Journal of Volcanology and Geothermal Research最新文献

Magma migrations frequently trigger seismic swarms, resulting in seismic events that overlap in time and hinder real-time phase picking commonly used for hypocenter location. Addressing this challenge, seismic amplitude ratio analysis (SARA) allows identification of seismic migrations in real-time by simply tracking the relative seismic amplitude between a pair of seismic stations.

This paper aims to identify key statistical features of the seismic network array locations that improve their ability to detect seismic migrations using SARA. We evaluated the capability to detect the most frequently oriented magma migrations in over 100 volcanoes, using a criterion previously proposed to study vertical magma migrations in Piton de la Fournaise. Additionally, we investigate the influence of vent-station proximity on magma conduit coverage and identify the distance ratio that yields improved detection.

Furthermore, we estimate the seismic network efficiency by calculating the detection capability volume per station. We then use the random forest regression algorithm to identify which statistical features of the seismic network location contribute more to the efficiency disparity among different volcanoes. Notably, our findings reveal that optimizing seismic network coverage entails maximizing the standard deviation of relative pair station distances, while maintaining a prescribed minimum separation distance between station pairs. Our results reveal important criteria that can be used to optimize seismic network location design.

This research presents an extensive inventory of 103 small-volume monogenetic phreatomagmatic volcanoes (PVs) along the Trans-Mexican Volcanic Belt (TMVB), aiming to evaluate the influence of the external environmental parameters in phreatomagmatic volcanism. The formation of PVs (maar-diatremes, tuff rings, and tuff cones) is facilitated by the interaction of small volumes of magma and available water, conditions supported by frequent small-volume distributed volcanism and inter-montane lacustrine basins in the TMVB, a Plio-Quaternary continental volcanic arc with over 3000 monogenetic volcanic structures, of which only about 3% are PVs. The inventory was analyzed, dividing the structures into two groups based on their surface morphology: maar-diatremes and tuff rings (MD-TR, 81%), and tuff cones (TC, 19%). Morphometric correlations allow differentiation between these groups, although there is an overlap that could be caused by the presence of magmatic eruptive phases in some PVs. The type of aquifer host is the only environmental parameter with some discernible influence on the size of PVs.

Most of the PVs are clustered in three specific areas: Valle de Santiago, Serdán-Oriental, and Los Tuxtlas. The PV clusters highlight the combinations of environmental parameters under which phreatomagmatism is most successful in terms of frequency and size. Less frequent sets of parameters are reflected in the scattered PVs. The magmatic flux, presumably low, is considered the first-degree influence on the conditions for a phreatomagmatic eruption, provided that there is water availability. This availability is determined by the local climate as second-degree influence and by the local hydrogeological configuration as third-degree. The hydrogeological configuration parameters involve the aquifer host, permeability, spatial distribution and hydraulic gradient. If these conditions, enhanced by a humid climate, facilitate the development of an extensive aquifer in an area of small-volume volcanism, it is more likely that a PV cluster will form. This inventory serves as a foundation for future research on phreatomagmatic volcanism in the TMVB, emphasizing the need for multidisciplinary studies to fill the existing gaps in knowledge regarding internal parameters and the interaction between magmatic and environmental factors.

