Bulletin of the Geological Survey of Japan，voL42（12），p・703－743，1991

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Makoto YuAsA＊，Fumitoshi MuRAKAMI＊，Eiji SAITo＊＊

and Kazuaki WATANABE＊＊

YuAsA，Makoto，MuRAKAMI，Fumitoshi，SAITo，Eiji andWATANABE，Kazuaki（1991）Subma－ rine topography of seamounts on the volcanic front of the Izu－Ogasawara Arc。B％ll． G601．S％7∂．ノ41）αη，vol。42（12），P．703－743，33fig．，2tab，

ALstract：Geomorphology of submarine volcanoes along the volcanic front（Shichito－Iwo－ jima Ridge）of the Izu－Ogasawara Arc was stu（1ied based on the results obtained during marine geologic researches by Geological Survey of Japan． Morphology of the volcanic edifices on the Shichito－Iwojima Ridge shows regularity fQr the height，basal diameterland spacing of them between the northem and southem parts of the Shichito－Iwojima Ridge，respectively．They are summarized as follows：

Northern part Southern part Height 10W high Elevation decrease to south decrease to north Spacing narrOW（irregular） wide（constant） Height of basement decrease to south decrease to north Basal diameter small large Submarine caldera abundant few

The different regularity of each part is resulted originally from the different crustal thickness between them and their independency of each other in their development history． The irregularity of spacing found in the northem part seems to be caused by the structural discontinuity in the arc．The seamomts in the Nishinoshima Trough make an independent group and are subdivided into three types b3sed on the topographic and seismic profiling features。 Existence of submarine calderas producing acidic volcanics is a special feature in the northem part of the Shichito－lwojima Ridge．These calderas are presumed to be submarine pumice cones based on their morphologic and geophysical features．Pumice cone is difficult to be formed under the subaerial condition but easy under the subaqueous situation．

arc including submarine parts is about1200 1．亙鵬ro咽臆ct童o盟 km long from Oshima to Minami－lwojima islands and400km wide，being comparable Izu－Ogasawara（Bonin）Arc is one of the with the Honshu Arc of Japan。From west to arc－trench systems in the westem Pacific and east，the arc consists of the Nishi－shichito， is situated along the eastern edge of the Keywords l submarine topography，seamomt，submarine Philippine Sea Plate．The size of the whole volcano，height，basal diameter，spacing，submarine caldera， pumice cone，Nishinoshima Trough，Shichiyo Seamomt ＊Marine Geology Department Group，Sofugan Tectonic Line，Izu－Ogasawara（Bonin）Arc， ＊＊Geological Infonnation Center Shichi－to－lwojima Ridge，along arc variation

一703一 Honshu，Japan 1、、継 350N ρ 『 † 冒畷4 ・一一 一 3 纏蓉詣 240N る 聡曝 30N 蕊郵。 300N 轟ひ o 》 黛 So如gan 留 謁． 盲 βQ Q 短 籍 昏 鹸 o。 地 庵・QO ㌧ 諺グ 340 二梅 舞 爆q 電 ，鞭⑤D 230 一奢 1410E ざ◎ ・ 気 駄慰 bu ◎ 29 寸『酬砂 2go “鞭 悶匠 5［ξげ8訟凝 雌 ミ 魁 翁 叉R 融 漏診； ・燐搭 命、 ◎ oo 。 § q ’亀q ．』、甑lhわj融 勇、も． 魁景 黛 330 220N ．才一o彬麟 ’ （＞ 葛 岬。 β ぷ 黛獄 ・㌧ 曳 鳴 。⑥簿軸 28 280 O O o 1420 1430E so 。6Φ瞭 ・ 睾 G ＠ の ご 旨 へ禽13。亀 鵯． ミ 、 閥類 ぽ ゆ けレロむ 320 〉 のN ミ ぎ 惣。，。溜、気 27 270 這 “ 1390E販 乱 も呼 慧 趣 ／0 蜷 勲 O ぷ 窮 識・ 0 ミ ～ ・鉱 旨 。ρ．岬 310 軌 § ψ D 26 つ 26。 鵜、尋 1390E 1400 ぎ ト 蕊 拷 臓＞ 食

q 冬 300N 250 25 コー 誌 麺） 1、鱒亀 一〇 § 尋 1400E l

24N 240N 1410E

Fig．1 Submarine topography of the Shichito－lwolima Ridge．The depth contours（500m）reproduced from submarinetopographicmapsoftheHydrographicDepartmentoftheMaritimeSafetyAgencyofJapan （the area north of Hachijojima Island）and Geological Su四ey of Japan（the area south of the island）。 軽 S66z％zoz〃zお o％ 孟h6 ∂olαz銘づoノンo％渉，1≧z6－Qg召s召z｛7ごz名ヒz／176 似。y％ごz＆z6渉ごzl．ノ）

Table1 List of volcanoes along the Shichito－Iwojima Ridge。 Elevation shows height above or depth below the sea level．Height，N－S and E－W diameters are measured on the basis of transitional point on the slope of volcanic edifice．Volcano spacing is shown by the inten7al from center to center between a given volcano and its neighboring（10wer）one．

IsIand－！seamount latitude Elevation Height Basai diameter Spacing CanyonITrough m m （N－S）km （E－W）km km Hakone☆ 1438 76 SagamiTrough Oshima★ 34．44 755 1955 28 26 25 Omurodashi 34．33 一28 872 26 20 48 NiijimaCanyon Miyakejima★ 34．05 814 2014 41 24 25 M獣uralima 33．52 851 2051 22 18．5 53 Minami－mikuraCanyo Kurose☆ 33．24 一114 1286 31．5 28 35 Hachijojima★ 33．08 854 2052 41 33 74 AogashimaCanyon Aogashima☆ 32．27 423 1723 35 33 39 Myojin KnoII★ 32．06 一451 950 22 18．5 22 Bayonnaise Rocks脅 31．53 10 1410 28 31．5 58 Myojinsho Canyon Sumisujima★ 31．26 136 935 22 22 20 Daisan－Sumisu Kno凹★ 31．16 一350 850 16．5 18．5 90 T◎rishima★ 30．29 394 1794 31．5 31．5 74 Sofu Canyon Sofugan 29．47 99 2350 30 28 33 Nichiyo Seamount 29．30 一1150 1650 31．5 24 22 GetsuyoSeamount 29．18 一625 2675 22 28 28 Kayo Seamount 29．03 一820 2680 27 21 56 Suiyo Seamount 28．34 一1418 1380 15 31．5 30 NishinoshimaTrough Mokuyo Seamount★ 28．18 一920 1780 35 39 33 Kin曹yoSeamount 28．03 一640 2660 26 28 39 DoyoSeamount 27．40 一860 2340 28 26 48 Nishinoshima 27．14 38 2538 54 44．5 65 Kalkata Seamount☆ 26．42 一150 2350 52 44．5 67 Kaitoku Seamount 26．06 一10 2485 55．5 52 78 lwojimaTrough Kita－iwojima 25．25 792 3282 70 55．5 71 lwojima 24．48 161 2161 48 52 64 Minami－iwojima 24．13 916 2916 55．5 30 24 Fukutoku Seamount 24．04 一201 2100 20 20 37 Kita－hiyoshi Seamoun 23．44 一214 1986 31．5 28 42 Minamトhiyoshi Sm匙 23．30 一30 2470 37 30 60 Nikko Seamount 23．05 一391 2909 41 26 178 Fukujin Seamount 21．56 一217 2833 41 48 Ohmachi Seamount 129．13 一1700 1750 59 31．5 Sawa Seamount 27．41 一690 2800 72 26

曹’accompaningwith caldera

Shichito－lwojima and Ogasawara（or Bonin） to the Ogasawara Trench，Geomorphologic ridges．Subaerial parts of the Shichito－lwo－ outlines of these volcanoes are shown in Fig．1． jima Ridge are recognized as a chain of many Some quantitative topographic properties of volcanic islands from Oshima to Minami－iwo－ volcanic islands including their subaqueous jima islands．The ridge corresponds to the parts and seamounts on the Shichito－Iwojima volcanic front of the Izu－Ogasawara Arc and Ridge are listed in Table1．Definition of terms continuestotheMarianaRidgewitha chainof used here are shown in Fig．2． Several active submarine volcanoes like Minami－ seamounts located between Sofugan and Ni－ hiyoshi，Nikko seamomts，etc．These volcanic shinoshima islands are not regarded as islands and submarine volcanoes mn paralle1 Quaternary volcanoes on the basis of the

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characteristic features of seismic profiles and Nishinoshima and Minami－iwojima islands． bathymetric map as mentioned later．In the table，however，these older seamomts are also 2．l S朋］bm我r豊恥e ca且dler我s listed for reference』 Nine submarine calderas are recognized in Seismic profiles by single chamel air－gm the Shichito－Iwolima Ridge．Myojinsho and and submarine topographic data compiled Sumisu calderas are accompanied with recent from12kHz PDR，which are used in this paper volcanic events（Morimoto and Ossaka，1955， are all obtained during the 1980 and 1984to 19701Kuno，1962〉．Eight of the calderas are in 1987R／V Hakurei－maru cruises on the marine the northern part of the ridge and they are all geologic research for hydrothermal activity in accompanied with acid volcanics．High 嚢 the Izu－Ogasawara region by Geological Sur－ apPearance rate of caldera is characteristic of 嚢 霧 veyofJapan． the area．On the other hand，in the southern 馨・ 萎 part of the ridge there is only one caldera， Kaikata Caldera，which consists of andesitic 熱 2．：Descri調樋o£se騰o腿醜ts 馨 volcanic rocks with basaltic central cone．No There are thirteen volcanic islands and fif－ acid rocks are found in this caldera．

察．1 teen major seamomts on the volcanic front 2．1．1 Kurose Hole 叢・ （Shichito－lwojima Ridge）of the Izu－Ogas－ Kurose Hole is located at lat．33。24’N，long． awara Arc． Of these islands，Bayonnaise 139。41’E on the Kurose Bank，30km north of lIl Rocks，Sumisujima，Torishima，Nishinoshima， H：achijojima Island（Figs．3and4）．The sum－ Kita－iwojima，Iwojima and Minami－iwojima mitofKuroseBankis107mbelowthesea islands form a part of individual seamomt， leveL This caldera was previously mapped by respectively．Such seamomts including each Hydrographic Department in1979（Oshimaα island as a peak on them are described here in α1．，1981）．The Kurose Hole is a nearly circu－ addition to other usual seamounts．However， lar caldera of about5－7km diameter．The the islands corresponding to only one major calderaflooris2kmacrossand600－700m peak of a volcanic edifice （e．g．，Oshima， depth．The floor inclines to north and the Miyakejima islands，etc．）are excluded in the maximum depth of the floor is about760m． description because they have been previously Reliefofthecalderawallisabout500mand studied in detai1．In the following descriptions inclination of the wall is14－20。．The somma are made for1）submarine calderas which isflat－topped，shallowerthan200mdepth． characterize most of the seamounts in the Northem part of the somma increases its northern part，2） seamounts in the Nishino－ depthtoabout300mandacanyon－1ikefea－ shima Trough，and3）seamounts between ture is developed here．

Island Caldera Seam◎unt ElevaUon Sea level Diameter ’ノ7》 ンンンノレ ’ンンンンンンンンンンンンンンンンン： Depth ヨ¢ight Re、餓 ＝： ノ』’・’。■ 触 慧！…i…1＿

ずドBasaldiam⇔しer Basal diameber 二引“

Fig．2 Definition of topographic terms for volcanic edifice．

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301

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33。201N kurOse－nishi Hole 房⑨ ＼」〉 500 o 0 、ooo ρ

139。301E 401

Fig．3 Submarine topographic map of Kurose and Kurose－nishi Hole（after Saito6厩」．，1988）．Contour interval 100m．

Kurose O s

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1

2

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｛ 裸 罫 i 譜』・ まヨ 劃 塁i ぷi ll 考l li 閣 麹 ピ！ 」 ド ll 鴇！ 梨 考1 ll ll 艶1 到 薯 マ1． じi穀 ユi §L l 蕊 ．剛 讐 W 0 10km E

Fig．4 Single channel air－gun reflection profile across the Kurose Caldera west to east obtained by GSJ，GH 80－4 Cruise．

High free－air gravity anomaly is observed Kurose－nishi Hole is located at lat．33。18’N， on the caldera，though the central part is lower long．139。33’E，20km southwest of Kurose Hole in anomaly than the surroundings． No （Figs．3and5）．The diameter of caldera rim remarkable magnetic anomaly isrecognized in is about5－6km．The shallowest part of the this caldera． rim is about400m depth．The floor is as deep 2．1．2 Kurose－nishi Hole as1500m，but its precise topographic feature

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Kurose－Nishi Hole Os 麟移茗 ド ．難 納 レマゐ 馨響 鷺 侵瞬・爾 難彦談 饗萎 輯… 『封 臣 難11． 署・ド 一1 膵“ に“，権 繋．u 卜 1，三 … 判1 一窒 羅． 劇銀 i 馨墾 i l 1’欝し穀 l i 欝勤磨，瀞穣難1 引lll・・ き灘鐵 I甲鮎十 …1 I I 覧 。 ．粥・ 卜羅 Ii 籔 妻i T蟻熱… 1 譲 懸 － l i I l 2 l l i I l l ！ l l導1㌧周i㌧

