ῒΐ 臂 /* 膆 (,**/) 臂 , 膞 1+ῐ2/ 臞 腏腔腕腊腒腂腀腁腄腅腈腆腇腍腑腌腉腎腃腓腋腐 膺臩 膰 腈 腂臃膘膙膫 腈腈 腆,**- 臕 / 膔 +, 臔膩臗腀 ,**/ 臕 - 膔 . 臔膩臬腇 Eruptive History of Satsuma Iwo-jima Island, Kikai Caldera, after a 0./ ka Caldera-forming Eruption Fukashi MAENO ῍ and Hiromitsu TANIGUCHI ῍῍ Kikai Caldera, +1 km wide and ,* km long, is a Quaternary volcano located, nearly submerged, in the East China Sea, southern Kyushu. Two volcanoes, Iwo-dake (a rhyolitic volcano) and Inamura-dake (a basaltic volcano) on Satsuma Iwo-jima Island at the marginal part of this caldera, were formed after the great caldera-forming eruption of 0./ ka known as the Akahoya Eruption. We made a geological study to understand the eruptive history of Satsuma Iwo-jima Island after the caldera formation. The volcanic activity in the island after the Akahoya Eruption is divided into three main stages and ten sub-stages based on volcanic edifice development and tephra deposits ; the old Iwo-dake stage (stage OIo-ῌ-῍), the Inamura-dake stage (stage In-ῌ-῏), and the young Iwo-dake stage (stage YIo-ῌ-῏). The old Iwo-dake stage was initiated by phreatomagmatic eruptions and pumice fallout (stage OIo-ῌ), followed by the e#usion of rhyolitic lava with continuous ejection of ash and lithic fragments, resulting in building up an old volcanic edifice (stage OIo-῍). In the stage OIo-῍, intermittent explosive eruptions also occurred. The Inamura-dake stage is characterized by the e#usion of basaltic lava flows and scoria-cone building (stage In-ῌ-῍). After that, at the western foot of the cone, phreatomagmatic explosions occurred (stage In-῎) and andesitic lava flow e#used (stage In-῏). In the young Iwo-dake stage, the explosive eruption occurred at the beginning (stage YIo-ῌ), followed by the e#usion of multiple rhyolitic lava flows from the summit crater and the formation of hyaloclastite in shallow sea that made up the marine terrace (stage YIo-῍). During the past +*** years, the volcanic activity changed to intermittent ones with pumice and bomb fallout (stage YIo-῎-῏). Total phenocryst contents of the Iwo-dake lavas increased up to 2 vol.ῌ through the young stages. The magma of the young Iwo-dake stage is di#erent from that of the Akahoya Eruption and the old Iwo-dake stage, bordered by the Inamura-dake stage. Key words : Kikai Caldera, Iwo-dake volcano, rhyolite lava, eruptive history, bimodal volcanism +ῌ ῐ῎ῑ῏ 膊膃腥腭腩腬腜腀 膏膪膠臓臄腛膟膾臥腍腟自 .* km 臓 膵腚腲臇腑腀 臡腍腟腻膌臑腂膬臰腀 腰臯腀 腯膾腙腘腛臁 膒腥腭腩腬腎臝腝膧膥臏臆膛腛臓臠腶臋膮腚腲臇腒腡腁 膗膡腀 膊膃腥腭腩腬腛臁臙臛腜膱臢腑腕腌腠腀 膢臣臮腹 臏腀 臈臏腙腘腎腥腭腩腬臟腗腑腕膁膮腚膗腢腕腋腡臌臎 腖腊腡腎腀 臐膵 ,* km, 臓臡 +1 km 腚腌腞腝臁膒腥腭腩 腬腖腊腡 (Matumoto, +3.- ; Figs. +a, b)腁 腏腛腥腭腩腬腜 腾膎膲 +* 臤臕膇腚腣腔腡臚膲膀腛臁膉臧臜腽腚腞腠膓 膳腐腢腀 0./ ka 腚腜 腄腤腥腪腫腽膣膂腅 腖臅腟腢腡膠膯腛 腥腭腩腬臜腽腎臖膴腑腔 腆臊臍腂膯腳腀 +312 ; 膬臩腂膽腀 +32, ; Kitagawa et al., +33/ ; 腸臩腀 ,**,腇腁 膢臣臮腹臏臮 腹膅 腆臭臨膈膨腇腀 腴膼膅 腆膖臘膈膨腇腀 膭臱臮腹臏 腆臭 臨膈膨腇腀 腓腑腕膁膱臦腺腚膻膡腒腡臉腷腽膘膋膑腎腀 腤 腥腪腫臜腽腱膝腚膳臋腑腔膯腑腋腽膣膿腖腊腡腁 膬臩腂 膽 (+32,)腀 腸臩腂膽 (,***)腀 Kawanabe and Saito (,**,) 腈 腉32*῍2/12 膷臀膤膶臫膐膜膆膦膶臫 臐臡臁膄臁膄腵臬膄膕膍腼臆膄膸膚 Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku Uni- versity, Aramaki-Aza-Aoba, Aoba-ku, Sendai 32*῍2/12, Japan. 腈腈 腉32*῍2/10 膷臀膤膶臫膐膹臒 臐臡臁膄臐臡腤腦腤膕膍腧腮腨腃 Center for Northeast Asian Studies, Tohoku Uni- versity, Kawauchi, Aoba-ku, Sendai 32*῍2/10, Japan. Corrsponding author : Fukashi Maeno E-mail : [email protected]
