アソ カザン ニ オケル カコ ヤク 9マンネン カン ノ コウカ カルイシ タイセキブツ

Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is difficult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sakakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clino- pyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several 14C ages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4 ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones