1999 年 44 巻 1 号 p. 1-12
A systematic K-Ar age determination has been made on thirty-seven samples from the Older Ontake volcanic products, Ontake Volcano, central Japan, with analytical uncertainties mostly within ±5% (1σ level). The “Kuragoehara Lava”, which the previous researchers used as a key bed for stratigraphic correlation, gives significantly different eruption ages in each disthbution area, and some lavas classified into the same stratigraphic units among each area also show quite diffrent K-Ar ages. Hence, a revised volcano-stratigraphy is proposed based on the radiometric data obtained in the present study. The Older Ontake Volcano may be a compound volcano consisting of more than four stratovolcanoes with different vents. The first one, “;Tobu Volcano Group”, erupted at around 750-650 ka on the wide eastern side. The second one, “Tsuchiurazawa Volcano”, erupted at around 680-570 ka mainly in the western area. The third one, “Uetawarayama Volcano”, produced lavas at around 540-520 ka, and the products filled along valleys in the western area. The last one, “Mikasayama Volcano” erupted at around 440-420 ka in the southeastern area. The inactive period between the Older and Younger Ontake volcanic products continued for more than three hundred thousand years. The present study has revealed that the K-Ar age determination is effective for estimating a time gap between the Quatermary volcanic products whose stratigraphic relation can not be well defined in field.