Understanding the detailed eruptive history and conditions during the buildup to catastrophic caldera-forming eruptions is essential for understanding the evolution of caldera volcanoes and predicting eruptive hazards. Towada Volcano is an active caldera volcano in the northern part of the Northeast Japan Arc. At least two catastrophic caldera-forming eruptions (eruptive episodes N and L) occurred during its caldera-forming stage (61–15.7 ka). A detailed geological survey identified three small vulcanian tephra layers that were erupted during the caldera-forming stage. The tephra layers are blue–gray ash fall deposits that consist mainly of fresh blocky dacite–rhyolite fragments. In ascending order, these deposits are assigned to eruptive episodes O′, N′, and M′. Each eruption had a volume of <0.11 km³, which is much smaller than that of other eruptions during the caldera-forming stage. The eruptive ages of episodes O′, N′, and M′ are estimated to be ca. 40, 23.1, and 17.2 ka, respectively, based on new ¹⁴C ages and paleosol thicknesses data. At least three cycles of a medium- to large-scale (3–20 km³) explosive eruption, preceded by a small vulcanian eruption (<0.11 km³), occurred during the latter half of the caldera-forming stage. The small vulcanian eruptions (episodes O′, N′, and M′) occurred after relatively long periods of quiescence (4000–14,000 years) and were followed by medium- to large-scale explosive eruptions (episodes N, M, and L) 1500–4000 years later. This cyclic activity probably reflects changes in overpressure in the magma reservoir. As the overpressure increased, episodes O′, N′, and M′ probably occurred when the overpressure had not increased sufficiently to trigger a medium- to large-scale explosive eruption, and episodes N, M, and L occurred at higher overpressures. These cycles characterize the fermentation phase of Towada Volcano, and terminated with the formation of the Towada Caldera during episode L at 15.7 ka. The magma composition and eruption frequency changed abruptly after episode L, and a small stratovolcano was formed through intermittent eruptions of mafic magma. This change suggests that the caldera collapse during episode L changed the entire shallow magmatic system that existed during the fermentation phase, and the system shifted to a recovery (post-caldera) phase. Based on eruptive volumes and frequencies, the present Towada Volcano has not yet reached the conditions that existed during the late caldera-forming stage and is therefore unlikely to produce a catastrophic caldera-forming eruption in the near future.

Strain accommodation in the Main Ethiopian Rift has been localized since the Quaternary in axial magmatic segments that contain magma intrusion, volcanic complexes, and fault zones. However, the crustal structure and magmatic plumbing features of the individual volcanic complexes within these magmatic segments are poorly constrained. In this study, gravity data from the Global Gravity Model plus2013 was used to interpret the crustal structure and subsurface volcanic network at and near the Boku Volcanic Complex (Boku VC). Two-dimensional gravity models and an upward continuation map analysis of the upper crust reveal a gravity maximum that is interpreted as mafic intrusion at depths between 5 and 10 km beneath the Boku VC. A circular gravity maximum on the upward continued and residual gravity anomaly maps over the Boku VC and adjacent segments suggest the shallow plumbing systems beneath the segments are discrete, but that they merge into the deeper crust. The gravity models suggest that below 5 km beneath the center of magmatic segments nearly all the extension over the last 2 My can be accounted for by magmatic intrusion. Our models require faults in the uppermost crust which likely contribute to extension and may serve locally as conduits for the conveying melts or hydrothermal fluids. Our gravity analysis supports petrological studies that indicate a two-level magmatic plumbing system beneath the Wonji fault belts in which a melt supply from the upper mantle moves to mid-crust and then to shallow upper crust where the magma fractionates into more siliceous magma within smaller magma chambers.

“Phreatoplinian” is an explosive phreatomagmatic eruption style that is defined by the fragmentation of magma and widespread dispersal of the resulting fine ash and accretionary lapilli. These eruptions pose significant future risks at caldera volcanoes that host lakes and abundant groundwater. There have been no direct observations of a phreatoplinian eruption, therefore, constraining the detailed mechanisms and sequences of such events relies on studying the deposits of previous eruptions. In order to advance our understanding of these hazardous phenomena we conducted a case study of the 40 ka caldera-forming eruption (Kp I) from Kutcharo volcano in eastern Hokkaido, Japan. We subdivided Kp I eruption deposits into 7 units (Units 1 to 7 in ascending order). Units 1 to 6 are air fall deposits consisting of alternating thin pumice and thick silty ash layers with abundant spherical accretionary lapilli. Stratigraphically higher ash fall units are thicker, finer in grain-size, and more widely distributed. The maximum eruption column height and mass-discharge rate were calculated to be 40 km and 1.4 × 10⁹ kg/s, respectively. Unit 7 is a climactic ignimbrite (76 km³), which is distributed widely over the area north of Kutcharo caldera.

