| 1. |
今井 亮, 下司 信夫, 自然硫黄のスピニフェックス組織:北海道知床硫黄山の溶融硫黄流噴火の冷却生成物, Resource geology, 10.1111/j.1751-3928.1999.tb00035.x, 49, 2, 99-104, 1999.06. |
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N Geshi, Fractionation and magma mixing within intruding dike swarm: evidence from the Miocene Shitara-Otoge igneous complex, central Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/S0377-0273(99)00188-2, 98, 1-4, 127-152, 2000.05, The analysis of intrusion pattern and petrological character of the central dike swarm in the Miocene igneous complex of the Shitara district, central Japan clarified that magma mixing between a strongly differentiated magma and a less-differentiated magma occurred within a dike swarm. The dike rocks have a wide compositional variation ranging from 5.5 to 0.7 wt.% MgO. They are divided into P1- and P2-types. The P2-type rocks provide many lines of evidence for magma mixing such as reversely zoned phenocrysts, bimodal composition distribution, and dissolution texture, whereas P1-type rocks do not. Phenocryst compositions of P2-type suggest that the magma mixing occurred between a less-fractionated phenocryst-poor magma and a strongly fractionated crystal-rich magma. Concentration ratios among incompatible elements show that the mixing end components were derived from a similar parental magma common to P1-type by fractional crystallization in a near closed system. The dikes with evidence for the magma mixing (P2-type) are distributed only in the southern marginal part of the dike swarm, whereas Pi-type dikes do not show any such localization. The distribution and the intrusion direction of the dikes indicate a nearly horizontal outward flow of magmas in the southern part of the dike swarm and accompanied magma mixing in the dike during intrusion. The fractionated end component is inferred to be a product of crystal fractionation within small and ephemeral magma pockets in the dike swarm. Magma mixing is thought to have occurred when a newly intruded dike ruptured the magma reservoir. The frequency of magma mixing was controlled mainly by competition between the lifetime of ephemeral magma reservoir and frequency of dike intrusions. The condition of magma mixing was satisfied only in the southern part of the dike swarm affected by the preceding volcanic activities. (C) 2000 Elsevier Science B.V. All rights reserved.. |
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N Geshi, Melt segregation by localized shear deformation and fracturing during crystallization of magma in shallow intrusions of the Otoge volcanic complex, central Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/S0377-0273(00)00271-7, 106, 3-4, 285-300, 2001.05, Basaltic dikes and sheets in the Miocene Otoge volcanic complex in central Japan have thin trachyte veins, which exhibit systematic variation regarding spacing with distance from the dike wall. Compositional relationship between the trachyte veins and surrounding basalt shows that the trachytic melt filling the veins was formed by in situ fractionation and segregated from the surrounding basalt. Mass balance calculation suggests that a residual liquid of trachyte composition formed when about two thirds of the parent basaltic magma was crystallized. Preferred orientation of groundmass crystals around the trachyte veins and echelon alignment of veinlets show that localized shear caused the deformation and fracturing in the incompletely crystallized magma and the interstitial residual melt was drawn into the fractures to form trachyte veins. The parallel development of trachyte veins and the increase of spacing toward the inner portion of the dike were controlled by the balance of cooling from the dike wall and segregation of interstitial melt. In a mobile magma body, shear will be concentrated in a crystal-mush layer in marginal parts, and the shear deformation could be an important mechanism to separate strongly-evolved residual melt from a mushy crystallizing layer. (C) 2001 Elsevier Science B.V. All rights reserved.. |
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N Geshi, T Shimano, T Chiba, S Nakada, Caldera collapse during the 2000 eruption of Miyakejima Volcano, Japan, BULLETIN OF VOLCANOLOGY, 10.1007/s00445-001-0184-z, 64, 1, 55-68, 2002.03, A collapsed caldera, 1.6 km in diameter and 450 m in depth, was formed at the summit of Miyakejima Volcano during the 2000 eruption. The collapsed caldera appeared on 8 July, with a minor phreatic eruption, 12 days after seismic activity and magma intrusion occurred northwest of the volcano. Growth of the caldera took from 8 July to the middle of August, with seismic swarms associated with the continuous intrusion of magma northwest of the volcano. The growth rate of the caldera was about 1.4X10(7) m(3)/day, and the final volume of the collapsed caldera was about 6X10(8) m(3). Major phreatomagmatic eruptions produced a total of about 1.6X10(10) kg (1.1X10(7) m(3)) of volcanic ash after caldera growth. The caldera structure, and the nature of the eruptive materials of the first collapse on 8 July, suggest that the surface subsidence was caused by the upward migration of a steam-filled cavity, with stoping of the roof rock above the magma reservoir. The diameter of the stoping column was estimated to be 600-700 m from circumferential faults that developed in the caldera floor, and the collapse of the caldera wall enlarged the diameter of the caldera to 1.6 km. The total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1.6-2.1 km.. |
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Development of the Middle Miocene Otoge volcanic complex, Shitara district, central Japan
Igneous activity of the Middle Miocene Otoge volcanic complex, Shitara district, central Japan, is divided into the cauldron-forming stage with emplacement of the Otoge pyroclastic rocks, and the post-cauldron stage with emplacement of dike and sheet swarm. Crosscutting relationship shows the igneous sequence in the post-cauldron stage : the Kamasawa brecciated intrusions and Kamoyamagawa trachyte dike were formed at the beginning of the post-cauldron stage and the formation of the Otoge cone sheets followed. The Shitara central dike swarm was formed in the final stage. Intrusion structure of these intrusions changed from irregular-shaped peperite to coherent tabular dike reflecting the consolidation of the host Otoge pyroclastic rocks. Most part of the the Otoge volcanic complex consists of alkaline rocks and their compositional change during the post-cauldron stage suggests the repletion of less-fractionated magma into a reservoir beneath the cauldron and progress of crystal fractionation within the reservoir. Input of high-temperature mafic magma to reservoir caused melting of its wall rocks and formed calc-alkaline dacite of the Otoge stocks.. |
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N Geshi, Structural development of dike swarms controlled by the change of magma supply rate: the cone sheets and parallel dike swarms of the Miocene Otoge igneous complex, Central Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2004.11.002, 141, 3-4, 267-281, 2005.03, Dike and sheet swarms of the post-cauldron activity of the Miocene Otoge igneous complex, central Japan, show systematic structural and petrological evolution. A concentric cone sheet swarm (Otoge cone sheets) measuring 10-15 km(3) was formed at the beginning of the post-cauldron stage and was later succeeded by the formation of a parallel dike swarm (Shitara central dike swarm) measuring 3-5 km(3). The Otoge cone sheets have a less fractionated character, with little compositional variations, whereas the Shitara central dike swarm has a fractionated character with wide compositional variations, this despite that both swarms reflect a single fractionation trend. The compositional change from the Otoge cone sheets to the Shitara central dike swarm can be explained by fractional crystallization due to the decline in the supply of less fractionated hot magma into the reservoir. The structural change from the Otoge cone sheets to the Shitara central dike swarm reflects the change in the stress field from a local compressive field to a regional extensive one, with the decline of magma replenishment into the reservoir during the final stage of volcanic activity. The concentrated distribution of the cone sheets suggests the sill-like or flat-top shaped magma reservoir beneath the center of the Otoge cone sheets. (c) 2004 Elsevier B.V. All rights reserved.. |
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Preface for the Special Issue on "Water ; from the Earth's Core to Planetary Space". |
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O. Ishizuka, N. Geshi, J. Itoh, Y. Kawanabe, T. Tsujino, The magmatic plumbing of the submarine volcanic chain of an oceanic island arc volcano: Long distance lateral magma transport?, GEOCHIMICA ET COSMOCHIMICA ACTA, 71, 15, A431-A431, 2007.08. |
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Minoru Urai, Nobuo Geshi, Thomas Staudacher, Size and volume evaluation of the caldera collapse on Piton de la Fournaise volcano during the April 2007 eruption using ASTER stereo imagery, GEOPHYSICAL RESEARCH LETTERS, 10.1029/2007GL031551, 34, 22, 2007.11, The scale of summit collapse during the April 2007 eruption of Piton de la Fournaise volcano, Reunion Island, western Indian Ocean, was evaluated using before- and- after differential digital elevation models (DEMs) derived from nadir and backward- looking images from the Advanced Spaceborne Emission and Reflection radiometer (ASTER) instrument. The dimensions of horizontal length and width, volume and depth of the depression were estimated as 1,100 x 800 m, 9.6 x 10(7) m(3) and 320 m, respectively. These estimates show excellent agreement with field survey data. A ring shaped thermal anomaly (diameter of about 500 m) was found at Dolomieu Crater during or just after the collapse on nighttime ASTER SWIR image. It may correspond to the high temperature areas associated with the cross section of a hydrothermal zone at a constant altitude inside the crater. Our analysis of ASTER orbital data documents topographic and related thermal changes as a result of the 2007 eruption and demonstrates the power of ASTER as a volcanological tool.. |
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IMAI AKIRA, GESHI NOBUO, SHIMANO TAKETO, NAKADA SETSUYA, Implication of the temporal sulphur isotopic variation during the 2000 eruption of Miyakejima Volcano, Japan, The Island arc, 10.1111/j.1440-1738.2007.00549.x, 16, 1, 83-92, 2007.03. |
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Nobuo Geshi, Susumu Umino, Hidenori Kumagai, John M. Sinton, Scott M. White, Kiyoyuki Kisimoto, Thomas W. Hilde, Discrete plumbing systems and heterogeneous magma sources of a 24 km(3) off-axis lava field on the western flank of East Pacific Rise, 14 degrees S., EARTH AND PLANETARY SCIENCE LETTERS, 10.1016/j.epsl.2007.03.019, 258, 1-2, 61-72, 2007.06, The largest known mid-ocean-ridge off-axis lava flow field occurs slightly off-axis near 14 degrees S along the East Pacific Rise (EPR). It comprises at least four volcanologically discrete units. We collected lava samples from 24 sites within the off-axis lava field and 7 sites on the adjacent ridge crest using the Shinkai-6500 submersible. The lava field comprises at least three distinct MORB compositions, all of which are different from the lavas collected from the nearby ridge axis. The east and west cones and the northern lobe of the lava field consist of normal MORB (N-MORB) lavas with a low concentration of incompatible elements and low LILE/HFSE and LREE/HR_EE ratios. By contrast, the samples from the west plain of the field have a higher concentration of incompatible elements and higher LILE/HFSE and LREE/HREE ratios indicating T-MORB character. The lava samples collected from the summit of the east cone show the highest concentration of LILE elements and LREEs among the lava field (E-MORB lava). The N-MORB of the off-axis lava field are more depleted in incompatible elements than the adjacent EPR axis lavas, possibly reflecting the re-melting of the residual mantle in the off-axis region. The E-MORB lava was probably derived from fertile mantle that did not undergo melting beneath the spreading center. T-MORB, which occupies the main part of the 14 degrees S lava field, is the product of magma mixing between N-MORB and E-MORB magmas. (C) 2007 Elsevier B.V All rights reserved.. |
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Surface Faults and Mass-movements Associated with the Iwate-Miyagi Nairiku Earthquake in 2008 in Northeastern Japan : Flash Report
A strong earthquake of M7.2 (MW6.7-6.9) occurred on 14 June 2008 in the north-central part of Northeastern Japan. It was named the Iwate-Miyagi Nairiku Earthquake in 2008. Several seismic models show a 25-30km long reverse fault in the NNE-SSW direction and a westward inclination. Although some geological thrusts with a similar trend are known, as shown in Fig. 1, none of them has been recognized as an active fault. Surface faults associated with the earthquake were found at several points along the geological thrusts. The quake induced many mass-movements in the mountainous and hilly area, which is composed mostly of Neogene to Pleistocene volcaniclastic rocks (Fig. 1), near the seismic fault and particularly on the western side (upper block). Most of the damage caused by the earthquake was closely connected to mass-movements.
