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Complex crystal cargo in basaltic eruptions has the potential to yield diverse insights on pre- and syn-eruptive timescales of magma storage and transport. Research on eruption products from the 1959 eruption from Kīlauea Iki Crater at Kīlauea volcano (Hawai'i) demonstrates that time-series samples collected during an eruption can yield a wealth of information not accessible by studying the fall deposit alone. Major element zoning in olivine crystals illustrates four environments of magma storage that were variably mixed and progressively involved in the eruption. Diffusion timescales retrieved from crystals < 1 mm in size are typically much shorter than those from crystals > 1 mm, illustrating the additional complexity of information recorded by different grain sizes and olivine populations. The timescales can be divided into two groups: (1) t > 100 days reflect longer-term magma recharge into Kīlauea's deep (8–10 km) reservoir system and (2) t < 100 days broadly correspond to a period of unrest and inflation beginning ~ 3 months prior to eruption. Some timescales reflect syn-eruptive processes, where diffusion began after the eruption onset. Progressive changes in zoning populations in the time-series scoriae samples illustrate how quickly diffusive re-equilibration can erase older magmatic histories and information and underscores that studies on fall deposits can lead to an incomplete record of magmatic processes. Thus, diffusion timescales from fall deposits alone should be cautiously interpreted with the caveat that they may be missing a substantial part of the total eruptive event and, therefore, the record of magmatic histories inferred from crystal cargo. [ABSTRACT FROM AUTHOR]

Jeju Island comprises numerous tuff rings and tuff cones and their reworked deposits in the subsurface, which formed on the ca. 120-m-deep Yellow Sea continental shelf under the fluctuating Quaternary sea levels. Tens of meter-thick and massive deposits were found by chance during groundwater drilling at three sites. These deposits are interpreted as either syn-eruptive diatreme-filling deposits or post-eruptive crater-filling deposits, both of hydromagmatic volcanoes. The diatremes were cut into shelf sediment, 70 to 250 m thick, and developed generally within it below the Quaternary sea levels. Abundant external water was therefore available for explosive magma-water interactions at shallow levels. The diatreme deposit in one core shows some features attributable to extreme wetness or water saturation of the diatreme fill, such as the matrix support of larger clasts, meager vertical changes in matrix content, and an absence of features related to particle adhesion. Fluidally shaped clasts with delicate reentrant margins in the core suggest minimal particle abrasion and breakage in a water-saturated and highly fluid slurry of tephra and water that was probably filling a shallow bowl-like diatreme, which is distinguished from both phreatomagmatic and kimberlite diatremes. The diatreme deposits in other cores comprise blocky and angular clasts in a sideromelane ash matrix, suggesting phreatomagmatic explosions at a deeper level. One of the cores contains collapsed deposits of thinly stratified tuff emplaced by pyroclastic surges, indicating that the diatreme is associated with an emergent tuff ring. Both Surtseyan and phreatomagmatic eruptions are therefore interpreted to have occurred on the shelf under the controls of fluctuating Quaternary sea levels. The subsurface diatremes suggest that there can be a variety of diatremes with different sizes, shapes, and material characteristics beneath the craters of hydromagmatic volcanoes, including not only maars but also tuff rings and tuff cones. [ABSTRACT FROM AUTHOR]

Rock resistivity measurements under non-wet conditions are essential for interpreting subsurface shallow resistivity structure. This study investigated the resistivity of rock samples collected from several locations around the Nakadake craters of Aso Volcano, Japan. The resistivity of rock samples exhibited a logarithmically linear relationship with changes in absolute humidity, corresponding to possible seasonal variations and correlating with the porosity of rocks. As the samples maintained an identical cylindrical shape and size, the observed variations in humidity were unrelated to the external surface area but were caused by the internal pore structure of rocks. Furthermore, the external surface area of homogeneous granite was artificially varied using slits and its measured resistivity did not vary in the presence of slits. The measured distribution of electrical potential was well explained by a homogeneous model, and less so by a two-layer model, indicating that the region of resistivity change was not restricted to the surface of the sample, but rather covered the entire sample. Further, changes in resistivity in response to absolute humidity were caused by the adsorption of water molecules onto minerals at internal pore surfaces, as evidenced by the measured temporal scale of resistivity change based on a simple concentration–diffusion process of gaseous water molecules caused by humidity changes in the external environment. These findings imply significant variations in the resistivity of shallow subsurfaces owing to humidity changes; thus, the consideration of humidity is important for the interpretation of subsurface shallow resistivity structures. • Resistivity of granite and Aso volcanic rock under non-wet conditions were measured. • Resistivity showed a logarithmically linear relationship with humidity changes. • Changes in resistivity caused by the adsorption of water molecules onto minerals. • Humidity fluctuations can cause variations in the resistivity of shallow subsurfaces. • Humidity should be considered for understandings of subsurface shallow structures. [ABSTRACT FROM AUTHOR]

