著者

井口 正人 為栗 健 平林 順一 中道 治久

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.64, no.2, pp.33-51, 2019-06-30 (Released:2019-07-06)

参考文献数

50

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In order to find an empirical event branch logic from abnormal phenomena to following volcanic activity for forecasting scale and type of eruption, the magma intrusion rate prior to eruptions of Sakurajima volcano is examined using ground deformation mostly from observation data and partially based on legends, for eruptions after the 20th century: the 1914 eruption starting with plinian eruption followed by effusion of lava, the 1946 eruptions with lava effusion, eruptions at the summit crater of Minamidake during the period from 1955 to 2005, and vulcanian eruptions at Showa crater east of the summit from 2006 to 2017. Prior to the 1914 eruption, it is estimated that the magma intrusion rate attained a level of approximately 108m3/day and was on the order of 106m3/day during the effusion of lava in the 1946 eruption. During the eruptive period of Minamidake summit crater, three types of eruption occurred: vulcanian eruption, strombolian/lava fountain and continuous emission of volcanic ash. In cases of intrusion of magma forming a new conduit, the intrusion rate immediately before the 1914 eruption exceeded 108m3/day, but only 106m3/day in the dyke-forming event of August 15, 2015. Magma intrusion rate into a pre-existing conduit prior to eruptions at Minamidake summit crater are ordered as follows: vulcanian eruption (1×105 to 8×105m3/day)>continuous emission of volcanic ash (approximately 1×105m3/day)>strombolian/lava fountain (0.2×105 to 2×105m3/day). The magma intrusion rate prior to vulcanian eruptions at Showa crater is smaller (approximately 104m3/day) than for eruptions at Minamidake summit crater. However, the rate reached an order of 105m3/day prior to lava fountain on August 22, 2017. Magma intrusion rates well correspond to the scale and type of eruption. In the case of magma intrusion under detection, the change of volcanic gas and increase in the heat discharge rate are available for the empirical event branch logic.

著者

吉田 明夫 細野 耕司

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.6, pp.727-738, 2003-01-08 (Released:2017-03-20)

参考文献数

28

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We investigated how the intermediate-depth seismicity changes with depth in the Pacific slab beneath Hokhaido and the Kanto district using JMA data since Oct. 1997. We found that seismicity in the upper seismic plane decreases noticeably at about the depth of 100 km and on its deeper side both in Hokkaido and the Kanto district. In Hokkaido the volcanic front is located nearly above the line where the seismicity in the upper seismic plane starts to decrease (the D line). This feature is the same as that seen in the Tohoku district (Hosono and Yoshida, 200la). In the Kanto region, however, the D line is located several tens km to the east of the volcanic front, the cause of which may be attributed to the subduction of the Philippine Sea plate above the Pacific plate. We think the noticeable decrease of seismicity in the upper seismic plane at about the depth of 100 km which is commonly observed in the Pacific slab beneath Japan and the correspondence between the location of the D Iine and the volcanic front in Hokkaido and the Tohoku district indicate that the decrease of seismicity in the upper seismic plane may be related to the genetic process of magmatic bodies in the subduction zone.

著者

橋本 武志

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.4, pp.489-500, 2022-12-31 (Released:2023-01-30)

参考文献数

18

著者

高橋 良 伊藤 久敏

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.65, no.3, pp.69-82, 2020-09-30 (Released:2020-10-13)