A large-scale block glide amounting to about 7×107m3 in the volume of dislocated mass (Figs. 2, 3) occurred in the northern half of the pre-existing landslide area composed of poorly-consolidated pyroclastic flow deposits with a welded cap around 5Ma (Np3 in Fig. 1). The almost horizontal slip surface of the glide is considered to have been formed in the underlying lacustrine beds. A large collapse, occurring at a slope on which snow remained on Kurikoma Volcano, composed of Quaternary andesitic lavas and pyroclastics (Qm2 in Fig. 1), formed a debris flow (Fig. 4) which buried a lodge at Komanoyu Spa. Many landslides checked river channels. One of them was observed at Ogawara, where a slide of a spur composed of Pleistocene pyroclastic deposits checked the Ichihasama River (Fig. 5). The collapse of Matsurube Bridge, a three-span 95 m-long bridge over a tributary of the Iwai River, was also entirely caused by a landslide at the north-facing slope of a narrow ridge (Fig. 6). A surface fault appeared in rice fields at Mochikorobashi on the side of a tributary of the Koromo River (Fig. 7). The trace of the water's edge and lines of planted rice indicate a co-seismic uplift and a right-lateral offset with a clockwise turn of the trend (Fig. 8).. |
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Nobuo Geshi, Vertical and lateral propagation of radial dikes inferred from the flow-direction analysis of the radial dike swarm in Komochi Volcano, Central Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2008.01.001, 173, 1-2, 122-134, 2008.06, Dikes with lateral and outward intrusion directions (named L-type dike) and dikes with vertical and upward intrusion directions (V-type dike) are recognized in a radial dike swarm of Komochi Volcano, Japan, by the combined analysis of the preferred orientations of deformed vesicles and anisotropy of magnetic susceptibility (ANIS). Intrusion directions of magmas were examined for 41 dike outcrops. Among them, 19 dikes were classified as L-type dike and 15 dikes were as V-type. The L-type dikes distribute over the area of the dike swarm radiating from the central conduit, which is named Daikokuiwa stock, and the V-type dikes occur mainly in the peripheral portion of the volcano. The L-type dikes are characterized with evolved whole-rock compositions similar to those of rocks consisting of the central conduit, whereas the V-type dikes have less-fractionated composition and are enrich in mafic phenocryst as compared to the L-type dikes. The outward intrusion directions of the L-type dikes and their petrological similarity to the rock of the central conduit indicate that these dikes intruded from the shallow part of the central conduit where the magmas underwent fractional crystallization and degassing. The petrological characteristics of the V-type dikes suggest that the less-fractionated magma intruded directly from a magma chamber, in which mafic phenocryst crystals accumulated. The larger dike thickness and higher magnetic foliation of the V-type dike as compared to the L-type dike indicate higher magmatic overpressure, which was possibly result of the direct connection to the pressurized magma chamber and vertical-growth of the dikes with buoyant magma. Solidification of the central conduit is favorable for the accumulation of internal excess pressure in the magma chamber to break the wall rock. Fissure eruptions independent of the central conduit have occurred in many volcanoes and these fissure eruptions might be fed by the dikes directly propagated from the deeper magma chamber. (C) 2008 Elsevier B.V. All rights reserved.. |
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Osamu Ishizuka, Nobuo Geshi, Jun'ichi Itoh, Yoshihisa Kawanabe, Taqumi TuZino, The magmatic plumbing of the submarine Hachijo NW volcanic chain, Hachijojima, Japan: Long-distance magma transport?, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 10.1029/2007JB005325, 113, B8, 2008.06, [1] Recent geophysical observations on basaltic composite volcanoes in Izu-Bonin arc reveal the process of long-distance lateral magma transport within arc crust. To clarify a long-distance magma transport system of the basaltic arc volcano from geological and petrological aspects, we investigated 20-km-long submarine volcanic chains (Hachijo NW chain and Hachijo-kojima chain) and cones on the northeastern slope ( NE edifices) as well as subaerial satellite cones nested Hachijo Nishiyama volcano in the northern Izu arc front. Basalts from Hachijo NW chain have more primitive composition than those from other edifices. The composition of the Hachijo NW chain basalts is controlled by fractional crystallization, while plagioclase accumulation occurred in NE edifices and subaerial satellite cones. Trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the volcano. This leads us to consider that magma was transported long distances between the Nishiyama volcano and the Hachijo NW chain. Primitive magma was laterally transported NNW for at least 20 km in the middle to lower crust (10-20 km deep) from Nishiyama volcano with only minimal crustal level modifications and formed Hachijo NW chain. On the other hand, magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath Nishiyama volcano seems to have been transported in a short distance (< 5 km) and formed NE trending edifices and subaerial satellite cones. The long-distance magma transport seems to be controlled by a regional extensional stress regime, while short-distance transport may be controlled by local stress regime affected by load of main volcanic edifice.. |
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Osamu Ishizuka, Nobuo Geshi, Jun'ichi Itoh, Yoshihisa Kawanabe, Taqumi TuZino, The magmatic plumbing of the submarine Hachijo NW volcanic chain, Hachijojima, Japan: Long-distance magma transport?, Journal of Geophysical Research: Solid Earth, 10.1029/2007JB005325, 113, 8, 2008.08, Recent geophysical observations on basaltic composite volcanoes in Izu-Bonin arc reveal the process of long-distance lateral magma transport within arc crust. To clarify a long-distance magma transport system of the basaltic arc volcano from geological and petrological aspects, we investigated 20-km-long submarine volcanic chains (Hachijo NW chain and Hachijo-kojima chain) and cones on the northeastern slope (NE edifices) as well as subaerial satellite cones nested Hachijo Nishiyama volcano in the northern Izu arc front. Basalts from Hachijo NW chain have more primitive composition than those from other edifices. The composition of the Hachijo NW chain basalts is controlled by fractional crystallization, while plagioclase accumulation occurred in NE edifices and subaerial satellite cones. Trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the Volcano. This leads us to consider that magma was transported long distances between the Nishiyama volcano and the Hachijo NW chain. Primitive magma was laterally transported NNW for at least 20 km in the middle to lower crust (10-20 km deep) from Nishiyama volcano with only minimal crustal level modifications and formed Hachijo NW chain. On the other hand, magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath Nishiyama volcano seems to have been transported in a short distance (<
5 km) and formed NE trending edifices and subaerial satellite cones. The long-distance magma transport seems to be controlled by a regional extensional stress regime, while short-distance transport may be controlled by local stress regime affected by load of main volcanic edifice. Copyright 2008 by the American Geophysical Union.. |
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Nobuo Geshi, Teruki Oikawa, Phreatomagmatic eruptions associated with the caldera collapse during the Miyakejima 2000 eruption, Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2008.04.013, 176, 4, 457-468, 2008.10, The 2000 AD eruption of Miyakejima was characterized by a series of phreatomagmatic eruptions from the subsiding caldera. Six major eruptive events occurred, and they can be divided into the first and second periods separated by a 25-day hiatus. The phreatomagmatic eruptions produced a total of similar to 2 x 10(10) kg of tephra, which mainly comprised fine-grained volcanic ash. The tephra layers could be divided into six fall units corresponding to the six major eruptive events.
The grains of the 2000 tephra consisted mainly of accessory fragments derived from the previous edifice. juvenile fragments were identified in four major units. The angular and blocky shape of the juvenile fragments showed rapid cooling of magma by external water, indicating the mechanical mixing of ground water and magma.
