著者

小野 晃司 渡辺 一徳 星住 英夫 高田 英樹 池辺 伸一郎

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.3, pp.133-151, 1995

参考文献数

41

被引用文献数

21

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We describe in this paper the character of ash eruption of Nakadake volcano presently going on and maintain that the products of the volcano during the recent geologic past are the main constituents of ashy soils distributed around, especially to the east of, the volcano. Nakadake volcano, the only active central cone of Aso caldera, mainly discharges black, sandy essential ash of basaltic andesite during its active period. The ash is the most voluminous constituent of the products of Nakadake through its activity of recent more than 20,000 years. We call ash eruption for the activity which discharges dominantly ash. The ash is divided into three groups : block-type ash, splash-type ash, and free crystals. The block-type ash, most common, is polyhedral surrounded by a few flat planes. It is formed by brittle fracturing of semi-solid top part of the magma column. While, the splash-type ash, discharged only during the very active phase in active periods, is derived from liquid magma which underlies the semi-solid top of the column. The ash is transported by gas stream from the magma in a quasi-steady state or intermittently, and is distributed around Nakadake volcano in a near-circular pattern by a low eruption column usually less than 1,000 m in height. Long-term thickness contours of the ashy soil from Nakadake volcano in three time intervals, separated by the present earth surface and three ash or pumice layers, of recent more than 20,000 years also show near-circular pattern but a little elongated to the east. 'Loam beds', mainly composed of decomposed and argillized volcanic ash, are said to be formed by accumulation of aeolian dusts during periods of no eruption in volcanic districts. But, loam beds (Akaboku) and black humic soils (Kuroboku) distributed around Aso volcano are composed mainly of primary fall-out deposits of ash or pumice along with aeolian dusts. Ash eruption of Nakadake mostly produces sandy ash rather continuously without long time break but in small rate of discharge. A correlation of detailed columnar sections eastwards of Nakadake crater shows most single strata of ashy soils, light brown- to black-colored, thin according to the increase of distance from the source crater. The ash, especially very fine ash distributed in the distal area, easily decompose and lose primary stratification to form a massive layer which is hardly discernible from aeolian loam beds by their close resemblance. Not only fine ash layers but Plinian pumice fall layers form 'loam' beds which are not distinguishable with adjacent aeolian beds by further decomposition. The thickness contours of the Kanto loam elongate and thin eastward from Fuji volcano over the Kanto plain. The thickness of loam beds changes regionally, thicker in volcanic areas and thinner in non-volcanic areas. These facts suggest contributions of primary pyroclastic falls for thickening of 'loam beds'

著者

星住 英夫 渡辺 一徳 阪口 圭一 宇都 浩三 小野 晃司 中村 武

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 (ISSN:24335320)

巻号頁・発行日

vol.1997, 1997

著者

星住 英夫 宮縁 育夫 宮城 磯治 下司 信夫 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.1, pp.91-112, 2022-03-31 (Released:2022-04-26)

参考文献数

71

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Aso volcano produced four huge ignimbrite-forming eruptions named Aso-1, 2, 3 and 4 in ascending order, among which Aso-4 is considered the largest eruption in Japan in the last 1 million years. This paper describes the tephra sequence between the Aso-4 and Aso-3 eruptions (Aso-4/3 tephra group). The reconstruction of the eruptive history for Aso-4/3 tephra group presented here provides a valuable contribution to the understanding of caldera volcanism by outlining the preparatory processes of a catastrophic ignimbrite eruption. The eruption sequence of the Aso-4/3 tephra group, which is composed of at least 37 units of pumice-fall, scoria-fall, and ash-fall deposits, is divided into five stages. Stage 1 is characterized by the eruption of mafic scoria (VEI 3-4) during 133-114.1 ka, after the eruption of Aso-3. Stage 2 is characterized by the frequent eruptions of mafic scoria and ash (VEI 3-4) during 114.1-108.4 ka. The magma composition became more felsic during explosive eruptions (VEI 3-4) from 108.4-104.7 ka (Stage 3). During the most active stage from 104.7-97.7 ka (Stage 4), voluminous felsic pumice-falls erupted (VEI 4-5). The ABCD tephra (97.7 ka) is the largest plinian pumice-fall deposit of Aso volcano. Stage 5 (97.7-88 ka) is a relatively dormant period, during which only a biotite dacite pumice-fall was deposited (VEI 4). The low number of eruptions during stage 5 suggests that the magma supply rate decreased during the 10 thousand years that preceded the Aso-4 ignimbrite eruption. The estimated total tephra volume for the Aso-4/3 tephra group is 23 km3, which corresponds to 10 km3 in dense rock equivalent (DRE). The estimated the long-range tephra discharge rate (0.23 km3 DRE/ky) is similar to that in the post-caldera stage of Aso-4 (0.2 km3 DRE/ky).