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Eruptive History of Satsuma Iwo-jima Island, Kikai Caldera,
after a 0./ ka Caldera-forming Eruption
Fukashi MAENO�and Hiromitsu TANIGUCHI��
Kikai Caldera, +1 km wide and ,* km long, is a Quaternary volcano located, nearly submerged, in the East
China Sea, southern Kyushu. Two volcanoes, Iwo-dake (a rhyolitic volcano) and Inamura-dake (a basaltic
volcano) on Satsuma Iwo-jima Island at the marginal part of this caldera, were formed after the great
caldera-forming eruption of 0./ ka known as the Akahoya Eruption. We made a geological study to understand
the eruptive history of Satsuma Iwo-jima Island after the caldera formation.
The volcanic activity in the island after the Akahoya Eruption is divided into three main stages and ten
sub-stages based on volcanic edifice development and tephra deposits ; the old Iwo-dake stage (stage OIo-�-�),the Inamura-dake stage (stage In-�-�), and the young Iwo-dake stage (stage YIo-�-�). The old Iwo-dake
stage was initiated by phreatomagmatic eruptions and pumice fallout (stage OIo-�), followed by the e#usion ofrhyolitic lava with continuous ejection of ash and lithic fragments, resulting in building up an old volcanic edifice
(stage OIo-�). In the stage OIo-�, intermittent explosive eruptions also occurred. The Inamura-dake stage is
characterized by the e#usion of basaltic lava flows and scoria-cone building (stage In-�-�). After that, at the
western foot of the cone, phreatomagmatic explosions occurred (stage In-�) and andesitic lava flow e#used (stageIn-�). In the young Iwo-dake stage, the explosive eruption occurred at the beginning (stage YIo-�), followed bythe e#usion of multiple rhyolitic lava flows from the summit crater and the formation of hyaloclastite in shallow
sea that made up the marine terrace (stage YIo-�). During the past +*** years, the volcanic activity changedto intermittent ones with pumice and bomb fallout (stage YIo-�-�). Total phenocryst contents of the Iwo-dake
lavas increased up to 2 vol.� through the young stages. The magma of the young Iwo-dake stage is di#erentfrom that of the Akahoya Eruption and the old Iwo-dake stage, bordered by the Inamura-dake stage.
(+33/) AMS +.C dating of the varved sediments from
Lake Suigetsu, central Japan and atmospheric +.C change
Fig. 3. Variations of modal abundance of phenocryst in volcanic rocks for the past 0/** years. The rock of the
stage OIo-�is more crystal-rich (¹,* vol.�) than that of other stages (º,* vol.�). In the Young
Iwo-dake stage, the modal abundance increased gradually with decreasing age.
»% ¼±²³´µ84
during the late Pleistocene. Radiocarbon, -1, -1+�-12.Kobayashi, T. and Hayakawa, Y. (+32.) Geology of Kikai
caldera (source of the Koya Ignimbrite), Japan. In Vol-
canology of the Koya ash flow, A progress report of the U.
S.-Japan cooperative science program (Ui, T. and Walker,
G. P. L. eds), +-�+..Machida, H. (+333) Quaternary widespread tephra catalog
in and around Japan : recent progress. Quatern. Res., -2,+3.�,*+.�� ������ (+312)������� �������������������� !"# +1#+.-�+0-.�� ������ (+33,)����$�%� �&'(��)# ,10 p.
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