S o 10km N W E

Fig。5 Single channel air－gun reflection profile across the Kurose－nishi Hole and Kurose west to east obtained by GSJ，GH80－4Cruise．

isnot confirmed because offew somding lines． bly overlying the acoustic basement of the 2．1．3 Higashi－aogashima Caldera seamount on the acoustic profiles crossing this This caldera is located at lat．32。27’N，10ng． caldera（Fig．7）．This suggests that the age of 139。54’E，and12km east of Aogashima Island， this caldera is older than the others． It rests on a NNE－SSW trending seamomt Bouguer gravity anomaly is highest at the with two peaks，Daini－higashi－aogashima central part of the caldera．Although the high Knoll to the north and Daisan－higashi－aoga－ gravity anomaly area is slightly biased to shima Knoll to the south（Figs．6and7〉。The south，this caldera seems to be a high gravity shape of the caldera is ellipsoidal and the size anomaly type one（lshihara，1987），A remar－ is5．4（E－W）×9．9（N－S）km across．The kable dipole magnetic anomaly，negative to caldera floor is4－7km across with600－800m the north and positive to the south，is found on depth．The maximum depth of the floor is805 the southern part of the caldera． matthesouthempart．Thesommaconsistsof 2．1．4 Kita－Bayonnaise Caldera the two knolls as above mentioned．The shal1－ Thi言caldera is located at lat．32。06’N，10ng． owestpointsoftwoknollsare180mdepthon 139。51’E，andNNWofBayonnaise Rocks．The the north and275m depth on the south．West caldera rim is nearly circular and5－7km in col ofsomma decreases its depth to600－700m． diameter（Figs．8and9）．The caldera floor is Relative height from col to floor becomes only about1300mdepthand5－6kmacross．The 100－200mandtheheightislowerthanthe floor is divided into two parts，northwest and other calderas。Inclination of the caldera wall southeast，by a central cone。The somma is is characteristically smal1（14－20。）．There are nat－toppedandgenerally600－700mindepth， recognized well stratified layers unconforma一 although the west part of somma named 舞

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Fig，6 Submarine topographic map of the Higashi－aogashima Caldera（after Saito窃認。， 1988）．Contour interva1100m．

Daisan－Higashi Daini－Higashi South of Aogashima Aogashima Kno” Aogashima KnoII O S

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Fig．7 Single chamel air－gun reflection profiles across the Higashi－aogashima Caldera west to east（1eft）and south to north（right）obtained by GSJ，GH84－2Cruise．

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Fig，8 Submarine topographic map of the Myojin Knoll（Kita－Bayonnaise Caldera）and Myolinsho Caldera （after Saitoαα」．，1988），Contour interval100m．

Myolin Kno” O S （Ki重abayonnaise Caldera） 騰 謬： 灘払 1 譲 襲i 麟 賎 寒 事 乗 襲鑛一粥 蕪 撫驚 撃．魏欝 2 吊 ♂恒

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Fig．9 Single channel air－gun reflection profile across the Myojin Kno11（Kita－Bayomaise Caldera）west to east obtained by GSJ，GH84－2Cmise．

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Myojin Knoll has a shallow point of364m tectonic movement（e．g．，Morimoto and Ossa－ depth．Relief of the caldera wa11is about700－ ka，1955〉．Recent submarine geologic survey， 900mandtheimerslopeisassteeperas20－ however，confirmed that Myojinsho is a 30。。 northeast peak of the somma and the caldera Low gravity anomaly is observed on the rim is well preserved．The caldera rim is eastem outer slope，whereas the central part nearly circular and7－9km in diameter．The of the caldera is relatively high．Accordingly， caldera floor is5－6km across and900－1000m the caldera seems to be of the high gravity deptk．Relief of the caldera is650to900m．A anomaly type． No remarkable magnetic largecentralconewithadepthof330matthe anomaly is recognized in this caldera． top was built on the central part of caldera 2．1．5Myojinsho Caldera floor，and the floor became a narrow and ring Myojinsho（Myojin Reef）is an active sub－ －shaped depression．The deepest point of the marine volcano．It is enclosed by a circle with flooris1102minthewestempart，whereasthe 10mile diameterdescribed”Dangerous area” northeast part becomes sha110wer to600m on the chart published by the Hydrographic depth by development of the flank of Myojin・ Department，Maritime Safety Agency of sho．The northem part of the somma deepens Japan． to700mdepth．Theslopeofthecalderawall The caldera is located at Iat，31。53’N，long． is steep and inclines30。． 139。59’E，about4km e歌st of the Bayomaise 2．1。6 Sumisu Caldera Rocks（10m above sea leve1），which is a Sumisu Caldera is located at lat．31。19’N， westem peak on its somma（Figs．8and10）。It 10ng．140。03’E，6kmnorth ofSumisujima Island had been thought that Myojinsho is a central （136mabovesealeve1），whichisapeakonits cone and the northeastern part of the caldera somma．The caldera forms part of a large rim surrounding it has been lost by volcanic or seamount with a basal diameter of about20

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Fig，10 Single channel air－gun reflection profile across the Myojinsho Caldera west to east，obtained by GSJ， GH87－3Cmise．

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Fig．11 Submarinetopographic map ofthe Sumisu Caldera and Daisan－sumisu Knol1（Minami－sumisu Caldera） （after Saitoαα1．，1988）．Contour interva口00m。

km（Fig．11）．This caldera is also nearly cir－ 11）．The caldera floor is l km across with a cular and about6－9km in diameter．The maximum depth of842m．The summit of caldera floor is5－6km across and800－900m somma is300－500m in depth，although a peak in depth．Maximum depth ofthe noor is about risesupto269minthenorthwestpart．lncli． 969m．Relief ofthe caldera is600to700m．A nation of the ca監dera wa1夏is20－40。（Fig．13）． centralconewithrelativeheightof250mand The caldera is accompanied with several 2km of diameter exists in the eastem part of satellite cones on the nank and the adjacent the floor．Slope of the caldera wall is very sea floor，Two larger cones are mapped onthe steep，32。in average（Fig．12）． east and west sides of the caldera（Fig．11）。 This caldera is of the high gravity anomaly 2。1。8 Torishima Submarine Caldera type，about25mga1（Bouguer）higher than This℃aldera is located at lat．30。32ンN，long． surroundings．A remarkable dipole magnetic 140。20’E just north of Torishima Island（Fig． anomaly，negative to the north and positive to 14）．The caldera isellipsoidal and6（E－W）× the south，is observed in the caldera． 8（N－S〉km in diameter．The caldera floor is 2．1。7 Minami－sumisu Caldera about l km across．The maximum depth of the A small caldera is fomd at20km south of flooris660．mandreliefofthecalderais480m． Sumisujima Island（Taylorαα1．，1990）．It is There is a small central cone on the eastem located at lat．31。16’N，10ng．140004’E．The size floor，and Torishima Island is on the south of of caldera is sma11，2－4km in diameter（Fig。 the somma．The northem part of the somma

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Sumisu Caldera O S 層 擁 霧 ．蕊 毒 欝藝、“． 饗難 」撫 1 繁灘 蝦環繋 i 嚢灘 ㌔撫．、熱 鱒藝

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Daisan－Sumlsu Kno” OS （Minami－sumisu Caldera）

ll 翼 1 農 1 薦 i 旨 響 轟蕪

s N W E O 10km － Fig．13Single channel air－gun reflection profile across the Daisan－sumisu Knol1（Minami－sumisu Caldera） west to east，obtained by GSJ，GH87－3Cruise．

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40’

◎

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30030‘N ㊥⑨・rishima 1「

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109 1400201E

Fig．14Submarine topographic map of the Torishima Caldera（after Bathymetric chart No，6553，Torishima by HydrographicDepartmentofMaritimeSoftyAgencyofJapan）．Contourinterva1100m．

恥rishima Caldera O s

1、．

懸 ． 1 轟 爵離劉1 幾 糊 1

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塑

ノ き 一線碗一『 llぞ茎≡訓 0 1・km／・ ／ W E ／ Fig．15 Single chamel air－gun reflection profile across the Torishima Caldera west to east，obtained by GSJ， 薩 GH84－2Cruise． 匿

decreases its depth to700m where the caldera 2。1。9 Kaikata Caldera 馨 wall is obscured．Inclination of the caldera Above mentioned eight calderas all belong wall isremarkably（20。）steeper than the outer to the northem half of the arc，while Kaikata slope（12。）（Fig．15）． Caldera is only one caldera existing in the southern part．

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26040『

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〆 ザ 26。34N 1400509E 1410009 141010－ 0 10km Fig．16Submarine topographic map of Kaikata Seamount（after Saito6’α1，，1986，partly modified）．Four peaks （called KC，KN，KM and KS）are recognized，Eastem peak KC accompanies the caldera．Contour interval100m．

O S

Kaikata SeamoUnt ． 轟， 1

2 繕撫

§ 3

麹 ぎ 彗 1レ

0 10km

嘗 箋1 罫 W E Fig．17 Single channel air－gun reflection profile across the Kaikata Seamount west to east，obtained by GSJ， GH85－1Cruise．

一715一 磯 羅…

Z3z6116渉ヵz6ゾ孟h6 G60Jogiαzl Sz6プz招ッの〆ノ4望）ごz％，Vbl．42，No．12

The caldera is Iocated at lat。26。42’N，10ng． available seismic profiles crossing each 141。05E on the Kaikata Seamomt，65km seamount． south of Nishinoshima Island．This caldera is Ohmachi Seamomt（Fig．19a）was regarded nearly circular and about3km in diameter as an old seamomt（Paleogene to Early （Fig．16）．Alargecentralconeexistsonthe Miocene〉on the basis of seismic profile ana1－ northwestem part，and the caldera floor is not ysis as shown in the marine geologic map of circular but crescent，1．6km long and O．8km the area by Honza 切 σ1． （1982）． Recently， wide。Thefloorisratherflatandabout920m Yuasa6渉α1．（1988a）sampled volcanic and depth．The caldera wall is very steep and sedimentary rocks from the seamomt and inclines27－30。（Fig．17）．Relief of the caldera dated two samples of andesite with the results is about400to500m． of about32－33Ma．The age of the Sawa Positive gravity anomaly over l20mgal is Seamount is not known，but it may be regar－ recognized in this caldera． It is not clear ded older since the seamount is covered by whether the central gravity anomaly of the thick stratified sediment layers and cut by caldera is relatively higher or lower than the many faults as shown in the seismic profiles surroundings．A remarkable dipole magnetic （Fig．19j）． anomaly，negative to the north and positive to The rest seven seamomts are divided into the south，is found on the caldera． two groups on the basis of bathymetric map and seismic profiles．One group of seamounts 2．2 Sea朧o腿虚s w亘出亘醜盛e N五sh亘齢曲ima has a conical shape with a sharp slope，and no Tr偲9恥 stratified layer is obser▽ed on the flank（Figs． Nishinoshima Trough shows a kind of rift 19c，d，h and g）．These seamomts are regard－ morphology cutting across the Izu－Ogasawara ed as younger volcanoes． Getsuyo，Kayo， Arc between Sofugan and Nishinoshima Kin7yo and Doyo seamounts belong to this Islands．An interval distance between islands group． The height of these seamomts is is widest here throughout the Shichito－lwo－ higher than 2000m． The other three jima Ridge and no islands are observed over seamounts are covered by thick stratified about290km．Here is also the deepest point of layers and some fault pattems are observed on the ridge． the flank．Their summits are not sharp but There are nine large seamomts，with para－ rather flat．Many small valley－1ike patterns sitic highs in some cases between Sofuga且and are incised on the flank．They are possibly Nishinoshima islands．Of these seamounts， older seamomts than the above four．Nichiyo， seven conical and sub－conical seamounts dis－ Suiyo and Mokuyo seamounts belong to this tribute linearly along the Shichito－lwojima older group，Their height is lower than2000

Ridge and other two large seamomts exist on m． the outer and inner sides of the ridge（Fig．18）． According to the magnetic anomaly map by These seamomts have been named Nichiyo Yamazaki6∫召1．（1991），the above relations （Sunday〉，Getsuyo（Monday），Kayo（Tues－ are also well recognized except for a day〉，Suiyo（Wednesday），Mokuyo（Thurs－ seamomt．They distinguished four types of day）， Kin’yo （Friday）， Doyo （Saturday） magnetic anomaly caused by the seamounts． seamounts for seven seamounts from north to The first type is characterized by a dipole with south，and Ohmachi and Sawa seamounts for a negative to the north of the summit and a two larger elongated seamomts at the north－ positive to the south．This pattem is normally east and southwest parts． Nichiyo to Doyo magnetized one for the middle latitudinal seamomts are totally called Shichiyo（Seven seamounts on the Northem hemisphere of Days）Seamomts Group．Table l lists the size recent age（after O．7Ma）．Kayo，Kin’yo and 菱 of these seamomts，and Figs．19a－j show some Doyo seamomts have this characteristic pat・

