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This paper examines phreatic eruptions which are driven by inputs of magma and magmatic gas. We synthesize data from several significant phreatic systems, including two in Costa Rica (Turrialba and Poás) which are currently highly active and hazardous. We define two endmember types of phreatic eruptions, the first (type 1) in which a deeper hydrothermal system fed by magmatic gases is sealed and produces overpressure sufficient to drive explosive eruptions, and the second (type 2) where magmatic gases are supplied via open-vent degassing to a near-surface hydrothermal system, vaporizing liquid water which drives the phreatic eruptions. The surficial source of type 2 eruptions is characteristic, while the source depth of type 1 eruptions is commonly greater. Hence, type 1 eruptions tend to be more energetic than type 2 eruptions. The first type of eruption we term “phreato-vulcanian”, and the second we term “phreato-surtseyan”. Some systems (e.g., Ruapehu, Poás) can produce both type 1 and type 2 eruptions, and all systems can undergo sealing at various timescales. We examine a number of precursory signals which appear to be important in understanding and forecasting phreatic eruptions; these include very long period events, banded tremor, and gas ratios, in particular H2S/SO2 and CO2/SO2. We propose that if these datasets are carefully integrated during a monitoring program, it may be possible to accurately forecast phreatic eruptions.

The Whakaari/White Island volcano, located ~ 50 km off the east coast of the North Island in New Zealand, has experienced sequences of quiescence, unrest, magmatic and phreatic eruptions over the last decades. For the last 15 years, seismic data have been continuously archived providing potential insight into this frequently active volcano. Here we take advantage of this unusually long time series to retrospectively process the seismic data using ambient noise and tremor-based methodologies. We investigate the time (RSAM) and frequency (Power Spectral Density) evolution of the volcanic tremor, then estimate the changes in the shallow subsurface using the Displacement Seismic Amplitude Ratio (DSAR), relative seismic velocity (dv/v) and decorrelation, and the Luni-Seismic Correlation (LSC). By combining our new set of observations with the long-term evolution of earthquakes, deformation, visual observations and geochemistry, we review the activity of Whakaari/White Island between 2007 and the end of 2018. Our analysis reveals the existence of distinct patterns related to the volcano activity with periods of calm followed by cycles of pressurization and eruptions. We finally put these results in the wider context of forecasting phreatic eruptions using continuous seismic records.

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

The structure and evolution of the magma plumbing system that produced the dacitic Yukiura Pumice (YP) at Iwate volcano, northern Honshu, are constrained by petrography and rhyolite–MELTS simulations. The basaltic Yukiura Scoria (YS) eruption immediately preceded YP, marking a bimodal shift in erupted compositions. For YS, whole-rock MELTS runs that reproduce the observed phenocryst assemblage support hydrous basalt (3–4.25 wt% H 2 O). Plagioclase (An 86–92 ) crystallized during 200–70 MPa decompression/cooling, followed by olivine (Fo 73–75 ) at 90–40 MPa. Plagioclase in YP dacite shows clear, normally zoned, patchy, and oscillatory textures. Correlating domains yields a thermal history: An fell from ~ 90 to ~ 60 with cooling, rose to 65–80 during a resorption event, then declined again; later, local fluctuations persisted within An 45–60 (mostly 50–55). Whole-rock MELTS calculations for YP match modal mineralogy and crystallinity but underpredict plagioclase An, demonstrating disequilibrium between crystals and bulk magma. Accordingly, most plagioclase—and cotectic mafic phases—are interpreted as antecrysts derived from less evolved magma. To reconcile petrography and chemistry, we model MECE (melt extraction with crystal entrainment): residual liquid is extracted from a high-crystallinity mafic mush while entraining crystals as antecrysts. Using groundmass as melt and observed phenocrysts as crystal phases, MELTS runs on a 90% crystalline, high-Fe/Mg basaltic mush (H 2 O > 3.5 wt%) at 350 MPa reproduce the observed phase relations, mineral chemistry, groundmass composition, and REE patterns. The preferred thermal path involves cooling from liquidus to 815 °C, episodic reheating and dehydration near the solidus, and final warming to 880 °C immediately before eruption. We infer that before the YS–YP sequence, basalt ascending beneath East Iwate intruded laterally at shallow depth and built a crystal mush that became the YP source. Injection of YS-type magma reheated this mush and triggered the extraction and eruption of the YP dacite. Graphical Abstract