Unit 6 is the largest air fall unit and can be considered to have been deposited by a phreatoplinian eruption, given its abundant accretionary lapilli, wide dispersion, and high degree of fragmentation. Unit 6 had the highest mass discharge rate (1.4 × 10⁹ kg/s), suggesting the interaction between magma and external water was most intense, and it is thought that a large eruption column covered eastern Hokkaido. In addition, Kp I eruption deposits commonly contain glass shards derived from fragmentation via both magma degassing and Molten Fuel Coolant Interaction (MFCI). To account for this observation, we infer that the conduit penetrated a large aquifer, and the margin of the ascending magma came into contact with this external water source. Due to repeated caldera-forming eruptions, intra-caldera filled deposits (hosting a large aquifer) likely played a key role in supplying external caldera lake water to a level near the fragmentation depth of H₂O-saturated felsic magma. The occurrence of these intra-caldera conduit and caldera-lake systems may provide the required conditions for phreatoplinian eruptions at continental arc caldera volcanoes in Japan and globally.

Astroni volcano in the Campi Flegrei caldera (southern Italy) is a 2 km wide, densely vegetated tuff ring and hosting several volcanic structures, including tuff cones, scoriae cones, lava domes, and three small lakes. Geochemical data of waters and dissolved gases from the lakes, coupled with microbiological analyses on lake water and sediments, were used to shed light on the possible relationship between the lakes and the hydrothermal fluid circulation system as suggested by previous geophysical surveys. Water chemistry was dominated by solutes, mainly Na⁺ and HCO₃⁻, deriving from fluids and CO₂-rich gases typically found in discharges located at the periphery of hydrothermal-volcanic systems. Lago Grande (LG) lake showed an anoxic hypolimnion with abundant non-atmospheric dissolved gases, consisting of biogenic CH₄ and CO₂, the latter having a twofold origin, biogenic and hydrothermal. The occurrence of anaerobic methanotrophs coupled with the lack of hydrogenotrophic methanogenic archaea along the whole vertical profile of LG suggested that CH₄ was mostly produced from degradation of abundant terrestrial organic matter within the deep lake sediments, and then consumed during its diffusion through the lake. Notwithstanding, the output rate of CH₄ from LG surface was anomalously high relative to those commonly measured in lakes. Carbon dioxide from the hydrothermal source and produced by CH₄ oxidation was partially fixed in the lake via the acetyl-CoA pathway. Accordingly, the CO₂ fluxes from the LG surface were relatively low, in the range of those measured in volcanic lakes dominated by biogenic CO₂. The dependence of the chemistry of the Astroni lakes on inputs from the Campi Flegrei hydrothermal system, besides on biogeochemical processes, offers a possible explanation for the anomalous increase of the LG water level occurred in the last years, which was not consistent with the recorded local rainfall but likely caused by an increasing hydraulic pressure related to the enhanced hydrothermal activity recorded at Campi Flegrei in the last decades. According to this hypothesis, the future evolution of the current volcanic unrest may govern the fate of the lake water level with important implications for the functioning of the precious Astroni ecosystem.

Phreatomagmatic eruptions in basaltic monogenetic volcanic fields are strongly influenced by their geological and environmental settings. Barriball Road volcano exemplifies the eruption processes associated with South Auckland Volcanic Field (SAVF), New Zealand. Stratigraphy and petrography reveal the complex eruptive history of this small-volume phreatomagmatic volcano. An initial phreatomagmatic phase formed two overlapping tuff rings from successive vents, and excavated lithics from a shell-rich Pliocene age aquifer (∼170 m depth). The first tuff ring was constructed mainly through pyroclastic fall and the second is dominated by pyroclastic surge (dilute pyroclastic density current) deposits. Transition to a terminal magmatic phase produced a nested scoria cone. Vent migration between the eruption of the tuff rings may have been induced by collapse of the soft substrate, restricting water and magma supply to the first vent. Regional block faulting is inferred to have strongly influenced magma ascent and vent alignment, as seen at many SAVF and other monogenetic field volcanoes.

