Abstract Kirishima volcano consists of more than 20 eruptive centers. Among them, Shinmoe-dake had magmatic eruptions in October 2017 and March 2018. Subsequently, another active cone, Iwo-yama, had phreatic eruptions in April 2018. These events were unique in that the 2018 eruption was the first effusion-dominated eruption of Shinmoe-dake and the first simultaneous activity of two cones of the Kirishima volcanic group ever documented. We report the detailed sequence of the events by combining areal photos, satellite images, and seismo-acoustic data analyses with the other published information. The seismo-acoustic data clarify the eruption onset and the transitions of the behaviors in three stages for each of the 2017 and 2018 eruptions. For both eruptions, we present regularly repeated tremors or ’drumbeat’ earthquakes in the second stage, which interpret as gas separation from magma, leading to the ash-poor plume in the 2017 eruption or the effusive eruption in the 2018 event. We also propose that the 2017 and 2018 eruptions of Shinmoe-dake and the 2018 eruption of Iwo-yama are sequential events linked by the degassing of magma beneath Shinmoe-dake. An eruption like the 2017–2018 eruptions of Shinmoe-dake would leave few geological records and could be captured only by modern techniques. Although Shinmoe-dake has been believed to be an example of less-frequent eruptions, effusive eruptions like the 2018 case might have occurred more frequently in the past , but the following eruptions had obscured their records. The timelines summarized in this study will be useful in future studies of Kirishima volcanoes and world equivalences. Graphical Abstract
The island-forming Nishinoshima eruptions in the Ogasawara Islands, Japan, provide a rare opportunity to examine how the terrestrial part of Earth’s surface increases via volcanism. Here, the sequence of recent eruptive activity of Nishinoshima is described based on long-term geological and geochemical monitoring of eruptive products. Processes of island growth and temporal changes in the magma chemistry are discussed. The growth of Nishinoshima was sustained by the effusion of low-viscosity andesite lava flows since 2013. The lava flows spread radially with numerous branches, resulting in compound lava flows. Lava flows form the coherent base of the new volcanic edifice; however, pyroclastic eruptions further developed the subaerial volcanic edifice. The duration of three consecutive eruptive episodes decreased from 2 years to a week through the entire eruptive sequence, with a decreasing eruptive volume and discharge rate through time. However, the latest, fourth episode was the most intense and largest, with a magma discharge rate on the order of 10 6 m 3 /day. The temporal change in the chemical composition of the magma indicates that more mafic magma was involved in the later episodes. The initial andesite magma with ∼60 wt% SiO 2 changed to basaltic andesite magma with ∼55 wt% SiO 2 , including olivine phenocryst, during the last episode. The eruptive behavior and geochemical characteristics suggest that the 2013–2020 Nishinoshima eruption was fueled by magma resulting from the mixing of silicic and mafic components in a shallow reservoir and by magma episodically supplied from deeper reservoirs. The lava effusion and the occasional explosive eruptions, sustained by the discharge of magma caused by the interactions of these multiple magma reservoirs at different depths, contributed to the formation and growth of the new Nishinoshima volcanic island since 2013. Comparisons with several examples of island-forming eruptions in shallow seas indicate that a long-lasting voluminous lava effusion with a discharge rate on the order of at least 10 4 m 3 /day (annual average) to 10 5 m 3 /day (monthly average) is required for the formation and growth of a new volcanic island with a diameter on km-scale that can survive sea-wave erosion over the years.
Abstract Phreatic and phreatomagmatic eruptions represent some of the greatest hazards occurring on volcanoes. They result from complex interactions at a depth between rock, water, and magmatic fluids. Understanding and assessing such processes remain a challenging task, notably because a large‐scale characterization of volcanic edifices is often lacking. Here we focused on Miyakejima Island, an inhabited 8‐km‐wide stratovolcano with regular phreatomagmatic activity. We imaged its plumbing system through a combination of four geophysical techniques: magnetotellurics, seismicity, self‐potential, and thermal image. We thus propose the first comprehensive interpretation of the volcanic island in terms of rock properties, temperature, fluid content, and fluid flow. We identify a shallow aquifer lying above a clay cap (<1 km depth) and reveal its relation with magmatic‐tectonic features and past eruptive activity. At greater depths (2–4.5 km), we infer a seismogenic resistive region interpreted as a magmatic gas‐rich reservoir (≥370°C). From this reservoir, gases rise through a fractured conduit before being released in the fumarolic area at ∼180°C. During their ascent, these hot fluids cross a ∼1.2‐km‐long liquid‐dominated zone causing local steam explosions. Such magmatic‐hydrothermal interaction elucidates (i) the origin of the long‐period seismic events and (ii) the mixing mechanism between magmatic and hydrothermal fluids, which was previously observed in the geochemical signature of fumaroles. Our results demonstrate that combining multidisciplinary large‐scale methods is a relevant approach to better understand volcanic systems, with implications for monitoring strategies.