参考文献数

32

被引用文献数

1

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We detected thick pumice layers by observations of cuttings taken from a 1,000 m deep hot spring well at the central Hakodate City, southwestern Hokkaido. In this study, we perform petrological investigations and U-Pb dating for these pumice layers and compare them with the eruptive products from the Quaternary volcanoes at the southern Oshima Peninsula. The pumice layers can be divided into several porphyritic pumice (depth 570-420 m, 310-283 m and 261-66 m) and aphyric pumice (depth 950-660 m, 420-380 m, 343-310 m and 283-261 m) layers. The porphyritic pumice is rich in plagioclase and hornblende phenocrysts and occasionally contains minor amounts of orthopyroxene, clinopyroxene, biotite, quartz and cummingtonite phenocrysts. The U-Pb ages of the lowermost and uppermost parts of the porphyritic pumice layers are 0.93±0.04 Ma and 0.81±0.06 Ma, respectively. Although lithic-rich part is partly observed, these pumice layers mostly consist of pumice clasts. In addition, pumices show variously welded textures at several depths. These features suggest that the thick pumice layers resulted from repeated pyroclastic flows and that reworked deposits are limited. This indicates that the source of the pumice layers should be located adjacent to Hakodate City, whereas the petrological features and U-Pb age of the pumice layers are inconsistent with the known eruptive products of the Quaternary volcanoes at the southern Oshima Peninsula. Therefore, our results indicate that unknown pumice eruptions repeatedly occurred around this area during 0.9-0.8 Ma. Although any pumice eruptions have not been detected in Hakodateyama volcano (1.5 km SW of the hot spring well), the eruption ages and mineral assemblage of the porphyritic pumice are similar to the lavas of the volcano, implying that the source of the pumices was Hakodateyama volcano or its adjacent area.

著者

奥野 充 井口 正人 三好 雅也 三浦 大助 小林 淳 橋本 武志 大場 武 佐藤 鋭一 宝田 晋治

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.251-254, 2022-09-30 (Released:2022-10-27)

参考文献数

18

著者

鈴木 由希

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.2, pp.233-244, 2022-06-30 (Released:2022-07-28)

参考文献数

27

著者

小山 真人

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.43, no.5, pp.349-371, 1998-10-30 (Released:2017-03-20)

参考文献数

53

被引用文献数

2

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Reevaluation of places, type, magnitude, and influences of the 800-802 A.D. eruption (Enryaku eruption) of Fuji Volcano, Japan, was made through tephrochronology and analyses of historical records. The Nishi Kofuji fissure on the northeastern slope is newly recognized as a crater of the 802 A.D. flank eruption. The Nishi Kofuji fissure ejected fallout scoria toward ENE and lava flows, which can be correlated with Takamarubi and Hinokimarubi 11 Lavas on the northeastern foot. The Tenjinyama-lgatonoyama fissure on the northwestern slope probably erupted during the Enryaku eruption and ejected fallout scoria and lava fiows. A series of historical documents and paintings (Miyashita documents), which are unauthorized, personal records and are regarded to be unreliable by many historians, includes many detailed descriptions of paleogeogra-phy around Fuji Volcano and of the Enryaku eruption. Although some of the descriptions were exaggerated and conflict with geological observations, some of them are concordant with geologic data. The Enryaku eruption probably gave serious damages to ancient traffic routes particularly on the northwestern-northeastern foot of Fuji Volcano. The Gotenba area, which is located on the eastern foot, was also damaged by thin ash-fall and probably by lahars. This caused a temporal, southward relocation of the offical trafiic route, which had passed through the Gotenba area.

著者

草野 有紀 及川 輝樹 石塚 吉浩

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.4, pp.327-346, 2021-12-31 (Released:2022-02-22)

