The Aso volcano produced four gigantic ignimbrites (Aso-1, Aso-2, Aso-3 and Aso-4) between 270 and 90 ka, after which it produced Aso central cones. Previously the volcanic products formed after the Aso-1 ignimbrites were defined as Aso volcanic products. However, Pre-Aso volcanoes had been formed until 400 ka around the Aso region. Pre-Aso volcanic rocks and Aso volcanic products have distinct SiO2-K2O chemical trends. In this study, we surveyed outcrops above and below the Aso-1 ignimbrites sequence in the eastern sidewall of the caldera at Sakanashi. Using volcanic stratigraphy and K-Ar dating, we identified a lava flow erupted just before the Aso-1 eruptions. This lava shows a chemical character similar to that of the Aso volcano. It is suggested that the activity of Aso volcano started before the emplacement of the Aso-1 ignimbrites. We also found that Aso-1B, consisting of scoriaceous (basaltic andesite) ignimbrite, overlies Aso-1A, which consists of welded ignimbrite, in the northwestern caldera wall at Matoishi. This succession indicates that the Aso-1 eruption cycle began with Plinian-type silicic eruptions (Aso-1P). This was followed by the major silicic ignimbrite Aso-1A and the minor mafic ignimbrite Aso-1B.
