The Mio-Pliocene Miyazaki Group in the Miyazaki coastal plain, southern Kyushu consists mainly of marine sediments which have well preserved tuff beds. Stratigraphy, lithology and petrography of the tuff beds were investigated. The tuff beds have mainly heavy mineral compositions of hornblende-orthopyroxene-opacity minerals, hornblend-orthopyroxene-clinopyroxene-opacity minerals and orthopyroxene-clinopyroxen-opacity minerals. The glass shards contained in these tuff beds consist of main sponge and fiber types, and minor Y and plate types. The representative 8 tuff beds as key bed in the studied area were dated by K-Ar method using mineral separates of plagioclase, biotite and hornblende, giving 6.5 to 2.0 Ma, being consistent with the biostratigraphy based on planktic foraminifera. The frequency of the tuff beds intercalated in the Miyazaki Group increases with time, having a maximum around 3.0 Ma. A clinopyroxene component of heavy mineral composition tends to increase after 3.5 Ma. The first appearance of seoria bed was confirmed in the upper part of the Group, being ca. 2.4 Ma in age. These characteristics for the tuff beds resemble with those of volcanic rocks distributed in the Hokusatsu area.

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The activity of Higashi-Izu monogenetic volcano group produced basalt, andesite, dacite, and rhyolite, which can be classified into rock types by whole-rock composition and phenocryst assemblage. It has been widely accepted that felsic upper crust has an important role in forming compositionally diverse ejecta; andesites are formed through the assimilation of felsic crust by the basaltic magma and the felsic magmas through remelting of the felsic crust. To reinvestigate genesis of felsic magma, I examine the physicochemical conditions of both pre-eruption felsic magmas and the felsic crust. Beside observed rhyolite, felsic magmas can be recognized in dacite as endmember of magma mixing. The pairs of the magnetite and ilmenite yield the temperature of 820-790℃ for the rhyolite and 850-820℃ for the dacite. The plagioclase geohydrometer shows that the rhyolitic magmas had PH20 of 2 kbar indicating their residence within the felsic upper crust at the depth of 6-15 km before eruption. Some andesite could be formed through bulk assimilation, a simple mixing between felsic crust and basaltic magma. The compositions of the felsic crust are obtained using mixing ratios of the felsic crust and basalt magrna. The felsic crust which could range from rhyolitic to andesitic, differs from those of the rhyolitic magmas; the rhyolitic magrnas are lower in HREE (Gd, Er, Dy) and Sr, and higher in LILE (Rb, Ba, K₂O) than those of the felsic crust. In addition, rhyolitic magmas cannot be formed by the fractional crystallization from the magma formed by the remelting of the felsic crust; Rb/K₂O ratio of the rhyolitic magmas is higher than that of the felsic crust. However, partial melting of the felsic crust, leaving amphibole and plagioclase as the residue could produce the rhyolitic magmas.

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A thermal model for a cylindrical magma chamber is considered in order to assess its influence on thermal structure of the surrounding crust. The calculated results show the following two possible influence patterns when a typical magma-supply rate for Japanese Quaternary volcanoes (0.1-1 km³/1000 yr.) is given to the system. The temperature of a small magma chamber (several cubic kilometers in volume) remains above solidus for a long time but result in small extent (a few kilometers) of thermal influence. On the other hand, a large magma chamber (several hundreds of cubic kilometers or more in volume) can only survive for several tens of thousand years but result in large extent (up to ten kilometers) of thermal influence at its peak stage. Comparing the thermal structures calculated based on this model with the geophysical observations relevant to crustal thermal structure for an active volcano is helpful to evaluate the applicability of depth and size of the magma chamber proposed for the volcano.

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