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1st stage
- Volcanic rocks
- Pleistocene (ca. 30 - 25 ka)
- Sanbe caldera formation stage
- Sedimentary facies: Crater lake-type caldera
- Lithofacies: Hornblende biotite dacite
- Eruptive volume: A few dozen km3
- Distribution (Diameter): 30 km or more
------ Conformity, ~~~~~~ Unconformity, ~~~--- Local unconformity, '''/,, Interfingered, :::::: Fault, /--/-- Thrust, +__+__ Ground surface, ?--?-- Unknown
- Distribution: Shimane pref.
- Hattori et al. (1983) Geology of the Sambesan District, Quadrangle Series, Scale 1:50,000, Geological Survey of Japan, 168p. (in Japanese with English abstract 7p.)