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500 閥

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D阿 28● 義

27● 140。 141。E

Fig．18 Submarine topographic map of the seamomts in the Nishinoshima Trough（Geological Survey of Japan， mpublished data）．West edge ofthe trough corresponds to the fault scarp of the Sofugan Tectonic Line （Yuasa，1985a）．Contour interva1100m． A：Nichiyo Seamount，B：Getsuyo seamount，C：Kayo Seamomt，Dl Suiyo Seamount，E：Mokuyo Seamomt，F：Kin’yo Seamount，G：Doyo Seamount，X：Ohmachi Seamomt，Y：Sawa Seamomt． tem．The second type has a weak anomaly than2000m．Thethird type is characterized by compared with the volume of their edifices． a unique anomaly distribution with a compli－ Nichiyo and Sawa seamounts belong to this cated pattem．In the case of Suiyo seamount， group．The height of the seamounts is lower a large negative anomaly is recognized to the

一717一 羅 驚

Bz6JJ6あ％oゾ孟h6060104αzl Sz67∂6ッ（ゾノ4ραη，『Vlol．42，No，12

east of the summit and two large positives floor． In fact，the basal diameter of these existto the northwest and south ofthe summit． seamounts is relatively larger in the Shichito－ In another case，Mokuyo seamount is char－ Iwojima Ridge．These six volcanoes are all acterized by a large positive anomaly on the Iarge composite volcanoes with multi－peaks summit．Although the origin of these compli－ as shown in Fig．1（center）．Since the islands cated anomaly pattems is not well known，it is form a part of the seamomt edifice，such suggested that both seamomts have been islands，including submarine part，are called affected by the magnetic field of different age here by the name of island attached with the from the recent field at this latitudinal terrain。 word“Volcano．” Getsuyo seamomt also belongs to this type 2．3．1 Nishinoshima Volcano and shows a dipole pattem with slightly shift－ Nishinoshima Volcano is an active volcano ed to northeast．The fourth type is character－ which erupted in1973－1974（Aoki and Ossaka ized by a broad monopole，or a flat pattem 1974）．This volcano consists of a main peak observed on the summit of Ohmachi which appears as an island and other two Seamount． subaqueous peaks which exist west and south The division based on the bathymetry and of the island．There is a small cone on the seismic profiles well corresponds to thatby the northeast flank of the main peak，which may magnetic anomaly data except for Getsuyo be a satellite cone．An elongated high trending seamount． That is，the sharp，conical in NNE to SSW is present on the west side of seamomts regarded as yomger volcanoes are westem peak．The high is covered by、thick comprised in the first type of magnetic anom－ sedimentary layers（Fig．20）． Since thick aly pattern that has been caused by the recent manganese oxide crusts of hydrogenous origin middle latitudinal magnetic field of the were recovered from the high，it is not regard－ ／ Northem hemisphere。The flat－topped or val－ ed as a part of recent Nishinoshima Volcano ley－incised seamounts regarded as older ones but an older high． are complicatedly or weakly magnetized， 2．3．2 Kaikata Seamount Getsuyo seamount is regarded as a young one There are no records or observations of from the seismic profile（Fig．19d）and mor－ volcanic activity at this seamount，though it phologic data （see Fig．32），but magnetic consists of andesitic and basaltic volcanic anomaly data show the characteristics slightly rocks and has a caldera on the east summit．A different from the other young seamounts hydrothermal alteration zone was found in the （Yamazaki6厩1．，1991）． caldera（Yuasa6齢1．，1988b）and disseminated Recently，the Hydrographic Department and sulfides were dredged from the caldera wa11 the submersible’ノShinkai2000”recognized a （Urabe6渉α1．，1987）．Reddish brown－colored caldera morphology and hot water（about40。 precipitates drifting were also observed in the C）seepage on the top of the Mokuyo seawater within the caldera and deposited on Seamount （Nagaoka 6渉α1．，1991）． The the wall and floor，and weak simmering of the hydrothermal activity has remained yet in the seawater on a wall，where thick precipitates seamomt． cover the outcrop of altered rocks（Yuasaα 召1．，1988b）．These phenomena show the pres－ 2．3 Larger seamo腿nts between Nishino・ ence of hydrothermal activity in the caldera shima a屈Minami一童wojima islan4s and there is a high possibility that the volcano In the southem part of the Izu－Ogasawara is active， Arc，a deep trough，Ogasawara Trough； Kaikata Seamount consists of major four develops on the east side of volcanic front．The peaks with several small satellite cones（Fig． eastem flank slope of each seamount or island 16）．As already mentioned，a caldera topo9－ continues deepen．ing downward to the trough raphy exists on the eastern peak of the

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Ohmachi Seamount 2S

鰺繋 懸繋’． 甕．灘、灘 ．・ 繋襲 3 ・一鴇灘 一・ ♪争哨’㌃ 署，獣デ、 騰灘灘嚢麟丼罰 Pロ 1か 、 願 灘難． 挿’「 尋 麟1 一 ， ■ 一 ． ず ．一吃ぶ一願一り 灘』．← ・占一’．＝ ㌦蟻灘 減還 i’ ～蕊勲一臼＝’ ？． 糞謹 ‘ ノ蒸 懸藷 檬鱒 』灘 4 縄蓼騰 羅プ諏恥 屍 ．羅譲 、驚畷 黛雛薄 匡、餅鱒31藩 ．くを 鞠欝、薄 ・ ’、筑『 ．…二輩藷灘 ・』・灘…・二 鰹弊 ”一 獣・一乙 手 ＼y顎・． ア 無～静 隅 ＝．＝、～唱rP “・礁鑓 一・簿 繁 、灘 i 煮ヌ脳欝 ざ・一轡 『．忌一．． 灘 、≦ メ」 5 畝臼ざ羅、糞こ際郵 豊 』、重二’『 憾纏4’ ．…蕪・』； 伊』究書一∈ 箋 ツ・一＾一一・F 、 曹甲r謡・・一弓』 f．5一燦i’熱 慧 鐘ll…1馨 ，凌藤 吻 ＝ウr’・レ” 喚臼1” ■一9 一 ・｛’・’諺 』む顔 ン婆・冶． 灘 一一メ ー ’・ i毒 、義 r＝き一 ・層 、 ， ． ・穰難 ！一 覇

餉＿一，∴r臨＝一． ひ 甲“一、．一＿一阜■ ♂旨ぐ一∴・・一 蓼棄 6 締一’… 配 』 一『一 彪、・ 簾驚潅

■，冒 一 ■” r ■ 0 10km ，髪’塁 鑛1…グヒ ’』’ w ． つ訊曽’』｝ンF ’ ： o ．． 犠1二 W E Fig．19a Single channel air－gm reflection profile across the Ohmachi Seamount in the Nishinoshima Trough west to east，obtained by GSJ，GH84－4Cruise，

Nichiyo Seamount 1S ↓ 講1 難、麟 2 i． 灘慧 総鞍 鞍 r軸P ず 甲 自 監 欝 髄 r 石一甲 ■ ■羅・…懸欝．、灘 Ohmachi Seamount 熱、甲糟． 斜 ・ ■ 3 ．毒 謙 § イ・・ 一磁》一 難難 ．難鍵、

．5 肇轡 繋． 講罪繋 7 唱1■卜 鍔． 驚 、麟7 4弼7s一 戴－一豊純 撫講麟 灘 吃 iド＿ ・ ； 灘、『難・ 叶r一一一一一’ 鷺 4 一 一 一 舳 一 一 『 鴨 諜r讐篭 纈 ｝ r ・’一 ”占一 ， 糠一 糠撫 』 一 一 一 r 語r・鼻：一・、 “、叢 一 ■＿一 ■一 ドー’ 」 ，’蔓・受． ∫1τ 一r一梢r『 イ1ぜ一輯・一

．鰯一 “’ 鱗｛i 癒毒．：ト． 喀． 髪” I Il ・亀 盤’ 6

W 暴『 0 10km． 望 鯉 客 嚢l 日蓋 雲 ．〆’ E Fig．19b Single chamel air－gun reflection profile across the Nichiyo and Ohmachi seamounts in the Nishino・ shima Trough west to east，obtained by GSJ，GH84－4Cruise．

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OS

Getsuyo Seamount

， ノ” 嘉 1

襲 Ohmachi Seamount 劃 2 劃

磨導 届輔『 舞、 一鑛繋懸 1蓑 三嚢P． 3 ギ ぼ 灘葦 難難 撫． 4 叢 ・義二、

穐 5 難藪’ 』嵐

廿 一曾 マ ゆ ミ 寒 イヌ！

0 10km 闘 一c 馨，、 W E 箋 Fig．19c Single chamel air－gm reflection profile across the Getsuyo and Ohmachi seamounts in the Nishino－ shima Trough west to east，obtained by GSJ，GH84－4Cruise． 翻

畢脚 ト防 Kayo Seamount P『 肇 ト 1S ド 轟 蟹 熊 ll、 iず 2

叢蕃 Ilr 斬 紮 Ohmachi Sea鵬Qunt 3 薫 ！1 1 ！ 竃 難 嚢1 頼

’誓、 4 り宰 1姦航 鷲 箋 5 馨馨翼 蒋 E W 0 10km 一 Fig．19d Single channel air－gun reflection profile across the Kayo Seamount and southem extension of Ohmachi Seamomt in the Nishinoshima Trough west to east，obtained by GSJ，GH84－4Cruise． 一720一 S64％zoz6盟オs o％ 孟h6 ∂oloαnづ6ノケoη！，ノ2z∫一（㎏αsα卿6z名o∠L76 磁． y初αs4 6渉召乙ノ

1 S

Suiyo Seamount ：漁 2 〆熱 慰

霧’．窒熱構 欝・ 熱 3 ・、臨 藩蓼 ・徽磯 灘饗灘灘 菱一簿慣 、灘馬．こ 4 り降 ■・一」腕㌦も ξ

’で㌧ 』 ▽夷 1L、 く ’ド W 0 10km E Fig。19e Single chamel air－gun reflection profile across the northem part of Suiyo Seamomt in the Nishino・ shima Trough west to east，obtained by GSJ，GH84－4Cruise，

Suiyo Seamount l S

．鍵 ㌔馨… 劉薄穿 臨 2 モ ロ と 』．レ1 l！ 無 『灘 i『1 ’、 ，講 奇『濃聖 ヨ ロ ほ ら ．講・ 舗 帽 コ覇 ‘こ』で訣。 ア 「 灘艶難 で辱 チ【叫 r F 麟、．、 犠難 へ プ

『』 讐 3 欝－1 遜 懸』 鍵隷

漂雛纂難 薫驚灘ヂ 「≧＿二 一一 、羅撫 4

一－一＝一・’デ，』一1・・？・『 一一も“隔＝ 卜F 耳一’ 1鷺1蟻’ 多 o． ’ r

ζ’㌻

肇 蕊 藁・ 婁 W E 0 10km

Fig．19f Single chamel air－gm reflection profile across the southem part of Suiyo Seamomt in the Nishino－ shima Trough west to east，obtained by GSJ，GH84－4Cruise。

一721一 B％♂髭虚％oゾ≠hεGεolo9∫o（zl Sz67∂ε：y oゾノ4りOl㌶，yioJ．42，No，12

Mokuyo Seamount 1S

2 A

乎・ 灸 顯 渥’「 3 ↓． ㍊護縛 壕 4 嚢罐 妻 垂 鑛 、繋 臥 懸 灘襯 5 ．、講 蜜錨

マ

導 6

毛 0 10km 婁 ド ノ W E Fig．199 Single channel air－gun reflection profile across the Mokuyo Seamount in the Nishinoshima Trough west to east，obtained by GSJ，GH84－4Cmise．

O S Kin－yo Seamount

織』！ 1

6 ゲ 慈 鷺“ 2

ξ雲ξ 鎌

『 ノ’㌔ 撫’ ｛ 3 『 畿 ’響ζ臼 デ マ 籔麟、1

灘 劇欝 劇 灘簿罵 灘臨 繕1 1 縛 遍 羅ぐ』 曽～暫、’一』＝・〆 4 ・・濃二 欝1｝護 ’灘顕 ’ 一．憾雌・ ｝ 一向＝甲i’”㌦ 厳 蕊慰・ ”｝『）1臼 一円’．一一・r一 ’ 驚llゴ ’遭 ” 染 『 選 ・， ヌ・轍襲，． ぜ ・ ，熱 ’li、’柄 雌1 5 ．v＝“、b，ひ ・き戴饗 一 P 一 一 回 あ宗，響・ ズ、 5 ．・｝・17・ 麟粥聾ら勧 。llン’ 機肇、 「’1』 ． 1 ，雛， も ．一＝卜甲r弗 ■＿

、』 讐 、＝・＝■ fl繋 ．駕 ，■ ド” 、 ミ’． 聾欝 b ■ 甲 A 彗 W∫ ，1’ 難 羅 0 10kmm F

Fig，19h Single channel air－gun reflection profile across the Kin’yo Seamount in the Nishinoshima Trough west一一 to east，obtained by GSJ，GH84－4Cruise，

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S6ごzηzoz6％お on オh6 z／ol（♪6zπ歪6．〃o館！，ノ2z6－09とzsα躍α名α／176 磁． 】匠鋸αsσ 6！6z乙ノ