The removal of the load on the aquifer by caldera subsidence and the intrusion of magma into the decompressed aquifer caused a series of phreatomagmatic eruptions from the subsiding caldera. The absence of surface water in the subsiding caldera during the eruption showed that groundwater participated in the phreatomagmatic eruptions. Rising magma along the ring faults intruded into the decompressed aquifer surrounding the caldera and caused phreatomagmatic explosions. The 2000 eruption of Miyakejima indicated that the fine-grained tephra abundant in hydrothermally altered rock fragments is a possible indicator of the phreatomagmatic eruptions in the hydrothermal system during the caldera formation in basaltic volcanoes. (C) 2008 Elsevier B.V. All rights reserved.. |
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Preface for the Special Issue on "Hydrothermal Fluid Circulation Systems of the Oceanic Crust and Interactions among the Lithosphere, Hydrosphere, and Biosphere". |
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Distribution and Flow Mechanisms of the 7.3ka Koya Pyroclastic Flow Deposits Covering Yakushima Island, Kagoshima Prefecture
A large pyroclastic eruption occurred around 7.3 ka from the Kikai caldera about 30 km north of Yakushima Island. Its pyroclastic flow and fall deposits covered the entire area of Yakushima Island and may have influenced the evolution of unique floras and faunas of Yakushima Island.
Detailed field survey revealed that the Koya pyroclastic flow deposit spread from NW to SE, covering almost the entire area of Yakushima. A part of the southern coastal area remained from the pyroclastic flow due to local alignment of topographic ridges and valleys, which acted as barriers to the pyroclastic flows. Possible tsunami deposits associated with the Kikai-Akahoya eruption were discovered in the area below ca. 50 m above sea level along the northern coasts of Yakushima and Kuchinoerabujima Islands.. |
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Nobuo Geshi, Asymmetric growth of collapsed caldera by oblique subsidence during the 2000 eruption of Miyakejima, Japan, EARTH AND PLANETARY SCIENCE LETTERS, 10.1016/j.epsl.2009.01.027, 280, 1-4, 149-158, 2009.04, Oblique development of the ring faults reflecting the structural heterogeneities inside the volcano formed many asymmetric structures of Miyakejima 2000 AD caldera. The asymmetry includes (a) offset location of the ring faults with respect to the associated shallow magma chamber, (b) unequal outward migration of the caldera wall 600 m at the southeastern rim but only 200 m at the northwestern rim, (c) development of tilted terrace only at the southeastern caldera margin, (d) eruption sites and fumaroles being confined to the southern part of the caldera.
Geophysical data, including ground deformation and seismic activity, indicates the offset of the location of the magma chamber about 2 km south of the caldera center on the surface. The ring faults propagated from the deflating magma chamber obliquely about 30 degrees toward the summit. The oblique subsidence of the cylindrical block formed a wider instable zone, particularly in the southeastern side of the ring fault that enhanced the larger outward migration of the caldera rim and also caused the formation of the outer half-ring fault bordering the tilting slope at the southern part. Ascending pass of the buoyant magma along the tilted ring faults was concentrated in the southern half of the caldera and consequently the distributions of the eruption sites and fumaroles are localized in the southern-half part of the caldera. The structure of the Miyakejima 2000 caldera with complete development of the ring faults, its high roof aspect ratio and oblique subsidence is clearly distinguishable from trapdoor-type caldera. The oblique development of the ring faults can be controlled by the mechanical contrast between the solidified conduits and surrounding fragile volcanic edifice. Asymmetric development of the Miyakejima caldera shows that the collapsed calderas are potential indicators of the heterogeneous structures inside of the volcano, particularly in the case of small-size caldera. (C) 2009 Elsevier B.V. All rights reserved.. |
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Preface for the Special Issue on "Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I)". |
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Preface for the Special Issue on "Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part II)". |
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Nobuo Geshi, Shigekazu Kusumoto, Agust Gudmundsson, Geometric difference between non-feeder and feeder dikes, GEOLOGY, 10.1130/G30350.1, 38, 3, 195-198, 2010.03, Feeder dikes bring magma to the surface; non-feeder dikes become arrested and never reach the surface. The differences, if any, between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non-feeder. Here we present measurements of feeder and non-feeder dikes exposed from depths of >200 m to the surface in the walls of the A. D. 2000 caldera collapse of the Miyakejima Volcano, Japan. A typical feeder thickness reaches a maximum of 2-4 m at the surface, decreases rapidly to similar to 1 m at a depth of 20-40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5-2 m at 15-45 m below the tip, and then decreases slowly with depth to 0.5-1 m at the bottom of the exposure. We propose that free-surface effects and magmatic overpressure (driving pressure) changes during the eruption cause the overall shape of a feeder to differ from that of a non-feeder.. |
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Preface for the Special Issue on "Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part III)". |
| 24. |
Nobuo Geshi, Karoly Nemeth, Teruki Oikawa, Growth of phreatomagmatic explosion craters: A model inferred from Suoana crater in Miyakejima Volcano, Japan, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2010.11.012, 201, 1-4, 30-38, 2011.04, A subvertical cross section of a maar-diatreme volcano is exposed in the wall of the AD. 2000 caldera on Miyakejima Volcano. The maar, Suoana, is one of the lateral vents of this volcano and it was inferred to be formed in the 7th century. The subvertical wall of the A.D. 2000 caldera truncated the Suoana maar crater at its center revealing the near comlete cross section of this small maar-diatreme volcano. Exposed in the cross section are a 400 m wide maar crater, an associated tuff ring with a maximum thickness of 20 m, a diatreme extending vertically to a depth of about 220 m from the floor of the maar crater, and a feeder dike connected to the base of the diatreme. The depth of the diatreme structure is about 260 m from the original ground surface. The outline of the diatreme resembles an upward-opening funnel with an almost vertical wall below 560 m asl and an upward flaring wall above 560 m asl. Coarse grained volcanic breccia fills the diatreme, the deposits of which can be divided into 6 units based on lithological and structural characteristics. The upper half of the diatreme is filled with landslide deposits, mainly derived from the surrounding crater wall. The bottom of the diatreme is occupied by massive explosion breccia. Some coherent blocks were detached from the wall of the diatreme and preserved in the diatreme fill.
The Y-shaped cross-sectional geometry of the Suoana diatreme is the result of a combination of an underground subsidence in the lower part of the diatreme and the surface landslide in its upper part. The inwardly-inclined stratification of pyroclastic rock units and development of many small faults in the diatreme-filling breccia indicate successive collapse and deformation of these materials within the diatreme during the eruption. The upper part of the diatreme was formed by the landslides of the crater wall induced by the subsidence of the crater floor. Discharge of tephra from the bottom of the diatreme caused infill subsidence, which induced sliding of the inner wall of the crater. As a result, the topographic diameter of the crater became much larger than that of the diatreme itself. The tephra ring surrounding the crater consists mainly of pyroclastic fall deposits. (C) 2010 Elsevier B.V. All rights reserved.. |
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J. Ruch, V. Acocella, N. Geshi, A. Nobile, F. Corbi, Kinematic analysis of vertical collapse on volcanoes using experimental models time series, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 10.1029/2012JB009229, 117, 2012.07, Volcanoes are often associated with vertical collapse, due to deeper magma withdrawal. Calderas are the most notable type of vertical collapse, on the summit of volcanoes. Caldera collapse has been observed and monitored only at Miyakejima (Japan; 2000), Dolomieu (Reunion; 2007) and Fernandina (Galapagos; 1968), highlighting our limited knowledge on its kinematic behavior. Here we use experimental models to investigate the kinematic evolution of calderas and vertical collapses in general. We extract velocity and strain fields using the Particle Image Velocimetry (PIV) technique, generating time series. Experimental vertical collapses undergoing constant subsidence velocity show three main kinematic behaviors: (1) continuous collapse, whose velocity is similar to the source subsidence velocity; (2) incremental collapse, with episodic (stepwise) accelerations along pre-existing ring structures; (3) sudden collapse, resulting from the upward migration of a cavity, only for T/D > 2 (T and D are the depth and width of the magma chamber, respectively) and without ring structures. The velocity in the collapsing column may increase up to four orders of magnitude with regard to the constant subsidence velocity of the source. Comparison to nature suggests that: (1) there are close kinematic similarities with monitored collapse calderas, explaining their incremental subsidence after the development of ring structures; (2) sudden pit crater formation is induced by the upward propagation of cavities, due to magma removal at depth and in absence of ring structures; (3) all these types of vertical collapses have a consistent mechanism of formation and kinematic behavior, function of T/D and the presence/absence of ring structures.. |
