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This book examines notable volcanic eruptions in history, explains why volcanoes erupt, and shows how scientists are working to understand and predict eruptions.

Anomalous earthquakes and crustal movements have been observed at Iwate volcano in northern Honshu, Japan, where phreatic eruptions have been feared since February 2024, and the volcanic alert level was raised from 1 to 2 on October 2, 2024. We collected fumarolic gases at two locations in Ojigokudani at Iwate volcano on September 5, 2024, and analyzed their chemical composition and hydrogen and oxygen stable isotope ratios (δD and δ 18 O) to assess the magmatic contribution to the hydrothermal system, which is useful information for evaluating the activity of volcanoes that produce phreatic eruptions. The characteristics of δD and δ 18 O indicate that the fumarolic gases are a mixture of local meteoric water and magmatic gas, with the meteoric water component being predominant. The fumarolic gases were poor in HCl, and the SO 2 /H 2 S ratio, which is high in high-temperature gases, were low at 0.01 and 0.02. The apparent equilibrium temperatures, which is useful for estimating the temperature of the gas source, were estimated to be 212 and 227 °C. These geochemical properties of fumarolic gases do not indicate that the contribution of magmatic gas was as intense as that during the 1998 unrest when the magmatic intrusion was suspected, but a failed eruption occurred. Graphical Abstract

\"Find out how scientists measure volcanic activity by following along with this exciting story.\"-- Provided by publisher.

The construction of ocean island basaltic volcanoes consists of a succession of eruptions, intrusions, and metamorphism. These events are often temporally ill‐constrained because the most widely used radiometric dating methods applicable to mafic volcanic rocks (K‐Ar or 40Ar/39Ar on whole rock or groundmass) are prone to inaccuracy when applied to slowly cooled, altered, or vesicular and aphyric products. Here, we adopt a multitechnique geochronology approach (including zircon U‐Pb, phlogopite 40Ar/39Ar, zircon and apatite (U‐Th)/He, and zircon double‐dating) to demonstrate its efficacy when applied to basaltic volcanoes. Taking the main volcano of Réunion Island (Piton des Neiges) as a case study, we establish the time of the major plutonic, metamorphic, and explosive events that had resisted previous dating attempts. We document four stages of pluton emplacement and metamorphism at 2,200–2,000 ka, 1,414 ± 8 ka, 665 ± 78 ka, and 150–110 ka, all coinciding with volcanism revival after quiescent intervals. We also date a major Plinian eruption at 188.2 ± 10.4 ka, coeval with the formation age of a large caldera, and, finally, we constrain the last eruption of Piton des Neiges to 27 ka, revising a previous estimate of 12 ka. By resolving several conundrums of Réunion's geological history, our multitechnique geochronology approach reveals that endogenous growth of a volcanic island proceeds as pulses at the beginning of renewed volcanism. We also demonstrate that crosschecking eruptions ages by diversified dating techniques is important to better assess the timing and recurrence of basaltic volcanic activity, with implications for hazard prediction. Plain Language Summary Dating techniques based on natural radioactivity span multiple isotopes and minerals. However, geochronology of volcanic islands remains challenging due to the extremely low radioactivity of erupted products—mostly basalts (including their mineral cargo). Consequently, many events in the geological history of these islands, like explosive eruptions or magma intrusions, remain challenging to date. To resolve this issue, we test the efficacy of six dating methods, unusually applied to a basaltic volcanic island and never in combination. Taking Piton des Neiges volcano (Réunion Island) as a natural laboratory, we demonstrate that combining these techniques is not only feasible, but also productive in terms of the recovered geological information. We obtain ages for many rocks of Réunion that resisted other techniques, or whose age has previously remained controversial. In particular, we date a major explosion at 188,000 years, and constrain the last eruptive activity of Piton des Neiges to 27,000 years, revising a previous estimate of 12,000 years. The new ages also show that significant volumes of magma must have remained stored inside the edifice at the beginning of active periods. Our combined approach thus offers a promising solution to reconstruct the history of volcanic islands and better predict their hazard. Key Points First application of zircon‐phlogopite‐apatite geochronology to a hotspot island Endogenous growth proceeds as pulses at the beginning of renewed volcanism Last volcanism of Piton des Neiges is 27 ka, not 12 ka as earlier proposed

\"This book is an update to a title published in 2007. Mount St. Helens is constantly erupting. It is pushing up a ridge of thick lava that is rebuilding the peak of the mountain that was blown off in 1980. The mountain is being monitored by geologists and volcanologists, all trying to answer the same question: Will it blow? Science is like detective work, and author Elizabeth Rusch presents the work of volcanology in a series of cases that need to be cracked, with Mount St. Helens as the central culprit, a master disguises, adept at sending out false clues. But through an understanding of earthquakes, gases that come from underground, infrared measurement of the earth's temperature, bumps and deformations on the surface of the earth, and kind of rock that is being formed in the crater, readers become volcano detectives. With sidebars about the latest gadgets and gizmos employed at the mountain and activities kids can enact, young people will learn the current science of volcanology and have fun at the same time.\"-- Provided by publisher.

