Mt Fuji had preferentially effused basaltic magma since its initiate of activity at about one hundred thousand years ago. In the Younger stage (5,600yrsBP～), however, it became to spout occasionally andesitic or dacitic materials to be involved in air-fall deposits or pyroclastic flows, such as, Hoei scoria or Osawa pyroclastic flows. Andesitic magma is solely found as melt-inclusions in phenocrystic minerals in the Older stage (100,000～10,000yrsBP). This suggests that some kind of changes might have occurred in the magma plumbing system of Mt Fuji. To investigate such possibility, we measured whole-rock chemical compositions of the representative scoria layers of the Younger stage erupted in and around the summit and Hoei craters, and interpreted the results based on our recent model of magma plumbing system of Mt Fuji, consisting of the deep basaltic and the shallow andesitic chambers. The compositions of the scoria layers of the younger stage showed high FeO^*/MgO ratio with high and varied TiO₂ and K₂O contents, which are the same as the general characteristics of Mt Fuji as pointed by previous studies. Detailed comparison with the scoria layers showed that compositions of the Younger stage had slightly but systematically higher SiO₂ content - occasionally basaltic andesite - than those of the Older stage. These chemical differences can be explained by existence of more differentiated magma having higher SiO₂ content in the shallow chambers in the Younger stage, compared to those in the Older stage, rather than by increase of the mixing ratio of the similar differentiated magma of the Older stage. Mixing such a differentiated magma with a basaltic magma newly raised from the deep chamber can generate basalts having slightly high SiO₂ content as erupted in the Younger stage. Although cause of generating SiO₂-rich magma in the Younger stage is unclear, it might have occurred associated with the inactive period suggested by the Fuji-black soil layer lying between the two stages. The shallow dacitic magma chamber assumed in the model of the Hoei eruption might be an extreme case where the magma in the shallow chamber evolved to be highly SiO₂-rich composition.

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The Kirishima volcanoes located in southern Kyushu are comprised of more than 20 volcanic edifices. The volcanoes occupy an elliptical area of approximately 330km² with the WNW-ESE direction. Among the different types of volcanic edifices, the typical ones are compound maars and lava flows in Ebinokogen. We studied the volcanic history of Ebinokogen by geological examination of tephra layers and lava flows. After the Karakunidake-Kobayashi plinian eruption, seven tephra were formed in this area. We determined the ages of those tephra and two lava flows. The magmatic eruptions, produced Tamakino B tephra, occurred after Karakunidake-Kobayashi tephra eruption. The first activity in Ebinokogen from about 9.0 cal ka BP generated Fudoike lava flow, and Fudoike-Tamakino A tephra erupted from Fudoike crater. Karakunidake north-Ebino D tephra was generated from the northwest flank of Karakunidake at 4.3 cal ka BP, with debris avalanche and lahars. Phreatic Fudoike-Ebino C tephra erupted from the Fudoike crater at 1.6 cal ka BP. Ioyama-Ebino B tephra eruption started from around the 16^th to 17^th century with lava flow. Phreatic Ioyama east-Ebino A tephra erupted from Ioyama east crater in 1768 AD. The Ebinokogen area is one of the active regions of Kirishima volcanoes explicated by geophysical observations. Our results indicate cyclical tephra depositions mainly produced by small magmatic and strong phreatic eruptions in this area after the Karakunidake-Kobayashi pyroclastic eruption. Furthermore, the vent locations were found to migrate with each eruption.

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Dozen volcano is located in the Japan Sea off the Shimane Peninsula, SW Japan, and is known as a caldera composed of volcanic rocks evolved from alkali olivine basalt magma. Many isotopic ages reported so far indicate the volcano is Pliocene in age, but the onset time of volcanic activity has remained uncertain, as the subsequent eruption products cover the initial products of the volcano. Dating was, therefore, made this time on the basal trachybasalt lava collected by drilling into a part of the somma, Nakanoshima Island. The plateau Ar-Ar age obtained for the groundmass is 6.382±0.018Ma, consistent with the underground stratigraphy in the drilling hole. The time span from this age of the basal lava to the K-Ar age of the central Takuhiyama pyroclastic cone is approximately 1 million years. A model calculation suggests that this long activity could have been sustained by a mantle diapir of ca. 5000km³ in volume and ca. 20km in diameter. Provided 10% of the melt fraction in the diapir and the estimated total eruption volume of 100-300km³, a large amount of the melt may have remained and solidified at depths beneath Dozen volcano. This can account for the high-gravity anomaly centering Dozen volcano, which suggests that a large mass ～20km across and denser than granitic rocks exists below the volcano.

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The Takahara volcano is a Quaternary stratovolcano located on the volcanic front in the southern part of the northeast Japan arc. Fujiyama is a dacitic lava dome in the northern part of the Takahara volcano, with dominant mafic inclusions that are andesitic in composition. A few phenocrysts lie across the boundary between host dacites and inclusions. Plagioclase and orthopyroxene phenocrysts in the host rocks and inclusions, respectively, have wide compositional variations; the whole-rock major and trace element compositions of these rocks yield linear data distributions. These results indicate that mixing between felsic and mafic magma occurred during the formation of the Fujiyama magma. The felsic end-member could be close in composition to the host dacite, whereas the mafic end-member is not similar to that of the eruptive products at the early stage of the Takahara volcano. It is suggested that the mafic inclusions were composed of andesitic magma formed by mixing in the chamber and then mingling in the conduit during ascent.

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