2 S Sawa Seamount 睡』，

Sma”high between Kinlyo and Doyo seamounts 3 誹 ，1轟瀦 蕪 聡 4 1－ し 馨 ｛

揺．．繕驚 劉 1 1 － “ 峯 5 蹴＝ ・、一鰺 P『マ ｝一 執 【r 藩驚 鰍蟹縢 二 轡■ ｝驚∫． 『畿．嚇 2鷲 醗至 奪 ギ ㌻“ 帯マγ ’一・謬 二 ． §噂 叫、’～ ’プ・ 鱒・エ 嚇 ．．欝 ＝』卜’） ↓1：＝ 、・’ξ購 も鴬曙も才創 軌・コ’ ’歯・ 1餐 ・”一だ 」r一 ＿二臼 」 一 一 警－＝ ∵』！ワ’－』 ’ F r ド、一

1司、一≧ 『 ’ 、 r ㎝が 頽 」し 一H 一r一冒 ．一表 ー一－一pら 一襲 一 ー デ顕肯 軸》》 9 掴 門 r』▼帰辱＝ご・ 、ヌ 」 ロ： 械マf・ ．試＿、¶ 7 凧≦一豪 4 r 或 解’徽．ブ 一 芦驚『誹胃 一’尋一一み離一＝ 一 甘 『 ・題iゴー ｝一一「 藝 訂、集 η〆 こ ． ’一 レ ’・ 響’ 匿’曽一遍一 一←・ ㌧ 1』膣 ． こ 価 、 て “P＿ ＿一 艶憎 『 1． ．’喜、』 ・や 営信F 気⇒＿ 『’ 一一〇km IE〆 1 F － ｛ 鈷Al・： ， 糟 誹 ろ・ 甲翻一イ＿ r 轡昏 き W， 一． 髄毫 鵡』 曙陶 晶 ら

Fig．19i Single chamel air－gun reflection profile across the Sawa Seamomt and a small knoll between Kin’yo and Doyo seamounts in the Nishinoshima Trough west to east，obtained by GSJ，GH84－4Cruise。

Sawa Seamount Doyo Seamount 1

議、 欝 2 藁 ．藩 3 雛蚕欝． 妻．｛． 鱗一

Ill』 1 4 懲鎖 灘． 灘脚 萎 搭 ＿≦榛 撫 5 騨一

1嫡』 瑚 覇 1、 貝 Lぐ 卜 0 10km 1， W iE Fig．19j Single chamel air－gm reflection profile across the Sawa and Doyo seamomts in the Nishinoshima Trough west to east，obtained by GSJ，GH84－4Cruise。

一723一 鰯

Bz61」6渉ヵz（ゾ云h6060109’6αJ Sz67∂6ッ（ゾノ4ρα％，9V；ol．42，No．12

North of Nishinoshima O S

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騨．【 難灘翻 3 購驚

驚｛，1 4

塞，蠣鐸藝響 ’1 難 ロ ■「 7 5 一攣警 癬 1 劃 享膚 ！ 鶴 き ■ 」 陸 ．、．一 £ 0 10km 蓑 、 毎 讐 蓮、．望，、張叢 薯 gl ／ W E Fig．20 Single chamel air－gm reflection profile across the northem part of Nishinoshima Volcano west to east， obtained by GSJ，GH85－1Cruise．

seamount．A minor ridge extends to the west named Kaitoku Seamount for the whole from northwestem somma of the caldera and seamounts． the ridge tur血s to southeast．This ridge forms Above three volcanoes are very similar in a horseshoe shape toge出er with the caldera size，about50km in basal diameter and about （Fig．16）．This feature may indicate a part of 2400m in height．All of them are composite larger somma of older caldera．Three peaks volcanoes consisting of three or four large except forthe caldera peak arrange in NNE to peaks．On the other hand，the following three SSW trend．An isolated high exists on the volcanoes，Kita－iwojima，Iwojima andMinami west of the volcano．As thick sediment layers －iwojima volcanoes，are also composite vol－ are observed in a seismic profile（Fig．17），the canoes but different from the above three in high may be as old as the elongated high on the their topographic features． west of Nishinoshima Volcano． 2．3．4 ：Kita－iwojima Volcano 2．3．3 Kaitoku Seamount A remarkable double chain of small cones Kaitoku Seamomt is an active volcano trending in NNW－SSE is recognized in the which empted in1984（Tsuchide6渉召1．，1985）． volcarlo（Fig．22）． Kita－iwojima Island is The volcano consists of three major peaks located at about10km east ofthe chain．There with some satellite cones（Fig．21）．East and areten ormoresmall cones on the volcano and west peaks which are separated by a deep of some of them have flat planes regarded as a 1000mdepthhavebeencalledHigashi－ wave－cut terrace．Submarine eruptions were kaitoku－ba and Nishi－kaitoku－ba，respective・ obselved at the Hmka－asane5km west ofthe 1y，by Japanese fishemen．When the subma－ island in 18th and 19th centuries． In recent rine eruption occurred at the Higashi－kaitoku years，discolored water has been often obser・ －ba in1984，the Hydrographic Department ved at the same point．

一724一 S66z7陀oz6％孟s o％ 孟h6 ∂olαz％げ6ノンo？z渉，1≧z6一（勾o熔ごz”ごzzごz／176 （7以． 】乙z66姶ごz6！召1．ノ）

Kaitoku Seamount O s

欝 轟 1 ！， 懇 難 雛 は壕． 集 2 。亀、

：・； ≒彦 糞、 嶽 蹴

： μ蹴輪難1 ，：1il渉’㌧ 』！、1 3 薦 鴫難甲 蒙鯵1…・ 暴裳 懇蝶 轟■ ■ 自ぎ 1 轄 ズ～ ド画 懇 掌1 馨 鯉 騨叢 轡 獅 馨籔 1ド 工建 ＿＿＿＿＿講礁＿伽 叢 4 一鰻 ミ…ll二il一’ φ1÷

瞬÷3 r一・一十穿 一

『二－”r一’i’ 1き1灘 『il業 縢 一・∴一：・1・一rテ 3 5

，一㌘ ミ↓ 闇・ 甲’一一i受il馳” 0 10km 翼i麩 、 W E Fig．21 Single chamel air－gun reflection profile across the Kaitoku Seamount west to east，obtained by GSJ， GH85－1Cmise．

South of Kita－iwojima O s

讐 璽

叢馨‘ 店『 馨 ヒ1 ぐ 1 、驚・ ［奏’ 舞 I i 乳 驚亀 薄

鷺 難．．… く 11 毫漫ぎ 醸藩、 2 灘 踏，㌫‡ ≦ 灘 i・F 鷺鞭辮 懇’÷一 ヤ蜜蘇 i 嚢・ 嘆瀧糖 ■ ｝ 1為rll 撫 曾轟捻1、’1 灘， 一一 1 メ蓼 餐罷駐 撚 辱 箆 類熱脳， 3 婆甘［繕｝・一？∴ 鰹 欄 ｛ タ←誤一・ ピ・誓護、 1鷺． ～ 就職益窄 P帰r 1 ． 礪二i 嚢蟻 鰐 1 ！， 一一唱一艶ぞ 爵 強、匿鼻 一 一う ： ‘卜〆轟「．、 拶騰難，辮擬建漁 騒 ’■ ・ 一P÷・ ・肇’ ，一・』先 lll驚葦 盆，i 欝曽 詳 ！ ， ＝・峯 ・一i 葺 b一 ，、鐸蕊尋悔 ン、謙駕撫 ． ？一 鑛、 ■舜耳．糞 騨’溜鱗ぢ 4 しち ン 』こ■ 爵’一嫉唱

夢ず∫・

r｛藩・ li桑曹 『・勘＝， 且 嘩 ぺ 9 。欝 1き＝婁』と 『 5

ヒ 0 10km Wl 鐸 垂 薯

Fig．22 Single chamel air－gm reflection profile across the southem part of Kita－iwojima Volcano west to east， obtained by GSJ，GH85－1Cruise．

一725一 騨

8z6JJ6あ初oゾ云h6 060JogJαzJ Sz67％ly6ゾノ4ραn，Vbl．42，ハ石o．12

North of Iwojima O S

縄

1 瓢．駄 難；

糞馨 2 lI・1 弄 鉱，司 ド， 1

卜 3

孕 ～ 陸 旨 含 ロ 面 慧 ド 黎 ド 豊 4 1，瓢／ 10km

W ’E 蠣 Fig．23 Single chamel air－gm renection profile across the northem part of Iwojima Volcano west to east， obtained by GSJ，GH85－l Cruise．

2．3．5 1wojima Volcano major highs join with each other to form a This volcano has a dome structure in central large elongated volcano with a NNW－SSE part of Iwojima Island．It contrasts markedly trend． One is the Minami－iwojima Island with the elongated feature of Kita－iwojima above sea level．There are several small cones Volcano．Aflat－toppedhighcalledKaise－ aroundthenorthemhigh（Fig．24）． Volcanic nishinoba exists on the WNW flank of Iwo－ activity of this volcano has been often obser－ jima Island．The flat－top is about200m depth ved on a small bank called Fukutoku－okanoba and considered to be a wave－cut terrace（Fig． northeast of the Minami－iwojima Island。The 23）．This may be an older high because the bank grew into an islet during the eruption in 霧 dredged rocks are coated by a thin ferroman－ 1904and 1914，and named “Shin－iwojima ganese oxide crust．Many satellite cones are Island”（Ogura，1915）．This island disappear－ present on the slope around Iwojima Island。 ed several months later（1904eruption）or Kato and Ikeda （1984） and Kaizuka α 6zl． within two years（1914emption）．The latest （1985） described a caldera around the eruption of the Fukutoku－okanoba occurred in Motoyama of Iwojima Island on the basis of 1986and a new volcanic island appeared again bathymetric feature that several highs sur－ within a short time．Recently discolored water round the Motoyama in a circle．Volcanic is often observed． There are two isolated activity of Iwojima Volcano is characterized highs west and east of the volcano．It is not mainly by phreatic emptions on land，and clear whether these highs belong to the mem－ many fumaroles，hot springs，alteration zones ber of the volcano or not．From the eastem and high temperature areas are present on high，benmoreitic trachyte lava was dredged． Iwojima Island． It is similar to the rocks recovered from Fu－ 2。3．6 Minami－iwojima Volcano kutoku－okanoba and Iwo∫ima． This volcano shows an elongated topogra－ As described above，there is a remark：able phy similar to Kita－iwolima Volcano．Two difference between Kita－iwojima and Minami

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O S South of Minamトiwojima

1

藝…屍、， 難緯 妻 腿護駅F， 盛 総鉱レ・ 2 ド 雛．、 、 申騨欝髪 ．雛勝 ‘ 鰯 撰鷺輝難き ｝ ．．擁 爬 “購 羅欝 一霧霧難難．…讐！、． r電『一

・驚磯、＾玉 量 ’二馨難 3 F 餐 一 r 懸議 ，4轟’之曹’へ ，鎌野織 1 譲 ギ藩・ 、藻、 ・マノレ■，5 1婁欝 翻霧二薄 脾泌㌦二 ’ 翻I 旨 訓鍵 4 麟警 I ド’ 一麟・悟’会 辱 F 辞錫ご監 秘 ［肖

ε 5 ・潔 鴬ζ〉櫓■ 一 覧嫁葦・’ ”癩さ趣 10km0一 浅雛1 ，黎多一・τ． ’ρ4 鰯 1 W で熱τ E Fig．24 Single chamel air－gm reflection profile acrossthe southem part of MinamHwojima Volcano west to east，obtained by GSJ，GH85－1Cruise．

一iwojima volcanoes and Iwojima Volcano in 1974），or magma supply rate or age of volcano shape。The former two have large elongated （Bloomer6！α1．，1989）． Ben－Avraham and features with55－70km in long axis and30－55 Nur （1980） indicated that the elevation of km in short axis，while the latter has a dome－ volcanoes is approximately constant within 1ike feature with a basal diameter about50 individual arc on the oceanic crust but varies km．The height of Kita－iwojima and Minami on the continental cmst，while edifice heights －iwojima volcanoes is higher（2900－3300m） are greatly variable on the oceanic crust but than Iwojima Volcano． ratherconstantonthecontinentalcrust．Basic relations among the elevation，height and 3．C恥ar癩er亘s伽sof面st曲櫨亘o醜，髄e宜幽t cmstal property may be generalized as above． a翻麟c量踊go飾o且c躍oes＆且膿gt恥evo且e麟c Other aspects of the relations in the arc vo1－ 貨o聡t o歪臨e亘z腿一〇9＆saw麗＆Are canoes may be also revealed by detailed It has been often pointed out that there may examination within an arc． Here，we will be some regularity regarding the dimensions present the regularity among the height，basal （height，diameter and volume），spacing and diameter and spacing of volcanoes on the distribution of arc volcanoes （e．g．，Vogt，1974 Shichito－lwojima Ridge，the volcanic front of a，bl Ben－Avraham and Nur，19801Katsui， the Izu－Ogasawara Arc，and discuss the origin 1981；Bloomer6ピ認．，1989）．Such regularity of the regularity． was discussed in connection with the nature of 3．1Dist曲盟t量膿＆翻s腿ei髄go£vo且c蹴oes the underlying crust，continental or oceanic， The arrangement of volcanoes on the vol－ （Ben－Avraham and Nur，1980），the crustal canic front shows an almost linear distribution thickness of arc（Vogt，1974a），the depth of pattem parallel to the trench．The linear dis－ partial melting （Lingenfelter and Schubert， tribution，however，is intermpted by some

