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Stage IIa
- Type locality: Mount Chokai, Yamagata prefecture - Akita prefecture
- Stratovolcano
- West Chokai edifice
- Volcanic rocks
- Pleistocene (0.12 Ma, K-Ar)
- Lithofacies: Calc-alkaline andesite
- Lithofacies: Lava
- Stage II
- [ Upper ]
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- Stage IIc
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- Formation of West Chokai horseshoe-shaped caldera
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- Stage IIb
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- Stage IIa
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- [ Lower ]
------ Conformity, ~~~~~~ Unconformity, ~~~--- Local unconformity, '''/,, Interfingered, :::::: Fault, /--/-- Thrust, +__+__ Ground surface, ?--?-- Unknown
- Distribution: Mount Chokai, Yamagata - Akita pref.
- Hayashi and Yamamoto (2008) Jour. Geol. Soc. Japan, vol.114, Supplement, pp.87-95 DOI
- Nakano and Tsuchiya (1992) Geology of the Chokaisan and Fukura District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 138p. (in Japanese with English abstract 7p.)