著者

安井 真也 高橋 正樹 石原 和弘 味喜 大介

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.52, no.3, pp.161-186, 2007-06-29 (Released:2017-03-20)

参考文献数

49

被引用文献数

4

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The 1914-1915 Sakurajima eruption was the largest eruption in Japan in the 20th century and erupted andesitic magma was about 1.5km3 DRE (Dense Rock Equivalent) in volume. Pumice fall and lava flows were generated from the fissure vents on the western and the eastern flanks of the volcano and pyroclastic cones were formed around the vents. Eruptive style changed with time. It is divided into three stages. After the initial, vigorous, Plinian eruption of about 36 hours (Stage 1), extrusion of lava associated with intermittent ash-emitting eruptions with or without detonations lasted for about 20 days on both sides (Stage 2), followed by an outflow of lava for more than 1.5 years on the eastern side (Stage 3). Consequently, the vast lava fields, which consist of a number of flow units formed on both sides of the volcano. Some units of lava show evidence of welded pyroclastic origin, suggesting clastogenic lava. In the western lava field, surface blocks characteristically consist of pyroclastic materials which show variable degrees of welding even within a single block. Typical eutaxitic textures and abundant broken crystals are also recognized under the microscope. Some flow units can be traced upstream to a pyroclastic cone. These features indicate that many flow units of lava on the western flank are clastogenic, which were generated by the initial, Plinian eruption of Stage 1. In the eastern lava field, evidence of pyroclastic origin is rarely discernable. However, the content of broken crystals varies widely from 20% to 80% in volume. Most lava flows, which were erupted in Stage 2 associated with frequent ash-emitting eruptions, contain broken crystals more or less than 50%. This fact indicates that magma in the conduit experienced repetitive fragmentation and coalescence due to intermittent explosions prior to outflow. Lava flows of Stage 3 contain much smaller amounts of broken crystals indicating gentle outflow of coherent lava. Relatively large-scale lava deltas developed toward the sea in the eastern lava field. Eyewitness account at that time reports that ocean entry of lava from several points started several months after the beginning of Stage 3. Although small-scale breakouts formed at the flow fronts of some lava on both sides, a large volume of the deltas can not be accounted for by secondary breakouts of ponded lava within the precedent flow lobes. It is considered that lava tube system fed lava to form the lava deltas.

著者

後藤 芳彦

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.56, no.4-5, pp.137-145, 2011-09-30 (Released:2017-03-20)

参考文献数

22

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北海道東部知床半島の天頂山は,安山岩質溶岩からなる小型の火山で,山頂部には北東-南西方向に配列する爆裂火口列がある.本論では,天頂山の爆裂火口列を形成した降下テフラ(Ten-a)の分布と年代を明らかにした.Ten-aテフラは天頂山の山頂部から知床半島の東海岸に分布し,東北東方向に伸長する分布主軸を示す.テフラは,新鮮~変質した安山岩質の石質岩片と新鮮な軽石からなり,マグマ水蒸気噴火の噴出物であると考えられる.テフラ直下の土壌層から得られた放射性炭素年代値は,1930±40 years BP(1960-1810 cal BP)である.天頂山は約1900年前に噴火し,山頂部に爆裂火口列を形成したと考えられる.

著者

第四紀火山カタログ委員会

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.44, no.6, pp.285-289, 1999-12-29 (Released:2017-03-20)

参考文献数

4

著者

小林 哲夫 奥野 充 ミラブエノ マリア ハナ

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.59, no.4, pp.299-303, 2014-12-31 (Released:2017-03-20)

著者

佐藤 博明

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.59, no.1, pp.25-29, 2014-03-31 (Released:2017-03-20)

著者

石崎 泰男 森田 考美 岡村 裕子 小池 一馬 宮本 亜里沙 及川 輝樹

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.59, no.3, pp.185-206, 2014-09-30 (Released:2017-03-20)