Sakurajima Muography Observatory captured the formation of a volcanic plug beneath the Showa crater with a spatial resolution of ∼60 m in accordance with the end of the eruption episode of Showa crater and the reactivation of Minamidake crater. The increase of average density was observed with above 3 σ standard deviation for both above the floor of Minamidake crater and beneath the floor of Showa crater, respectively, being interpreted as volcanic ejecta deposition and the formation of a volcanic plug laterally extended within a few hundreds of meters. Plain Language Summary: Muography is an imaging technique that provides information about the amount of material across kilometer‐thick geological structures by the measurement of the yield of cosmic rays, similarly to X‐raying of human body. We applied muography to capture two muon images of Minamidake and Showa craters of Sakurajima volcano before and after the shift of eruption activities occurred between the two craters at the end of 2017. The comparison of the images revealed the presence of increased material across the activated Minamidake and beneath the deactivated Showa crater. Muography detected the formation of a magma plug in the conduit of Showa crater that was not detected earlier by any other geophysical technique. These results demonstrate the applicability of cosmic muon imaging for monitoring of magma movements inside active volcanoes. Key Points: Two muographic images were taken across the erupting Sakurajima volcano with a spatial resolution of 60 mIncreased average densities were observed beneath the deactivated crater after the shift of eruption sequenceMuography data suggest that uprising magma formed a plug in the conduit of Showa crater [ABSTRACT FROM AUTHOR]

A lengthy period of eruptive activity from the summit craters of Mt. Etna started in January 2011. It culminated in early December 2015 with a spectacular sequence of intense eruptive events involving all four summit craters (Voragine, Bocca Nuova, New Southeast Crater, and Northeast Crater). The activity consisted of high eruption columns, Strombolian explosions, lava flows and widespread ash falls that repeatedly interfered with air traffic. The most powerful episode occurred on 3 December 2015 from the Voragine. After three further potent episodes from the Voragine, activity shifted to the New Southeast Crater on 6 December 2015, where Strombolian activity and lava flow emission lasted for two days and were fed by the most primitive magma of the study period. Activity once more shifted to the Northeast Crater, where ash emission and weak Strombolian activity took place for several days. Sporadic ash emissions from all craters continued until 18 December, when all activity ceased. Although resembling the summit eruptions of 1998–1999, which also involved all four summit craters, this multifaceted eruptive sequence occurred in an exceptionally short time window of less than three days, unprecedented in the recent activity of Mt. Etna. It also produced important morphostructural changes of the summit area with the coalescence of Voragine and Bocca Nuova in a single large crater, the “Central Crater”, reproducing the morphological setting of the summit cone before the formation of Bocca Nuova in 1968. The December 2015 volcanic crisis was followed closely by the staff of the Etna Observatory to monitor the on-going activity and forecast its evolution, in accordance with protocols agreed with the Italian Civil Protection Department. [ABSTRACT FROM AUTHOR]

Gravity-induced pyroclastic density currents (PDCs) can be produced by the collapse of volcanic crater rims or due to the gravitational instability of materials deposited in proximal areas during explosive activity. These types of PDCs, which are also known as “glowing avalanches”, have been directly observed, and their deposits have been widely identified on the flanks of several volcanoes that are fed by mafic to intermediate magmas. In this research, the suitability of landslide numerical models for simulating gravity-induced PDCs to provide hazard assessments was tested. This work also presents the results of a back-analysis of three events that occurred in 1906, 1930 and 1944 at the Stromboli volcano by applying a depth-averaged 3D numerical code named DAN-3D. The model assumes a frictional internal rheology and a variable basal rheology (i.e., frictional, Voellmy and plastic). The numerical modelling was able to reproduce the gravity-induced PDCs' extension and deposit thicknesses to an order of magnitude of that reported in the literature. The best results when compared with field data were obtained using a Voellmy model with a frictional coefficient of f = 0.19 and a turbulence parameter ξ = 1000 m s − 1 . The results highlight the suitability of this numerical code, which is generally used for landslides, to reproduce the destructive potential of these events in volcanic environments and to obtain information on hazards connected with explosive-related, mass-wasting phenomena in Stromboli Island and at volcanic systems characterized by similar phenomena. [ABSTRACT FROM AUTHOR]