参考文献数

54

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Nikko-Shirane Volcano located on the border of Gunma and Tochigi prefectures had the largest eruption on the historic records in AD 1649. We reconstructed the eruption event based on the geological mapping of the pyroclastic fall deposit and craters at the summit, 14C dating of soil underlying the pyroclastic fall deposit and interpretation of historic records. The pyroclastic fall deposit is observed in a 10×6 km area around Nikko-Shirane Volcano and thickens to the summit of Mt. Shirane. The pyroclastic fall deposit is preserved at>4 km east from the summit and observed 5-8 cm thick around Lake Yunoko and 20 cm thick in maximum around the southern part of Senjogahara. Based on the historic records of the 1649 eruption, the craters with about 220 m in long axis diameter and 30 m deep located next to a small shrine at the summit were opened. Thus, the 1649 eruption is considered to occur at the summit of Mt. Shirane and pyroclastic materials fell east to southeast ward. The total mass of pyroclastic fall deposit is estimated at 2×107-3×107 m3 which is a digit larger than the previous report, and it is comparable to Volcanic Explosive Index=3 and Magnitude=3.4-3.6. The pyroclastic material contains essential vesicular vitreous particles consisting 1-48 % (mean 19 %) of component in 250-2000 μm fraction. Combination of the essential particles in the 1649 pyroclastic materials suggests that a magmatic eruption was occurred during the 1649 eruption. The essential particles are concentrated in three principal distribution axes of the pyroclastic fall deposit extending to the east, southeast and west. However, the pyroclastic fall deposit is composed of a lot of fine particles, indicating that the 1649 eruption would be possible of a phreatomagmatic eruption triggered by magma intrusion to an aquifer below the volcanic body. Around the time of the eruption, lahar occurred at the western valley of Mt. Shirane and flowed through Ohirogawara to the Nigamatazawa River.

著者

小山 真人

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.43, no.5, pp.323-347, 1998-10-30 (Released:2017-03-20)

参考文献数

74

被引用文献数

5

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All available historical documents, which reeord abnormal phenomena relating (or possibly relating) to the activity of Fuji Volcano, Japan, were re-examined and classified aceording to the reliability of each document. Comparisons of the reliable descriptions with geologic evidence were executed and several new correlations between historical records and eruptive deposits are proposed. Volcanic activity of Fuji Volcano was in high-level from the 9th to I Ith century; in this period at least 7 reliable and 5 possible eruptions occurred. Although only 2 reliable and 1 possible eruption records exist from the 12th to the early 17th century, this low-level activity may be apparent because of lack of enough historical records. After the middle 17th century, enough historical records suggest that the activity is generally low except for the 1707 eruption, which is one of the most voluminous and explosive eruptions in the history of Fuji Volcano. At least thirteen large earthquakes (M 8 and possible M 8 class) have occurred near Fuji Volcano (in east Nankai and Sagami Troughs) since the 9th century. Eleven of these 13 earthquakes were accompanied with volcanic events (eruption, rumbling, or change in geothermal activity) of Fuji Volcano before and/or after each earthquake (in ±25 years).

著者

山本 裕朗

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.48, no.1, pp.11-25, 2003-03-18

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The lava effusion process from a cinder cone and its mechanism are discussed based on the field observation of Ojika-Jima Monogenetic Volcano Group (OMVG). The cinder cones of OMVG are classified into two types, C-type and D-type cones, based on the mode of lava effusion from the cone. In the C-type cone, lava overflowed from the central crater, whereas in the D-type cone, lava flowed out from the flank. These types are related to the morphology and internal structure of the cone. The ratio of cone height (H_<co>) to width (W_<co>) of the C-type is smaller than that of the D-type, and the part of the dense welding is widespread around the cone. On the other hand, the welding area of the D-type is within the limits to the central part of the cone. The D-type is further divided into two types; Dc-type is accompanied by a mountain body collapse with lava effusion and Dp-type does not have this collapse. The majority of Dc-type cones are larger than Dp-type cones, although the ratios of H_<co>/W_<co> are similar. In the OMVG, a thin dike (less than 1 m thick) is generally observed inside the cone. However, if a dike intrusion does not have enough stress to collapse a mature cone, a branched dike system could cause a much larger load to the slope of cone and push a sector of the cone outward. Therefore, a branched dike system seems to control in cone breaching. The dike system is always observed inside Dc-type cones, while it is rare inside Dp-type cones. Considering the concept of crack propagation in an elastic body, the dike branches off under the condition that the breaking strength of the deposit around the tip of a feeder dike is low. Accordingly, the collapse of a cinder cone caused by a branched dike system is incident in the larger-scaled cinder cone, especially when the welded area is restricted to the central part of the cone and altitude difference between the lava lake in the crater and the top of the dike is large. It has been assumed in previous works that the density difference between the lava and cinder cone is the main controlling factor for the mode of lava effusion from the cinder cone. In this paper, the author concluded that the degree of welding around the feeder dike and total volume of the cinder cone are the major controlling factors in the dike propagation process.