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Nantai volcano (2,486m a.s.l.), a near-conical stratovolcano with a summit crater ~1km wide, is located along the volcanic front of NE Japan. To date, the eruptive history and characteristics of this volcano have been poorly studied, except for an explosive eruption that occurred at ~17 cal. ka BP (Stage 2 eruption). In this paper, we present the results of investigation of the stratigraphy of recent proximal eruption products, the tephrostratigraphy of the northeastern foot of the volcano, and new radiocarbon ages. The results show that at least six eruptions of Nantai volcano have occurred after Stage 2 eruption, and we refer to these as Stage 3 eruptions. We identify four tephra layers and one pyroclastic flow deposit in the soil sections above the Stage 2 pumice flow deposit, at the northeastern foot of the volcano. These are classified in the ascending order as: (1) Nantai-Bentengawara Tephra 4 (Nt-Bt4), (2) Bentengawara Pyroclastic Flow Deposit, (3) Nt-Bt3, (4) Nt-Bt2, and (5) Nt-Bt1. The Nt-Bt2 is phreatic fallout with no juvenile material; the other tephra layers are phreatomagmatic fallouts containing juvenile pyroclasts together with ash aggregates. Six Stage 3 eruption products are identified within and around the summit crater: (1) a blocky lava flow (Osawa Lava) exposed on the northern crater wall, (2) a partly dissected scoria cone and (3) a poorly-preserved tuff ring (and its resedimented deposits) on the steep crater floor, (4) a subaqueous lava and associated lava fragments sandwiched by lacustrine deposits, (5) a tuff breccia containing hydrothermally-altered lava block and clayey matrix, and (6) a stratigraphically uppermost phreatomagmatic tephra (Nantai-Yudonoyama Tephra). All the proximal eruption products, except for the subaqueous lava, can be correlated with the tephra layers and the pyroclastic flow deposit on the northeastern foot based on stratigraphic positions, lithologies, and geochemical affinities. Our study reveals that five tephra-forming eruptions (ca. 14, 12, 8, 7.5, and 7 cal. ka BP) and one non-explosive subaqueous eruption (between 12 and 8 cal. ka BP) occurred during Stage 3, from a discrete eruption center inside the summit crater. Moreover, the tephra-forming eruptions were diverse in style, with strombolian (12 cal. ka BP), phreatomagmatic (14, 8, and 7 cal. ka BP), and phreatic (7.5 cal. ka BP) eruptions. Eruption style was determined primarily by vent position and spatiotemporal variations in local hydrological factors (e.g., the presence or absence of a crater lake, wet lacustrine deposits, and streams).

著者

松本 哲一 小林 武彦

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.44, no.1, pp.1-12, 1999-03-05 (Released:2017-03-20)

参考文献数

12

被引用文献数

4

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A systematic K-Ar age determination has been made on thirty-seven samples from the Older Ontake volcanic products, Ontake Volcano, central Japan, with analytical uncertainties mostly within ±5% (1σ level). The “Kuragoehara Lava”, which the previous researchers used as a key bed for stratigraphic correlation, gives significantly different eruption ages in each disthbution area, and some lavas classified into the same stratigraphic units among each area also show quite diffrent K-Ar ages. Hence, a revised volcano-stratigraphy is proposed based on the radiometric data obtained in the present study. The Older Ontake Volcano may be a compound volcano consisting of more than four stratovolcanoes with different vents. The first one, “;Tobu Volcano Group”, erupted at around 750-650 ka on the wide eastern side. The second one, “Tsuchiurazawa Volcano”, erupted at around 680-570 ka mainly in the western area. The third one, “Uetawarayama Volcano”, produced lavas at around 540-520 ka, and the products filled along valleys in the western area. The last one, “Mikasayama Volcano” erupted at around 440-420 ka in the southeastern area. The inactive period between the Older and Younger Ontake volcanic products continued for more than three hundred thousand years. The present study has revealed that the K-Ar age determination is effective for estimating a time gap between the Quatermary volcanic products whose stratigraphic relation can not be well defined in field.

著者

筒井 智樹

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.50, no.Special, pp.S101-S114, 2005-12-20 (Released:2017-03-20)

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Recent achievements in seismic studies of volcanic structure are summarized in this article. Seismic studies of volcanic structure was enhanced and advanced extensively in 1990's. The advance of the study was supported by new compact and light weighted digital recorders and by popularization of 3-D travel time inversion algorithm. Seismological characters in volcanic structure were revealed and were represented as high and low velocity anomalies, concentrated seismicity and intra-crust seismic reflectors. Important problems for seismic studies of volcanic structure are presented as more penetration, more resolution and more quantification of seismic characters in active volcanoes, which should be solved in next decade.

著者

勝井 義雄

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.12, no.3, 1967-12-01

著者

勝井 義雄

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.17, no.1, 1972-06-01

著者

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.2, pp.43-59, 1998-04-30 (Released:2017-03-20)

参考文献数

22

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A large amount of pyroclastic materials (ca. 1.7 km3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.

