Akagi volcano situated in the North Kanto district of central Japan is a Quaternary volcano. The eruptive history of this volcano during the last 200, 000 years is clarified by the tephrochronological study.
The plinian pumice fall deposits derived from Akagi volcano are as follows in ascending order; the Moka Pumice (MoP), Akagi-Mizunuma Pumice-16. …-12, -10…-1 (MzP-16…-12, -10…-1), Namekawa Pumice-2, -1 (Nm-2, -1), Yunokuchi Pumice (UP) and Kanuma Pumice (KP) (Fig. 1). Stratigraphy, distributions and petrographic characteristics of these tephras are described (Figs. 5, 6, 8, 9 and Tables 1, 3).
The MoP pumice fall deposit covering the most part of the eastern part of the North Kanto district, erupted in the penultimate glacial stage preceding the Last Interglacial Stage. Moreover, the stratigraphic relations of the MzP-10…-1, Nm-2, -1 and UP with the well dated widespread tephras, which are the K1P-7 (
ca. 130 ka), DPm, On-Pm I (
ca. 80 ka), K-Tz (
ca. 75-80 ka), Aso-4 (
ca.. 70 ka) and DKP (
ca. 45-48 ka), are clarified (Fig. 7). These data can give the chronological framework for the eruptive history.
The MzP series, Nm-2, Nm-1 and UP erupted during the stage called the younger stratovolcano (YS) of the Akagi volcano in previous work (Fig. 10). The total volume of the plinian pumice fall deposits from the MzP-10 to the UP amounts to 28 km
3. This corresponds to the discharge rate of the pumice equivalent to 0.33 km3/1, 000 years and the frequency of the plinian eruption 0.15/1, 000 years.
Before the formation of the central cone, it occurred the most eruptive episode of Akagi volcano. This is represented by the members : the KP pumice fall deposit (
ca. 31-32 ka) and the Mizunuma lithic (chert lapilli) fall deposit (CLP). The volume of the KP deposit which amounts to 25 km
3, is the largest volume of the plinian pumice fall deposits derived from Akagi volcano.
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