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 Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and ca ad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km 3 ), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions.
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.
草津白根火山の本白根火砕丘群は,本白根西火砕丘,古本白根火砕丘,新本白根火砕丘,鏡池火砕丘,鏡池北火砕丘から構成される複合火砕丘である.これらの火砕丘はほぼ南北に連なり,新本白根火砕丘を除く火砕丘では溶岩流出から火砕物を噴出する爆発的噴火に推移した.また,火砕丘の表層部には隣接する火砕丘から放出された火砕物が堆積しており,火砕丘本体形成後にも爆発的噴火が起きたことがわかる.火砕丘を形成したマグマ噴火の年代は,直下の堆積物や炭化木片から推定され,鏡池火砕丘では約4800cal yr BP,鏡池北火砕丘では約1500cal yr BP以降である.火砕丘毎に噴出したマグマの組成は固有の組成変化傾向をもっており,これは単一のデイサイト質マグマのマグマ溜りに多様な組成の苦鉄質マグマが注入・混合することにより生み出されたと考えられる.
Sidescan sonar and sub-bottom reflection surveys off Shikabe town, along the Pacific coast, show the extent and volume of the submarine debris-avalanche deposit caused by the AD 1640 eruption of the Hokkaido-Komagatake volcano. The avalanche deposit extends from the subaerial part of the Shikabe lobe to as far as 20 km seaward. The subaqueous deposit, which reaches to 80 m of water depth, is 15 km wide and covers 126 km2 of the seafloor. Hummocks of the subaqueous deposit decrease in height and width with distance from the source. The margin of the deposit lacks hummocks. The ratio of the collapse height to the traveled distance (H/L) is 0.06, suggesting that the debris avalanche of origin was more mobile than those of most subaerial debris-avalanche deposits. The subaqueous volume of the avalanche deposits, estimated by extrapolating the pre-eruptive topography of the surrounding area, ranges from 0.92 to 1.20 km3.
The trace element and Sr–Nd isotopic compositions of Quaternary magmas from the Pre–Komitake volcano were investigated. The Sr and Nd isotope ratios ranged from 0.703320–0.703476, and 0.512885–0.513087, respectively, which are very similar to those of the lavas from Fuji and Komitake volcanoes that erupted subsequently. Enrichment of large ion lithophile elements, Pb and Sr, can be seen in the primitive mantle–normalized multi–element diagram of the Pre–Komitake, Komitake, and Fuji lavas. These collectively show island arc lava signatures; however, the middle to heavy rare earth elements are more depleted in the Pre–Komitake lavas, compared to those from Fuji. Positive Eu anomalies are observed, although the extents of these anomalies decrease with increasing SiO2 in the Pre–Komitake lavas, whereas this is not observed in Fuji lavas. The Sr/Y ratios of Pre–Komitake lavas increase from basalt to basaltic andesite, but decreases through andesite to dacite. This occurs in combination with a rapid increase in La/Yb ratios, followed by a more gradual increase. A gradual decrease in Dy/Yb ratios is also seen over the entire compositional range. These data suggest deep (>12 kbar) fractionation of garnet and amphibole followed by shallow (i.e., ~ 5 kbar) fractionation of amphibole and plagioclase. Such variations are not observed in the Komitake and Fuji lavas, for which deep fractionation of clinopyroxene and shallow fractionation of plagioclase have been suggested. All three lavas, including those from the Pre–Komitake volcano, show similar isotopic, major, and trace element compositions in the unfractionated basalts. The differing geochemical trends found in the Pre–Komitake lavas are likely to be due to different mineral fractionations occurring in the hydrous Pre–Komitake basalts compared to the dry Fuji and Komitake basalts.