著者

宮縁 育夫 星住 英夫 渡辺 一徳

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.49, no.2, pp.51-64, 2004-05-20 (Released:2017-03-20)

参考文献数

33

被引用文献数

5

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The stratigraphy and chronology of late-Pleistocene (after deposition of AT ash) tephra layers erupted from post-caldera central cones of Aso Volcano, SW Japan, are evaluated through study of the thick tephra sequence preserved mainly atop the pyroclastic-flow plateau east of the caldera. Twenty andesite to basaltic-andesite scoria-fall deposits (YmS20-YmS1 in ascending order) and two pumice-fall deposits (NbP2 and NbP1) were identified as maker beds for the interval ca. 29-13ka (calibrated 14C age). The tephra layers were produced mostly by scoria and ash eruptions of Nakadake, the only active post-caldera central cone, and their bulk volumes calculated from isopach maps range from 0.01 to 0.9km3. The radiocarbon ages of buried soils just below individual tephra layers reveal that YmS20 to YmS15 were erupted at ca. 22-21 ka and YmS20 to Ym-S1 at ca. 18-16ka. Total tephra volume during 29-13ka is estimated at about 1.3km3 (DRE). Production of tephra was mainly concentrated in two periods : 22-21 ka (0.8km3) and 18-16ka (0.5km3).

著者

早川 由紀夫 中島 秀子

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.43, no.4, pp.213-221, 1998-08-31

被引用文献数

4

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The 1108 eruption of Asama is the largest among numerous eruptions of the volcano during the Holocene. The magnitude is twice as large as that of the notorious 1783 eruption, which killed about 1,400 people. It is also the oldest written eruption of Asama. Chuyuki, which was written in Kyoto, 300 km SW of Asama, describes that the eruption started on September 29, 1108, by the Julian calendar, and that fields of rice and other crops were severely damaged. Many fatalities are strongly suspected by the distribution of the Oiwake ignimbrite, but no description is given for human loss in Chuyuki. A thin pumice layer intercalated between the 1108 scoria and the 1783 pumice can be correlated to a record of Pele's hair-fall in Kyoto in 1596. As many as 800 fatalities at the summit in 1598 described in Todaiki cannot be true. Tenmei Shinjo Hen'iki, which describes that a number of villages along the Jabori River were swept away by hot lahars in 1532, is not a contemporary document. It was written in the late 18th century. Fifteen fatalities at the summit in 1721 can be true. After the 1783 eruption, Asama had been relatively quiet for 100 years. During the early and middle 20th century, Asama had been very active with a peak of 398 times vulcanian explosions in 1941. About 30 Iives were lost at the summit, in the 20th century, by 12 explosions among the total about 3,000 explosions.

著者

栗谷 豪

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.3, pp.147-156, 2021-09-30 (Released:2021-10-29)

参考文献数

35

著者

長谷川 健 柴田 翔平 小林 哲夫 望月 伸竜 中川 光弘 岸本 博志

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.3, pp.187-210, 2021-09-30 (Released:2021-10-29)