著者

宮地 直道

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.29, no.1, pp.17-30, 1984-04-01

被引用文献数

3

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"Fuji 1707 Tephra" was thrown out from the three craters on the upper southeast flank of Fuji Volcano on December 16 to 31, 1707 (TSUYA, 1955). It is composed of scoria beds named Ho-II to -IV and lowest two pumice beds named Ho-I. Ho-I and -II are probably originated from the second and the third craters and Ho-III and -IV are from the first crater on the basis of distribution of ejecta and characteristics in apparent densities. Ho-I_a, the lower Ho-I, is studied with respect to its mode of eruption, transportation and sedimentation. This bed is composed of white to pale brownish pumice, and is readily recognizable in the field and traceable over 130 km eastward from the source. Ho-I_a is the Plinian pumice-fall deposit and its volume is calculated to be 0.067 km^3 by the method of "Cross section area" (SUZUKI et al., 1973). The axis of maximum pumice diameters (MP), maximum lithic fragment diameters (ML) and median diameters of grains (Md) run along the southern margin of the distribution area. The same feature is recognized in the apparent density distribution for pumice fragments. More than 130 km eastward from the source, such an asymmetric distribution for MP is preserved. These facts suggest that the eruption column of Ho-I_a was affected by the strong westerlies in the upper atmosphere and the local SW winds in the lower one. The present meteorological data support this interpretation. Consequently, it is concluded that the irregular distribution of this Plinian tephra was originated by the differences in wind direction and velocity due to the altitude.

著者

鎌田 清吉

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.18, no.2, 1973-08-01

著者

中野 俊 伊藤 順一

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.3, pp.123-126, 1998-06-10 (Released:2017-03-20)

被引用文献数

1

著者

安井 真也

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.62, no.3, pp.117-134, 2017-09-30 (Released:2017-10-11)

参考文献数

25

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The Asama-Maekake volcano has been active for about 10,000 years. Although the many pyroclastic fall deposits, which mainly consist of pumice fall layers, are indicative of past sub-Plinian eruptions, the stratigraphy of the deposits over the entire history of the Asama-Maekake Volcano has not yet been revealed. This is because the pyroclastic fall deposits are distributed in the various directions from the crater. Moreover, the deposits predating the 12th century have a similar occurrence and petrography, making it difficult to correlate deposits among different localities. Therefore, little information is available for the older eruptions. The pyroclastic fall deposit in the direction not leeward of the dominant wind is focused in this study. A pyroclastic fall deposit called Miyota pumice (referred to as As-My hereafter), which is characteristically distributed in the southern direction from the crater, was targeted. The distribution of As-My has barely been mapped owing to the difficulty resulting from its poor exposure. The 14C ages of the samples of black humus soil immediately beneath As-My at two localities were dated to 5720±30yBP and 5530±30yBP. These ages are almost the same as those of the pyroclastic fall deposits As-UB (Ubagahara) and As-Kn (Kuni) distributed on the northern flank. Since the stratigraphic relation among these deposits is difficult to determine, it is not clear whether As-My, As-UB, and As-Kn are products from a single eruption or different eruptive activities. At least, it can be considered that multiple sub-Plinian eruptions occurred around 6,000 years ago. In addition, black humus soil and intercalating four pumice fall deposits including As-My at a major outcrop on the SSE flank slope (Locality M72) provided information on the activity over the last 8,000 years, back to the earliest stage of the volcano. The 14C ages of samples of black humus soil taken from immediately beneath the pumice fall deposits were 3,830±30yBP, 4,710±30yBP, 5,530±30yBP (just beneath As-My), and 7,470±30yBP. This is the first time that such sequential data concerning eruptive ages on the flank slope has been obtained. Black soil contains angular lithic fragments, which are similar to the ash grains from Vulcanian eruptions (e.g., the 2004 eruption). This suggests that Vulcanian eruptions have occurred between sub-Plinian eruptions since the early stage of the Asama-Maekake volcano. Such information from outcrops on the flank slopes with various directions is required to reconstruct the detailed eruptive history.

著者

中川 光弘 星住 英夫

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.38, no.4, pp.139-144, 1993-09-30 (Released:2017-03-20)

参考文献数

3

著者

工藤 崇

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.55, no.2, pp.89-107, 2010-04-30 (Released:2017-03-20)