著者

宝田 晋治 星住 英夫

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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火砕流は,火山体周辺に多大な災害をもたらす.特に大規模火砕流の場合は,1883年のクラカタウ火砕流による犠牲者数36,400人などで明らかなように被害も甚大となる.国内の大規模火砕流としては,90kaに発生した阿蘇4火砕流は到達距離が 160km以上に達しており,分布・体積の正確な把握や流動堆積機構の解明が重要となってきている.阿蘇4火砕流の詳細な影響範囲の把握のため,既存文献,ボーリングデータを元に,現地調査結果を加え,堆積物の分布を明らかにした上で.噴火当時の復元分布図を作成した.また,5kmのメッシュごとに層厚を復元し,高精度に噴出量を算出した.さらに,現地調査により,大規模火砕流堆積物の岩相変化.軽石及び岩片の最大粒径の変化に基づく,流動堆積機構の検討を行った.現存する堆積物の分布については,産総研の5万分の1地質図幅を基本とし,刊行されていない地域に関しては20万分の1地質図幅や表層地質図の他,出版済みの文献を参照した.これらから阿蘇4火砕流堆積物の分布をGIS上でトレースし,現存分布図を作成した.また,噴火直後の推定分布図は.地形状況と噴火時点での地質を考慮した上で,文献情報,ボーリング情報を元に再現した.現存堆積物の分布は,火砕流が全方向に広がり,給源から北北東160km以上の萩市周辺にも到達し,南は,人吉盆地,宮崎市周辺まで到達したことを示している.現存堆積物の面積は,約2,500km2となった.層厚については,地質図,露頭データ.ボーリング柱状図を用いて,火砕流堆積物の上端高度,下端高度を5kmメッシュ毎に数点以上読み取った上で,メッシュごとの平均層厚を算出した.平均層厚は,カルデラリム周辺で最大約100mを示し,中流域では,0.2〜50m前後,下流域では0.01〜10m程度となった.GISソフトウェア上で,メッシュ毎の分布面積を算出し.層厚を乗じてメッシュ毎の見かけ体積を算出した.その上で,溶結,非溶結の量比などを勘案した上で,メッシュ毎の堆積物の平均密度を算出し,火砕流堆積物の体積(DRE)を算出した.その結果,降下テフラ分を除く阿蘇4火砕流堆積物の体積は,20-60km3 (現存体積),50-140km3(復元体積)となった.阿蘇4火砕流の流動堆積機構の解明のため,火口近傍から160km遠方の露頭まで,カルデラから東方向と北北東方向の流域で現地調査を行い,岩相変化,軽石と岩片の最大粒径の変化を明らかにした.最大粒径は,ラグブレッチャ以外の火砕流本体に対して,各露頭毎に軽石と岩片についてそれぞれ10個長径と短径を測定し,最大と最小のサンプルを除いた8サンプルの算術平均から,各地点での最大粒径を求めた.流走距離ごとに,軽石の最大粒径をプロットする(図)と,給源(カルデラの中心付近, 中岳第1火口を仮定)から16kmまでの地点では,3〜9cmと比較的小さく,17〜20km地点では約28cm,26km地点の火砕流到達前の原地形の傾斜変換点付近(小国町周辺)では47cmと最大値を示し,その後,72km地点まで次第に減少し,3cmとなる.海を渡った山口県内では,最大粒径は,132〜162km地点で0.4〜0.9cmと非常に小さくなる.岩片の最大粒径は給源から6.5km地点では1〜2.5cmと比較的小さく,16km付近で11.2cmと最大になり,その後は単調に減少し,72km地点で0.6-0.9cmとなり,北九州の117km地点の折尾の露頭では,0.3cmと非常に小さくなる.山口県内の露頭では,肉眼で測定可能な岩片はほとんど含まれていない.これらの結果は予察であり,今後より詳細なユニット対比,岩相変化,粒径変化等の現地調査を予定している.給源付近で軽石や岩片の最大粒径がやや小さいことは,大規模火砕流発生時に,この付近は噴煙柱の内部もしくは近傍で,乱流度が高く.火砕流の運搬能力が十分高かったことを示唆している.軽石の最大粒径が傾斜変換点の26km地点付近で最大となっていることは,火砕流が傾斜変換点に達し,ハイドローリックジャンプ等の現象で急激に運搬能力が落ちたため,運びきれなくなった軽石を多量に落としたことが原因である可能性が高い.軽石や岩片の最大粒径が単調に減少することも,乱流状態の火砕流の基底部から順次より大きい軽石や岩片が堆積したと考えることが可能である.堆積物の内部構造として,火砕流の内部には,逆級化した層厚20〜70cm程度の弱い層理構造が見られる場合がある.このことは,軽石同士の堆積時の相互作用を示唆し,火砕流の基底部に比較的高濃度な密度流が形成され,堆積サブユニットを形成しつつ順次堆積したモデルでうまく説明できる.海を渡った山口県内の火砕流は層厚10cm〜6m程度であり,軽石の最大粒径は1cm以下で非常に小さく,肉眼で認識できる岩片はほとんど含まれていない.部分的にやや高度の高い部分では,サージ状の岩相を示す.このことは,遠方まで運ばれた火砕流は,最後まで残った比較的低密度で細粒部分のみが160km以上の地点まで到達したことを示唆する.

