8220;silent VLPs&amp;#8221;). These VLPs do not correspond with lava effusion (Marchetti and Ripepe, 2005; Ripepe et al., 2015). F2 have a higher dominant period (8-10 s) compared to F1 (3-4 s). The repeating VLPs are part of a broadband signal and the higher frequencies start after the VLP. VLP peak amplitudes are generally larger for F1 events. The dip of the VLP particle motion roughly locates the F1 and F2 VLP source centroids beneath the active crater and are stable throughout the dataset. Both VLP displacements show a small outward, large inward, and subsequent large outward motion from the crater. The lack of explosions relative to repeating VLPs does not support the slug model, where a slug rises through a conduit, generates a VLP through interactions with changes in conduit geometry, and then bursts at the lava free surface. Our observations support the plug model (Suckale et al., 2016). We suggest the &amp;#8220;silent&amp;#8221; VLPs are generated when the gas bubbles interact with and move into the semipermeable plug. Then the plug behaves as a mechanical filter for gas escape and allows for passive and explosive escape mechanisms.&lt;/p&gt;

Processes culminating in the 2015 phreatic explosion at Lascar volcano, Chile, evidenced by multiparametric data

Small steam-driven volcanic explosions are common at volcanoes worldwide but are rarely documented or monitored; therefore, these events still put residents and tourists at risk every year. Steam-driven explosions also occur frequently (once every 2–5 years on average) at Lascar volcano, Chile, where they are often spontaneous and lack any identifiable precursor activity. Here, for the first time at Lascar, we describe the processes culminating in such a sudden volcanic explosion that occurred on 30 October 2015, which was thoroughly monitored by cameras, a seismic network, and gas and temperature sensors. Prior to the eruption, we retrospectively identified unrest manifesting as a gradual increase in the number of long-period (LP) seismic events in 2014, indicating an enhanced level of activity at the volcano. Additionally, sulfur dioxide (SO2) flux and thermal anomalies were detected before the eruption. Then, our weather station reported a precipitation event, followed by an increase in steaming and a sudden volcanic explosion at Lascar. The multidisciplinary data exhibited short-term variations associated with the explosion, including (1) an abrupt eruption onset that was seismically identified in the 1–10 Hz frequency band, (2) the detection of a 1.7 km high white-gray eruption column in camera images, and (3) a pronounced spike in SO2 emission rates reaching 55 kg s−1 during the main pulse of the eruption as measured by a mini-differential optical absorption spectroscopy (DOAS) scanner. Continuous carbon dioxide (CO2) and temperature measurements conducted at a fumarole on the southern rim of the Lascar crater revealed a pronounced change in the trend of the relationship between the CO2 mixing ratio and the gas outlet temperature; we speculate that this change was associated with the prior precipitation event. An increased thermal anomaly inside the active crater as observed in Sentinel-2 images and drone overflights performed after the steam-driven explosion revealed the presence of a ∼50 m long fracture truncating the floor of the active crater, which coincides well with the location of the thermal anomaly. This study presents the chronology of events culminating in a steam-driven explosion but also demonstrates that phreatic explosions are difficult to predict, even if the volcano is thoroughly monitored; these findings emphasize why ascending to the summits of Lascar and similar volcanoes is hazardous, particularly after considerable precipitation.