The large-scale eruptions in history took place in 788 at Ohachi and 1716-1717 at Sinmoedake. Based on chemical composition of volcanic products of Ohach and Sinmoedake, magma composition became more siliceous and oxigen pressure of the magma increased with the times during about 1,200 years. Wee can estimate the next eruption may occur at Ohachi or Sinmoedake with the silica rich magma of SiO2 over 60%, and it is presumed that increase of oxigen pressure may generate destruction of the volcanic edifice

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Our previous K-Ar chronology and volcanostratigraphic investigations of the Minami Zao volcano revealed that it started its volcanic activity about 1.2 Ma in the northwesten part of the region (Mt. Sugigamine and Mt. Maeyama area), and major activity started 0.6 Ma. There exist, however, several disagreements between the K-Ar ages and volcanostratigraphic sequences. Newly obtained K-Ar ages suggest that the 1.2 Ma activity also occurred in the southern part of the volcano. Reverse magnetization of the sample is consistent with the 1.2 Ma age. We name this "the old-Fubo" activity. The activities between 1 to 1.2 Ma are also found in the North Zao (Mt. Ryuzan) and Central Zao (Roba no Mini iwa=Donky ear rock). The second period of the activity started around 0.5 Ma. From the volcanostratigraphy view, subsequently occurred the third period of the activity, but no gap was found between these activities from the K-Ar study. The most active period was around 0.3 Ma. Disordance between K-Ar ages and volcanostratigraphic have been lessen but still exists.

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