The first historical eruption of Kambalny volcano began on March 24, 2017 with the powerful ash emission from the summit crater reaching as high as 6 km above sea level. The explosive activity continued without interruption from March 24 to March 30. The most powerful ash emission was registered on March 25-26, when the ash plume drifted several thousand kilometers SW, S, and SE from the volcano. On April 2 and April 9, after several calm days, powerful ash explosions occurred generating ash plumes up to 7 km high. The area of the land and sea over which the ash plume drifted during the day of March 25, was 650000 km2; the area of the ash accumulation on the land that was formed from March 24 to April 9, exceeded 1500 km2. These parameters were measured using the satellite-based data in the VolSatView information system. Domination of the silty fraction and the presence of secondary minerals (pyrite, gypsum, sulfur, and others) in the ash point to the phreatic character of the volcanic eruption. [ABSTRACT FROM AUTHOR]

This study systematically characterizes normal faults in tephra rings at Weizhou and Xieyang volcanic islands in southern China. Detailed field observations, measurements and analyses indicate that these faults were likely formed during phreatomagmatic eruptions based on the following evidence: i) faults in tephra rings are closely spaced and fault cores commonly have no fault gouge, debris and secondary infill, ruling out long-term faulting mechanisms in extensional settings; ii) faults are usually accompanied by soft-sediment deformation structures, indicating most volcanic layers were in soft during faulting; iii) faulted tephra deposits are consistently thicker on the downdropped hanging-wall than on the uplifted footwall, indicating that the faults formed during the deposition of the tephra; iv) unlike late-stage strike-slip faults, which crop out in many places on the two islands, the normal faults generally occur adjacent to the volcanic craters, implying local control. Repeated phreatomagatic eruptions produced alternating and unstable ash and lapilli deposits on the tephra rings. These layers failed during the eruptions, resulting in the formation of syn-eruptive normal faults in the tephra rings. • Syn-eruptive faults in tephra rings dip steeply and usually form grabens and horsts. • Syn-eruptive faults mainly occur in tephra rings proximal to the eruptive crater. • Syn-eruptive faults may form from the instability of volcanic pyroclastic beds on slopes. [ABSTRACT FROM AUTHOR]

The seismic signatures, recorded during transition from passive degassing to slow growth of a new lava dome in January–February 2016 at Volcán de Colima, gave us the information allowing reconstruct this process. The study is based on seismic data collected by the seismic network of Colima University (RESCO). Processing of seismic signals was performed using the unfiltered records of vertical component of digital broadband station EZ5 situated at a distance of 4 km from the crater. Two types of seismic signals, with explosions followed by pyroclastic flows and incandescent rockfalls (PFIRs) (type 1) and without PFIRs (type 2), define the vent-clearing and a dome growing processes. Totally were selected 154 seismic signals which represented these two types of waveforms. Events of type 1 dominated during January and the first days of February. Events of type 2 began to appear actively beginning from 4 to 6 February. The process, generating Vulcanian explosions and corresponding seismic signals, is considered to follow two-stage conceptual model which consists of the ascent of fragmented magma to the surface (stage 1), and a subsequent explosion (stage 2). Explosive waveforms of the events of both types demonstrated the similarity in the range of seismic energy of the explosive phases and their spectral frequency. Similarity in distributions of durations of the phases, corresponding to the depths of initial ascent of fragmented magma to the surface before explosion, indicated the generation of both types of explosions at the same fragmentation zone within the conduit. Existence of two-phase and three-phase explosive waveforms showed the complex collection of the magma bubbles within fragmentation zone. It was noted a tendency of decreasing of the spectral amplitudes of PFIR signals in time during the period of study, which illustrated the development of the vent-clearing process and defined the period of a birth of new lava dome in the crater of volcano. Initial growth of a new lava dome may be considered within the time interval from 4 to 6 to 24 February, during a break in a vent-clearing process. The full filling of the crater with magma continued for about 8 months. • Seismic signals, produced by explosions during two months of transition from passive degassing to the lava dome growth were analyzed. • Two types of seismic signals, with explosions followed by PFIRs (type 1) and without PFIRs (type 2) were recorded. • Type 1 events were associated with vent-clearing process before the lava dome growth. • Waveforms of both types of events demonstrated the similarity in the range of seismic energy of the main phases and their spectral frequency. • Tendency of decreasing of the spectral amplitudes of PFIR signals of the events of type 1 during the total period of study was noted. [ABSTRACT FROM AUTHOR]