Developing techniques to monitor volcanic activity from safe distances is crucial for advancing scientific knowledge while protecting the safety of field personnel. One of the most demanding tasks in this context is the measurement of soil gas emissions, which offer valuable insights into fluid migration through the shallow crust and act as an early indicator of volcanic unrest and potential eruptive activity. Traditional soil degassing measurements commonly require two operators to be physically present with the instrument, sometimes exposing them to hazardous conditions. In this study, we present a new method for performing soil degassing measurements from a safe distance, using a customized Remotely Piloted Aircraft System (RPAS). This drone-based approach was designed to carry out accumulation chamber measurements in hazardous or otherwise inaccessible areas. We tested the system at four locations around the active crater of Poás Volcano in Costa Rica, where we collected data on CO 2 and H 2 O fluxes, along with soil temperature and moisture. Our results reveal spatial variability in gas emissions and surface conditions across the study sites. A site located on the crater rim (Site 1) showed the highest CO 2 and H 2 O fluxes, indicating active gas release possibly associated with structural features. A second site, located within the crater (Site 2), exhibited elevated H 2 O flux without detectable CO 2 , suggesting localized processes related to moisture transport. Our experiment on another crater site (Site 3) produced a complete and high-quality dataset, demonstrating the operational success of the method. In contrast, measurements at the last crater site (Site 4) were affected by chamber sealing issues and potentially by the influence of volcanic gas plumes. While the experiment faced several challenges, including imperfect ground-sensor contact as well as occasional telemetry interruptions, it successfully demonstrated the feasibility of using drones for soil degassing surveys. Based on these findings, we identify specific areas for improvement and propose future directions to enhance the system reliability and performance. Overall, this method offers a promising tool for extending soil gas measurements to hazardous or hard-to-reach environments, contributing to safer and more comprehensive monitoring of active volcanic systems. Graphical Abstract

In volcanic regions, active earthquake swarms often occur in association with volcanic activity, and their rapid detection and analysis are crucial for volcano disaster prevention. Currently, these processes are ultimately left to human judgment and require significant time and money, making detailed real-time verification impossible. To overcome this issue, we attempted to apply machine learning, which has been successfully applied to various seismological fields to date. For seismic phase pick, several models have already been trained using a large amount of training data (mainly crustal earthquakes). Although there are some cases in which these models can be applied without any problems, regional dependence on pre-trained models has been reported. Since this study targets earthquakes in a volcanic region, applying existing pre-trained models may be difficult. Therefore, in this study, we compared three models; the publicly available trained model (model 0), a model which was trained with approximately 220,000 P- and S-wave onset reading data recorded at the Hakone volcano from 1999 to 2020 with initialized parameters (model 1) using the same architecture, and a model fine-tuned with the aforementioned Hakone data using the parameters of model 0 as initial values (model 2), and evaluated their phase identification performance for the Hakone data. As a result, the seismic phase detection rates of models 1 and 2 were much higher than those of model 0. However, small-amplitude signals are often missed when multiple seismic events occur within a detection time window. Therefore, we created training data with two earthquakes in the same time window, retrained the model using the data, and successfully detected events that previously would have been missed. In addition, it was found that more events were detected by setting the threshold to a low probability value for detection, increasing the number of seismic phase detections, and filtering by phase association and hypocenter location.

At Tokachidake volcano, Japan, inflation of the crater area was observed alongside a decrease in fumarolic plume height during 2006–2017, which was attributed to conduit sealing. The accumulation of excess pressure owing to continuous sealing can lead to phreatic eruptions and edifice collapses. To reveal the sealing process, this study investigates the characteristics of hydrothermal alterations in the volcanic conduit by analyzing altered materials from the edifice interior and associated changes in porosity and permeability. The weakly altered rock retained some of its primary minerals and can be interpreted as having formed in an environment, where the supply of volcanic gases and hydrothermal fluids was limited. In contrast, the conduit-filling lava and scoria fragments were intensely leached and cemented by fine alunite and amorphous silica, resulting in consolidated aggregates (strongly altered rock). Porosity and permeability tended to increase as hydrothermal alteration advanced. However, the strongly altered rocks were originally unconsolidated aggregates of lava and scoria fragments. Our data suggest that the consolidation associated with hydrothermal alterations reduced the permeability of the conduit by three to four orders of magnitude, leading to conduit sealing. This decrease in permeability would create excess pore pressure within the conduit, with our estimates ranging from <1 to 13.5 MPa. Such conduit sealing is likely to occur repeatedly in the future. Therefore, to elucidate the present state of the volcano and support hazard mitigation, it is essential to correctly interpret the phenomena occurring beneath the crater area based on our results. Graphical Abstract

Eruption models developed using the waveform inversion method for explosion earthquakes differ across studies, and a unified model has not been established. Here, Vulcanian eruption processes of the Sakurajima volcano, Japan, were clarified using seismic waveform inversion analysis in the long- and very-long period bands for two specific events: one with a clear preceding infrasound phase (July 17, 2022) and the other with almost no preceding infrasound phase (July 24, 2022). The waveform inversion analysis used the layered velocity structure obtained from artificial seismic experiments and the detailed topography of the Sakurajima edifice. In both events, the single-force component, in addition to the moment tensor, was a necessary parameter for explaining the eruption process. The source location of the explosion earthquake on July 17 was 480 m below sea level (1140 m below the crater), while that of the explosion earthquake on July 24 was 440 m above sea level (220 m below the crater). In the former event, the magma reached supersaturation at depth by the time of the explosion. Magma ascent and bubble growth appeared as the expansion and contraction of the crack in the volcanic conduit beneath the crater. In addition, as the magma rose, vertical drag and reaction forces were exerted on the conduit. In the latter event, magma supersaturation remained shallow within the conduit. Following rapid magma ejection from the shallow part of the conduit and the expulsion of volcanic bombs, the vesiculation level of magma in the conduit decreased, resulting in gradual ash ejection. The preceding phase of infrasound was seen in the July 17 event but not in the July 24 event; the presence or absence of the preceding phase did not seem to determine the strength of the lava plug since there were six events between the two events, and the rate of SO 2 emission during that period showed an increasing trend. Overall, these findings highlight the necessity of considering the single-force component and the moment tensor to explain the eruption process. This study also provides insights into the source locations of explosion earthquakes, as well as the dynamics of magma ascent and bubble growth. Graphical Abstract

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions-where, when, how big, how long, and the consequences. Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.