The active Campi Flegrei caldera in southern Italy has a remarkably long history of coexistence between volcanism and human settlements, and it is famous for its peculiar slow ground movement called bradyseism, i.e. episodes of inflation and deflation of the caldera floor due to magmatic and/or hydrothermal processes. This natural phenomenon has interacted with the civilization that inhabited this strategic and fertile area, especially in Roman times, when the sinking of the coast hindered the flourishment of Puteoli and Baiae coastal towns. The drowning of a large part of Republic-early Imperial Roman coastal buildings, west of the modern Pozzuoli town, is classically used to illustrate the bradyseism activity. In this paper, we investigate the spatial variability and the role of this phenomenon, demonstrating that the caldera deflation alone cannot account for the submersion of Roman facilities in the western sector where the harbour structures of Portus Iulius and luxury villas of the Baianus Lacus presently lie beneath sea level. On the contrary, the sinking of this area is mainly the result of the activity of volcano-tectonic faults. We restored the topography to 100 BCE using archaeological and high-resolution topographic data. Results show that the several metres of vertical displacement recorded in the Baia area in the last 2100 yr were mainly produced by the activity of normal faults and secondarily by caldera deflation, the former including the long-lived Baia Fault and the younger normal faults associated with the Monte Nuovo eruption at 1538 CE.

The textural indices of eruption products (pumice, scoria and lavas), number density of crystals or bubbles, or characteristics of their size distributions, have been proposed to estimate dynamic properties such as magma ascent rates, on the basis of theoretical and experimental studies. To clarify the applicability and limitation of these indices, I review the fundamental mechanism of nucleation and growth of crystals and bubbles during change of temperature and pressure, together with discussion of applications of these methods to case studies. Discrimination between homogeneous nucleation (HON) and heterogeneous nucleation (HEN) are critically important in interpreting rock texture such as microlite number density (MND) and bubble number density (BND) in matrix-textures. In HON cases, traditional rate-meters for matrix-texture are applicable, whereas in HEN cases, these meters should be used carefully, consulting natural observation such as compositional zoning of crystals, laboratory experiments, and geological occurrence. Pheno-textures such as phenocrysts and pheno-vesicles (-bubbles) which can be expected to carry information about the state of deeper processes in magma chamber and magma supply to magma chamber from the mantle source regions are still limited to constrain magma dynamics. As an example of study on phenocrysts, the slope of crystal size distribution as an index of magma supply rate to a shallow magma chamber from deeper region is discussed with a simple model and existing experimental data, which allow us to estimate deeper conduit cross-sectional area and ascent velocity from deeper magma source region in mantle beneath Sakurajima volcano. Pheno-vesicle content as an index for eruption volume is proposed on the basis of recent numerical experiments reproducing cumulative volume evolution (Step-diagram) of the last 500 years at Sakurajima volcano and textural observation. Pheno-bubble number density can also be leveraged to evaluate the decompression rate during caldera-forming eruptions as the index of decompression rate during magma evacuation when caldera-forming eruption occurs. Graphical Abstract

Ocean bottom electromagnetometers (OBEMs) installed on the seafloor around Nishinoshima Island (Bonin Islands) were missing after a December volcanic eruption. In February 2021, one was found on a beach on Iriomote Island (Ryukyu Islands), implying that it drifted westward for 1700 km. The reason(s) for the disappearance of the OBEMs and the path followed by the recovered OBEM while drifting are important information for future ocean bottom observations and seafloor volcanology in general. We conducted particle drifting simulations with and without horizonal eddy diffusion to estimate the possible drift path and duration of the recovered OBEM. Our simulations show that particles arriving at Iriomote Island have a 7–10% probability of having been transported from Nishinoshima; thus, such transport is not a rare occurrence. Transport durations in our simulations varied widely between 140 and 602 days depending on the drift paths. More detailed insight into the path and duration of drift of the OBEM will require further comparison between drifting simulations and growth histories of barnacles attached on the OBEM. A similar drift duration and path was reported for pumices that erupted from Fukutoku-Oka-no-Ba submarine volcano (southern Bonin Islands) during 18–21 January 1986 and arrived in the Ryukyu Islands in late May 1986. Such drifting simulations may prove useful for identifying the sources of drift pumices, and thus otherwise undetectable eruptions. Finally, the Fukutoku-Oka-no-Ba submarine volcano erupted on 13 August 2021, producing abundant pumice rafts that, based on our results, would likely arrive in the Ryukyu Islands. In fact, the beginning of October 2021, they began to arrive in the Ryukyu Islands.

