Clastic dike structure is developed in Membo rhyolitic lava flow at the southwestern margin of Kozushima, Izu-Mariana Arc. In the present study, the geological descriptions of the dikes and the discussions on the formation process of the dikes have been done. The main results are as follows : The dikes are composed of tabular to rounded chilled blocks of lava with interstitial essential tuffs. Quenched glassy layer and chilled joints are found in the inner wall of the dikes. Dip and strike of the dikes are nearly perpendicular to the measured flow direction. Hyaloclastite structure developes only at the lower part of the lava flow. Block structure is common on the surface of the lava flow without any evidence of quench texture or water-laid sediment. These evidences suggest that this lava flow was formed just at shallow sea water region and soaked only its bottom part. The clastic dike has been formed by explosive disruption of the hot lava by volcanic gas separated from the lava and steam generated from sea water. Thus, the dike is felsic equivalent of spiracle described in some basaltic lava flows.

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A new method on the correlation of tephras using the lattice parameters of the rock-forming minerals was examined. Each rock-forming mineral was separated from 172 specimens which were collected from various places of Japan. The lattice constants of titanomagnetites, orthopyroxenes, clinopyroxenes, hornblendes and cummingtonites, 2θ₁₀₄ values of hemoilmenites, d₁₃₀ values of olivines and ⊿2θ=2θ₁₃₁-2θ₁₃₁ values of plagioclases were measured using powder X-ray diffractometer. The lattice constants of titanomagnetites of 166 specimens varied widely from 8.3912(11) to 8.4327(8)Å and the wide distribution of their occurrence was found. The 2θ₁₀₄ values of hemoilmenites of 94 specimens were between 32.68 and 32.91°. The d₁₃₀ values of olivines of 57 specimens ranged from 2.7742 to 2.7846Å and their distribution was fairly narrow. The ⊿2θ values of plagioclases of 139 specimens were largely centralized between 1.90 and 2.05°. The lattice constants of b axis of orthopyroxenes of 127 specimens varied widely from 8.868(1) to 8.946(2)Å and those of clinopyroxenes of 97 specimens varied from 8.909(1) to 8.944(2)Å. The lattice constants of a and b axes of hornblendes of 57 specimens changed from 9.793(3) to 9.852(2)Å, and from 18.059(4) to 18.113(6)Å, respectively. Cummingtonites were found only in 8 specimens among 144 specimens. From the above mineralogical examinations, useful minerals for the correlation of tephras were considered to be (ferromagnetic minerals)＞(amphiboles)＞(pyroxenes)＞(feldspar)＞(olivine) in order. Correlations of some marker tephras were tried using this method. Almost all of marker tephras were able to be easily distinguished from each other using these data. It was concluded that this method was effective to correlate tephras and has advantages to rapidly and precisely obtain various data to correlate tephras compared with previous correlation methods.

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Sakurajima Volcano consists of two main stratovolcanoes, Kitadake and Minamidake, which are composed of pyroclastic rocks and lava flows of pyroxene andesites and dacites. The basement rocks are composed of sedimentary rocks such as shale, sandstone, and acidic tuff, welded tuff, and granitic rocks. Geomorphology, large-scale historic eruptions, and the classification of surface textures of andesitic lava flows are briefly summarized. Volcanic history on the basis of the correlation between tephrochronology and the welded pyroclastic deposits is also presented. Taisho and An-ei lavas extruded at the later stage of the eruptions contain microphenocrysts of olivine. Orthopyroxenes of Bunmei pumice of later stage are rich in En content than those of early stage. These facts suggest that the volcanic products of the large eruptions were derived from zoned magma chambers.

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The Sakurajima Volcano is continuously keeping its eruptive activity since October, 1955. Huge amounts of volcanic gases and ashes have been emitted from the summit crater of the Minamidake which is the most active point in the Sakurajima Volcano. Moreover, there are many active fumaroles at the bottom of sea in the northern Kagoshima Bay. On the southeastern part of the Sakurajima, several hot springs have been found. In this paper, geochemical studies are described on the Sakurajima Volcano, especially on the relationships between volcanic activity and changes of chemical compositions of volcanic gases and hot spring waters. Submarine volcanism in the northern Kagoshima Bay is also reported.

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