Many volcanoes show continuous but variable deformation over timescales of years to decades. Variations in uplift rate are typically interpreted as changes in magma supply rate and/or a viscoelastic response of the host rock. Here we conduct analogue experiments in the laboratory to represent the inflation of a silicic magma body at a constant volumetric flux, and measure the chamber pressure and resulting surface displacement field. We observe that dyke intrusions radiating from the magma body cause a decrease in the peak uplift rate, but do not significantly affect the spatial pattern of deformation or spatially averaged uplift rate. We identify 4 distinct phases: 1) elastic inflation of the chamber, 2) a gradual decrease in the rate of uplift and pressurisation, associated with the formation of visible cracks 3) propagation of a dyke by mode 1 failure at the crack tip and 4) a pressure decrease within the chamber. Phase 2 can be explained by either a) crack damage, which reduces the elastic moduli of the surrounding rock or b) magma filling pre-existing cracks. Thus these experiments provide alternative mechanisms to explain observed variations in uplift rate, with important implications for the interpretation of deformation patterns at volcanoes around the world.

Stromboli (Italy) is an open-vent volcano with persistent explosive activity producing up to five hundred mild explosions per day. Fluctuations in explosion intensity, varying even by orders of magnitude in terms of emitted volume and their subsequent impact on the surrounding regions, sometimes occur abruptly. Consequently, identifying precursors of larger eruptive activities, particularly for more intense (paroxysmal) explosions, is challenging. In order to search for anomalies in the pre-paroxysm activity related to the summer 2019 eruption, we applied a hybrid method to the automatic analysis of geophysical and geochemical time series. This approach is based on the combination of two methods: 1. the Empirical Mode Decomposition (EMD) and 2. the Support Vector Regression (SVR). The aggregation of these two methods allowed us to identify anomalies in the patterns of the geophysical and geochemical parameters measured on Stromboli in a ten-month period including the July–August 2019 eruption. The results of this study are encouraging for an improvement of the monitoring systems and for volcano early warning applications.

Bathymetry and acoustic imagery swath mapping, along with observations and samples from four manned submersible and four ROV dives, confirm that a seafloor slope break on the northern approaches to Kaiwi Channel, between the islands of Oʻahu and Molokaʻi, Hawaiʻi is a former shoreline, now submerged ∼800 m below present sea level. Subaerially emplaced, low-relief basaltic lavas above the slope break transition to submarine morphologies below. The entire region has been tilted about 1° to the SSE (150°), and is cut by an 8–15 m-high, north-facing scarp, 100–400 m south of the slope break. The distribution of platy, table-top, and rarer mounded branching corals indicates the former presence of fringing reefs around low-relief paleo-islands. We infer that the regional tilt resulted from loading by younger Hawaiian volcanoes, compounded by flexural uplift and back tilting away from the unloaded footwall of a flank landslide to the north.

Basalt samples collected from both above and below the slope break have petrography, chemical composition, and age (1.64–1.80 Ma) indicating correlation with the (late-shield) Lower Member of the East Molokaʻi Volcanics, rather than with the more proximal volcano of West Molokaʻi. The most likely source of the Kaiwi basalts is a submarine ridge (rift zone) that extends northwest away from ʻĪlio Point on West Molokaʻi. Although the submarine ridge was previously assumed to be an extension of West Molokaʻi's northwest rift, we conclude that regional bathymetry and gravity are consistent with this feature being an extension of the west rift of East Molokaʻi. A corallary of this interpretation is that the shoreline slope break (SSB 7 of Taylor, 2019) in this area is distinct from and younger than the southern SSB 7 formed on West Molokaʻi volcano (∼1.65 Ma vs. ∼1.8 Ma).