Interactions between the elastic magma chamber and the variable flow resistance of conduits control volcanic eruptions. Recent geodetic observations have revealed complex conduit deformations, especially in inclined dikes, which traditional models have struggled to capture. In this study, we introduce a simplified coupled model that explicitly accounts for magma flow within a nonuniform, elastically deformable inclined dike conduit under the influence of edifice loading. Dimensional analysis reduces the governing equations to an advection–diffusion type equation, controlled by two key dimensionless parameters representing the relative deformability of the conduit and the contribution of gravitational and pressure gradient forces. Numerical simulations demonstrate that these parameters govern the regime of pressure propagation. Specifically, deformation driven by large overpressure can significantly increase effusion rates and induce response delays between different depths. Applying this model to the 2018 Kilauea eruption, we show that the observed response delays and the spatial pattern of deformation (simultaneous expansion and contraction) are well-reproduced in a regime, where advective effects dominate. Our findings provide a refined theoretical framework for understanding volcanic conduit behavior in complex geometries and could explain observed non-uniform deformation phenomena. Graphical abstract

Hakone volcano, located in central Japan, produced a small phreatic eruption in June 2015. Although seismic activity in the Hakone region is generally low, the area experiences episodic earthquake swarms approximately once every few years. In the volcano, many small earthquakes were observed at very shallow depths near Owakudani geothermal area from May to July 2022. In contrast to regular volcano-tectonic earthquakes, these events had no clear P- and S-wave arrivals. In this study, we sought to identify these earthquakes in continuous data and locate their hypocenters. We identified 11,016 earthquakes with similar waveforms between 2014 and 2023 using the matched-filter technique. Many earthquakes occurred in 2015, when the phreatic eruption occurred; however, the shallow seismicity was also active in 2020–2022 at a time when no other volcanic activity including volcano-tectonic earthquakes and crustal deformation occurred. The earthquakes were sometimes triggered by volcanic activity and sometimes occurred ambiently. The hypocenters of the earthquakes were located based on amplitude source location method around the Owakudani geothermal area at depths of − 1 to 0 km below sea level, close to the surface. The hypocenters are located close to the crack that opened around the time of the 2015 phreatic eruption and close to a series of ancient craters inferred from the topography. Given the waveforms, locations, and timing of the earthquakes, we infer that they were caused by the movement of fluid and volcanic gas near the surface. Graphical abstract

Discrete explosive eruptions, including vulcanian eruptions, occur at various stages during the effusion of viscous lava. However, the differences in occurrence time and mechanisms are not yet well understood or categorized. We investigated high-frequency Himawari-8 infrared data and seismic data for the 2018 Shinmoe-dake activity, and compared the results with the 2019 Bezymianny activity. Each stage of the 2018 Shinmoe-dake activity showed characteristic thermal variations: the ash-plume emission stage had no thermal anomalies, the lava effusion stage had continuous large thermal anomalies, and the vulcanian eruption stage had a series of spike-like thermal anomalies. The timing of individual thermal anomaly spikes coincided with the onset of vulcanian eruptions. Each spike exhibited an asymmetric thermal pulse, consisting of an abrupt thermal increase followed by a gradual decrease, corresponding to the deposition of hot ejected materials and subsequent cooling, without any noticeable precursory thermal anomaly. In contrast, the 2019 Bezymianny activity generated a symmetric or two-sided thermal pulse, involving a precursory anomaly; the thermal anomaly increased with lava effusion, peaked with an explosive eruption, and decreased due to cooling. Thus, this study identified two types of explosive eruptions. In the post-effusion type (2018 Shinmoe-dake), the lava effusion continues without explosive eruptions until the cessation of effusion, eventually forming a lava bed with a solid surface covering the vent. This forms a cap rock and is the cause of subsequent vulcanian eruptions. These eruptions show an asymmetric thermal pulse with no precursory anomaly. In the syn-effusion type (2019 Bezymianny), in contrast, the lava effusion activity transitions to an explosive eruption while lava is still effusing. Accordingly, the discrete explosive eruption shows a symmetric thermal pulse involving a precursory thermal anomaly. Their difference can be related to whether the volatile components are efficiently released from the ascending magma during the lava effusion. Graphical Abstract