著者

小野 晃司 渡辺 一徳 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.40, no.3, pp.212-213, 1995-07-31

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

山﨑 誠子 星住 英夫 松本 哲一

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.3, pp.519-531, 2016-09-30 (Released:2016-11-08)

参考文献数

33

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Unspiked K-Ar dating procedure enables the correction for mass fractionated initial 40Ar/36Ar ratios, and has been successfully applied to young volcanic samples, especially younger than 0.5 Ma. We reconstructed the growth history of western to central part of Kuju Volcanic Group based on the unspiked K-Ar age data. The unspiked K-Ar ages for the small andesitic-dacitic volcanoes of Stage 1 (Kutsukakesan Volcano, Iozan Lava, Kuroiwasan Volcano, Narukoyama Volcano) provided with two age group of ca. 160-150 and ca. 90-80 ka, suggesting that Stage 1 may be divided into at least two sub-stages. During Stage 3 after the largest eruption associated with Handa pyroclastic flow deposit, some lava domes and stratovolcanoes in the central part (Ogigahana Volcano, Nakadake Volcano and Mimatayama Volcano) were built up from ca. 54 to 34 ka, which is older than the previous interpretation.Based on these K-Ar ages and estimated volume for each volcanic unit, we calculated the magma discharge rate during each volcanic stage and the whole period of the volcano history. The calculated magma discharge rate for Stage 1, 3 and 4 are 0.09, 0.12, 0.18 km3/ky, respectively, showing that the activity has slightly increased. The rate of 0.12 km3/ky for the whole period is similar to that for Aso and Unzen volcanoes lie to the west of this volcano.

著者

小林 哲夫 星住 英夫

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2017 (ISSN:24335320)

巻号頁・発行日

pp.127, 2017 (Released:2018-02-01)

著者

宮縁 育夫 星住 英夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.126, no.5, pp.581-593, 2017-10-25 (Released:2017-11-10)

