Hachijojima, one of the Izu Islands, is located on the Volcanic front of the Izu-Ogasawara (Bonin) Arc which lies south to north along the boundary of the Pacific plate and the Philippine Sea Plate. Hachijojima consists of two stratovolcanoes : Higashiyama and Nishiyama. There is a sharp contrast between Higahshiyama, which is a well-dissected mountain, and Nishiyama, which is cone-shaped. It is topographically clear that Higashiyama was formed earlier than Nishiyama. The following is a summary of the results of this study.
1) The eruptive history of Higashiyama can be divided into six ages : Sokodo I (40-50 ka), Sokodo II (ca
. 25-33 ka), Sueyoshi (ca
. 17-25 ka), Nakanogo I (ca
. 11-17 ka), Nakanogo II (ca
. 5.8-11) and Mitsune (ca
. 5.8 ka and later).
2) At Higashiyama, a large amount of dacitic magma erupted in the Sokodo I Age and the Sueyoshi Age, and formed a caldera. In the Sokodo I age, the early stratovolcanoes were subjected to landslides and collapsed, and the Higashiyama caldera emarged. In the Sueyoshi Age, the summit of Higashiyama stratovolcanoes were subject to landslides and collapsed to form the Nishihakuunzan caldera.
3) the Higashiyama stratovolcano (young stratovolcano I) was formed in the Sokodo II Age and Mihara stratovolcano (young stratovolcano II a, b) in the Nakanogo I Age by eruptions of basalt and andesite magma.
4) Higashiyama formed a central cone at the end of Nakanogo I Age and eruptions from the summit ended around ca
. II ka. Thereafter, it transformed into a flank eruption which issued andesite magma in the Nakanogo II age. In the Mitsune age, volcanic activities of Higashiyama receded.
5) In the Nishiyama region, eruptions became active from the Nakanogo I Age and issued mainly basaltic tephra and pyroclastic surges. However, dacitic tephra was also issued. Occasional finding of pumice fall deposit suggest the generation of daciticmagma.
6) On Higashiyama, deposits Aira-Tn ash (ca
. 24.5 ka) and Kikai-Akahoya ash (ca
. 6.3 ka) were transported by westerlies from the Kyusyu area. They prove to be important marker tephras clarifing the tephra stratigraphy and eruptive history in this region.
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