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Nobuo Geshi, Valerio Acocella, Joel Ruch, From structure- to erosion-controlled subsiding calderas: evidence thresholds and mechanics, BULLETIN OF VOLCANOLOGY, 10.1007/s00445-012-0617-x, 74, 6, 1553-1567, 2012.08, Collapse calderas evolve by increasing their depth/diameter ratio. To properly characterize caldera evolution, a structural S/D (ratio between structural subsidence and ring-fault diameter; S (s) /D (s) ), and a topographic S/D (ratio between topographic caldera depth and topographic caldera width; S (t) /D (t) ), are considered. We review the evolution of the A.D. 2000 Miyakejima caldera, with two concentric ring faults at earlier collapsing stages, and erosion of its wall, accumulating debris on the floor, at later collapsing stages. While S (t) /D (t) and S (s) /D (s) show a similar increase at initial stages, when S (s) /D (s) similar to 0.33 the S (s) /D (s) becomes significantly different from S (t) /D (t) : while continuous caldera subsidence monotonically increases S (s) /D (s) , the erosion of the wall and the filling of the floor decrease S (t) /D (t) . This evolution finds close similarities with recent caldera collapses of Krakatau (1883), Katmai (1912), Fernandina (1968), Tolbachik (1975-1976), Pinatubo (1991), and Dolomieu (2007). Analog experiments mimic the observed variation, evolving from a depression controlled by the activity of the double-ring faults to that controlled by the erosion of the wall and sedimentation at the floor. These natural and modeling results show that the control on the shape of mature calderas (S (s) /D (s) > 0.07) and approaching S (s) /D (s) = 0.3-0.4 passes from a mainly structural to a mainly erosional control. Both S (t) /D (t) and S (s) /D (s) are needed to describe the evolution of a collapse and the processes accompanying it. Evaluating S (t) /D (t) and S (s) /D (s) allows proper description of the precise evolutionary stage of a caldera and of the relative importance of the structural and erosional processes and allows making semiquantitative comparisons between evolutionary stages.. |
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Nobuo Geshi, Shigekazu Kusumoto, Agust Gudmundsson, Effects of mechanical layering of host rocks on dike growth and arrest, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2012.02.004, 223, 74-82, 2012.04, Many dikes intruding the layered host rock of Miyakejima and Piton de la Fournaise are arrested, segmented, and show variation in thickness. These geometric factors can be largely attributed to the mechanical layering of the host rocks that constitute these volcanoes. Some dikes are arrested at the base of stiff (high Young's modulus) rock layers, whereas others are segmented in the layers. We use finite-element models (FEM) to simulate a typical dike arrest at the base of a stiffer layer. The dike may become arrested when it reaches at the base of the stiffer layer for several reasons. First, the dike-induced tensile stress may not be high enough to break the rock. Second, the stiff layer may act as a "stress barrier". Third, the material toughness of the layers may be unfavorable for vertical dike propagation and result either in dike arrest or dike deflection. When the magmatic overpressure (driving pressure) is sufficiently high dike can intrude into the overlying layer. Some deflected dikes resume their vertical propagation, thereby generating offset segments. The layering of a volcano thus commonly controls the dike propagation paths and if and where the dikes become arrested. Our measurements show that the dikes in these volcanoes tend to be comparatively thick where they dissect "soft" pyroclastic layers, and thin where they dissect stiff lava flows and sills. Numerical FEM simulations indicate that the variation of dike thickness observed in Miyakejima and Piton de la Fournaise can be broadly explained in terms of layer-stiffness differences of one or two orders of magnitude. (C) 2012 Elsevier B.V. All rights reserved.. |
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Takahiro Miwa, Nobuo Geshi, Decompression rate of magma at fragmentation: Inference from broken crystals in pumice of vulcanian eruption, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, 10.1016/j.jvolgeores.2012.03.003, 227, 76-84, 2012.05, Decompression rate of magma at fragmentation (immediately before or after fragmentation) was determined using broken crystals found in the pumices ejected during the vulcanian explosion of Sakurajima volcano, Japan. The combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7.0 x 10(3)-7.8 x 10(4) Pa/s for the ejecta produced during the later phase of vulcanian explosion. This result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent. An understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation. (C) 2012 Elsevier B.V. All rights reserved.. |
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Takahiro Miwa, Nobuo Geshi, Hiroshi Shinohara, Temporal variation in volcanic ash texture during a vulcanian eruption at the sakurajima volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2013.05.010, 260, 80-89, 2013.06, We discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the Sakurajima volcano, Japan. Falling ash samples were collected every 2-50. min for petrological analysis. These ash samples contained juvenile particles, altered rock fragments, and fragments of crystals; here, we subdivide the juvenile particles into blocky, vesicular, and fluidal particles. Because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles, we refer to the vesicular and fluidal particles as low-crystallinity particles (LCPs) and the blocky particles as high-crystallinity particles (HCPs). The temporal variation observed in ash texture reveals that the ratio of LCPs to HCPs (LCP/HCP) was small at the initiation of the eruption and increased as the eruption progressed. The higher microlite crystallinity in the HCPs indicates crystallization under lower pressures for longer durations relative to the LCP magma. The increase of the LCP/HCP ratio during a single vulcanian eruption suggests that the emission of less viscous gas-rich magma under a cap rock increases as the eruption progresses. Such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption. © 2013 Elsevier B.V.. |
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Akihiko Tomiya, Isoji Miyagi, Genji Saito, Nobuo Geshi, Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan, Bulletin of Volcanology, 10.1007/s00445-013-0750-1, 75, 10, 1-19, 2013.10, We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO2 57 wt%) with a small amount of more silicic pumice (SiO2 62-63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake. © 2013 The Author(s).. |
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NAKADA Setsuya, UKAWA Motoo, NEWHALL Christopher G., MCNUTT Stephen R., WRIGHT Thomas L., ICHIHARA Mie, GESHI Nobuo, Preface, Earth, planets and space, 10.5047/eps.2013.06.002, 65, 6, 473-473, 2013.06. |
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Shigekazu Kusumoto, Agust Gudmundsson, Trine H. Simmenes, Nobuo Geshi, Sonja L. Philipp, Inverse modeling for estimating fluid-overpressure distributions and stress intensity factors from an arbitrary open-fracture geometry, Journal of Structural Geology, 10.1016/j.jsg.2012.10.004, 46, 92-98, 2013.01, We present a new solution for estimating the fluid overpressure (driving-pressure or net-pressure) acting on the walls of a fracture with an arbitrary opening displacement. In the paper, we first present a forward modeling solution, using Fourier cosine series, for the opening displacement of a fracture subject to an overpressure that varies irregularly along the length of a fracture. By changing the form of the solution, we provide a matrix equation for estimating the Fourier coefficients and thereby obtain the overpressure variation from the fracture geometry. As numerical tests of this inverse analysis, we estimated the overpressure variation from fracture-opening displacements given by well known analytical solutions, and found that this method can be used for overpressure estimates for a variety of fluid-driven fractures. We apply our solutions to a mineral vein hosted by gneiss (West Norway) and conclude from the aperture variation that, at the time of vein formation, the overpressure increased toward the vein tip. We also discuss the physical meaning of the Fourier coefficients by applying our results to man-made hydraulic wing fractures. The results indicate that the coefficients of n = 0 and n = 1 relate to the fluid overpressure and the critical stress intensity factor. © 2012 Elsevier Ltd.. |
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Shigekazu Kusumoto, Nobuo Geshi, Agust Gudmundsson, Aspect ratios and magma overpressures of non-feeder dikes observed in the Miyake-jima volcano (Japan), and fracture toughness of its upper part, Geophysical Research Letters, 10.1002/grl.50284, 40, 6, 1065-1068, 2013.03, We present a new method for estimating the length and maximum thickness (aperture) of a dike from the observed opening at one dike tip. We apply the method to 15 arrested non-feeder dikes (where the upper tip is known, the lower tip unknown) in the caldera walls of Miyake-jima, Japan, to estimate the length-thickness ratio, as well as the magma overpressure and fracture toughness. The calculated length-thickness ratio ranges from 61 to 246, with an average of 136. The ratios are low because the dikes are emplaced close to the surface in comparatively compliant (soft) rocks. Using these ratios and the appropriate elastic constants, the calculated magmatic overpressures of the dikes are between 2.3 and 8.9 MPa, and the stress intensity factors between 38 and 117 MPa m1/2. All these values are within the range of typical in situ estimates, supporting the validity of this new method. Key Points Magma overpressures of non-feeder dikes in Miyake-jima are less than 9 MPa Aspect ratios of dike reflect intrusion environment and host rock propertie Successful evaluation of the fracture toughness of the Miyake-jima volcano. ©2013 American Geophysical Union. All Rights Reserved.. |
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Mitsuru Okuno, Toshio Nakamura, Nobuo Geshi, Katsuhiko Kimura, Yoko Saito-Kokubu, Tetsuo Kobayashi, AMS radiocarbon dating of wood trunks in the pumiceous deposits of the Kikai-Akahoya eruption in Yakushima Island, SW Japan, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 10.1016/j.nimb.2012.05.015, 294, 602-605, 2013.01, Radiocarbon dating using accelerator mass spectrometry (AMS) was performed on numerous wood trunks from pumiceous deposits along the Nagata, Isso and Miyanoura rivers on the northern side of Yakushima Island, 60 km south of Kyushu Island. The obtained 14C dates were around 6.5 ka BP, which, in combination with the geological characteristics of the pumiceous deposits indicates that these specimens were buried during the Kikai-Akahoya (K-Ah) eruption from the Kikai caldera. However, the fact that they are not charred suggests that the origin of these deposits are not pyroclastic flows. Fourteen taxa (Pinus subgen. Diploxylon, Tsuga, Cryptomeria, Chamaecyparis, Myrica, Castanea, Castanopsis, Quercus subgen. Cyclobalanopsis, Trochodendron, Phellodendron, Lagerstroemia, Rhododendron, Myrsine and Symplocos) were identified through anatomical characteristics. This is the first discovery of forest species on the Yakushima Island before the devastating eruption. © 2012 Elsevier B.V. All rights reserved.. |
