抄録
The Takashima-oki core boring was carried out in the central part of Lake Biwa. The core samples are composed mainly of dark greenish grey homogeneous clay sediments ranging in age from the Middle Pleistocene to Holocene. Seventyfive ashes comprising 44 distinct volcanic ash layers and 31 invisible volcanic ash horizons, are intercalated in these sediments. In order to estimate the history of explosive volcanism in the source region of the ashes, the writers divided these sediments into 8 tephro-subzones, a to h in desending order, based on lithologic and petrographic properties and frequency of appearence of the ashes. Subzones h (400 to 280 ka),f (210 to 180 ka) and c (87 to 78 ka) are characterized by the dominance of crystal ashes which consist mainly of T-type glass, feldspar and amphibole. These ashes are the products of major eruptions of the Daisen volcanic zone consisting mainly of the Daisen, Sambe and Kuju volcanoes. Subzones g (280 to 210 ka) and d (140 to 87 ka) are characterized by the dominance of vitric ashes containing abundant glass and small amounts of orthopyroxene and clinopyroxene as mafic minerals. Most of these ashes are the products of large-scale eruptions of the Kirishima volcanic zone consisting mainly of the Aso, Kakuto, Aira, Ata and Kikai caldera volcanoes. Subzones e (180 to 140 ka) and b (78 to 26 ka) are characterized by the scanty of ashes. During the deposition of these subzones, explosive volcanic activities of the Daisen and Kirishima volcanic zones are practically extinct. Subzone a (26 to 0 ka) is characterized by the occurrence of amphibole bearing crystal ashes erupted from the Daisen volcanic zone and two-pyroxene bearing vitric ashes from the Kirishima volcanic zone.
