著者

小室 裕明 志知 龍一 和田 浩之 糸井 理樹

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.41, no.1, pp.1-10, 1996-03-05 (Released:2017-03-20)

参考文献数

31

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Sanbe Volcano is a large lava dome, 2 km in diameter, emplaced at the center of an elliptical caldera with a long axis of 7 km and a short axis of 5 km. One crater is exposed in the center of this dome, and four peaks surround this crater. However, the basement depth of the caldera has not been determined. We have made gravity measurements around Mt. Sanbe in order to investigate the caldera morphology. The residual gravity anomaly (which is obtained after the regional gravity trend is taken away from the Bouguer anomaly, assumed to be 2.67 g/cm3 in density) is low at the center of Sanbe Caldera. Mass deficiency, estimated by Gauss's theorem, indicates that Sanbe Caldera is one of the low anomaly type calderas, as large as the Mashu caldera. Basement depression under Sanbe Caldera has been determined by three dimensional analysis, when the density contrast between the basement rocks and Sanbe volcanic rocks is 0.43 g/cm3. The depression is square shaped, with a steep rim and a flat floor. The square outline suggests that the depression part may be subsided along normal faults ; accordingly this caldera may have originated in a cauldron. The western and eastern rims of the depression correspond to the somma, but the northern and southern rims are discordant. The poor topographic expression in these areas may be due to collapse of the caldera walls. The deepest part of the basement is centered on the east side of the present crater. This deep center may be the only vent of Sanbe Volcano, because other depressions are not observed beneath the lava dome.

著者

三宅 康幸 齋藤 美由紀 竹下 欣宏 及川 輝樹 齋藤 武士

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.54, no.4, pp.163-173, 2009-08-31 (Released:2017-03-20)

参考文献数

29

被引用文献数

1

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Nantai Volcano is a symmetrical stratovolcano, situated in the southern part of the Northeast Japan arc. Many geologic studies hitherto have suggested that the stratovolcano was formed during the Main stage, and the overlying pyroclastic materials and a lava flow were formed in the Later stage. Because no sedimentary gap is found between any deposits of the Later stage, it is inferred that all of the activity in the Later stage took place successively around 12ky BP (15-14 cal ka BP) and went dormant until now. However, we found a pyroclastic flow deposit named Bentengawara Pyroclastic Flow Deposit (BPFD) at the northeastern flank of the Nantai volcano about 2km from the summit crater. This deposit overlies an 80cm thick deposit of weathered ashy sediments that in turn overlies the Arasawa Pumice Flow Deposit, a member of the Later stage. The lower half of the BPFD consists of volcanic lapilli and ash that is remarkably fine-depleted while the upper half contains abundant scoria of mainly lapilli-block sized clasts. The deposit also includes a small number of breadcrust blocks and occasional accessory lava blocks and fragments of charred wood. The breadcrust blocks consist of a dense outer crust that is significantly fractured and a vesiculated interior. It is noteworthy that the edges of the cracks are sharp and never rounded, suggesting that the vaporization of the inner magma that produced these cracks took place just before or immediately following the settlement of the blocks. Paleomagnetic data from three breadcrust block samples indicate that the magnetic vectors of high temperature components are aligned with our present-day poles. Two pieces of charred wood were measured for their 14C ages with results of 12-11 cal ka BP. The whole rock chemistry of scoria and breadcrust blocks are determined to be significantly different from any of the rocks of the Later stage, but the accessory block in the BPFD has the similar chemistry to the Osawa Lava, the last product of the Later stage. We therefore suggest that the BPFD was deposited after the Later stage with a short (~3ka) dormant period between them. Since the age is possibly around 10ka, the Nantai volcano should be counted as active volcano based on the definition provided by the Meteorological Agency of Japan.

著者

後藤 芳彦 佐々木 央岳 鳥口 能誠 畠山 信

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.3, pp.461-472, 2013-09-30 (Released:2017-03-20)

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北海道クッタラ火山登別地熱地域,大湯沼の北東200m地点において,トレンチ調査(深さ6.3m)を行い,登別地熱地域の噴火史を解明した。トレンチ断面の層序は,12層の水蒸気噴火堆積物(厚さ3-100cm)と,それらに挟在するB-Tmテフラ,Us-bテフラからなる。12層の水蒸気噴火堆積物は,変質したデイサイト質石質岩片(最大粒径80cm)と粘土質のマトリクスから構成され,サグ構造を示すことから,登別地熱地域から噴出したと考えられる。各々の水蒸気噴火堆積物は土壌層を挟在し,12回の噴火が休止期を挟んで繰り返し起きたことを示す。水蒸気噴火堆積物の直下土壌層の放射性炭素年代測定値,および広域テフラとの対比により,水蒸気噴火は,約BC6450年,BC5370年,BC3980年,BC3440年,BC1990年,BC1710年,BC1280年,BC900年,BC200年,AD980年,AD1480年,およびAD1663年以降に起きたと推定される。登別地熱地域では,過去8500年間に12回以上の水蒸気噴火が起こり,噴火の頻度は700年に1回程度であると考えられる。