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Nobuo Geshi, Teruki Oikawa, The spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at Miyakejima volcano, Japan, Bulletin of Volcanology, 10.1007/s00445-014-0797-7, 76, 3, 1-14, 2014.03, Basaltic feeder systems exposed in the caldera wall of Miyakejima volcano are classified into three groups: (1) effusive feeders, (2) moderately explosive feeders, and (3) highly explosive feeders. The surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them, ranging from non-explosive lava effusions to violent explosive eruptions, despite the apparent lack of influence of external water. Effusive feeders are filled with coherent (non-fragmented) intrusive rock, indicating no significant fragmentation in the feeder system. The other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part. Their uppermost parts show an upward-flaring shape. The transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma. The depth of the coherent-pyroclastic transition is deeper (20-150 m) in highly explosive feeders than in the moderately explosive feeders ( |
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Hitomi Nakamura, Teruki Oikawa, Nobuo Geshi, Akikazu Matsumoto, Migration of a volcanic front inferred from K-Ar ages of late Miocene to Pliocene volcanic rocks in central Japan, Island Arc, 10.1111/iar.12073, 23, 3, 236-250, 2014.09, K-Ar ages have been determined for 14 late Miocene to Pliocene volcanic rocks in the north of the Kanto Mountains, Japan, for tracking the location of the volcanic front through the time. These samples were collected from volcanoes located behind the trench-trench-trench (TTT) triple junction of the Pacific, Philippine Sea, and North American plates. This junction is the site of subduction of slabs of the Pacific and the Philippine Sea plates, both of which are thought to have influenced magmatism in this region. The stratigraphy and K-Ar ages of volcanic rocks in the study area indicate that volcanism occurred between the late Miocene and the Pliocene, and ceased before the Pleistocene. Volcanism in adjacent areas of the southern NE Japan and northern Izu-Bonin arcs also occurred during the Pliocene and ceased at around 3 Ma with the westward migration of the volcanic front, as reported previously. Combining our new age data with the existing data shows that before 3 Ma the volcanic front around the TTT junction was located about 50km east of the preset-day volcanic front. We suggest that northward subduction of the Philippine Sea Plate slab ended at ~3 Ma as a result of collision between the northern margin of the plate with the surface of the Pacific Plate slab. This collision may have caused a change in the subduction vector of the Philippine Sea Plate from the original north-directed subduction to the present-day northwest-directed subduction. This indicates that the post ~3 Ma westward migration of the volcanic front was a result of this change in plate motion. © 2014 Wiley Publishing Asia Pty Ltd.. |
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Osamu Ishizuka, Nobuo Geshi, Yoshihisa Kawanabe, Itaru Ogitsu, Rex N. Taylor, Taqumi Tuzino, Izumi Sakamoto, Kohsaku Arai, Shun Nakano, Long-distance magma transport from arc volcanoes inferred from the submarine eruptive fissures offshore Izu-Oshima volcano, Izu-Bonin arc, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2014.08.006, 285, 1-17, 2014.09, Long-distance lateral magma transport away from volcanic centers in island arcs is emerging as a common phenomenon where the regional stress regime is favorable. It should also be recognized as an important factor in the construction and growth of island arcs, and a potential trigger for devastating eruptions. In this contribution, we report on recent investigations into the magma dynamics of Izu-Oshima volcano, an active basaltic volcano with an extensive fissure system.Izu-Oshima is flanked by numerous, subparallel NW-SE trending submarine ridges extending up to 22. km to the NW and the SE from the central vent. During a recent submersible survey we have identified that these ridges are fissures which erupted basaltic spatter and lava flows. Furthermore, lavas are petrographically similar along each ridge, while there are noticeable differences between ridges. The subparallel ridges are observed to transect a series of seamounts - the Izu-Tobu monogenetic volcanoes - which are dispersed across this area of the rear-arc. However, there are consistent petrographic and chemical differences between these seamounts and the ridges, indicating that they have different magma sources, yet, they are essentially bounding each other in dive tracks. The most appropriate scenario for their development is one where the Izu-Tobu Volcanoes are fed by an "in-situ" underlying source, while the NW-SE ridges are fed by lateral magma transport from Izu-Oshima.Magma erupted from each ridge is of a consistent geochemistry along its length, but has experienced crystal fractionation and some plagioclase accumulation. Compositions of the ridges are also very similar to lavas from the subaerial cones that can be traced down the flanks of Izu-Oshima. This implies that pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the storage system beneath Izu-Oshima. Hence, magma from this crustal reservoir moved upward to feed the on-edifice cones, as well as laterally to supply submarine fissures and dykes to a distance of 22. km. © 2014 Elsevier B.V.. |
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Nobuo Geshi, Joel Ruch, Valerio Acocella, Evaluating volumes for magma chambers and magma withdrawn for caldera collapse, Earth and Planetary Science Letters, 10.1016/j.epsl.2014.03.059, 396, 107-115, 2014.06, We develop an analytical model to infer the total volume of a magma chamber associated with caldera collapse and the critical volume of magma that must be withdrawn to induce caldera collapse. The diameter of caldera border fault, depth to the magma chamber, and volumes of magma erupted before the onset of collapse and of entire eruption are compiled for 14 representative calderas. The volume of erupted magma at the onset of collapse aligns between the total erupted volume of the other representative caldera-forming eruptions and the volume of eruptions without collapse during the post-caldera stage, correlating with the structural diameter of the calderas.The total volume of magma chamber is evaluated using a piston-cylinder collapse model, in which the competition between the decompression inside magma chamber and friction along the caldera fault controls the collapse. Estimated volumes of the magma chambers associated with caldera collapse are 3-10 km3 for Vesuvius 79 A.D. to 3000-10500 km3 for Long Valley, correlating with the cube of caldera diameters. The estimated volumes of magma chamber are always larger than the total volume of erupted magma for caldera formation, suggesting that the magma chambers are never completely emptied by the caldera-forming eruptions. The minimum volumes of erupted magma to trigger collapse are calculated from the correlation between the caldera diameters and the evaluated volume of magma chambers. The minimum eruptive volume for the collapse correlates with the square of the caldera radius r and the square of the depth to the magma chamber h, and inversely correlates with the bulk modulus of magma, which is mainly controlled by the bubble fraction in the magma. A bubble fraction between 5 and 10% at the onset of collapse may explain the distribution of the erupted volumes at the onset of collapse of the calderas in nature. © 2014 Elsevier B.V.. |
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Nobuo Geshi, Marco Neri, Dynamic feeder dyke systems in basaltic volcanoes: The exceptional example of the 1809 Etna eruption (Italy), Frontiers in Earth Science, 10.3389/feart.2014.00013, 2, Jul, 2014.07, © 2014. Geshi and Neri. The detection and understanding of the movement of magma at very shallow levels remains one of the most fascinating challenges of modern volcanology, because such information allows us to identify and circumscribe the most probable location where future eruptive vents will open. Unfortunately, it is rarely possible to observe any detail of the internal structure of the feeder system of recent eruptions; in only very few cases, geological observations in dissected volcanoes can help us imagine how magma moved and evolved inside the feeder system. In this paper, we describe the 1809 eruption of Mt. Etna, Italy, which represents one historical and rare case in which it is possible to closely observe the internal structure of the feeder system. This is possible thanks to the presence of two large pit craters located in the middle of the eruptive fracture field that allow studying a section of the shallow feeder system. Along the walls of one of these craters, we analyzed well-exposed cross sections of the uppermost 15-20 m of the feeder system and related volcanic products. Here, we describe the structure, morphology and lithology of this portion of the 1809 feeder system, including the host rock which conditioned the propagation of the dyke, and compare the results with other recent eruptions. Finally, we propose a dynamic model of the magma behavior inside a laterally-propagating feeder dyke, demonstrating how this dynamic triggered important changes in the eruptive style (from effusive/Strombolian to phreatomagmatic) during the same eruption. This is therefore an exceptional case to understand how basaltic magmas move during the propagation of an eruptive fissure, which furnishes fundamental elements for the modeling of superficial intrusive processes. Our results are also useful for hazard assessment related to the development of flank eruptions, potentially the most hazardous type of eruption from basaltic volcanoes in densely urbanized areas.. |
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Osamu Ishizuka, Rex N. Taylor, Nobuo Geshi, Teruki Oikawa, Yoshihisa Kawanabe, Itaru Ogitsu, Progressive mixed-magma recharging of Izu-Oshima volcano, Japan: A guide to magma chamber volume, Earth and Planetary Science Letters, 10.1016/j.epsl.2015.08.004, 430, 19-29, 2015.07, © 2015 Elsevier B.V. To discover how magmas move and interact beneath an arc we have examined the temporal and spatial evolution of the largest Izu-Bonin frontal arc volcano Izu-Oshima and the adjacent Izu-Tobu field of backarc volcanoes. Extensive 14C ages and geochemical analysis of subaerial satellite cones as well as other effusives has enabled us to construct a well-constrained ~14 ka record of Izu-Oshima volcanism.The geochemistry of Izu-Oshima is found to change systematically through the last 14000 yr. Ba/La, Pb/Ce, 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb all decrease between 10 ka and 5 ka before increasing between 5 ka and the present, while La/Yb and Nb/Zr show the reverse. These changes in composition match the addition of Izu-Tobu (backarc) magma to the Izu-Oshima plumbing system with a maximum of a 40% Izu-Tobu at around 5 ka. Progressive but asymptotically declining changes in composition through the 10-5 ka period are found to fit a model where pre-mixed magma is episodically added to, and mixed with, a chamber beneath Izu-Oshima. The 5-0 ka period reverses this trend, but is again progressive and declining, suggesting a switch to a progressive influx of pure Izu-Oshima frontal arc magma. Combining flux and eruption volume estimates with the observed geochemical mixing rates indicates that the accessible melt volume of the Izu-Oshima magma system is ~16 km3. Interaction and pre-mixing between the fluid-dominated frontal arc melt and the sediment-bearing backarc magmas must occur at deeper levels within the arc crust. This deep reservoir receives a continuous feed from the frontal arc mantle, but may periodically intercept rising magmas from the backarc source to produce episodes of magma mixing on timescales of ~5000 yr. This study demonstrates that interaction between frontal arc and backarc magma needs to be considered to achieve better understanding of material transfers and elemental budgets at subduction zones.. |