一727一 翼

βz61♂6云ヵz（ゾ云h6 G60Jo4αzl S％7％y6ゾノ4ρα％，Vわ乙 42，No。12

139。、．．．．．140。E 加購 噺 34腿舗 Kur・se紘舗iku「aCanyOn 誌戴論吻・

320一一一一 鰻shQ Sumisujima舎 Myojinsho Canyon △Daisan－sumisu knoIl l 310 ←一TQrishima Canyon も 今下orishima

噸一』一r Sofu Canyon 30’N 、 1 △Sofugan Aべ l B今ムX 29。 △ C 冶D Nishinoshima¶rough l F 一千←2ぴ ム ムG牽蜷・・一

Kaikata△ ～ Kaitoku△ 一巡1塑

lwolima△ ・、 Minami－iwojima△Fukutoku ，縄 24。 △ h極．

143。E

Fig．25 Distribution of volcanoes along the Shichito－Iwojima Ridge。Troughs and canyons crossing the ridge are also shown．Open triangles：Quatemary volcanoes，and solid ones：01der volcanoes．Alphabets A to G，X and Y are the same in Fig．18．Getsuyo Seamount（symbol B）is regarded as a younger seamomt in this figure． discontinuities and the segmentation of the submarine volcanoes on the Shichito－lwojima volcanic front is often observed in several arcs Ridge are shown by open triangle in Fig．25． （Stoiber and Carr，197310ide，1974；Marsh， The linear distribution of volcanoes forming 19791Carr，1984）．Volcanic islands and recent the volcanic front is defined here as the first

一728一 S召（z抄zo鋸アz孟s oη ごhε ∂oJ66z％ぎ（7〆ン07z孟，Z≧π一〇9αs（zz仇z名とz／1名6 （M． yiz6αsごz6ごα」．ノ order distribution of volcanoes．In the trace of shinoshima Trough．This segment，however， the volcanoes from the south to the north， is interrupted near at lat．28。north。The third there are at least three gaps among them．The segment starts from Kayo Seamount，the cen－ first order distribution is separated in each tral part of Nishinoshima Trough to Sofugan segment by these gaps and the arrangement of Island．The fourth segment comprises only volcanoes in each segment is called the second Torishima Island．Torishima Island is located order distribution． In the northward of slightly on the forearc side．Further north，five Minami－iwojima，a small gap is present segments are recognized as shown in Fig．25． between Kita－iwojima and Kaitoku Seamomt． That is，they are the segments consisting of That is，Iwojima and Kita－iwojima islands Daisan－sumisu Knoll to Sumisujima Island， make a segment with MinamHwojima Island． Myojinsho（or Bayomaise Rocks）to Aoga－ Next segment starts from Kaitoku Seamomt shima Island，Hachijojima to Kurose，Mikura－ to Nishinoshima Islands through Kaikata jima to Miyakejima islands，and Omurodashi Seamount and continues further north to to Oshima Island． Thus the first order Kin7yo and Doyo seamounts，south of Ni一 arrangement of volcanoes on the Shichito一

δ o Nishinoshima

27。 う馬 兎 の Kaikata o

Kaitoku o 26。

ぴ

Klta－iwojima

9’ 25。N D722 漆 Iwojima

醸D713 Minaml－iwolima O D819 24。

Minami－hiyoshi の 黒

謡o 価， ， Nlkko 、 23。 1410 Q躊 ・騰 魁 翁 賦

220

142。N Fukujin 143。

Fig，26 Distribution trends of highs（solid lines）on the larger composite volcanoes in the southem part of the Shichito－Iwojima Ridge．Note the remarkable difference in the trend between Nishinoshima－Kaitoku volcanoes and Kita－iwojima－MinamHwojima volcanoes．Crosses show the locafities of benmoreitic trachyte around Iwojima Island．A broken line indicates their arrangement．

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Bz61」6！1％（ゾ孟h6 G60108ガαzl S％7z／61y6ゾノ4ゆごz％，Vわ」．42，No．12

Table2 Volcano spacing for the younger volcanoes in the Nishinoshima Trough，

lsland／seamount Spacing Canyon ITrough km Sofugan 55 （Getsuyo Seamount） 28 Kayo Seamount 419 Nishinoshima Trough Klnlyo Seamount 39 Doyo Seamount 48 Nishinoshima

km 200 OD 卜

且o o 邸 島 ① ω

100 8 OD 田 ミ ミ 卜r r 8お トマ 田 。る 8 等 昭 3 qり 寸σ》 響 8 oり 8 田 鴇 鵠 縄 8

0 123456789101112131415161718192021222324252627

11Sagami Trough，3：Niijima Canyon，5：Mikura Canyon， 71Aogashima Canyon，10：Myolin Canyon，12：Torishima Canyon， 13：Sofu Canyon，16：Nishinoshima Trough（Sofugan Tectonic Line）， 20：lwojima Trough

Fig．27 Volcano spacings along the Shichito－Iwojima Ridge from north（left）to south（right）．Numerals on the columns are the interval in km，See also Table1．Nichiyo，Suiyo，Mokuyo，Ohmachi and Sawa seamomts are neglected in the figure。

Iwojima Ridge is divided into nine segments． trend，whereas those on the southem vo1－ There are some canyons and troughs on the canoes of Kita－and Minami－iwojima show boundary among these segments（Fig．25）． NNW－SSE trend（Fig，26）．The dikes on the There are characteristic arrangements in Kita－iwojima Island also show the same trend distribution of individual peaks on the large （Kikuchi and Imaizumi，1984）． composite volcanoes． This arrangement is The spacings of volcanoes are shown in Fig． defined asthe third order distribution．To give 27． It is remarkable that the spacings of a few instances，the distributions of individual southem volcanoes，Nishinoshima to Minami－ highs on the composite volcanoes of Nishino－ iwojima islands，are relatively constant，65－78 shima，Kaikata and Kaitoku show NNE－SSW km，whereas those of the northem volcanoes

一730一 S（3ごzηzoz6η孟s on 孟h6 ∂olαz％づ6ノ玩onJ，ノ2π一〇9ζzs召z侃z70／170 偬． y擁ごzsごz6！召」．ノ）

∈奪E E E vary greatly and have a wide range from20to の の の o 8σ888 二 90km．、The spacings only for the yomger o 987叩叩 o 〇 volcanoes within the Nishinoshima Trough ＆％＆ Z are shown in Table2．Maximum spacing is 0 about ll9km at the boundary between Kayo

and Kinフyo seamomts．The irregular spacing O 寸 in the northern volcanoes is represented by o ＼藁

both narrow（20－25km）and wide spacings（35 、 －90km〉as shown in Table l and Fig．27．The ｝ 、 ど の “ Φ 。㌔似。 ㊤ Φ 旨 distribution of volcanoes （Fig．25） and the ＞ q bO 邸 o ・一 r⇒ projected sections（Fig．28）show that there o 勉 ノ 一●目o o ，1 ，霞①o are some pairs of narrow spacing in the ≦曇ρ ＼ ℃ o のQ廟＜ northem arc and the interval between each 、 9おも邸 ΦFQ pair is wider． There are some canyons or 1 ℃き 邸 o ＝ 一調 q ！ ○ の Qイ troughs incising from the back－arc side or o 置1 の三～ trench side in the wider spacing part（Table l 々（％韓、 ｝ ℃ O bO韻φ 刷。あ℃ and Fig．25）．Of these canyons，the Minami－ ％8 曳 し 国8田ゆ ね mikura Canyon，for instance，incises along the ！ 日83 O イ ＝昌℃9 0 9 コ fault（Oshima8厩1．，19811Yuasa，1985b〉and o 、 3 旨図 ｝ 7“．9 the different distribution pattern of earth－ o％司々 ノ ．呂老お 要 嵩 邸 ① quake aftershock area is recognized between イ＼ 。2ρ‘ 瀦 Φ ←｝ the north and the south of the canyon （e．9．， 曳 （ノ）謂 O 輯 一 o 噛、 の い o 調 O bO Kasahara6！α1．，1974）．The canyon makes a o 々戦。 1 θ一．…i bO － b心 8 氏 9 邸 ⊆1 tectonic boundary here．There are the same 0 α o く possibilities for the other canyons or troughs． ダ・、D 罵拐で 」 の や o⊃ ■ ．⊆ 、ノ ノ 台3の If so，it may be inferred that the volcanic ⊆ 竃・9晟 oコ φ o o 、1 spacing within the same stmctural block is o 彦 Q oコ 翫2三 q o ⑩ ＆も2 E 」 の narrow，while the volcanic spacing on the ■ ，／’ oり 8§お block boundary is wider along the volcanic o o ．舅躍 こ ＼ 焉 田 闘慧 front of the northem Izu－Ogasawara Arc． 乏 ￥ 塵。三 o 回り 、 There is another remarkable example in the q L 房こ：雲 Nishinoshima Trough where the arc is divided も8寓 o ⊆詩8 into two parts，northem and southem。 A ＆％80％8 ・二り o o60 Ω、図 Q の 一 r→ major fault，Sofugan Tectonic Line，1imits the ・聾 の ゑ バΦΦロりbb o ト ℃．聾メq westem edge of the trough（Yuasa，1985a）． “ 8・qで’一 り の ト の ％級％昏 ．9台’βε The volcanic spacing is as wide as about119 0 9 邸 ■き8．の kmwhere the tectonic line crosses the volcanic Ω‘○ ω Φ ＜湯雲彊 front．These facts suggest that the fracture 巽 Qo controls the distribution of volcano in the q ，蝉 opposite sense of Vogt（1974a），i．e．，volcanoes 口 ＆ζ％吻 are not found for a long distance where the ’む砺 fracture crosses the arc． Oo The similar relations between volcano and “ ㌔錫ノ fracture are known in the Northeast Japan Arc．On the basis of the distribution map of volcanoes and tectonic line in Tohoku area， ニ o コ 寸 o Mukaiyama6渉α1．（1983）showed that the q E 可 E E E の ＞o Φ o o o o － o o o o α5 0 0 0 F Φ w 叩叩 の 一731一 Bz61」6あ％6ゾ孟h6 060104αzl Sz67∂6：y6ゾノ4ρごz多z，Vマ）1。42，ハめ．12

spacing ofvolcanoes in the same cmstal block volcanoes，such as those erupted acid volcanic bounded by the tectonic lines is narrow but the rocks and intermediate to basic volcanic spacing between volcanoes in the different rocks． In general，the height of volcanoes blocks is wide．Horikoshi（1979）examined erupted acid volcanics is lower than the others the relation between volcanic distribution and in the northem part，and the height of these basement geologic stmcture in the Kuril and volcanoes is nearly constant． This suggests Northeast Japan arcs and concluded that the that the basement height decreases toward き characteristic features of non－volcanic area Nishinoshima Trough． Fig．31shows the are continental suture（or plate boundary）or basement depth of volcanoes （subtracting subsidence zone． On the basis of Central elevation from height or adding depth of the American volcanoes Carr（1984）also emphas－ top of seamount to its height〉．The variation ized that at segment boundaries volcanoes are pattem indicated by the basement depth is widely spaced． similar to that by volcanic elevation。That is， the elevation of volcano reflects the basement き 3．2He直9恥t我賊S亘zeo蝕0且ea聡oes depth．The maximum depth of water on the 鞘 A projected section of the submarine topog・ Shichito－lwojima Ridge is near lat．29。N， raphy along the Shichito－lwojima Ridge is where water depth is over3400m．In the shown in Fig．28．This figure is a projection southem part，the basement height increases through the highest parts of the ridge，showing from this deep to the south．Nishinoshima a sky－line of the ridge observed from the east． Trough intersects the Shichito－lwojima Ridge The elevation（or depth）of each volcano is at this deepest point． The trough forms a somewhat meven as a whole．However，in the boundary between the north and the south of northem half it tends to decrease from Oshima the Shichito－lwojima Ridge．There are some to the south，while in the southem half it different features of volcanic topography increases from the Nishinoshima Trough to between them．For quantitative presentation the south．The variation in volcanic elevation of the topographic features， the relation shows a symmetry between the northem and between height and basal diameter of the the southem sides of Nishinoshima Trough（28 volcanoes on the Shichito－lwojima Ridge is －29。N〉．The elevation of volcanoes and depth shown in Fig．32． of submarine volcanoes are meridionally As easily recognized from this figure，these arranged in Fig．29．The elevation shows a volcanoes are classified into some groups of zigzag pattem and decreases from north to geographic distribution or volcano types in south in the northern part，then it increases terms of the relation between the height and southward in the southem part．On the other basal diameter．There are seven groups in・ hand，height of each of the volcanoeS indicates cluding the Northem Mariana volcanoes as different pattem as arranged in Fig．30．Here shown in Fig．32． Four groups are distin－ the height is taken as the distance from top of guished by the geographic classification，such the volcano to the changing Point of slope as the northem and the southem parts of the inclination of the volcanic edifice． In this Shichito－Iwojima Ridge，the Nishinoshima figure the height is nearly constant for each of Trough，and northem Marianas．In the north・ the northern and southern parts divided by em Shichito－lwojima Ridge，caldera vo1－ Nishinoshima Trough and does not show the canoes with acid volcanic rocks（dacitic or tendency to change systematically toward the rhyolitic pumice） are distinguished from trough as represented by the variation in ele－ stratovolcanoes consisting of intermediate to vation． The zigzag pattem observed in the basic volcanics． The seamomts in the Ni－ volcanic heights of the northem part may be shinoshima Trough are divided into two types， caused by the mixture of different types of younger and older ones．Moreover，Ohmachi