Island arc low-K tholeiites are basaltic magmas erupting from frontal arc volcanoes of juvenile arcs associated with the subduction of old and cold plates. We investigated the origins of geochemical variation in volcanic rocks having multiple phase saturated liquid compositions from the Izu-Oshima volcano in the northern Izu arc. The geochemical variations in the liquids fall between two endmember trends, namely higher- and lower-Al/Si trends. Polybaric differentiation of H2O-saturated melts between a 4-km-deep magma chamber and degassed melts near the surface should be responsible for the observed variation in the liquids.
After publication of this article (Kaneko et al. 2018), it is noticed there is an error in Fig. 3b; the lower 2.3-µm band image is mistakenly removed. The correct Fig. 3 is given below.
The sudden eruption of Mount Ontake on September 27, 2014, led to a tragedy that caused more than 60 fatalities including missing persons. In order to mitigate the potential risks posed by similar volcano-related disasters, it is vital to have a clear understanding of the activity status and progression of eruptions. Because the erupted material was largely disturbed while access was strictly prohibited for a month, we analyzed the aerial photographs taken on September 28. The results showed that there were three large vents in the bottom of the Jigokudani valley on September 28. The vent in the center was considered to have been the main vent involved in the eruption, and the vents on either side were considered to have been formed by non-explosive processes. The pyroclastic flows extended approximately 2.5 km along the valley at an average speed of 32 km/h. The absence of burned or fallen trees in this area indicated that the temperatures and destructive forces associated with the pyroclastic flow were both low. The distribution of ballistics was categorized into four zones based on the number of impact craters per unit area, and the furthest impact crater was located 950 m from the vents. Based on ballistic models, the maximum initial velocity of the ejecta was estimated to be 111 m/s. Just after the beginning of the eruption, very few ballistic ejecta had arrived at the summit, even though the eruption plume had risen above the summit, which suggested that a large amount of ballistic ejecta was expelled from the volcano several tens-of-seconds after the beginning of the eruption. This initial period was characterized by the escape of a vapor phase from the vents, which then caused the explosive eruption phase that generated large amounts of ballistic ejecta via sudden decompression of a hydrothermal reservoir.
AbstractUnmanned aerial vehicles (UAVs) have recently received attention in various research fields for their ability to perform measurements, surveillance, and operations in hazardous areas. Our application is volcano surveillance, in which we used an unmanned autonomous helicopter to conduct a dense low-altitude aeromagnetic survey over Tarumae Volcano, northern Japan.In autonomous flight, we demonstrated positioning control with an accuracy of ~10 m, which would be difficult for an ordinary crewed vehicle. In contrast to ground-based magnetic measurement, which is highly susceptible to local anomalies, the field gradient in the air with a terrain clearance of 100 to 300 m was fairly small at 1 nT/m. This result suggests that detection of temporal changes of an order of 10 nT may be feasible through a direct comparison of magnetic data between separate surveys by means of such a system, rather than that obtained by upward continuation to a common reduction surface. We assessed the temporal magnetic changes in the air, assuming the same remagnetising source within the volcano that was recently determined through ground surveys. We conclude that these expected temporal changes would reach a detection level in several years through a future survey in the air with the same autonomous vehicle.Key words:: aeromagnetic surveygeomagnetismTarumae Volcanounmanned autonomous helicopter AcknowledgementsWe sincerely thank Muroran Development and Construction Department, HRDB, for cooperation in the field experiments by offering the use of their unmanned helicopter system. We are grateful to Yamaha Motor Co., Ltd, for their technical support in the field operation. We used the 10 m mesh digital elevation map published by Geospatial Information Authority of Japan, for the inversion of magnetic anomalies. Special thanks are extended to Satoshi Okuyama of Hokkaido University for his effort in pre-processing the DEM data. This study was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions. Comments and suggestions of Dr Mark Dransfield, two anonymous reviewers and Dr Mark Lackie, the Associate Editor, contributed to improve the manuscript.
Abstract Kirishima volcano consists of more than 20 eruptive centers. Among them, Shinmoe-dake had magmatic eruptions in October 2017 and March 2018. Subsequently, another active cone, Iwo-yama, had phreatic eruptions in April 2018. These events were unique in that the 2018 eruption was the first effusion-dominated eruption of Shinmoe-dake and the first simultaneous activity of two cones of the Kirishima volcanic group ever documented. We report the detailed sequence of the events by combining areal photos, satellite images, and seismo-acoustic data analyses with the other published information. The seismo-acoustic data clarify the eruption onset and the transitions of the behaviors in three stages for each of the 2017 and 2018 eruptions. For both eruptions, we present regularly repeated tremors or 'drumbeat' earthquakes in the second stage, which interpret as gas separation from magma, leading to the ash-poor plume in the 2017 eruption or the effusive eruption in the 2018 event. We also propose that the 2017 and 2018 eruptions of Shinmoe-dake and the 2018 eruption of Iwo-yama are sequential events linked by the degassing of magma beneath Shinmoe-dake. An eruption like the 2017--2018 eruptions of Shinmoe-dake would leave little geological record and could be captured only by modern techniques. Although Shinmoe-dake has been believed to be an example of less-frequent eruptions, effusive eruptions like the 2018 case might have occurred more frequently in the past. The timelines summarized in this study will be useful in future studies of Kirishima volcanoes and world equivalences.
