In 1986, Takashima-oki core boring was carried out in the central part of Lake Biwa, as shown in Fig. 1. The core samples are about 141 meters in total length and composed mainly of dark greenish grey silty clay sediments ranging in age from the middle Pleistocene to Holocene. Approximately 45 distinct volcanic ash layers and 31 volcanic ash-horizons which do not form megascopically distinct layers are found in the core sediments. Stratigraphy, lithology and petrography of the volcanic ashes are investigated, and the following results are obtained. 1) Tephrostratigraphy and properties of each volcanic ashes are made clear, and summarized in Figs. 2, 3 and Table 1. 2) On the basis of the stratigraphic positions and properties of volcanic ashes, the Takashima-oki and the 200m boring core samples from the bottom of Lake Biwa are correlated with each other as shown in Fig. 4, Sixteen corresponding volcanic ashes are found in these cores. 3) Five well-known widespread volcanic ash layers are discovered in these core sediments from Lake Biwa such as Kikai-Akahoya, Ulreung-Oki, Aira-Tn, Aso-4 and Kikai-Tozurahara volcanic ashes.

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Geological study on Asakusa volcano located on the southern part of Moriyoshi volcanic zone in the Northeast Japan arc has been carried out to construct the volcanic history. The volcano had been active in the middle Pleistocene age arid ejected about 6km^3 of the volcanic materials. Volcariism of this volcano began with eruption of minor amounts of lava flows (early stage lava flows), followed by development of the principal constituents of the volcano. The volcanic history of its principal constituents is summarized as follows. Stage 1: Major lava flows and associated pyroclastic flows erupted from a central vent to form a cone-shaped stratovolcano. The estimated volume of the volcanic product is about 2.5km^3. The rocks are composed mainly of andesite and basalt, with augite, hypersthene and sometimes olivine phenocrysts, which is accompanied by a small amount of dacite with phenocrysts of hornblende, augite and hypersthene. Stage II: Central eruption of large amounts of pyroclastic flows and subordinate lava flows also occurred and the stratovolcano built by the first stage volcanism was further grown. The volume of the volcanic edifice is estimated to be about 3 km^3. Andesite with phenocrysts of hornblende, hypersthene and augite was erupted in the early period of this stage, which was followed by eruptions of pyroxene-olivine basalt and pyroxene andesite. The central vents were situated in the uppermost stream of the Tadamizawa Valley throughout the first to second stage. Stage III: After completion of the stratovolcano, small eruptions of lavas composed largely of olivine bearing pyroxene basalt and andesite, and pyroxene andesite occurred from several vents and these lava flows covered the northern and southern flanks of the volcano. Stage IV: Small eruptions of lava flows and pyroclastic flows occurred and their products mainly covered the northern flank. The erupted materials are basalt and andesite containing phenocryst minerals same as those of the rocks in Stage III. Total volume of the volcanic edifice in Stages III and IV amounted roughly to 0.5km^3.

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The rocks from Asakusa volcano, Northeast Japan, are described, and the representative 76 samples were chemically analyzed. Asakusa Volcano Collaborative Research Group (1991) divided the geologic history of the volcano into main two stages, early and late stages. The late stage activity formed the principal edifice of the volcano and is further subdivided into four stages. The rocks of the volcano are composed mostly of the rock suites ranging from basalt to andesite with minor amounts of dacite and all the samples are porphyritic with 20-50 vol. % of phenocrysts. The early stage rocks are composed of andesite of Kuno's (1950) hypersthenic rock series (H series), containing phenocrysts of hornblende together with pyroxene. The rock suite of the first stage consists mainly of andesite and basalt, belonging to both Kuno's pigeonitic rock series (P series) and H series, with minor amounts of dacite of H series. Hornblende phenocrysts are also found in the dacite. The second stage volcanic products are composed largely of andesite and basalt, corresponding to both P and H series. Hornblende phenocrysts are present in andesite erupted in the early period of this stage, but phenocrystic hydrous minerals are absent in one formed in the late period. The volcanic products of both the third and fourth stages are made up basalt and andesite without phenocrystic hydrous minerals. They are of both P and H series. The major and trace element chemical characteristics of the rocks from Asakusa volcano are as follows: 1) In the FeO^*/MgO-SiO_2 diagram, the rocks of each stage mostly plot in the field of the tholeiitic series regardless of P or H series and represent similar differentiation trend. 2) They generally plot on around the boundary between the two fields of the low-alkalitholeiite and high-alkalitholeiite series by Kuno (1968) in total alkalies vs. SiO_2 diagram. 3) All of the rocks also plot in the midway region of the field of Gill's (1981) medium-K andesites and its extention in K_2O vs. SiO_2 diagram. 4) The basaltic rocks from the volcano show the incompatible element characteristics typical of island are basalts, indicating the positive Rb, K and Rb anomalies and the negative Zr, Ti and Y anomalies on the N-type MORE normalized pattern for incompatible elements. Although Asakusa volcano is situated in the Moriyoshi volcanic zone, the volcano has the petrographical and chemical characteristics similar to those of both Moriyoshi and Chokai volcanoes according to the comparative examination of LIL element compositions (Na_2O+K_2O, K_2O, Rb, Sr) and petrography among three volcanoes.

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Quaternary volcanic rocks in the Genbudo area include the Genbudo lava and the stratigraphically overlying Akaishi lava with subordinate amounts of scoria beds. These two lavas are completely different from each other in mineralogy, geochemistry and isotopic composition. The Genbudo lava consists of alkaline basalt with phenocrysts of olivine and minor plagioclase. The groundmass consists of plagioclase, olivine, clinopyroxene, magnetite and ilmenite with or without phlogopite. The major element chemistry of the Genbudo basalt is similar to the Quaternary basalts surrounding the Genbudo area, such as the Kannabe and Takurayama basalts. A whole-rock K-Ar age of 1.61 Ma has been reported from the Genbudo lava, which retains reversed remnant paleomagnetism of the Matsuyama epoch. The Akaishi lava has a porphyritic texture with phenocrysts of clinopyroxene and minor amounts of olivine, phlogopite, phosphate-carbonate mineral, orthopyroxene, hornblende and quartz. The groundmass consists of volcanic glass, anorthoclase, amphibole, phosphate mineral, magnetite and ilmenite. The Akaishi lava is very poor in SiO2 (35-43 wt. %) and enriched in alkalis (Na2O : 0.9-1.5 wt. % ; K2O : 0.6-1.2 wt. %) and high P2O5 (2.2-2.6 wt. %) and light REEs. Such unusual chemical features of the Akaishi lava are likely resulted from hydration and alteration of the groundmass glass. The original chemical compositions of the Akaishi lava before alteration were estimated as basalt or basaltic andesite compositions with high alkalis and P2O5 and low Al2O3. The Akaishi lava has a reverse-remnant magnetization probably corresponoding to the Matsuyama reversed epoch. Each of the Genbudo and Akaishi lavas was derived from individual primary magma, and both of them erupted in the Genbudo area during early Pleistocene.

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