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2000 m

＝ ・一 1000o 邸 》 ① 国 、 0 』 ユ ① o －1000 ｝ 一2000 21 23 25 27 29 31 33 35 1at（。N）

Fig．29 Elevation and depth of volcanoes along the Shichito－lwojima Ridge．Soli（1triangles show the vo1－ canoes erupted acid volcanic rocks in the northern part．Nishinoshima Trough crosses the volcanic front between the range shown as NT．Arrow indicates the position where the Sofugan Tectonic Line intersects the volcanic front．Nichiyo，Suiyo，Mokuyo，Ohmachi and Sawa seamounts are neglected in the figure．

4000 m

3000

三 〇 2000 ’5 ＝

1000

O 21 23 25 2ア 29 31 33 35 lat（。N）

Fig．30 Height of volcanoes along the Shichito－lwojima Ridge．Solid triangles show the volcanoes empted acid volcanic rocks in the northem part．Higashi－aogashima Caldera is plotted separating from Aogashima Volcano．NT and arrow same as in Fig，29，Nichiyo，Suiyo，Mokuyo，Ohmachi and Sawa seamounts are neglected in the figure．

m 0

一1000

』 ユ ① ’ロ ー2000

① の 邸 m －3000

一4000 21 23 25 27 29 31 33 35 1at（。N）

Fig，31 Variation in depth of the base of volcanic edifice for the volcanoes along the arc axis of the Shichito－ IwojimaRidge．NT andarrowarethesameasinFig．29．Nichiyo，Suiyo，Mokuyo，OhmachiandSawa seamounts are neglected in the figure．

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▲ 3000

2500

－ ’152000o 工

1500

1000 ロ ロ ロ

霧

500 一■一一 一 一一一一r一一一 一－丁一 km 10 20 30 40 50 60 70 80 Basal diameter

囲 Northem part of Shichito－Iwojima Ridge（inter．一basic）

ロ Northempart of Shichito－lwojima Ridge（acid）

◆ Younger seamounts within Nishinoshima Trough

◇ Older and complicated seamounts within Nishinoshima Trough

▲ Kita・iwolima－Minamトiwojima voIcanoes

Nishinoshima－Kaitoku volcanoes△

Volcanoes of Northern Mariana area×

Fig．32 Relation between height and basal diameter of volcanoes． G：Getsuyo Seamount，Oh：Ohmachi Seamount，Sw：Sawa Seamomt， Sf：Sofugan Volcano，F：Fukutoku Seamount．

and Sawa seamounts off the axis of the vo1－ that of the southern volcanoes tends to be canic front in the trough form another group． larger with increasing of the basal diameter． Sofugan Volcano including subaqueous part Forth，the height of yomger seamounts within plots near to the younger volcano group in the the Nishinoshima Trough is higher than that trough rather than the group of stratovol－ of the older ones． Ohmachi and Sawa canoes in the northern part of the Shichito－ seamounts are larger than those of the others Iwojima Ridge．From these classifications，the in the Trough．These seamounts have ceased following regularity can be shown in the their activity since long geologic time ago so properties of seamount geomorphology． that their volcanic features were wom out by First，the height of volcanoes in the northern structural movement，sedimentation and ero－ part ofShichito－lwojima Ridge is smaller than sion． that in the southem part。 This fact corre－ sponds to the difference in the size of basal 3．3 The origin of reg既larity of the vo亘嗣 diameter between them，i．e．，the basal diame－ canic topography ter of the southern seamounts of Shichito－ Regularity of the volcanic topography above Iwojima Ridge is larger than that of the mentioned shows conspicuous differences northern ones． Second，the height and basal between the north and the south areas of the diameter of caldera volcanoes with acid vo1－ Sofugan Tectonic Line． canics areboth smaller than the others．Third， 3．3．1 The differences in topography the height of stratovolcanoes with intermedi－ between northem and southem volcanoes ate to basic volcanics on the northern Shichito The differences between the two parts are －lwojima Ridge is relatively uniform，whereas summarized as followsl

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Height 10W high Elevation decrease to south decrease to north Spacing narrow（irregular） wide（constant） Height of basement decrease to south decrease to north Basal diameter smal1 large

In addition to the above，there is difference height of volcanoes is constant for each and it in volume of volcanoes between the northem is not a suitable example for that the height of and the southem parts．As the volume can be volcanoes on the oceanic crust varies conspic－ regarded as a function of its height （Francis uously． The same consideration has to be and Abbott，1973），the difference in volume is pointed out for the different topographic fea－ not listed above． The existence of caldera tures between the northem and the southem volcanoes consisting of acid volcanic rocks is parts of the Izu－Ogasawara Arc as summar・ characteristic of the northem arc． These ized earlier，i．e．，the northern and southern calderas form a group of low－height volcano。 parts are of different units． The origin of them is discussed in the next The height of volcanoes for each unit of the paragraph。 arc may not be related to whether the under－ Ben－Avraham and Nur（1980〉indicated 1ying crust is oceanic or continental． In each that the elevation of volcanoes is constant unit，the elevation （or （iepth〉 of volcanoes within arcs on oceanic crust but varies on depends onthe basement leve1（Fig．31）。In the continental cmst，while edifice heights are case of the Izu－Ogasawara Arc （Shichito－ greatly variable on oceanic crust but constant Iwojima Ridge），two mits are distinguished on continental cmst，In the Izu－Ogasawara and each unit extends about500－600km along Arc，the height of both northem and southem the axis． The misunderstanding of Ben－ volcanoes is nearly constant，which corre－ Avraham and Nur（1980）may be caused sponds to the characteristics of arc volcanoes mainly from their disregarding of the subma－ on the continental crust． In addition to the rine volcanoes and division of arc unit． height，the elevation or depth of volcanoes is Submarine volcanism has taken place mder variable，corresponding aIso to those on the the different environment from the subaerial continental cmst。This contradicts their own condition．The rest angle of submarine vo1－ description of the arc．They treated the vo1－ canic edifice is supPosed to be larger than that canoes on the Izu－Ogasawara and Mariana of subaerial one，and the submarine edifice arcs as a continuous arc on the oceanic crust grows up higher，if other conditions are the and indicated that this is the best example same．For t五isreason，pumice cones which are which shows the pattem of oceanic volcanoes． rarely found on land exist often under the sea， They showed in their figure that the height of as will be mentioned in the next para玄raph． volcanoes decreases northward from the On the other hand，the size and growth rate of Mariana to the Izu－Ogasawara arcs．In the subaerial volcanic edifice is smaller because figure，however，if the volcanoes are divided its small rest angle and erosion by wind and into two parts，Izu－Ogasawara and Mariana rain，if other conditions are the same．Thus，it arcs，it is also possible to interpret on the basis is expected that the height of volcanic edifice of their plots that the height of volcanoes in on the subaqueous arc becomes higher with theMarianaArcishigherthanthat intheIzu－ increasing the proportion of its subaqueous Ogasawara Arc，but the height of volcanoes in part． each arc is almost constant．That is，if we There are some discussions on what the regard the two arcs are independent，the spacing of volcanoes means．Vogt（1974a）

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noticed the relation between the spacing and ma source depth，i．e，，the deeper partial melt－ crustal thickness and pointed out that the wide ing，the wider spacing．As already mentioned spacing of volcanoes is found on the arc with in Yuasa and Tamaki（1982），there are some thicker crust and vice versa。 In the Izu－ differences in chemistry of volcanic rocks on Ogasawara Arc，volcano spacing is narrow in the volcanic front between the north and the the northem part and wide in the southem south of the arc，i．e．，low－alkali tholeiite oc－ part．According to Vogt（1974b〉，the under－ curs in the northem part whereas high－alkali lying crust may be expected to be thinner in tholeiite－alkaline volcanic rocks occur in the the northern part than the southern part． southern part．This suggests that the（iegree of Crustal sections have been obtained near32。N partial melting of mantle source may be lower （Hotta，1970）and23．5。N（Murauchi6渉召1．， in the southern part than in the northern part． 1968）in the Izu－Ogasawara Arc．The thick－ As will be shown in other volume（Yuasa and ness of the crust on both sections is almost the Nohara，in prep．〉，SB systematic diagram same each other and shows about15－17km shows the same trend of the degree．Tatsumi thick．Although the similar thickness is re－ αα1．（1983）shows that the degree of partial ported for the crust between the northem and melting is considered to depend on the tem－ the southem parts，the following facts suggest perature in the mantle wedge，and the degree the difference between both cmsts． increases with rising the temperature，i．e．，the In the northem part of the arc，many ca1－ transfer to low pressure side in the wedge． deras which erupted acid volcanics exist．If This indicates that the higher degree of partial we considered that the acid rocks have been melting corresponds to the shallower depth of derived from remelting of crustal rocks like in the melting。Therefore，it is possible that the those of Niijima and Kozushima islands depth of partial melting tends to be shallower （Onuma召∫α♂．，1983），the heat source of in the north an（i deeper in the south． remelting may be caused more from the This corresponds to the argument by Lin－ thicker crust rather than the subducting plate， genfelter and Schubert （1974）concerning the Because there are no different in age of sub－ spacing． That is，the narrower spacing of ducting plates between the northem and the volcanoes in the northern part indicates the southem parts．The abundant occurrence of shallower depth of partial melting of mantle rocks belonging the hypersthenic－rock series， source than in the southern part of the arc． as will be mentioned elsewhere（Yuasa and 3．3．2 0rigin of submarine caldera Nohara，in prep．〉，also indicates the thicker There are many volcanoes with calderas in cmst of the northem part．Miyashiro（1974） the northern part of the volcanic front，but emphasized the positive correlation between only one caldera is known in the southem part the crustal thickness and the proportion of （Table1）．Recently，several submarine cal． calc－alkali series in whole volcanic rocks of deras have been newly found to the east of each arc．The vast of arc mass shown by the Aogashima Island，to the north of Bayonnaise 2000misodepthlinemayreflecttheexistence Rocks，on the north of Sumisujima Island，etc． of thicker crust in the northem part．These （Yuasa and Murakami，19851Murakami and facts suggest that the crust of the northern Ishihara，1985）．The calderas concentrate in part is thicker than that of the southern part． the northem part of the Izu－Ogasawara Arc The argument by Vogt（1974b）is inconsistent （Yuasa and Murakami，1985）and are recog・ with the above． nized at 10 0f 16volcanoes on the volcanic Lingenfelter and Schubert （1974） argued front from Oshima Island to Nishinoshima that the spacing of arc volcanoes reflects the Trough． The appearance rate of caldera depth of partial melting and the length of exceeds60％here，which is remarkably high spacing does not exceed the distance of mag一 relative to the average rate of about20％for

一736一 S6ごz彫oz6％お oη 云h6 z／ol6ごz盟づo〃o錫！，12％一（噛αsごzz仇z名8／176 似． 】晦ごzsごz 6渉ごzl．ノ the world’s arcs（Suzuki，1977）．Six of the ten low gravity anomaly type．Examples of the calderas are accompanied with eruption of former type are Oshima and Hawaii which acid volcanic rocks（dacitic or rhyolitic pum－ empted mafic lavas．The high gravity anom・ ice）and these calderas all occur under the sea． alies of these calderas are caused by the They are Kurose，Higashi－aogashima，：Kita－ accumulation of high density lava into the Bayomaise，Myolinsho，Sumisu and Minami－ crater．Shikotsu，Toya，Aso and Aira calderas sumisu calderas from north to south．On the belong to the latter，low gravity type caldera． other hand，in the southern part of the arc These calderas were formed by the explosion there exists only one caldera associated with erupting huge acidic volcanics （pumice）． Kaikata Seamomt．The seamount consists of From these view points，the above submarine andesitic and basaltic rocks， and no acidic calderas are the high gravity anomaly type volcanic rocks have been obtained yet from caldera in terms of gravimetric data，although the seamount． the products of these calderas have character－ As mentioned above，the calderas producing istics of low gravity anomaly type caldera．In acid volcanics in the northem Shichito－lwo－ this area，there exists the typical high gravity jima Ridge all occur as the submarine vo1－ anomaly type caldera，Oshima Volcano（Yo－ canoes．Their basal diameter is very small koyama，1969）， relative to that of the subaerial caldera vol・ Remark：able magnetic anomaly which canoes which empted a large volume of pyro－ reflects the dense lavas accumulating under clastic flow （Moriya，1979〉，whereas the the caldera noor is recognized in the volcano height ofthe submarine calderas is higher than （Kato6厩1．，19621Nakatsuka，6抱1．，1980）． that of the subaerial calderas （Figs．32and The amplitude of magnetic anomaly is char－ 33）．The floor of these submarine calderas is acteristically small in these submarine ca1－ almost at the same level of the surroundings。 deras，and any dipole anomaly pattem corre－ This shape may correspond to the feature of sponding just to the caldera is not reco9．nized pyroclastic cones．Their dimension，however， （Oshima6厩1．，1981；Murakami and Ishihara， is much larger than the subaerial cones 1985）．Even if there are some dipole anomaly （Wood，1980）．Stratified layers are often pattem near the caldera，the anomaly corre・ observed in seismic profiles of the caldera sponds only to a part of somma． flank（e．g．，Figs．4，7and9〉，This suggests Although they are not accompanied with the that volcaniclastic materials cover the vo1－ loss of mass as observed in Shikotsu Caldera， canic edifice．Ishihara（1987）showed that the it is also difficult to consider the accumulation density（2．43and2．38）of caldera seamomts of high density lava into the crater for the （Higashi－aogashima and Sumisu calderas）is compensation for the loss as observed in the lower than the other non－caldera volcanoes（2． Oshima Volcano．From the above geological 62－2．97）． This supports that the caldera and geophysical features，it is concluded that seamounts are possibly constructed by light the edifice is a huge submarine pumice cone。 materials． Submarine pumice eruptions have been According to the geophysical observations， often observed as the pumice rafts on the sea these submarine calderas are associated with or on the beach（Gass6渉α1．，19631Kato，1988）． high gravity anomalies but in many cases their However，the eruption under the condition of magnetic3nomalies show narrow amplitude． deep water causes the deposition of the rela．一 Such calderas with the above mentioned fea－ tively high density pumice （for instance， tures are not included in the general classifi－ woody pumice of Kato，1987〉near the crater， cation of calderas proposed by Aramaki and their repetition will form the submarine （1983〉．Yokoyama（1974）classified calderas pumice cone．In the case of subaerial pumice into two types，high gravity anomaly type and eruption the activity is explosive and the pum・