著者

石毛 康介 安田 裕紀 志水 宏行 富島 千晴 大橋 正俊 関根 大輔

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.1, pp.37-42, 2017-03-31 (Released:2017-03-28)

参考文献数

6

著者

大沢 信二 三島 壮智

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.1, pp.31-36, 2017-03-31 (Released:2017-03-28)

参考文献数

15

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Free CO2 gas of deep (mantle+subducting slab) origin has been discharging accompanied by cold spring water of Hyoshimizu Hot Spring located at the eastern coast of Himeshima island, which is a Pleistocene volcanic island located northeast of Kyushu, Japan. On the western side of this island, it can be also observed free gas upwelling off the coast of Nishiura village. Therefore, we sampled upwelling bubble gas for the first time and report chemical and isotopic compositions of the sample gas. The major chemical component of the bubble gas from the Nishiura submarine gas discharge (Nishiura SGD) is CO2 as in the gas associated with the Hyoshimizu Hot Spring. The He-Ar-N2 relative composition and the 3He/4He-4He/20Ne relation of the Nishiura SGD gas are common with those of the free gas of Hyoshimizu Hot Spring and indicate that the Nishiura SGD gas will be of deep origin. However, the Nishiura SGD gas is rich in CH4 while its concentration in the free gas in Hyoshimizu Hot Spring is negligibly small. The carbon isotope composition (δ13C) of CH4 in the Nishiura SGD gas suggests that it should be of abiogenic origin. Since we found gases released on the eastern and western coasts of Himeshima island are both derived from deep, it is expected that such deep-originated gas seeps out to the atmosphere from ground inland of Himeshima island.

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.60, no.2, pp.269-274, 2015-06-30

著者

風間 卓仁 栗原 剛志 山本 圭吾 井口 正人 福田 洋一

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.61, no.4, pp.593-604, 2016-12-31 (Released:2017-01-13)

参考文献数

24

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Continuous time variations in relative gravity and tilt were observed by a CG-3 M relative gravimeter at Arimura, Sakurajima Volcano (Southern Japan) during the rapid inflation event on August 15, 2015. The gravity/tilt signals were retrieved from the original data by correcting several disturbances such as instrumental drift and tidal effect. The retrieved gravity change is -5.86±0.27μGal;its amplitude is smaller than the typical uncertainty of relative gravimeters (∼10μGal), but the continuous measurement of relative gravity in a one-minute interval contributed to the detection of the small gravity change in the case of Sakurajima Volcano. The tilt change of 55.9μrad is also retrieved from the CG-3 M’s tilt data. The success in detection of the tilt change shows that the gravimeters can be utilized as portable tiltmeters as long as significant tilt variations are expected at volcanic areas. The observed gravity change is consistent with one of the dike intrusion models provided by Geospatial Information Authority of Japan, if the density value in the dike of 0.98±0.37g/cm3 is assumed.