参考文献数

57

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Based on detailed fieldwork, petrological and paleomagnetic investigations, we present a revised stratigraphy of deposits from the 7.6 ka eruption at Mashu volcano and the formation process of its summit caldera, eastern Hokkaido, Japan. As previously described, the eruption products consist of an initial phreatomagmatic unit (Ma-j) and the overlying three pumice-fall layers (Ma-i, -h, and -g), which are in turn overlain by pyroclastic-flow deposits (Ma-f). In the present study, we divide Ma-f into 4 subunits: Ma-f1/2, Ma-fAc, Ma-f3a and Ma-f3b in descending order. Ma-f3b is a valley-ponding, pumice-flow deposit with limited distribution. Ma-f3a comprises clast-supported facies (fines-depleted ignimbrite: FDI) and matrix-supported (normal ignimbrite) facies, the two changing across topography. The FDI is characterized by a gray, fines-depleted, lithic-breccia-rich layer with materials incorporated from the substrate. Impact sag structures from large (>50 cm) dacite ballistic blocks were recognized at the base of the Ma-f3a within 10 km from the source. Ma-fAc is a minor eruption unit consisting of accretionary lapilli. Ma-f1/2 is a most voluminous (8.8 km3), widely distributed and weakly stratified ignimbrite. Both Ma-f3a and Ma-f1/2 can be classified as “low aspect ratio ignimbrite (LARI)”. Dacite lithic fragments are ubiquitously observed throughout the sequence and are not considered to be juvenile; they have distinctly different chemical compositions from the pumice fragments in the early pumice-fall (Ma-g~Ma-i) and pyroclastic-flow (Ma-f3b) deposits, but those of pumice clasts in the late pyroclastic-flow units (Ma-f3a and Ma-f2) lie between the two on a FeO*/MgO vs. SiO2 diagram. The 7.6 ka caldera-forming eruption of the Mashu volcano was initiated by Plinian fall (Ma-j~-g), and then, a small-volume high aspect ratio ignimbrite (Ma-f3b) was deposited by a valley-confined pyroclastic flow that was generated by partial column collapse. After that, a violent pyroclastic flow was generated probably during a strong explosion of a dacite lava edifice on the summit of Mashu volcano. This flow emplaced Ma-f3a. The caldera collapse that followed the explosion generated a climactic pyroclastic flow that emplaced Ma-f1/2. Ma-f3a flow was extremely fast. Ma-f1/2 flow was related to sustained flow due to low settling velocity and high discharge volume. These are supported by field observations and numerical simulation that shows the ability of the flow to surmount high topographic obstacles and spread widely. The 7.6 ka caldera-forming process of Mashu volcano was driven not only by subsidence of roof block but also by violent explosions.

著者

湯浅 真人

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.40, no.4, pp.277-284, 1995-09-20 (Released:2017-03-20)

参考文献数

29

被引用文献数

1

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Many caldera volcanoes occur in the northern part of the Izu-Ogasawara Arc. These submarine calderas produced a large amount of pumice and show high gravity anomalies in the central parts along with small amplitude of magnetic anomalies on the volcanic edifices. They have no equivalent in the usual classification of on-land calderas as far as known. Elsewhere I proposed that the submarine "calderas" associated with pumice in the arc were pumice cones which were difficult to form under subaerial conditions but easy under subaqueous conditions. The Myojin Knoll is one of the typical pumice cones with calderas in the arc. It is located between Aogashima Island and Myojinsho Reef. The size of the knoll is about 18 km in diameter at the base and 950 m in height. The knoll has caldera structure on its summit. The nearly circular caldera rim is 5-7 km in diameter. The caldera floor is about 1400 m deep and 5-6 km across. A central cone rises from the floor. Relief of the caldera wall is about 700-900 m with the inner slope being at about 20-30°. The submersible study by Shinkai 2000 reported here shows that the top of knoll is constructed mainly of stratified pumice deposits underlain by rhyolitic lava. These rocks have characteristic chemical compositions with high SiO2 (more than 71 wt%) and low K2O (less than 0.86 wt%), and show similar range to the rocks from Aogashima Rift volcanoes to the west of the knoll rather than those from adjacent Myojinsho Reef Volcano. The eruption under the deep water conditions caused the deposition of pumice near the crater. The repetition of such eruptions formed the submarine pumice volcano. Acid volcanism under subaqueous conditions in the northern part of the arc formed the volcanoes with pumice cones and calderas.