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50 100 Basal diameter km

Fig．33 Comparison between submarine caldera volcanoes and the Quatemary on－1and volcanoes of Japan （after Moriya，1979）．Volcano types are classified by the difference in their development。Submarine calderas erupting pumice Qn the Shichito－Iwojima Ridge are plotted by open circle symbols in this diagram．Kaikata and Torishima calderas are also plotted by solid symbols． ice is dispersed far from the vent as pumice zima）described by Tanakadate（1935）is an fall and flow．So it is difficult to form a steep example of this case．Omurodashi，south of pumice cone mder the subaerial condition Oshima Island，may be such an eroded rem－ （Moriya，1984）．Under the submarine condi． nant of pumice cone． tion，however，hot pumice just erupted imme－ As mentioned above，submarine calderas diately precipitates and deposits at the nearest associated pumice in the northem part of the place． The experiment by Whitham and Izu－Ogasawara arc are the pumice cones Sparks （1986〉has revealed that water is which is difficult to be formed under the sucked into most of the vesicles immediately subaerial condition but easy under the suba－ in the hot condition and even pumice with a queous condition． The edifice seems to be density of O．2g／cm3will sink if the tempera． constructed mainly of pumice deposits around ture exceeds700。C．Then the hot pumice does the crater．Therefore these calderas are not not float on the surface of water as far as the accompanied with low gravity anomaly in eruption is kept in submarine condition．This spite of pumiceous eruption．It is concluded is the reason why the pumice cone is easily that acid volcanism occurs mder subaqueous fomed under the submarine condition．When condition in the northern part of the Izu－ the pumice cone grows near the surface of Ogasawara Arc as shown in Fig．29and they water，the top of cone is to be easily eroded by form the pumice cones with large crater which w＆ve．In this case，the growth rate of the cone is recognizable as caldera． Therefore the becomes small． If pumice eruption occurs appearance rate of caldera is remarkably high under the shallow water condition，the erupted in the northem part of the Izu－Ogasawara pumice partly or mainly goes out of water and Arc． begins to noat above mentioned．When the As compared with the above pumice ca1一 edifice itself grows above the surface ofwater， （1era，the caldera of the Kaikata seamount is explosive pumice eruptions occur under the different． It consists of andesitic lavas，dikes subaerial condition and no pumice cone grows and volcanic breccias．Diameter of the caldera more．At the same time，the edifice is eroded is as small as about3km．In the northwestem out by wave within a relatively short period． part of the noor，a basaltic central cone exists． The submarine eruption and formation of a The dikes intruding in the caldera wall run new island near the Satsuma－iwojima（io一 parallel to the wall（Yuasa6渉α1．，1988）and

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they may form a ring－dike system though it is Marianas．In the northem Shichito－lwojima not ascertained yet。There are some indica－ Ridge，caldera volcanoes with acid volcanic tions of hydrothermal activity，and altered rocks are distinguished from the others con－ volcanic rocks with sulfide minerals were sisting of basic volcanics．The seamomts in obtained（Urabe et a1。，1987）．Submersible the Nishinoshima Trough are divided into two study by”Shinkai2000”showed that some types，younger and older ones by chl’onological dikes intrude the alteration zope but they are relation． not altered themselves （Urabe，personal 3） Regularity of the volcanic topography communication〉，suggesting that hydrother－ shows conspicuous differences between the mal alteration occurred prior to the dike north and the south areas of the Sofugan intmsion．Although the caldera may be as－ Tectonic Line．These two parts are regarded signed to the Haruna－type（or funne1－type）of essentially as independent units of the arc． Aramaki（1983）because of its small diameter， There is each characteristic feature for the it is not clear whether the gravity anomaly height of edifice of each unit．It does not seem observed on the caldera is high or low．The to be related to whether the underlying crust is caldera wall retreats in several places and a oceanic or continental．It is expected that the large amomt of talus breccia is deposited on height of volcanic edifice on the subaqueous the base of wa11． The dikes parallel to the arc becomes higher with increasing the pro－ wall are exposed in the canyon formed by wall portion of its subaqueous part． In each unit， retreating．Accordingly，the original caldera the elevation of volcanoes depends on the is considered to be smaller than the present． basement level． The difference of edifice spacing between both mits of arc may reflect 4． Conc恥阻sio11しs the depth of partial melting of mantle source． 4）Topographic features of seamounts in the 1）Morphology ofthevolcanic edifices onthe Nishinoshima Trough are distinguished from Shichito－lwojima Ridge，the volcanic front of the nearby seamomts in the relation between the arc，shows certain difference for the height and basal diameter．These seamomts height，basal diameter and spacing of vol－ are subdivided into three types，and it coin－ canoes between the northem and southem cides with the other division by magnetic parts of the arc．The height and basal diame－ anomaly and seismic profiling features． ter of the northem volcanoes are low and 5）Existence of submarine calderas produc－ small，while those of the southem volcanoes ing acidic ejecta is a special feature in the are high and large．Spacing between the edi－ northem Shichito－lwojima Ridge．These ca1－ fices isnarrow and irregular in the former，and deras have higher edifices and narrower basal wide and regular in the latter．The irregular－ diameter than the subaerial Crater Lake－type ity of spacing in the northern volcanoes seems calderas． Low density determined from to be caused from the structural discontinuity， gravimetric data and weak magnetic ampli－ e．g．，the volcanic spacing within the same tude shows that the caldera seamounts are stmctural block is narrow，while that on the possibly constructed by light and acidic vo1－ block boundary is wider． canic materials．The caldera floors are nearly 2）The volcanoes on the volcahic front are at the same level of the surromdings．The classified into seven groups in terms of the shape of the caldera seamomts corresponds to relation between the height and basal diame・ the feature of pyroclastic cone， especially ter． Four groups are distinguished by the pumice cone in this case．Pumice cones are geographic classification，i．e．，the northern and difficult to form under the subaerial condition the southem parts of the Shichito－lwojima but easy under the subaqueous condition． Ridge，the Nishinoshima Trough，and northem

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Bz61」6痂z（ゾ孟h6 060Jo8づαzl S％7z76ッ（1ゾノ4ゆα％，V『ol．42，No．12

Ae㎞ow團g贈蝋s：We wouldliketo express 23． our sincere thanks to Emeritus Professor Y． Gass，1．G．，Harris，P．G．and Holdgate，M．W． Katsui，Hokkaido University，for his continu－ （1963）Pumice eruption in the area of the ous guidance，encouragement and a critical South Sandwich Islands．G601．〃4g．，vo1． 100，p．321－330． reading of the manuscript． Professors Y． Honza，E．，Tamaki，K．，Yuasa，M．，Tanahashi，M． Hariya，M．Kato and S．Yui，Drs．Y．Ikeda and and Nishimura，A．（1982）G60Jog加1吻砂 T．Watanabe，Hokkaido University，T， ρブ 云h6 ハ呂oπh6ηz Ogごzsζzz〃απど／1γ6ンSαzJ6 去 Moritani，Sumitomo Construction Co。，LTD．， 1，000，000，Marine Geology Map Series K．Fujioka，Japan Marine Science and Tech－ 17，Geol．Surv．Japan． nology Center，and Prof．1．Moriya，Kanazawa Horikoshi，E．（1979）Downward extension of the University，are also acknowledged for reading Kurosegawa tectonic zone based on the the original manuscript and giving many help・ distribution of Quaternary volcanoes． ノと）z6z． G601．So6．ノψ召銘，vol．85，P．427－734． ful suggestions． （JE） We are also grateful to the following Per－ Hotta，H．（1970）Acrustalsectionacrossthelzu sons of the Geological Survey of Japan for －Ogasawara Arc and Trench，ノ∂％7．P物s． their help and cooperation with the field sur－ Eごz7云h，vo1．18，p．125－141． vey：K．Iizasa，T。Ishihara，K．Kishimoto，T． Ishihara，T。（1985）Geomagnetic anomaly．in1984 Miyazaki，S．Nakao，A．Nishimura，M．No－ F．7． 名4》oア渉 ノわ7 ノ勿∂6s渉z誓召あo％ （ゾ h64∂ツ hara，Y．Okamura，T．Urabe，A．Usui and T． ηz（3孟とzl 名εso％766s ごzo60ηz1）ごz7zづ（34 zσ髭h szのηz6z－ 7z’n6 h夕4名o云h67耀1 ごz6渉づz疹砂， G601． Sz67zノ． Yamazaki． ノ吻）σ％，Geol．Surv．Japan，P．21－23．（J） （1987） Gravimetric determination of Re蛋ere遡ces densities of seamounts along the Bonin Arc．in S召召郷o観応，乞s♂観4s，伽4 諺olZs， § （」）：in Japanese，（JE）：in Japanese with English Keating， B。H． 6オ 召1． eds．， Geophys． き abstract． Monogr．Series，vo1。43，p．97－113． Aoki，H．and Ossaka，」．（1974） 。4窺ッs彪η6ゾ Kaizuka，S．，Kato，S．，Nagaoka，S．and Miyauchi， szめ窺αη●％6 z70」αz％06s 一 云h6 6幼Jo名とzあo％ T．（1985） Geomorphology of Iwo－jima 名60074s （ゾ1、κshづ％oshづ耀 （瓦zあ6魏ごzzごz多z no and surrounding sea floor．∫oz67． G6087．， ハ危βo）．Tokai Univ．Press，250p．（」） vol．94，p．424－436． Aramaki，S．（1983）Some problems on the genesis Kasahara，J．，Koresawa，S．，Nakamura，1．， of Japanese calderas．ノ協’7zズ％g G601．⑦）66乞ごzl Tsumura，K．and Nagumo，S．（1974） 途sz66，vol．11，p，139－154．（JE） Foreshocks and aftershocks of an earth－ Ben－Avraham，Z．and Nur，A．（1980）The eleva－ quake east of Hachijo－jima，December4， tion of volcanoes and their edifice heights 1972．ノ；oz67．S¢∫s％zoJ．Soo．ノ4メ）α鴛，ser．2，vo1． at subduction zone．ノbz6名 σ602）hys．石～6＆．， 26，p．326－335．（JE） vol．85，P．4325－4335． Kato，S．and Ikeda，T．（1984）Submarine topog－ Bloomer，S．H．，Stem，R．and Smoot，N，C．（1989） raphy around Iwo－zima、Rの云．助47097． Physical volcanology of the submarine 1～6s。，vo1．19，p。59－73．（JE） Mariana and Volcano Arcs。動ll．γol－ Kato，Yoshio，Matsuo，T．and Takagi，A．（1962） 6σ％oJ．，vol．51，P．210－224． Aeromagnetic surveys over the Oshima Carr，M．J．（1984） Symmetrical and segmented Island．S6」．Rの孟．Toho肋U勉∂．，5th Ser． variation of physical and geochemical Geophys．，vo1．14，p．65－80， characteristics of the Central American Kato，Yuzo（1987）Woodypumicegeneratedwith volcanic front．∫oz躍． 1呂oJoσ箆ol．Gεo孟hε7仰z． submarine emption．10％7．G601．Soo．μ勿％， ム～6s．，vo1．20，P．231－252． vo1．93，p．11－20． ，Rose，W．1．and Stoiber，R．E．（1982） （1988） Gray pumices drifted from Fu－ Central America．in ∠4編6s吻s，R．S． kutoku－oka－no－ba to the Ryukyu Islands， Thorpeed．，JohnWiley＆Sons Ltd．，P．149 β％IJ．1茜oJαz％ol．Soo．ノψごzη，ser．2，vo1．33，P． －166． 21－30．（JE） Francis，P．W．and Abbott，B．M。（1973）Size of Katsui，Y．（1981） Distribution of volcanoes in conical volcanoes。．〈乙ごz孟z6名召，vo1．244，P．22一