• New thunderstorm antennas allow long-term electrical monitoring of active volcanoes. • Thunderstorm detectors efficiently detect relatively small ash-rich eruptions. • Electrical discharges mark the inception of consecutive explosion pulses. • Electrical activity increases overall with increasing plume height. • Fracto- and tribo-electrification dominate in Vulcanian plumes. Very low frequency and wide-band lightning detection networks can detect major volcanic plumes via their intense electrical and lightning activity. However, the high number of non-detected explosive episodes confirmed by direct observations, reveals the limits of these systems in the detection of the more frequent smaller ash-rich explosive events. Here, we use a data-efficient thunderstorm detector to observe electrical discharges generated from July 2018 to January 2020 by the persistent Vulcanian activity of Minamidake crater at Sakurajima volcano in Japan. Two thunderstorm detectors recorded the electrical activity produced by explosions at Minamidake crater from a distance of 3 and 4 km from the active vents. The instruments measured the induced current due to the change in electric field with time within the extremely low frequency range (1-45 Hz). Using a volcanic lightning detection algorithm together with the catalogue of volcanic explosions compiled by the Japan Meteorological Agency (JMA) and Tokyo Volcanic Ash Advisory Center (Tokyo VAAC), the number of electrical discharges, the electrical discharge rate and the total amount of measured voltage were determined for each individual explosive event. In addition, the start of the electrical discharges was compared to the explosion onset as provided by the JMA (with a one-minute time resolution). The sensors detected electrical discharges in 71% of the 724 recorded explosions. Our detection algorithm successfully recognises the presence/absence of electrical discharges with an accuracy of 73%. We find a non-linear positive correlation between the number of discharges and the plume height. Moreover, we find that the maximum electrical discharge rate and the maximum amount of measured voltage by a single discharge also increase with plume height. Fracto- and tribo-electrification appear to be the dominant plume electrification mechanisms. Even for the few explosive events that exceeded the −10 °C isotherm, the timescale of electrical activity seems to be too short for ice nucleation to make a significant contribution to the plume electrification. Finally, for 12% of the electrically-active explosive events, discharges were detected by the sensors more than a minute before the JMA explosion onset. Our results show the capability of our detectors in pinpointing the inception of electrified explosive episodes in real-time and in providing an indication of the magnitude of each explosion, demonstrating their effectiveness as a cost- and data-efficient instrumentation for the monitoring of explosive ash emissions at active volcanoes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Volcanic ejecta were studied from the 7 March 2012 eruption on Ioto Island, Japan. • Alteration minerals in the volcanic ejecta were identified using Raman spectroscopy. • The alteration minerals were derived from the acidic alteration zone (150–190 °C). • Five reference clay minerals were analyzed using Raman spectroscopy. We used micro-Raman spectroscopy to analyze volcanic ejecta from the 7 March 2012 phreatic eruption at Old Crater (Million Dollar Hole) on Ioto Island, Izu-Bonin arc, Japan, to assess the ability of this technique to identify clay and other alteration minerals in volcanic ejecta. We also analyzed five reference clay minerals (kaolinite, dickite, pyrophyllite, montmorillonite, and saponite) using the same technique. Dickite, montmorillonite, gypsum, pyrite, marcasite, quartz (α-quartz), and anatase were identified by comparison with spectra of the reference materials and published databases. These minerals occur in close spatial association with each other. Marcasite, quartz, and anatase in aliquots of the sample were not observed by a previous powder X-ray diffraction (XRD) study. The presence of dickite, montmorillonite, gypsum, quartz, marcasite, and anatase indicates that these alteration minerals were likely derived from the acidic alteration zone (150–190 °C) below the crater on Ioto Island. Our results demonstrate that Raman microspectroscopy, compared with conventional XRD, is a more sensitive technique for identifying and characterizing alteration minerals present in the volcanic ejecta. However, owing to the strong fluorescence from clay minerals during Raman analysis, the presence of other minerals in the sample may be overlooked. Therefore, the use of the two techniques in combination can be a powerful tool for obtaining a full understanding of mineral phases in volcanic ejecta from phreatic eruptions, thereby providing more accurate information on the hydrothermal conditions below volcanic craters. [ABSTRACT FROM AUTHOR]