著者

片山 郁夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.61, no.1, pp.69-77, 2016-03-31 (Released:2017-03-20)

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Water plays an important role for magma genesis and frictional properties; consequently, water circulation systems contribute to the variation of magmatic and seismic activity at subduction zones. Although subducting plate transports a large amount of water, most of water is released into the mantle via dehydration reactions at elevated temperature during subduction. Aqueous fluids released from the subducting plate then migrate along the plate boundary due to permeability anisotropy developed in the highly sheared serpentinite. Based on laboratory data, we estimated the fluid migration velocity to be〜7cm/year, which is close to the descending plate velocity, suggesting that polarity of water migration can be different in subduction systems. In northeast Japan, fluid migration velocity is slower than the subduction velocity, and hence water is transported downward into the deeper portions trapped by the mantle corner flow. In contrast, in southwest Japan where the fluid velocity is higher than the subduction velocity, water could be returned to the shallow regions along the subducting plate interface. This model can explain the seismic low velocity anomalies and geochemical signatures in these regions, in which the hydration of the plate interface is observed in shallow mantle wedge in southwest Japan, but is limited to the deeper parts of the mantle in northeast Japan. Water transported to deep levels could contribute to the active arc volcanism in northeast Japan, whereas water circulating at shallow levels in southwest Japan could trigger slow earthquakes due to fluid pressure build-up at the plate boundary.

著者

源内 直美 平松 良浩 河野 芳輝

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.47, no.5, pp.411-418, 2002-11-29 (Released:2017-03-20)

参考文献数

26

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A three-dimensional density structure in the shallower crust beneath the Hida Mountains, central Japan, is estimated by using gravity data. We employed seismic velocity structure models both as an initial condition and constraints of gravity structure models. We estimated that an extremely low-density body (density is smaller than 2.1 g/cm3) exists at 4-8 km depth (2 to 6 km below the sea level) beneath Mt. Tateyama (3,015 m) and also along the Hida mountains. Estimated horizontal extent of the body is about 14 km in east to west, about 28 km in north to south directions, respectively, and about 4 km in thickness. The volume of the body is about 1,000 km3. Spatial distribution of the extremely low-density body is well consistent with a low velocity region estimated from studies of seismic tomography.

著者

安井 真也 高橋 正樹 島田 純 味喜 大介 石原 和弘

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.58, no.1, pp.59-76, 2013-03-29

被引用文献数

1

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桜島火山の歴史時代の大規模噴火である安永噴火(1779-1782年)と大正噴火(1914-1915年)の噴出物の岩相や層序,地形,噴火当時の記録を比較した.両噴火では山頂をはさんだ両測山腹で割れ目火口列が活動した.割れ目の推定の長さは大正噴火で約2.3km,安永噴火では5kmに及ぶ.噴火開始後数10時間の大正噴火と安永噴火の噴火様式は共通しており,プリニー式噴煙柱から火口近傍への大量の火砕物降下により斜面上に火砕丘を形成しながら火砕成溶岩をもたらした.引き続く数週間には両噴火とも溶岩流出が繰返されて溶岩原が形成された.その後は,大正噴火が陸上での溶岩流出を主としたのに対し,安永噴火では北東沖で海底噴火が起きて安永諸島を形成した点で大きく異なる.両噴火とも噴火初期に割れ目火口近傍へ著しい火砕物降下があることが特徴的である.これは火山体形成の観点からは,両噴火では山頂部の地形変化はほどんどないが,山腹斜面が成長したことを意味する.また桜島の大規模噴火の減災という観点では,居住地域近くまで到達しうる割れ目火口の活動への迅速な初期対応の重要性を示している.