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Annalisa Cappello, Nobuo Geshi, Marco Neri, Ciro Del Negro, Lava flow hazards-An impending threat at Miyakejima volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2015.10.005, 308, 1-9, 2015.12, © 2015 Elsevier B.V. The majority of the historic eruptions recorded at Miyakejima volcano were fissure eruptions that occurred on the flanks of the volcano. During the last 1100 years, 17 fissure eruptions have been reported with a mean interval of about 76-78 years. In the last century, the mean interval between fissure eruptions decreased to 21-22 years, increasing significantly the threat of lava flow inundations to people and property. Here we quantify the lava flow hazards posed by effusive eruptions in Miyakejima by combining field data, numerical simulations and probability analysis. Our analysis is the first to assess both the spatiotemporal probability of vent opening, which highlights the areas most likely to host a new eruption, and the lava flow hazard, which shows the probabilities of lava-flow inundation in the next 50 years. Future eruptive vents are expected in the vicinity of the Hatchodaira caldera, radiating from the summit of the volcano toward the costs. Areas more likely to be threatened by lava flows are Ako and Kamitsuki villages, as well as Miike port and Miyakejima airport. Thus, our results can be useful for risk evaluation, investment decisions, and emergency response preparation.. |
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Koshun Yamaoka, Nobuo Geshi, Takeshi Hashimoto, S. E. Ingebritsen, Teruki Oikawa, Special issue "the phreatic eruption of Mt. Ontake volcano in 2014" the Phreatic Eruption of Mt. Ontake Volcano in 2014 5. Volcanology, Earth, Planets and Space, 10.1186/s40623-016-0548-4, 68, 1, 2016.12. |
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S. Kósik, K. Németh, G. Kereszturi, J. N. Procter, G. F. Zellmer, N. Geshi, Phreatomagmatic and water-influenced Strombolian eruptions of a small-volume parasitic cone complex on the southern ringplain of Mt. Ruapehu, New Zealand: Facies architecture and eruption mechanisms of the Ohakune Volcanic Complex controlled by an unstable fissure eruption, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2016.07.005, 327, 99-115, 2016.11, © 2016 Elsevier B.V. The Ohakune Volcanic Complex is a late Pleistocene tuff ring – scoria/spatter cone complex located south of Ruapehu volcano. This small-volume volcano consists of an outer E-W elongated compound tuff ring edifice, three inner scoria-spatter cones and further volcanic depressions, located on the Ohakune Fault. We quantified accurately the variations of the eruptive styles and processes through time by systematic sampling of key stratigraphic marker beds at proximal and distal locations, and the determination of grain size distribution, componentry, density and vesicularity. Using a Digital Terrain Model coupled with stratigraphic data, we also determined the spatial distribution and volume of each identified unit and individual edifices within the Ohakune Volcanic Complex. Activity began with a shallow phreatomagmatic phase characterized by an almost continuous generation of a low eruptive column, accompanied by wet pyroclastic density currents, together with the ejection of juvenile fragments and accidental lithics from the surrounding country rocks. Subsequent activity was dominated by a variety of Strombolian eruptions exhibiting differing intensities that were at times disrupted by phreatic blasts or phreatomagmatic explosions due to the interaction with external water and/or sudden changes in magma discharge rate. At least three major vent-shifting events occurred during the eruption, which is demonstrated by the truncation of the initial tuff ring and the infilling of the truncated area by several coarse grained surge units. Our study indicates that approx. 12 × 106 m3 DRE magma erupted within maximum 2.5 to 5 months through multiple vents. The erupted magma ascended from a depth of 16–18 km, and reached the surface within approximately 50 h. Alternating eruption styles, frequent vent-shifting and a variety of emplacement mechanisms inferred from the deposits of the Ohakune Volcanic Complex demonstrate the unpredictable nature of small-volume volcanism, contributing to our understanding of hazards attributed to this type of activity. Future small-volume eruptions within the Taupo Volcanic Zone may be of similar nature.. |
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Nobuo Geshi, Teruki Oikawa, Orientation of the eruption fissures controlled by a shallow magma chamber in Miyakejima, Frontiers in Earth Science, 10.3389/feart.2016.00099, 4, 2016.11, © 2016 Geshi and Oikawa. Orientation of the eruption fissures and composition of the lavas of the Miyakejima volcano is indicative of the competitive processes of the regional tectonic stress and the local stress generated by the activity of a magma plumbing system beneath the volcano. We examined the distributions and magmatic compositions of 23 fissures that formed within the last 2800 years, based on a field survey and a new dataset of 14 C ages. The dominant orientation of the eruption fissures in the central portion of the volcano was found to be NE-SW, which is perpendicular to the direction of regional maximum horizontal compressive stress (σHmax). Magmas that show evidence of mixing between basaltic and andesitic compositions erupted mainly fromthe eruption fissures with a higher offset angle from the regional σHmax direction. The presence of a dike pattern perpendicular to the direction of maximum compression σHmax is an unusual and uncommon feature in volcanoes. Here we investigate the conditions possibly controlling this unexpected dike pattern. The distribution and magmatic compositions of the eruption fissures in Miyakejima volcano highlight the tectonic influence of shallow magma chamber on the development of feeder dikes in a composite volcano. The presence of a shallow dike-shaped magma chamber controls the eccentric distribution of the eruption fissures perpendicular to the present direction of σHmax. The injection of basaltic magma into the shallow andesitic magma chamber caused the temporal rise of internal magmatic pressure in the shallow magma chamber which elongates in NE-SW direction. Dikes extending from the andesitic magma chamber intrude along the local stress field which is generated by the internal excess pressure of the andesitic magma chamber. As the result, the eruption fissures trend parallel to the elongation direction of the shallow magma chamber. Some basaltic dikes from the deep-seated magma chamber reach the ground surface without intersection with the andesitic magma chamber. These basaltic dikes develop parallel to the regional compressive stress in NW-SE direction. The patterns of the eruption fissures can be modified in the future as was observed in the case of the destruction of the shallow magma chamber during the 2000 AD eruption.. |
| 45. |
Shinji Takarada, Teruki Oikawa, Ryuta Furukawa, Hideo Hoshizumi, Jun'Ichi Itoh, Nobuo Geshi, Isoji Miyagi, Estimation of total discharged mass from the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, Earth, Planets and Space, 10.1186/s40623-016-0511-4, 68, 1, 2016.12, © 2016 The Author(s). The total mass discharged by the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, was estimated using several methods. The estimated discharged mass was 1.2 × 106 t (segment integration method), 8.9 × 105 t (Pyle's exponential method), and varied from 8.6 × 103 to 2.5 × 106 t (Hayakawa's single isopach method). The segment integration and Pyle's exponential methods gave similar values. The single isopach method, however, gave a wide range of results depending on which contour was used. Therefore, the total discharged mass of the 2014 eruption is estimated at between 8.9 × 105 and 1.2 × 106 t. More than 90 % of the total mass accumulated within the proximal area. This shows how important it is to include a proximal area field survey for the total mass estimation of phreatic eruptions. A detailed isopleth mass distribution map was prepared covering as far as 85 km from the source. The main ash-fall dispersal was ENE in the proximal and medial areas and E in the distal area. The secondary distribution lobes also extended to the S and NW proximally, reflecting the effects of elutriation ash and surge deposits from pyroclastic density currents during the phreatic eruption. The total discharged mass of the 1979 phreatic eruption was also calculated for comparison. The resulting volume of 1.9 × 106 t (using the segment integration method) indicates that it was about 1.6-2.1 times larger than the 2014 eruption. The estimated average discharged mass flux rate of the 2014 eruption was 1.7 × 108 kg/h and for the 1979 eruption was 1.0 × 108 kg/h. One of the possible reasons for the higher flux rate of the 2014 eruption is the occurrence of pyroclastic density currents at the summit area.. |
| 46. |
Nobuo Geshi, Yasuo Miyabuchi, Conduit enlargement during the precursory Plinian eruption of Aira Caldera, Japan, Bulletin of Volcanology, 10.1007/s00445-016-1057-9, 78, 9, 2016.09, © 2016, Springer-Verlag Berlin Heidelberg. Increase in magma flux as the result of conduit enlargement is one of the key processes that triggered caldera collapse and eruption of the Ito ignimbrite from Aira Caldera at ~29 ka. We examine the total volume of the pumice fall deposit, vertical variations in grain size of pumice, and the lithic content in the Osumi pumice deposit to investigate the trigger for caldera collapse. Wider distribution of the later-stage unit and the upward coarsening of grain size throughout the Osumi pumice fall deposit indicate an increase in magma discharge toward the onset of collapse. The total volume of lithic fragments in the Osumi pumice fall deposit is estimated as ~1.6 km3, based on the lithic content in several representative outcrops and the total volume of the Osumi pumice fall deposit. The lithic fragments in the Osumi pumice fall deposit indicate intense mechanical erosion of the conduit during the Plinian eruption prior to caldera collapse. Caldera collapse requires decompression of the magma chamber by withdrawal of magma; effective enlargement of the conduit diameter during precursory eruptive phases is one of the important processes that subsequently allow the rapid discharge of a large volume of magma, which in turn facilitates decompression of the reservoir and induces caldera collapse.. |
| 47. |