• The fine-sized mica in Allophanic Andosols is not originated from volcanic ashes. • The δ18O values of fine-quartz in the Andosols is similar to those of Asian dust. • The quartz content associatively increased with mica as further away from a volcano. • Asian-dust-derived mica is crucial to increase RCs retention ability in Andosols. Deposition of Asian dust, i.e. mica-bearing aeolian particles, throughout geological times has contributed greatly to the increased ability of soil to specifically retain radiocesium (137Cs), particularly in soils derived from mica-deficient parent materials. Allophanic Andosols, volcanic mica-deficient soils, in Japan were hypothesized to contain fewer micaceous minerals proximal to volcanoes due to the dilution of Asian dust by thicker volcanic ash depositions. To test this hypothesis, we collected soils from stratified horizons mainly composed of mafic volcanic ash at four sites near the volcanic crater of Mt. Aso, Japan. The profiles were in volcanic ash soil dominated by poorly ordered clay minerals, and classified as Allophanic Andosols. Radiocesium interception potential (RIP) and mineral content were determined on soil particles fractionated to < 20 µm effective spherical diameter. The RIP for these particles ranged widely from < 0.1 to 4.5 mol kg soil−1, showed a strong-positive correlation with mica and quartz content, and increased with distance away from the crater. Isolated quartz grains, ranging from 2 to 20 µm, were analyzed for oxygen isotopic ratio (δ18O) as an indicator of Asian dust contribution. The average δ18O was 16.0 ± 0.4‰, very close to those of fine quartz in the Gobi Desert, while clearly differentiated from high temperature formation environments of SiO 2 grains from volcanic origins, δ18O < 10‰. Results showed that RIP of Allophanic Andosols is primarily controlled by the proportion of Asian dust incorporated into volcanic ashes, which increased with distance from the main volcanoes. These findings improve predictions of the spatial distribution patterns soil radio cesium (RC) and the soil-to-plant transfer of RCs in Andosols in Japan. [ABSTRACT FROM AUTHOR]

Silica-rich coatings of up to 660 μm thickness were found on the juvenile volcanic lapilli and blocks of the 1783 CE pyroclastic flow deposits 1.9–3.6 km northeast of the Maekake crater of Asama volcano in Central Japan. These coatings are composed of fine-grained volcanic ash particles with silica-rich cement. Raman spectroscopic measurements revealed that the interstitial silica-rich phase is hydrous amorphous silica and is considered to be a reaction product of ash particles with acidic volcanic rain and fog like volcanic smog (vog) as it has been recorded in Hawaii. The formation of amorphous silica without jarosite and the preferential alteration of plagioclase compared with pyroxene indicate that the reaction occurred with a sulfuric acid solution of pH < ~3. The porous structure and large surface area of the ash layer would have been favorable for the dissolution of sintered volcanic ash and the precipitation of amorphous silica. The thickness of the coating and degree of alteration in the plagioclase phenocrysts adjacent to the coating tend to decrease with increasing distance from the volcanic crater, which is consistent with the absence of fumarolic areas in the flank. The Asama-Maekake volcano has repeated Vulcanian and phreatic eruptions since 1783 CE, especially in the first half of the 20th century. The amorphous silica coating recognized in this study is assumed to have formed mainly during this period, which can thus be an indicator of the acidity and distribution of past acidic volcanic rain and fog. • Silica-rich coating on flank deposits of the 1783 CE eruption at Asama volcano • The coating is composed of ash particles and amorphous silica cement • The acidity of the reactant solution was constrained to be pH < ~3 • Sulfuric acidic volcanic rain and fog reached at least 3.5 km from the crater • The coating formed after 1783 CE, possibly in the first half of the 20th century [ABSTRACT FROM AUTHOR]