著者

小幡 涼江 海野 進

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.4, pp.201-216, 1999-08-30

被引用文献数

1

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Aokigahara Lava Flow erupted in A.D. 864 covers the northwestern foot of Fuji volcano. It consists of two flows: Ishizuka Lava erupted from a parasitic vent at the foot of Ohmuroyama, and Nagaoyama Lava effused from a cinder cone of Nagaoyama. Ishizuka Lava flowed down to Motosuko Lake, spreading laterally like an alluvial fan in front of the lake. Approximately 36 flow lobes that protruded from the aa front, pinch and swell, occasionally bifurcating into several branches from swollen portions of the lobes. Most lobes have a few longitudinal clefts with similar features to crease structures of rhyolitic and andesitic lava domes and to be banded on cleft walls on pahoehoe tumuli. The flow lobes are classified into three types on the basis of surface structures. Type-I lobes are characterized by well-developed clefts, which exhibit two distinct features: Stripe-type has alternating stripes of red and grey on the cleft wall parallel to the elcngation of the lobes. The stripes exist only for a depth a cm from the surface of the walls. The groundmass of the grey stripes contains abundant dendritic crystallites, while that of the red stripes consists of palagonitized glass; Blister-type develops blisters with pipe vesicles on the cleft wall. It does not have stripes, however, they may be present at different levels on the same cleft wall. Type-II lobes have clinkers with stripes similar to those of Stripe-type clefts of Type-Ilobes, however, they do not develop clefts. Type-III lobes are thinner than Type-I obes and are covered with red platy clinkers, giving the whole lobe a reddish appearance. Clefts are poorly developed. Continuous supply of lava into a flow lobe, which ceased advancement, causes tensile stress on solidifying crusts. We suggest that stepwise opening of cracks resulted in the difference in crystal density and habit of stripes on the clefts. Occasional opening of the crack and quenching hot semi-molten lava exposed at the crack tip under water resulted in the fomation of red stripes, followed by solidification of the new crust beneath the crack tip. Subsequent fracturing of newly formed brittle crust resulted in the formation of the grey stripes. The Blister-type clefts were formed when the supply of lava temporaily stopped and reslarted again. Solidification of the crust and the formation of blisters proceeded when lava within the lobe was static. We conclude that Type-I was produced by intermittent supply of lava into a static flow lobe. Type-II lobes were moving during inflation of the lobe. As a result, cleft walls with stripes were disrupted to produce clinkers. Type-III was produced by breaking of crust immediately after its fomationd uring rapid emplacement of the lobe. Therefore, the shear rates during the fomation of flow lobes increase from Type-I, II, to III lobes.

著者

三村 弘二 金谷 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.46, no.1, pp.17-20, 2001

参考文献数

11

被引用文献数

2

著者

後藤 芳彦 松塚 悟 亀山 聖二 檀原 徹

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.60, no.1, pp.17-33, 2015-03-31

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我々は,北海道洞爺カルデラ中島火山の火山地質を明らかにするため,ヘリコプター搭載型の高分解能レーザースキャナを用いたレーザーマッピングと,地表地質調査を行った.レーザーマッピングは中島の全域を含む3×3kmで行い,計測データから3次元のデジタル地形図を作成した.地表地質調査は3次元デジタル地形図を用いて中島の全域で行い,詳細な産状記載と岩石記載を行った.デジタル地形図と地表地質調査から,中島の詳細な火山地質と形成史が明らかになった.中島は,デイサイトおよび安山岩質マグマが噴出して形成した8個の溶岩ドーム(東山ドーム,西山ドーム,北西ドーム,北山ドーム,南西ドーム,観音島ドーム,弁天島ドーム,饅頭島ドーム),デイサイトマグマが湖底堆積物を押し上げて形成した潜在ドーム(北東岬ドーム),およびデイサイト質のマグマ水蒸気噴火により形成したタフコーン(東山火砕丘)からなる.中島の北東部と南西部には,泥岩と砂岩からなる湖底堆積物が分布しており,中島の火山活動がカルデラ底の隆起を伴ったことを示す.中島は,洞爺カルデラ中央部のリサージェントドームの形成と,それに伴うデイサイト〜安山岩質マグマの噴出により形成されたと考えられる.高分解能レーザースキャナによる地形計測と3次元デジタル地形図を用いた地質調査は,火山地質の解明に極めて有効である.

著者

後藤 芳彦

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.56, no.4, pp.137-145, 2011-09-30

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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

宮地 直道 中川 光弘 吉田 真理夫

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.45, no.2, pp.75-85, 2000-05-10

被引用文献数

8

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Recent eruptive episodes since the last 2200 years of Rausudake volcano, east Hokkaido, are revealed by tephrochronology, geological survey of volcanic edifice and petrology of eruptive products. Eruptive ages of these episodes are estimated by ^<14>C age dating, presence of wide spread tephras, Ma-b from Mashu volcano (about 1 ka) and Ta-a from Tarumai volcano (AD 1739), and thickness of soil between tephra. We identify three major eruption episodes occurring in ca. 2200, ca. 1400 and 500-700 y. B. P. In each episode, plinian eruption associated with generation of pyroclastic flows and possibly with effusion of lava flows and domes had occurred from the summit area. Volcanic explosive index (VEI) of each eruption is 2-4. Tephra identified as the deposits of ca. 1400 y. B. P, eruption had spread widely and has been found in Kunashiri Island which locates about 60 km east of Rausudake volcano. In Shiretoko Peninsula, east Hokkaido, Mt. Shiretoko-Iouzan has been recognized to be an active volcano. We should note that Mt. Rausudake is also active volcano that repeated its magmatic eruptions at intervals of ca. 800 years.