一740一 S＆z窺oz〃z云s o％ 云h6 z701αz％∫6ノ㌃o箆渉，ノ2z6一（忽召＆zz仇z7zz／1名6 偬． yz6召sごz6！ごzl．ノ）

island arcs－a review．B％ll．仏olo醐ol． Nakatsuka，T．，HorikawaシY．，Okubo，Y．，Tanji，K． Soo．ノ4かz％，ser．2，vo1．26，P．221－233．（JE） and Nakai，J。（1980） Total intensity Kikuchi，T．and Imaizumi，T．（1984）Geology and aeromagnetic map off the coast of geomorphology of volcanic island，Kita－ Sagaminada －Izu shoto area．Total iwo－jima． 09召sσωα7α R6s．， To紗o intensity aeromagnetic maps XXVII－3，1： 漉跡oρol渉．Uη吻．，nos．10＆11，P．1－24．（JE） 200，000，Geo1．Surv．Japan． Kuno，H．（1962） G召云とzJogz昭（ゾ云h66zoあ∂6z／olo召ノzo6s Ogura，T．（1915）On the new Iwo－zima eruption． cゾ 孟h6 卿oπ4 勿zol％4わzg so肋如ηz ノ彦9614s， 石～4）云．1ηゆ。Eα7孟hg．1勿z／6s孟．Co〃zηz．，vo1．79， 勲πX正．血勿盟，ル勿伽催4伽吻郷． p．4－16，（」） with English abstract（The Intem．Assoc．Volcano1。，Roma，332p． eruption of Shin－Iwo－jima），p．1－4． Lingenfelter，R．E．and Schubert，G．（1974） Hot Oide，K．（1974） Tectonic fractures and linear spot and trench volcano separations． arrangementS Of VOICanOeS．且SSOO．（｝601． 〈乙ごz！％名6，vo1．249，P．820－821． Colめδ． ノ41）αη， ノ灰）麗097．， vo1．18， P．63－73． Marsh，B．D．（1979）Island arc development：Some （JE） observations，experiments，and specula－ Onuma，N．，Hirano，M．and Isshiki，N．（1983） tions．ノヒ）％名0601．，vo1．87，P．687－713． Genesis of basalt magmas and their Miyashiro，A．（1974） Volcanic rock series in derivatives under the Izu Islands，Japan， island arcs and active continental mar－ inferred from Sr／Ca－Ba／Ca systematics． gins．／1ηz，ノioz67．56づ．，vo1．274，P．321－355， ノ∂％名 1V6」αz錫ol．G60孟h67ηz．Rθs．，vol．18，P． Morimoto，R．andOssakaナ」。（1955）The1952－1953 511－529． submarine eruption of the Myojin Reef Oshima，S．，Tozaki，T．，Onodefa，K．，Kaneko，T． near the Bayonnaise Rocks，Japan （1）． and Ueda，Y．（1981）Geomagnetic anom・ B％ll．Eα7飾（1．1～εs．乃zs孟．，vol．33，P．221－250． alies at sea around central part of Honshu and （1970）The1970activity of Japan．ム～（ψ渉．理ソ4zog飢ム～6s．，vo1．16，p．25－45。 Myojin reef．力％7．06097．，vol．79，p．301－ （JE） 320．（JE） Saito，E．，Watanabe，K．，Miyazaki，」．and Mura・ Moriya，1．（1979）Geomorphological developments kami，F．（1988）Bathymetry ofsubmarine and classification of the Quatemary vol－ calderas in the Izu－Ogasawara Arc． in canoes in Japan．G609γ．ノ～6z7．ノ4ゼ）ごzn，vol．52， ヱ987 F．】r． zの）oπ ノわ7 1％z尼sあ9とzあo銘 （ゾ

p．479－501． h6ごzz2ツ ηz6云ごzl 名6soz6γ66s ごzo60％zρα％勿6！ ω鉱勉 （1984） Pyroclastic cone．in／16短ごzlカho60－ s％6〃zσπn6 h夕4名o孟h67㎜1 σoあ毎砂，Geol．

9名αψh （ゾ ∂olo召多zづo 渉01）09名冴ゆh夕 〃z ／4ρα％， Surv．J’apan，p．10－15．（」） Volcano1．Soc．Japan ed．，p．88－91．（」） ， ，and Murakami，F．，（1986） Mukaiyama，H．，Nakamura，M．，Inoue，M．and Submarine topography of the Izu－Ogas－ Kimura，T．（1983）Geotectonism and post awara and northem Mariana arcs． in －Miocene volcanism in the Northeast ヱ985 F．y：． 名召ρoπ ノb7 1nz／6sガ8とzあo多z （ゾ Japan．Bz611． 『V～）1αz％ol． Soo．ノ4望）召多z，ser．2， h6ごzzノ：y ％z6たzl z6so％766s α660〃zρごzη164 z｛ノづ孟h vo1．28，p．395－408．（JE） sz6動昭πじ％ hッ伽o孟h67㎜1 αoあ擁砂，Geo1． Murakami，F．and Ishihara，T．（1985）Submarine Sur▽．Japan，p。8－10。（」） calderas discovered in the northern part of Stoiber，R．E．and Carr，M．J．（1973）Quatemary Ogasawara Arc．E召πh伽n孟h砂，voL7，p． volcanic and tectonic segmentation of 638－646．（」） Central America．B％ll．γひlo襯ol．，vo1．37， Murauchi，S．，Den，N．，Asano，S．，Hotta，H．，Yoshii， p．304－325． T。，Asanuma，T．，Hagiwara，K．，Ichikawa， Suzuki，T．（1977）Volcano types and their global K．，Sato，T．，Ludwig，W，J．，Ewing，」．1．， population percentages． Bz611． Vてoloα％ol． Edgar，N．T．and Houtz，R．E．（1968） So6．ノ4ρ∠z多z，ser．2，vo1．22，p．27－40．（JE） Crustal structure of the Philippine Sea． Tanakadate，H，（1935） Evolution of a new vo1－ ノ∂％7．Gθoρhタs．R6s．，vo1．73，P．3143－3171． canic islet near Io－zima．P名oo．加ρ6プ． Nagaoka，S．，Okino，K．and Kato，S．（1991） Land－ 且αz4．，vol．11，P．152－154． forms of submarine volcanoes in central Tatsumi，Y．，Sakuyama，M．，Fukuyama，H．and part of the Izu－Ogasawara Arc，by multi－ Kushiro，1．（1983） Generation of arc beam sounding system．ノ～砂07∫功4響7． basalt magmas and thermal structure of ノ～6s．，vo1．27，p．145－172．（JE） the mantle wedge in subduction zones．

一741一 羅

Bz61」6渉勿zの〆孟h6 （｝60Jo4αzl Sz67z／6ッ（ゾ忽）α％，Vbl．42，．〈り．12

∫o％7．G601）h夕s．ノ～6s．，vo1．88，P．5815－5825。 Yokoyama，1．（1969）The subsurface structure of Taylor，B．，Brown，G．，Fryer，P．，Gil1，エB．，Hoch－ Oosima Volcano Izu．力％7．jp勿s．励励， staedter，A．G．，Hotta，H．，Leinen，C．H．M．， vol．17，p．55－68． Nishimura，A．and Urabe，T．（1990） （1974） Calderas and their formation． Alvin－SeaBeam studies of the Sumisu ノ1sso6．0601．Oolんzわ．ノψごz％，ノ吻箆097，，vol．18， Rift，Izu－Bonin arc， Eごzπh P伽？zα．Soづ． p．41－53． L6あ．，vo1．100，P。127－147． Yuasa，M．（1985a）Sofugan Tectonic Line，a new Tsuchide，M．，Kato，S．，Uchida，A．，Sato，H．，Koni－ tectonic boundary separating northern shi， N．，Ossaka，」． and Hirabayashi， 」。 and southem parts of the Ogasawara （1985）Submarine volcanic activity at the （Bonin） Arc，northwest Pacific．in Fo7一 Kaitoku seamount in1984．ノ～砂云．琢4zo8ア． 耀あo％6ゾ∠16伽6006碗ル勉怨初s，N．Nasu ノ～6s．，vol．20，P．47－82．（JE） α召1．eds．，Terra Pub。Tokyo，p．483－496． Urabe，T．，Yuasa，M．Nakao，S．and on－board （1985b） G60Jo9づoα1耀1）（ゾ 云h6 η07孟h6ごzs渉 scientists（1987） Hydrothermal sulfides 6ゾ伽oh勿’oヅづ耀．Marine Geology Map from a submarine calderain the Shichito－ Series26，Geo1．Surv．Japan。 Iwojima．Ridge，northwestem Pacific． and Nohara，M．（in prep．）Petrographic ル疫zη’多z6G601．，vo1．74，P．295－299． and geochemical features of volcanic Vogt，P．R．（1974a） Volcano spacing，fractures， rocks along the volcanic front of the Izu－ and thickness of the lithosphere．Eごz7孟h Ogasawara arc．Bul1．Geo1．Surv．Japan． P旋z％6云．S6づ．。乙6あ．，vo1．21，P．235－252． and Murakami，F．（1985）Geology and （1974b）Volcano height and plate thick・ geomorphology ofthe Ogasawara Arc and ness．1弛πh jp凌z多z6云．Soづ．五α孟．，vo1．23，P．337－ the Sofugan Tectonic Line．ノ∂％7．06097．， vol．94，p．115－136．（JE） 348． Whitham，A．G．and Sparks，R，S．」．（1986）Pum－ and Tamaki，K．（1982） Basalt from ice．Bz41．VてoJoのzol．，vo1．48，p．209－223． Minami－lwojima Island，Volcano Islands． Wood，C．A．（1980）Morphometric analysis of Bz611．G601．Sz∫7∂．ノ4φごz多z，vol．33，P．531－540． （JE） cinder cone degradation． ノ∂z47．Vblαz％ol． G60云h67ηz．ノ～6s．，vol．8，P．137－160． ，Uchiumi，S．，Nishimura，A．and Shibata， Yamazaki，T．and Ishihara，T． （1986） K．（1988a） K－Ar age of a forearc Geomagnetic anomaly in the southem Izu seamount adjacent to the volcanic front of －Ogasawara area （GH85－1and GH85－3 the Izu－Ogasawara Arc（abstract）．B％ll． Cruises）． in1985F．］r．名召汐oπノわ71％∂6sガー Vblαz％ol．So6．ノ41》ごz％，vo1．33，P．352－353。（J） 8iとzガo％ （ゾ h64zノツ nz6云π1 76soz6z66s ごz6－ ，Urabe，T．and Murakami，F．（1988b）A oo〃z1）召％づ64 卿髭h szめ澱zγ勿z6 h』y4アo孟h67卿zごzl submersible study ofhydrothermal field at ζzoあ擁砂，Geo1．Sur▽．Japan，1）。127－134．（」） Kaikata Seamount，Izu－Ogasawara Arc． ，Ishihara，T．and Murakami，F．（1991） T66h．R4）！．4渉h 5ン窺1）o．Z）66ゆ S6ごz 石～6s． ” Magnetic anomalies over the Izu－Ogasa－ Sh初h漉 2000，”JAMSTEC，p．129－139． wara Arc and northem Mariana Trough． （JE） B％IJ．G601．S％7z7．ノ41）召銘，vo1．42，P．655－686．

一742一 S6ごz郷oz6nな o多z 孟h6 ∂olo召％げ6かoη4 12κ一〇9αsαz仇z名π／170 偬． ｝勉召s4 θ渉ごz乙ノ

伊豆・小笠原弧火山フロントの海底火山地形

湯浅真人・村上文敏・斉藤英二・渡辺和明

要 旨

伊豆・小笠原弧の火山フロントである七島一硫黄島海嶺の海底火山地形を記載し，それらの配列，比高， 底径，山体の問隔等に見られる規則性を総括した．各火山および地域ごとのグループにみられる上記要 素に関する規則性は，伊豆・小笠原弧を南北に二分する構造線，嬬婦岩構造線を境に次のようにまとめ られる． 北部 南部 比高 低い 高い 高さ 南へ減少 北へ減少 間隔 狭い，不規則 広い，一定 基盤高度 南へ低下 北へ低下 基底の直径 小さい 大きい 海底カルデラ 多い 少ない 南北間に見られる上記の規則性の違いは，本島弧の南部および北部で島弧地殻の厚さが異なっている ことを暗示している． 南部および北部の問に存在する西之島トラフ内の海山は，その南北の火山とは独立した特徴を示して いる．これらは，海底地形，音波探査プロファイルにみられる特徴の違いに基づいて，火山フロント上 にならぶ新期および古期海山，および火山フロントをはさんで分布する大型の古期海山の，3つのタイ プに分けられる． 伊豆・小笠原弧北部には，酸性火山岩を噴出している海底カルデラが多く出現することが特徴となっ ている．これらのカルデラは，その形態および地球物理データから海底で生じた軽石丘と考えられる． 陸上では軽石丘の存在はまれとされているが，海面下という条件のもとでは比較的形成され易いのであ ろう．このため伊豆・小笠原弧北部のカルデラ出現率が高くなっている．

（受付：1991年7月12日；受理：1991年10月7日）

一743一