ῒΐ 臢 /+ 膒 (,**0) 臢 + 膰 -+ῌ.* 腉 ῎ῒ῍῏ῌ K-Ar ῑῐ 至膼臯 膣 腆 腂膀臱膮膶 腆腆 腂臇 腐 臮腽 腆腆腆 腄,**/ 臼 0 膤 ,1 臺臂腃腀 ,**0 臼 + 膤 +* 臺臂腔腅 K-Ar Dating of Kimpo Volcano, Northern Ryukyu Arc Kiyoshi TOSHIDA ῍ , Kozo UTO ῍῍ and Akikazu MATSUMOTO ῍῍῍ Kimpo volcano is a distinct volcano in central Kyushu, western Japan, and is located -* km west of the active volcanic front of Ryukyu arc. In order to determine the growth history of the volcano, systematic K-Ar dating has been performed. The activity of Kimpo volcano has been divided into three stages. The oldest activity formed the andesitic stratovolcano in the southern part (Older stage). Six ages are concentrated between +.-2 and +.+/ Ma. The horseshoe shaped caldera was formed before the middle stage activity. The middle stage activity formed the andesitic volcano in the northern part (Sannotake-Ninotake). Ages between *./2 and *./* Ma are obtained from six samples. The youngest activity formed a dacite lava dome (Ichinotake), which is located within the caldera of the first stage activity. Two samples gave consistent ages of *., Ma. The total duration of the volcanism at Kimpo volcano exceeds one million years. However, long periods of repose existed between each active stage : about 0**,*** years between the first and the second stages, and about -**,*** years between the second and the third stages. These inactive periods are much longer than the durations of volcanic activities of each stage. It is therefore important to determine the duration of each active and inactive period of the volcanoes that have longer than a few thousand years of growth history. Key words : Kimpo volcano, K-Ar dating, Ryukyu arc, Quaternary, long term eruption rate +ῌ ῐ῎ῑ῏ 膠臃臦腎腛腕膜膪腚腏臤腆腙腻臧腏腀 膑臷臭膊膷腋臹 腏腙膃腝腧膊膷腰腶腷腮腩膭臏腑腧腽腎腖腀 臵臑腙膆臥 腑腧腊腅ῌ致膨臦膯膄腉腙腢臢膹膘膊膷腋腀腚腏腕腉腧腁 膟腍膊膷腛腸臙膊膷腚臑腎腓 -* km腀 膁臕膊膷腗腚臨膓 腙腻臧腏腀 膡腐膻臑腆腩腇腄臨臋腗腑腧臢膹膘臏臚膊膷 腖腈腧 (Fig. +)腁 膩膴腛腈膗腣膊膷臎臦臍臶腀 膊膷膑臷 腚臩膫腛腽臔臻腡腥腨腘腉腁 腕膜膪腙臬腟膳腠腰腪腴腯 腷膍腱腵腃腮腛腀 膊膷腰腶腷腮臫膇腖臌臲腓 +** km 腙 臣腏腓腋腦腊腥臬腟膳腠膎臲腋膛臜腏腀 膟腍膊膷腚腻臧 腖腛臌臲腓 -** km 腙臣腏腀 腒腨腤腦臑腖腛臌腂臦臍腩 腂臐腏腘腌腘腧 ( 臒膨腂膚臯腀 +33,腅腁 腍腚腓腡腀 膟腍膊 膷腤腦臑腎腙腻臧腑腧膁臕膊膷腣臟腖膐膊膷腚膇腖腛腀 腰腪腴腯腷膍腱腵腃腮腙腤腧臌腂臦臍腛腂臐腏腕腉腘 腉腁 膟腍膊膷腚膔臒腛腀 膎臖臒腹膷膔腩臁臠腗腑腧腍腗 腣膈膏臘臏腚臸自腋腀 腊腅ῌ致膨臦膯膄腉腙腇腄腑腧膠 臄膊膷腣膁臕膊膷腗腙膽腑腧腍腗腋腌膲腎腨腕腉腧 ( 臨 臯腀 +320腅腁 膟腍膊膷腚臠臓腛腓 ,/ km - 腖腈腦腀 腞腓腀 膢臏膾膖腙 腔腉腕腀 Takai et al. (+32.) 腛腰腪腭腬腲腷腂腮腳腭腫 (FT) 臼臡腗臦膿膗腎腻腩腢腗腙腀 腓 +** 腑臼臗腊腥腓 +/ 腑臼臗腞腖 2* 腑臼膓腺臈腖腈腧腗腛腐腓腁 腍腨腩臞 腆 腆腆 腇,1*ῌ++3. 膋臝膺膻膋臝膺 +0.0 腄膵腅臰腘臨膅膦膝臆 臦膜膬膏膦膝臆臦膥膉膏腗腼 Geosphere Science Sector, CRIEPI, +0.0 Abiko, Abiko-shi, Chiba ,1*ῌ++3., Japan. 腇-*/ῌ2/02 腾臉膧腔腌腜膻腁膂 +ῌ+ῌ+ 腔腌腜臨膅 臢 , 膸膞膙臅臛膱膦膝臆膕膌腐腆 Planning Headquarters, AIST, Tsukuba Central ,, +ῌ+ῌ+ Umezono, Tsukuba-shi, Ibaraki -*/ῌ2/02, Japan. 腆腆腆 腇-*/ῌ2/01 腾臉膧腔腌腜膻臵 +ῌ+ῌ+ 腔腌腜臨膅臢 1 膸膞膙臅臛膱膦膝臆 臦臀臊腌膦膝腆腒 Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 1, +ῌ+ῌ+ Higashi, Tsukuba-shi, Ibaraki -*/ῌ2/01, Japan. Corresponding author : Kiyoshi Toshida e-mail : [email protected]
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K-Arkazan.or.jp/cgi-bin/dpdf.cgi?q=51.1.3.pdf · Fig. +. Topography and geologic map of Kimpo volcano. (a) Location of Kimpo volcano in Kyushu. (b) Bird’s eye view of Kimpo volcano
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Kimpo volcano is a distinct volcano in central Kyushu, western Japan, and is located -* km west of the activevolcanic front of Ryukyu arc. In order to determine the growth history of the volcano, systematic K-Ar dating
has been performed.
The activity of Kimpo volcano has been divided into three stages. The oldest activity formed the andesitic
stratovolcano in the southern part (Older stage). Six ages are concentrated between +.-2 and +.+/Ma. The
horseshoe shaped caldera was formed before the middle stage activity. The middle stage activity formed the
andesitic volcano in the northern part (Sannotake-Ninotake). Ages between *./2 and *./*Ma are obtained fromsix samples. The youngest activity formed a dacite lava dome (Ichinotake), which is located within the caldera
of the first stage activity. Two samples gave consistent ages of *.,Ma.The total duration of the volcanism at Kimpo volcano exceeds one million years. However, long periods of
repose existed between each active stage : about 0**,*** years between the first and the second stages, and about-**,*** years between the second and the third stages. These inactive periods are much longer than the durations
of volcanic activities of each stage. It is therefore important to determine the duration of each active and inactive
period of the volcanoes that have longer than a few thousand years of growth history.
Key words : Kimpo volcano, K-Ar dating, Ryukyu arc, Quaternary, long term eruption rate