長崎県壱岐島,岳ノ辻火山の噴火史

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The Iki island is late Miocene to Pleistocene volcanoes including scoria cones, shield volcanoes and lava plateau of alkalic and subalkalic rocks in the southern end of Japan Sea. Takenotsuji volcano located in the southern part of the island is one of the youngest volcanoes in the island. The volcano is 212m high and is composed of lava flows and scoria cones of alkalic basalt. Through description and interpretation of the volcanic sequence and the K-Ar dating, we reconstruct the eruptive processes and history on Takenotsuji volcano. We recognize and describe a total of 14 units formed successively with total bulk eruptive volume of approximately 0.18km^3 in the eruption. There are three vent positions for the units. According to the vent position, all of the units are grouped into three stages: Umezu, Shiwara, and Nagata. First, Umezu stage the vent of which is the west of summit of Takenotsuji, is composed of Umezu scoria flow(Usfl), Umezu lava flow (U-1)which widely covered pre-Takenotsuji volcanoes formed lava plateau, Nishi-Takenotsuji scoria fall (U-2) which formed western part of Takenotsuji cone. Second stage, Shiwara stage, occurred from fissures and craters striking E-W in the northern flank of Takenotsuji. This stage is dominated by pyroclastics forming and phreatomagmatic eruptions: Tanakabure scoria flow(S-1), Shiwara scoria fall(S-2), Shiwaraminamibure pyoclasitc surge(S-3), and Shiwaranishibure scoria flow and pyroclasic surge (S-4) . Last Nagata stage alternated lava flows and scoria falls from the central vent and constructed eastern part of Takenotsuji cone. The products in this stage are Nagata lava flow(N-1), Nanto lava flow(N-2), Hokuto lava flow(N-3), Higashi-Takenotsuji scoria fall(N-4), Chosuichi lava flow(N-5), and Sancho scoria fall(N-6). The K-Ar dating indicates that these three stage occurred in 0.9-0.6Ma.
2003-06-30

長崎県壱岐島,岳ノ辻火山の噴火史

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