Measurements of volcanic tephra fallout deposits provide useful information about the magnitude and intensity of explosive volcanic eruptions and potential for remobilization of deposits as dangerous volcanic flows. However, gathering information in the vicinity of erupting craters is extremely dangerous, and moreover, it is often quite difficult to determine deposit thickness proximal to volcanic craters because the thickness of the deposit is too great to easily measure; thus, airborne remote sensing technologies have generally been utilized during the intermission between eruptions. As an alternative tool, a muographic tephra deposit monitoring system was developed in this work. Here we report the performance of this system by applying the muographic data acquired at Sakurajima volcano, Japan as an example. By assuming the average density of the deposit was 2.0 g cm−3, the deposit thicknesses measured with muography were in agreement with the airborne results, indicating that volcanic fallout built up within the upper river basin, showed its potential for monitoring the episodic tephra fallouts even during eruptions. [ABSTRACT FROM AUTHOR]

Sedimentary tuffs comprise important tight oil reservoirs in the Middle Permian Tiaohu Formation in the Santanghu Basin. This paper describes three research advances made on these reservoirs. (1) There are many sets of sedimentary tuff in the second member of the Tiaohu Formation and they are the "sweet section" for tight oil exploration. Vitric, crystal-vitric, and argillaceous tuffs are the main reservoir types, of which the first two are more favorable. (2) A model was established to show how the volcanic edifice controls the type and distribution of the volcanic lacustrine tuff deposition. The crystal-vitric tuff is distributed in the lacustrine deposits nearest to the volcanic crater, and the vitric tuff is distributed on both sides of the active volcanic belt a certain distance from the crater, and this is the most favorable distribution area for sedimentary tuff. Later devitrification resulted in formation of a large number of micro-pores, and this is the main reason for the presence of favorable reservoirs. (3) The sedimentary tuff reservoir has undergone successive geological processes from formation of secondary dissolution pores, then hydrocarbon charge and accumulation, to compaction. The tight oil accumulation is the result of the coupling of the diagenetic evolution of the tuff in the Tiaohu Formation and the hydrocarbon generation and charging from the shale in the Lucaogou Formation. • The most typical and famous sedimentary tuff tight oil reservoir in China. • A model showing how volcanic edifice controlling the type and distribution of the volcanic lacustrine tuff deposition. • The coupling effect of the diagenetic evolution of the tuff and the hydrocarbon generation and charging from the shale. • Vitric tuff and crystal-vitric tuff are two types of favorable reservoirs. [ABSTRACT FROM AUTHOR]

Volcanic tremor is often observed to be associated with an increase in volcanic activity and during periods approaching eruptions. It is therefore of crucial importance to study this phenomenon. The opening of a new vent and subsequent ash-gas emissions was observed in the active crater (Nakadake crater) of Aso volcano, Japan, in January 2014. These events were considered to be associated with phreatomagmatic activity similar to the small events of 2003-2005. During the period from December 2013 to January 2014, a significant variation in the amplitude of continuous seismic tremors was observed corresponding to surficial volcanic activity. We estimated the tremor source locations for this two-month period by a three-dimensional grid search using the tremor amplitude ratio of 5-10 Hz band-pass filtered waveforms. The estimated source locations were distributed in a roughly cylindrical region (100-150 m in diameter) ranging from the ground surface to a depth of 400 m. Migration of the estimated source location was also identified and was associated with changes in volcanic activity. We assumed that the source locations coincided with a conduit system of the volcano, consisting of networks of fractures. This area is likely situated above the crack-like conduit proposed in previous studies. Before the 2014 event, an increase in gas-dominated volcanic fluid first caused an enlargement of the conduit zone, followed by the migration of further magmatic fluid through other pathways, which resulted in a subsequent ash-gas emission. Although we do not have sufficient information to discuss the causal relationship between these processes, it seems reasonable that continuous tremors might change the conduit conditions. [ABSTRACT FROM AUTHOR]