参考文献数

34

被引用文献数

2 1

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Several volcaniclastic deposits discovered at the foot of the eastern wall of Aso caldera, central Kyushu, southwestern Japan, are divided into two types: lahar deposits (Lh1-Lh5 in descending order), which contain abundant subangular to subrounded lithic clasts ( 3.5 m in diameter) set in a sandy to silty matrix, and debris avalanche deposits (DA1 and DA2), which include numerous plastically deformed fragments of tephra (ash and scoria) and soil layers in a homogenous silty to clay matrix. DA2, which underlies a paleosol dated at 5.4 ka (calibrated 14C age), is the largest volcaniclastic deposit observed in the section (more than 2.5 m thick and about 70 m wide). Because the debris avalanche deposits display no evidence that they were transported by water, they are likely to have originated from landslides triggered by intense earthquakes. Tephra chronology and 14C-dating on paleosols along the succession suggest that lahars occurred once over 900 years (6.3-5.4 ka), three times over 1400 years (5.4-4 ka), once over 400 years (4-3.6 ka) and twice (including the 2012 lahar) in the last 3600 years. This evidence indicates that the lahars occurred at an interval of 400-1800 years. In contrast, two debris avalanche deposits exist in the same succession spanning the last 6300 years. Including landslides and associated debris avalanches triggered by the 2016 Kumamoto earthquake (Mj 7.3), landslides generating debris avalanches in the Aso caldera occurred at least three times in the last 6300 years. This may suggest the frequency of large earthquakes triggering debris avalanches in the central Kyushu region, which has many active faults.

著者

鎌田 浩毅 檀原 徹 山下 透 星住 英夫 林田 明 竹村 恵二

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.100, no.11, pp.848-866, 1994-11-15 (Released:2008-04-11)

参考文献数

80

被引用文献数

23 26

著者

松本 哲一 星住 英夫 廣田 明成

出版者

特定非営利活動法人日本火山学会

雑誌

日本火山学会講演予稿集

巻号頁・発行日

vol.2012, 2012-10-14

著者

伊藤 順一 星住 英夫 川辺 禎久 鎌田 浩毅

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 (ISSN:24335320)

巻号頁・発行日

vol.1997, 1997

著者

宇都 浩三 中田 節也 星住 英夫 清水 洋

出版者

一般社団法人 日本地質学会

雑誌

日本地質学会学術大会講演要旨 (ISSN:13483935)

巻号頁・発行日

vol.2003, 2003

著者

中田 節也 宇都 浩三 吉本 充宏 杉本 健 嶋野 岳人 黒川 将 星住 英夫 後藤 芳彦 野口 聡 小栗 和清

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 (ISSN:24335320)

巻号頁・発行日

vol.2004, 2004

著者

宝田 晋治 星住 英夫

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2016 (ISSN:24335320)

巻号頁・発行日

pp.26, 2016 (Released:2017-02-07)

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

鎌田 浩毅 星住 英夫 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

日本火山学会講演予稿集 1987.2 (ISSN:24335320)

巻号頁・発行日

pp.85, 1987-10-05 (Released:2017-02-10)

著者

宮縁 育夫 星住 英夫 高田 英樹 渡辺 一徳 徐 勝

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.2, pp.195-214, 2003-05-20

被引用文献数

12

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Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is difficult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sasakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenrigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clino- pyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several ^<14>C ages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones.

著者

宇都 浩三 風早 康平 斎藤 元治 伊藤 順一 高田 亮 川辺 禎久 星住 英夫 山元 孝広 宮城 磯治 東宮 昭彦 佐藤 久夫 濱崎 聡志 篠原 宏志

出版者

学術雑誌目次速報データベース由来

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.2, pp.257-270, 2001

被引用文献数

7 13

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To understand the eruptive mechanism of the 2000 Miyakejima volcanic activity, we conducted intensive geological, petrographic, and mineralogical studies on the pyroclastics of the August 18 eruption. Volcanic ashes, which were rich in accretionary lapilli, covered most of the islands. Cauliflower-shaped bombs and lapilli were ejected along with accidental lava blocks. Black-colored angular scoriaceous particles with abundant vesicles 10 -100 μm in diameter are found among ashes, comprising about 40 wt. % of total constituents. These bombs, lapilli, and black ashes have identical bulk chemical compositions and constituent mineral compositions, suggesting a common origin. Existence of oxidized ashes and accretionary lapilli attached to a large flattened bomb and chemicallyreacted anhydrite particles trapped in the voids of bombs suggest that bombs were still hot and ductile when they were emplaced on the ground. We, therefore, conclude that the August 18 eruption was a phreatomagmatic eruption and cauliflower-shaped bombs and black ashes were essential magmatic materials. Significant SO<SUB>2</SUB> emissions from the volcano after August 18 also suggest convective upwelling of magmas to a shallower level beneath the volcanic edifice. We propose a magma-ascending model in which vesiculating magmas continuously ascend through the wall of subsided piston-like blocks.