参考文献数

39

被引用文献数

1

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Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. Ogurayama Lava Dome (OLD), which is a dacitic lava dome located near the center of Towada volcano, has been regarded as a product of the latest eruptive episode A. In this paper, the author reports that the OLD is older than previously thought and that it was formed at the end of eruptive episode D'. The OLD overlies pyroclastic deposits of the eruptive episode E and is overlain by pyroclastic deposits of the eruptive episode C. These stratigraphic relations restrict the eruption age of the OLD to 9.2-6.2cal kyr BP. Within this time interval, two eruptive episodes (D' and D) are recognized as tephra fall deposits in the distal area. The distribution of Herai Ash from the eruptive episode D' shows that the source vent is located in the vicinity of the OLD. Furthermore, the petrological features of the OLD closely resemble those of the Herai Ash. These observations indicate that the OLD is the product of the eruptive episode D' (7.5cal kyr BP). The probable eruption sequence of the eruptive episode D' is as follows. Intermittent phreatomagmatic eruptions occurred in the earliest stage. These eruptions produced the lower part of the Herai Ash. Subsequent lava eruptions formed the OLD and accompanied intermittent vulcanian eruptions produced the main part of the Herai Ash. The source vent of the eruptive episode A is not the Ogurayama, because the Ogurayama was formed before this episode. Since the only crater topography currently recognized in the Towada volcano is the Nakanoumi crater (NC), the source vent of the eruptive episode A is considered to be the NC. Since the NC has been the main crater throughout the post-caldera stage, future eruptions will probably occur in the NC. There is the current NC at the bottom of the lake of 320m in depth. A detailed examination of probable eruption style in the future will be required for predicting volcanic hazard of Towada volcano.

著者

藤井 敏嗣

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.211-223, 2016

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The main stream of the researches for prediction of volcanic eruption in Japan has been promoted through the national program which was established in 1974. The research has been advanced based on the development of basic volcanology and on the accumulation of practical knowledge obtained through the occasional volcanic eruptions. As it was shown in the review of the recent volcanic eruptions in Japan and the measures taken to mitigate the disasters caused by these eruptions, the prediction of volcanic eruptions in Japan is, however, on the stage of empirical pattern recognition. It is still far away from prediction based on the models of the underlying dynamics of volcano. Even in such situation, public society asks when and where eruption will occur, and how long the eruption will continue. It is difficult to answer these questions; however it is necessary to provide useful information based on the monitoring of eruption and the application of the available knowledge for the mitigation of volcanic disaster. In this context, several issues which might be concerned in the research of prediction of volcanic eruption and in developing the measures to mitigate volcanic disasters are described.

著者

鹿野 和彦

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.50, no.Special, pp.S253-S272, 2005-12-20 (Released:2017-03-20)

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This paper gives a brief review on the gravity flows sourced from volcanoes on land and under water. Pyroclastic flows are supported by internal gas and the air incorporated during flowage and run out a long distance as density currents. Ash-cloud umbrella is a special case of density current and the particle fallout from the umbrella is a transition to a dilute, pyroclastic density current. Subaqueous equivalents of pyroclastic flows are supported by internal gas and/or the water incorporated during flowage and are thus interpreted as either subaqueous pyroclastic flows in the strict sense or eruption-fed density currents. Debris avalanches and lahars are also important elements of volcaniclastic gravity flows both on land and under water. These pyroclastic and volcaniclastic gravity flows are thought to transform into traction-dominated flow, particle dispersion-dominated flow (grain or granular flow), fluid escape-dominated flow, or debris flows during flowage in response to the changes mainly of flow velocity, particle concentration, and shear stress. The details of these processes still remain in debate. The role of the heat in pyroclastic density current and subaqueous eruption-fed density current is a future subject to be solved.

著者

津久井 雅志 荒牧 重雄

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.35, no.2, pp.231-248, 1990-07-02

被引用文献数

5

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22,000 years ago, about 100 km^3 of magma erupted from the northrn end of Kagoshima Bay, southern Kyushu. The eruption produced 5 units of pyroclastic deposits; (1) 98 km^3 of airfall pumice (Osumi pumicde fall, OS), (2) 13 km^3 of oxidized, fine-grained Tsumaya pyroclastic flow (TSU), (3) Kamewarizaka breccia (KM) of the new vent-opening and enlargement stage, (4) 250 km^3 of Ito pyroclastic flow (ITO) at the climactic stage, (5) >50 km^3 of co-ignimbrite ash fall (AT ash). Phenocryst mineral assemblage throughout the whole sequence is ubiquitously plag+qtz+opx(Mg#45-60) +mt+il. One exceptional sample (ITO 11c) carries Fe-rich oliv (Fo 26-28) and cpx beside other phases. Fifty-five new XRF analyses of 10 major and 15 trace elements show that the majority of the erupted magma consisted of a remarkably homogeneous, high-silica rhyolite with SiO_2 74-76.5%(H_2O free and 100% normalized). The maximum fluctuation found both in major and trace elements is ±40%. These variations can be explained by the crystal-liquid separation near the roof of the magma reservoir. Mt-il temperatures and opx-mt-qtz pressures show narrow ranges, i e., 770±20℃ and 3-5 kb, respectively. Although the sample ITO 11c shows similar temperature, its calculated pressure is close to 0 kb. The bulk and mineral chemistry and the temperature-pressure estimation suggest that the magma reservoir was not distinctly zoned but was very homogeneous throughout prior to eruption.