G. Giordano, M. L. Carapezza, G. Della Monica, M. Todesco, P. Tuccimei, G. Carlucci, A. A. De Benedetti, A. Gattuso, C. Lucchetti, M. Piersanti, M. Ranaldi, L. Tarchini, N. M. Pagliuca, T. Ricci, S. Facchini, F. D'Ambrosio, M. Misuraca, A. Bonamico, N. Geshi, Conditions for long-lasting gas eruptions: The 2013 event at Fiumicino International Airport (Rome, Italy), Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2016.06.020, 325, 119-134, 2016.10, © 2016 Elsevier B.V. A hazardous gas eruption from two very close shallow boreholes occurred near the Fiumicino International Airport of Roma (Italy) from August to December 2013. The erupted gas was mostly CO 2 of deep origin and gas output was high and sustained over time reaching values of nearly 20 t day•1. After 3 months, the gas flux was still above 5 t day•1 and was only stopped in December 2013 by long and expensive works of closure of the boreholes. The gas eruption was uncommon as being associated with the building of two mud volcanoes. This style of sustained deep CO 2 eruptions contrasts with the more common short-lived eruptions of shallow biogenic methane-dominated gas pockets. In this work, we present the chronology of the event, the results of geological, geochemical, and geophysical monitoring and a numerical modeling. We propose that the August-December 2013 sustained and prolonged event does not relate to the simple degassing of a shallow, isolated pocket of gas. On the contrary, it reflects very specific conditions in a shallow reservoir (hosted in a 10 m thick gravel layer at –40 m within the Tiber river delta deposits), related to the interplay between the total pressure and the fraction of free CO 2 initially present, across very narrow value ranges around 0.59 MPa and 0.18, respectively. The coexistence of short-lived and long-lived eruptions from the same reservoir suggest that these conditions are not achieved everywhere in the gas reservoir, despite its homogeneous properties. This consideration implies either a pressure compartmentalization of the reservoir, or the occurrence of a transient, possibly associated with an impulsive release of gas from greater depths. The involvement of deeper and larger gas reservoirs connected along faults is evidenced by geophysical investigations. This conceptual model bears significant implications for gas hazard studies. |
| 48. |
Hiroshi Shinohara, Nobuo Geshi, Nobuo Matsushima, Genji Saito, Ryunosuke Kazahaya, Volcanic gas composition changes during the gradual decrease of the gigantic degassing activity of Miyakejima volcano, Japan, 2000-2015, Bulletin of Volcanology, 10.1007/s00445-017-1105-0, 79, 2, 2017.02, © 2017, Springer-Verlag Berlin Heidelberg. The composition of volcanic gases discharged from Miyakejima volcano has been monitored during the intensive degassing activity that began after the eruption in 2000. During the 15 years from 2000 to 2015, Miyakejima volcano discharged 25.5 Mt of SO2, which required degassing of 3 km3 of basaltic magma. The SO2 emission rate peaked at 50 kt/day at the end of 2000 and quickly decreased to 5 kt/day by 2003. During the early degassing period, the volcanic gas composition was constant with the CO2/SO2 = 0.8 (mol ratio), H2O/SO2 = 35, HCl/SO2 = 0.08, and SO2/H2S = 15. The SO2 emission rate decreased gradually to 0.5 kt/day by 2012, and the gas composition also changed gradually to CO2/SO2 = 1.5, H2O/SO2 = 150, HCl/SO2 = 0.15, and SO2/H2S = 6. The compositional changes are not likely caused by changes in degassing pressure or volatile heterogeneity of a magma chamber but are likely attributed to an increase of hydrothermal scrubbing caused by large decrease of the volcanic gas emission rate, suggesting a supply of gases with constant composition during the 15 years. The intensive degassing was modeled based on degassing of a convecting magma conduit. The gradual SO2 emission rate that decrease without changes in volcanic gas composition is attributed to a reduction of diameter of the convecting magma conduit.. |
| 49. |
Nobuo Geshi, Fukashi Maeno, Shojiro Nakagawa, Hideto Naruo, Tetsuo Kobayashi, Tsunami deposits associated with the 7.3 ka caldera-forming eruption of the Kikai Caldera, insights for tsunami generation during submarine caldera-forming eruptions, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2017.09.015, 347, 221-233, 2017.11, © 2017 Elsevier B.V. Timing and mechanism of volcanic tsunamis will be a key to understand the dynamics of large-scale submarine explosive volcanism. Tsunami deposits associated with the VEI 7 eruption of the Kikai Caldera at 7.3 ka are found in the Yakushima and Kuchinoerabujima Islands, ~ 40 km south -southeast of the caldera rim. The tsunami deposits distribute along the rivers in their northern coast up to ~ 4.5 km from the river exit and up to 50 m above the present sea level. The tsunami deposits in the Yakushima area consist of pumice-bearing gravels in the lower part of the section (Unit I) and pumiceous conglomerate in the upper part (Unit II). The presence of rounded pebbles of sedimentary rocks, which characterize the beach deposit, indicates a run-up current from the coastal area. The rip-up clasts of the underlying paleosol in Unit I show strong erosion during the invasion of tsunami. Compositional similarity between the pumices in the tsunami deposit and the juvenile materials erupted in the early phase of the Akahoya eruption indicates the formation of tsunami deposit during the early phase of the eruption, which produced the initial Plinian pumice fall and the lower half of the Koya pyroclastic flow. Presence of the dense volcanic components (obsidians and lava fragments) besides pumices in the tsunami deposit supports that they were carried by the Koya pyroclastic flow, and not the pumices floating on the sea surface. Sequential relationship between the Koya pyroclastic flow and the tsunami suggests that the emplacement of the pyroclastic flow into the sea surrounding the caldera is the most probable mechanism of the tsunami.. |
| 50. |
A. Todde, R. Cioni, M. Pistolesi, N. Geshi, C. Bonadonna, The 1914 Taisho eruption of Sakurajima volcano: stratigraphy and dynamics of the largest explosive event in Japan during the twentieth century, Bulletin of Volcanology, 10.1007/s00445-017-1154-4, 79, 10, 2017.10, © 2017, Springer-Verlag GmbH Germany. The 1914 Taisho eruption of Sakurijima volcano was Japan’s highest intensity and magnitude eruption of the twentieth century. After a 35-year period of quiescence, the volcano suddenly rewoke a few days before the eruption, when earthquakes began to be felt on Sakurajima Island. The eruption began on January 12, 1914, from two fissures located on opposite sides of the volcano, and was characterized by a complex time evolution and changes in eruptive styles. The eruption began with a subPlinian explosive phase in which two convective columns rose from the two fissures. Both plumes were sustained for at least 2 days. This resulted in deposition of a widely dispersed tephra sequence. After this phase, the eruption evolved to a final, waning phase, shifting toward effusive activity that lasted until April 1914. During the first weeks, effusive activity was also accompanied by ash emission. The complex sequence of events, characterized by contemporaneous explosive and effusive activity, is typical of several recently observed mid-intensity eruptions, such as during the 2011 eruption of Cordón Caulle, Chile. The stratigraphic sequence of the eruptive deposits from the Taisho eruption comprises alternating coarse-to-fine lapilli beds with ash beds dispersed toward the ESE and SE. These deposits can be subdivided into three lapilli-bearing units (Units T1, T2 and T3, which correspond to the subPlinian phase) and one ash-bearing unit (Unit T4, which corresponds to the final ash venting, accompanying the first day/weeks of lava flow activity). Grain size analyses from each unit reveal a marked polymodal distribution generally described by the sum of two or three Gaussian subpopulations. Both the modes and the relative amounts of the coarse subpopulations vary with distance from vent, with those of the fine subpopulation remaining nearly constant. Within the vertical sequence, component analysis shows a progressive increase in lithic fragments, suggesting that conduit enlargement continued until the final stages of the eruption. The estimated volume of the tephra deposit of the subPlinian phase of the eruption is 0.33 ± 0.11 km3 (dense rock equivalent (DRE) volume = 0.09 ± 0.03 km3). The height of the eruption column was also assessed by using four different isopleth maps compiled based on different strategies for the characterization of the largest clasts. The maximum height attained by the eruption column is estimated at 15.0 ± 1.2 km above the vent, resulting in a maximum mass discharge rate of 3.6 ± 1.2 × 107 kg s−1 (calculated taking into account the strong effect of wind advection). Finally, different classification schemes were applied to classify the eruption, which generally straddles the fields between Plinian and subPlinian.. |
| 51. |
Adriano Nobile, Valerio Acocella, Joel Ruch, Yosuke Aoki, Sven Borgstrom, Valeria Siniscalchi, Nobuo Geshi, Steady subsidence of a repeatedly erupting caldera through InSAR observations: Aso, Japan, Bulletin of Volcanology, 10.1007/s00445-017-1112-1, 79, 5, 2017.05, © 2017, Springer-Verlag Berlin Heidelberg. The relation between unrest and eruption at calderas is still poorly understood. Aso caldera, Japan, shows minor episodic phreatomagmatic eruptions associated with steady subsidence. We analyse the deformation of Aso using SAR images from 1993 to 2011 and compare it with the eruptive activity. Although the dataset suffers from limitations (e.g. atmospheric effects, coherence loss, low signal-to-noise ratio), we observe a steady subsidence signal from 1996 to 1998, which suggests an overall contraction of a magmatic source below the caldera centre, from 4 to 5 km depth. We propose that the observed contraction may have been induced by the release of the magmatic fluids feeding the eruptions. If confirmed by further data, this hypothesis suggests that degassing processes play a crucial role in triggering minor eruptions within open conduit calderas, such as at Aso. Our study underlines the importance of defining any eruptive potential also from deflating magmatic systems with open conduit.. |
| 52. |
S. Biass, A. Todde, R. Cioni, M. Pistolesi, N. Geshi, C. Bonadonna, Potential impacts of tephra fallout from a large-scale explosive eruption at Sakurajima volcano, Japan, Bulletin of Volcanology, 10.1007/s00445-017-1153-5, 79, 10, 2017.10, © 2017, Springer-Verlag GmbH Germany. We present an exposure analysis of infrastructure and lifeline to tephra fallout for a future large-scale explosive eruption of Sakurajima volcano. An eruption scenario is identified based on the field characterization of the last subplinian eruption at Sakurajima and a review of reports of the eruptions that occurred in the past six centuries. A scenario-based probabilistic hazard assessment is performed using the Tephra2 model, considering various eruption durations to reflect complex eruptive sequences of all considered reference eruptions. A quantitative exposure analysis of infrastructures and lifelines is presented primarily using open-access data. The post-event impact assessment of Magill et al. (Earth Planets Space 65:677–698, 2013) after the 2011 VEI 2 eruption of Shinmoedake is used to discuss the vulnerability and the resilience of infrastructures during a future large eruption of Sakurajima. Results indicate a main eastward dispersal, with longer eruption durations increasing the probability of tephra accumulation in proximal areas and reducing it in distal areas. The exposure analysis reveals that 2300 km of road network, 18 km2 of urban area, and 306 km2 of agricultural land have a 50% probability of being affected by an accumulation of tephra of 1 kg/m2. A simple qualitative exposure analysis suggests that the municipalities of Kagoshima, Kanoya, and Tarumizu are the most likely to suffer impacts. Finally, the 2011 VEI 2 eruption of Shinmoedake demonstrated that the already implemented mitigation strategies have increased resilience and improved recovery of affected infrastructures. Nevertheless, the extent to which these mitigation actions will perform during the VEI 4 eruption presented here is unclear and our hazard assessment points to possible damages on the Sakurajima peninsula and the neighboring municipality of Tarumizu.. |
| 53. |
Osamu Ishizuka, Rex N. Taylor, Nobuo Geshi, Nobutatsu Mochizuki, Large-volume lateral magma transport from the Mull volcano: An insight to magma chamber processes, Geochemistry, Geophysics, Geosystems, 10.1002/2016GC006712, 18, 4, 1618-1640, 2017.04, © 2017. American Geophysical Union. All Rights Reserved. Long-distance lateral magma transport within the crust has been inferred for various magmatic systems including oceanic island volcanoes, mid-oceanic ridges, and large igneous provinces. However, studying the physical and chemical properties of active fissure systems is difficult. Hence, this study investigates the movement of magma away from the Mull volcano in the North Atlantic Igneous Province, where erosion has exposed its upper crustal dike networks. Magmatic lineations within dikes indicate that the magma flow in the Mull dike suite changed from near vertical to horizontal within 30 km of the volcanic center. This implies that distal dikes were fed by lateral magma transport from Mull. Geochemical characteristics indicate that many |
| 54. |
G. Giordano, E. Zanella, M. Trolese, C. Baffioni, A. Vona, C. Caricchi, A. A. De Benedetti, S. Corrado, C. Romano, R. Sulpizio, N. Geshi, Thermal interactions of the AD79 Vesuvius pyroclastic density currents and their deposits at Villa dei Papiri (Herculaneum archaeological site, Italy), Earth and Planetary Science Letters, 10.1016/j.epsl.2018.03.023, 490, 180-192, 2018.05, © 2018 Elsevier B.V. Pyroclastic density currents (PDCs) can have devastating impacts on urban settlements, due to their dynamic pressure and high temperatures. Our degree of understanding of the interplay between these hot currents and the affected infrastructures is thus fundamental not only to implement our strategies for risk reduction, but also to better understand PDC dynamics. We studied the temperature of emplacement of PDC deposits that destroyed and buried the Villa dei Papiri, an aristocratic Roman edifice located just outside the Herculaneum city, during the AD79 plinian eruption of Mt Vesuvius (Italy) by using the thermal remanent magnetization of embedded lithic clasts. The PDC deposits around and inside the Villa show substantial internal thermal disequilibrium. In areas affected by convective mixing with surface water or with collapsed walls, temperatures average at around 270 °C (min 190 °C, max 300 °C). Where the deposits show no evidence of mixing with external material, the temperature is much higher, averaging at 350 °C (min 300 °C; max 440 °C). Numerical simulations and comparison with temperatures retrieved at the very same sites from the reflectance of charcoal fragments indicate that such thermal disequilibrium can be maintained inside the PDC deposit for time-scales well over 24 hours, i.e. the acquisition time of deposit temperatures for common proxies. We reconstructed in detail the history of the progressive destruction and burial of Villa dei Papiri and infer that the rather homogeneous highest deposit temperatures (average 350 °C) were carried by the ash-sized fraction in thermal equilibrium with the fluid phase of the incoming PDCs. These temperatures can be lowered on short time- (less than hours) and length-scales (meters to tens of meters) only where convective mixing with external materials or fluids occurs. By contrast, where the Villa walls remained standing the thermal exchange was only conductive and very slow, i.e. negligible at 50 cm distance from contact after 24 hours. We then argue that the state of conservation of materials buried by PDC deposits largely depends on the style of the thermal interactions. Here we also suggest that PDC deposit temperatures are excellent proxies for the temperatures of basal parts of PDCs close to their depositional boundary layer. This general conclusion stresses the importance of mapping of deposit temperatures for the understanding of thermal processes associated with PDC flow dynamics and during their interaction with the affected environment.. |
| 55. |
Shinohara Hiroshi, Geshi Nobuo, Yokoo Akihiko, Ohkura Takahiro, Terada Akihiko, Salt shell fallout during the ash eruption at the Nakadake crater, Aso volcano, Japan: evidence of an undergrounds hydrothermal system surrounding the erupting vent, Earth, Planets and Space, 10.1186/s40623-018-0798-4, 70, 70, 2018.03, A hot and acid crater lake is located in the Nakadake crater, Aso volcano, Japan. The volume of water in the lake decreases with increasing activity, drying out prior to the magmatic eruptions. Salt-rich materials of various shapes were observed, falling from the volcanic plume during the active periods. In May 2011, salt flakes fell from the gas plume emitted from an intense fumarole when the acid crater lake was almost dry. The chemical composition of these salt flakes was similar to those of the salts formed by the drying of the crater lake waters, suggesting that they originated from the crater lake water. The salt flakes are likely formed by the drying up of the crater lake water droplets sprayed into the plume by the fumarolic gas jet. In late 2014, the crater lake dried completely, followed by the magmatic eruptions with continuous ash eruptions and intermittent Strombolian explosions. Spherical hollow salt shells were observed on several occasions during and shortly after the weak ash eruptions. The chemical composition of the salt shells was similar to the salts formed by the drying of the crater lake water. The hollow structure of the shells suggests that they were formed by the heating of hydrothermal solution droplets suspended by a mixed stream of gas and ash in the plume. The salt shells suggest the existence of a hydrothermal system beneath the crater floor, even during the course of magmatic eruptions. Instability of the magmatic–hydrothermal interface can cause phreatomagmatic explosions, which often occur at the end of the eruptive phase of this volcano.. |
| 56. |
Pyroclastic density currents associated with the 2015 phreatomagmatic eruption of the Kuchinoerabujima volcano. |
| 57. |
Mitsuru Okuno, Toshio Nakumura, Minoru Sakamoto, Shinya Yatsuzuka, Teruki Oikawa, Nobuo Geshi, Yasuharu Hoshino, Toshihiko Takahashi, Eruption age of the Haruna Futatsudake Pumice (Hr-FP), central Japan, by radiocarbon wiggle matching with special reference to a 14C dataset developed from a Japanese tree, Quaternary International, 10.1016/j.quaint.2018.12.023, 527, 29-33, 2019.08, © 2019 Elsevier Ltd and INQUA Haruna volcano, located in central Japan, erupted the Hr-FA and Hr-FP tephras during a period of several decades around the 6th century. To estimate the precise age of the second eruption, we performed AMS radiocarbon dating on a charred wood trunk (29 rings) collected from a pyroclastic flow deposit of the Hr-FP, and obtained a year range of cal AD 424–444 (2σ) using 14C wiggle matching of seven dates with the IntCal13 dataset. However, this result is approximately 100 years older than the inferred calendar year from Sue pottery and Kofun (ancient tomb mound) chronologies and is also inconsistent with the stratigraphic relation of the Hr-FA and Hr-FP. For the 5−6th centuries when the Hr-FP might have erupted, 14C dating was performed on Japanese trees whose precise chronologies were established dendrochronologically. The 14C ages of the annual rings of these trees showed local offsets about 30 years older than those of IntCal13. This tendency has been observed in several studies on trees in Japan and is consistent with the results from the Japanese cedar tree. As a preliminary study, the Hr-FP data were wiggle matched with the existing local dataset for AD 340–545 obtained from one tree in central Japan. The year of the outermost annual ring of the sample tree collected in the Hr-FP tephra is estimated to be cal AD 538−559 (79.0% probability). This estimated age of the Hr-FP eruption is consistent with the archaeological chronology (Sue pottery and Kofun) and tephrochronology around Haruna volcano.. |
| 58. |
Report of the IAVCEI 5th Volcano Geology Workshop in New Zealand. |
| 59. |
Keiko Matsumoto, Nobuo Geshi, Shallow crystallization of eruptive magma inferred from volcanic ash microtextures: a case study of the 2018 eruption of Shinmoedake volcano, Japan, Bulletin of Volcanology, 10.1007/s00445-021-01451-6, 83, 5, 2021.05, AbstractThe occurrence of groundmass crystals reveals the shallow conduit process of magmas, which affects the behavior of eruptions. Here, we analyzed groundmass microtextures of ash samples from the 2018 eruption of Shinmoedake volcano, Japan, to evaluate the change of magma ascent conditions during the eruption sequence. The eruptive activity changed from ash venting (Phase 1: March 1–6) to lava effusion with continuous ash-laden plumes (Phase 2: March 6–9) and then shifted to Vulcanian explosions (Phase 3: March 10–April 5). Non-juvenile particles were abundant in Phase 1, whereas juvenile particles were dominant in Phases 2 and 3. Vesicular juvenile particles were more abundant in Phase 2 than Phase 3. The lower microlite crystallinity and groundmass SiO2 concentrations of the vesicular particles indicate that they were sourced from magma that ascended rapidly. Abundant nanolites were observed in the black interstitial glass of juvenile particles under an optical microscope, whereas few nanolites were observed in the transparent ones. The presence of nanolites can be explained by the dehydration of silicate melt, as well as cooling and oxidation between fragmentation and quenching. Temporal changes in the ash componentry show that the eruption activity started from the erosion of the pre-existing vent plug (Phase 1), shifted to the simultaneous eruption of bubble-bearing and outgassed magmas (Phase 2), and concluded with explosions of the stagnant lava (Phase 3), thereby demonstrating the sequence of vent opening and extrusion and stagnation of magma. Therefore, ash microtextures are valuable for monitoring the shallow conduit process of eruptive magma.. |