著者

早川 由紀夫 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.37, no.4, pp.167-181, 1992-10-01

被引用文献数

4

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The eruptive history of the Higashi Izu monogenetic volcano field for the past 32,000 years is revealed by tephrochronology and loess-chronometry. Morphology, color, and size of basaltic tephra grains are widely variable depending on the mode of the eruption; e.g., spinose red scoria are fallout from a strombolian eruption column when a scoria cone is established around the crater; yellowish green lapilli and hard tuff (kata) are products from phreatomagmatic explosions, the former being more magmatic than the latter. In the field, these discriminations are useful not only for identification of each tephra bed, but also for understanding the transition of eruptive styles during one eruptive event. Because vents are closed or sealed at the end of an eruption, an absolutely quiescence occurs between eruptive events in a monogenetic volcano field. This proves the validity of loess-chronometry. Some of the NW-SE or NE-SW trending alignments of volcanoes proved to be created by eruptive fissures; i.e., they are erupted simultaneously. Among them, the 11 km-long Iwanoyama-Iyuzan volcanic chain is the most conspicuous, which was active about 2,000 years ago. Eruptive events more than 10^9 kg of magma discharge are recognized 13 times during the past 32,000 years, so that the average frequency of eruption in this field is calculated one every 2,500 years. The last is the Iwanoyama-Iyuzan eruption. The discharge rate of magma is 100×l0^9 kg/ky for the past 32,000 years or 330×10^9 kg/ky for the past 5,000 years. The rate seems to be accelerated recently, however, it is still an order of magnitude lower than that of a polygenetic volcano such as Izu Oshima. The Kawagodaira eruption of 3,000 years ago is remarkable for two reasons: the largest with 765×10^9 kg of magma and the first appearance of rhyolite in the field.

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. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.29, no.3, pp.245-246, 1984-10-01

著者

宇井 忠英 吉本 充宏 佐藤 十一 橋本 勲 宮村 淳一

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.42, no.6, pp.429-431, 1997-12-26

被引用文献数

7

著者

藤 一郎 宇井 忠英

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.24, no.2, 1979-07-01

著者

宇都 浩三 小屋口 剛博

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.32, no.3, pp.263-267, 1987-10-15

被引用文献数

1

著者

早川 由紀夫 井村 隆介

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.36, no.1, pp.p25-35, 1991-04

被引用文献数

4

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The eruptive history of Aso volcano for the past 80,000 years is revealed by tephrochronology and loess-chronometry. Around the Aso caldera is a thick accumulation of loess, which is intercalated with numerous Aso tephra layers of limited dispersal as well as three widespread tephra layers of known age that are good marker horizons ; the Akahoya ash (6.3 ka), the Aira-Tn ash (22 ka), and the Aso-4 ignimbrite (70 ka). Loess-chronometry is based on the assumption that, in the Aso region, the accumulation rate of loess has been constant as 12 cm/ky from 80 ka to the present. Most of tephra layers after the caldera-forming Aso-4 eruption are composed of volcanic sand or scoria lapilli of basaltic andesite composition. However the 27 ka Kusasenri dacite (SiO_2 = 67%) pumice is a conspicuous exception. The large volume of 5.85 km^3 (bulk) and wide dispersal of this pumice suggests that it is a product of plinian eruption. From October 5 to the end of November 1989, the Nakadake crater of Aso volcano was in eruption. Ash was uninterruptedly emitted from a 500-1,000 m high eruption column coming out of the crater. The average discharge rate of ash was 5 × 10^7 kg/day. The total mass of ash discharged during the two months reached 3 × 10^9 kg. The penultimate eruption in recent history was June-August 1979, when 7.5 × 10^9 kg of ash was discharged. Outside the Aso caldera, the thickness of the 1989 ash is less than 1 cm. It is almost impossible to detect an old ash layer of thickness about 1 cm in a loess cross section, suggesting that sedimentary records 10 km away from a volcano are insufficient to reconstruct past eruptions smaller than 10^<10> kg. Eruptions smaller than 10^<10> kg can be determined only from proximal deposits. The history of eruptions of Aso volcano over the last few thousand years is tentatively determined from cross sections 2-4 km west of the Nakadake crater. After a 580-1,250 year dormant period, Aso volcano became active about 1,780 years ago. From then, small eruptions each with 10^9-10^<10> kg ash discharge have been repeated 48-88 times up to the present. The duration of each eruption was a few months, and the dormant interval between eruptions averaged 20-37 years.

著者

早川 由紀夫

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.2, 1984-07-01

著者

早川 由紀夫

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.25, no.4, pp.305-306, 1980-12-01