[Abstract]

Three lava flows: Nakahono, Nagano, Otogase lava flows and Otogase debris avalanche deposit were newly discovered on the western slope of Aso central cones, southwestern Japan. The Nakahono lava is biotite rhyolite (SiO2=73 wt.%) and is distributed on only a floor of the Otogasegawa River. Occurrence age of the Nakahono lava is older than that of ACP3 or ACP5. The Nagano lava is pyroxene dacite (SiO2=68 wt.%) and comprises a pyroclastic cone (ca. 600 m diameter). The Otogase lava is pyroxene rhyolite (SiO2=73 wt.%) and forms a lava plateau (ca. 1 km long and 200-300 m wide). The lava is interbedded between the Aira-Tn ash (AT, ca. 29 cal ka) and the Kikai Akahoya ash (K-Ah, ca. 7.3 cal ka). The Otogase debris avalanche deposit is possibly generated by a collapse of lava flow or dome on the vent of the Otogase lava. Based on the stratigraphic relationship with tephra deposits, the eruption age was estimated at ca. 20 cal ka. In the period of younger stage than approximately 30 cal ka, intermittent eruptions producing three rhyolite to dacite lava flows provide important information about eruptive history and magma evolution in the western part of Aso central cones.

Key words:

lava, rhyolite, dacite, debris avalanche, tephra