著者

植田 義夫 小野寺 建英 大谷 康弘 鈴木 晃 中田 節也

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.46, no.4, pp.175-185, 2001-08-30

被引用文献数

1

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The Myojin-sho volcano is one of the active submarine volcanos in the northern part of the Izu-Ogasawara arc about 400 km south of Tokyo. This volcano is a somma edifice of the Myojin-sho caldera, 6.5 km×8 km in diameter and 1000 m deep. The topography, seismic profiler, magnetic and gravity surveys around the Myojin-sho caldera were conducted by the Hydrographic Department, Japan (JHD) in 1998 and 1999. The geophysical structures of the caldera were derived, and the possible cause of the caldera formation is discussed. The residual gravity anomalies were calculated from the observed free-air anomalies by subtracting the gravity effect of 2-layer subbottom model structure, which amounts to 10 m Gals in a localized zone from the caldera to the northern somma. Bouguer gravity anomalies with the assumed density of 2.0 and 2.4 g/cm^3 also show the positive anomaly over the same zone, which is accompanied by the acoustic and magnetic basement depression. Moreover, it seems that the sediment volume nearby Myojin-sho caldera cannot compensate the volume loss of caldera (20 to 41 km^3). These features insist that the Myojin-sho caldera is caused by the collapse of the pre-caldera edifice rather than the explosion. The origin of the high gravity caldera may be ascribed to the magma pocket causing the depression, instead of the high density erupted material filling the caldera floor. The magnetization intensity of 4.8-5.3 A/m at the Myojin-sho volcano is derived from the magnetic anomaly, which may claim that the Myojin-sho volcano consists of andesitic to basaltic rock rather than dacitic rock. On the other hand, magnetization of the central cone of Takane-sho volcano is estimated to be 1.1-1.9 A/m, which is consistent with the fact that dacite pumices were sampled.

著者

藤野 直樹 小林 哲夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.42, no.3, pp.195-211, 1997-06-30 (Released:2017-03-20)

参考文献数

26

被引用文献数

3

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Kaimondake Volcano, situated in the Ibusuki Volcanic Region of southern Kyushu, is an undissected volcano which consists of a basal stratovolcano and a small central volcano. We established the eruptive history of this volcano by tephrochronology. Kaimondake Volcano started its eruption ca. 4 ka, and the latest eruption occurred in A. D. 885 (ca. 1.1ka). For about 2,900 years during this period, the volcano had been active, and 12 major eruption deposits (Km 1-Km 12) were recognized. The repose periods between these eruptions were estimated to range from 100 to 400 years. The mode of eruption of this volcano was mainly scoriaceous sub-plinian type, and was frequently associated with phreatomagmatic eruptions because the volcano originated from the shallow sea or near-shore environment. Lava flows were often associated with the scoria eruptions. Submarine lava flows which flowed southeastward are topographically divided into three; among them the lowest one is the most voluminous and is thought to have flowed out in the early stage, probably before Km6 eruption period. Among the 12 major eruption deposits, Km1, Km9 (ca. 2 ka), Km11 (ca. 1.5 ka), and Km12 (ca. 1.1 ka) were voluminous, and largely contributed to the formation of the volcanic edifice. During the latest eruption (Km 12), a central volcano was formed in the summit crater. This central volcano is not a simple lava dome, but a mound of complex volcanic materials with a composite structure. It consists of a basal scoria cone associated with fluid lava flows, which is later capped by viscous lava dome, and then subsequently penetrated by volcanic plug around the summit. The summit crater, which is named Hachikubo, had been thought to be a collapse crater, but it was geologically proved to be a large explosion crater which was successively enlarged during the eruption of Km12a. The total amount of volcanic products was calculated to be 3.1 km3 and 2.3 km3 for tephra and lava flows, respectively. Although there are no systematic relations between eruption volumes and the preceding repose period, the eruption materials containing tephra were more voluminous in the later stage than in the early stage, while those of lava flows were exceptionally large in the early stage of volcanic history.