The Upper Donzurubo Formation consists of pyroclastic-flow deposits, pyroclastic-surge deposits, hot reworked deposits, and epiclastic deposits. The lower zone of the Upper Donzurubo Formation consists mainly of epiclastic deposits, intercalated ash-fall deposits, wet surge deposits and pyroclastic-flow deposits. The middle zone consists of wet surge deposits and pyroclastic-flow deposits. The upper zone comprises epiclastic deposits and several units of wet surge and pyroclastic-flow deposits. At the beginning of deposition of the Upper Donzurubo Formation, epiclastic deposits derived from subaerial pyroclastic flows and surges were emplaced in a shallow water area. Afterwards, at least 20 pyroclastic flows and surges were emplaced on the epiclastic deposits which filled the shallow water area. Secondary vesicles and accretionary lapilli are common in the ash matrix of pyroclastic surge deposits, implying the surges were wet, generated by phreatomagmatic eruptions. Essential fragments of the pyroclasticflow deposits retain internally consistent one magnetization vector stable even at temperatures higher than about 500℃. The pyroclastic flow deposits have been supposed to have been emplaced in a subaqueous setting, but their emplacement at high temperatures in association with pyroclastic surge deposits is to be subaerial.

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Miyake-jima volcano, one of the tholeiitic volcanoes in Izu-Mariana arc, has erupted at least 14 times since 1085. After 66-year repose period since the 1874 AD eruption, the largest eruption during the historic period occurred in 1940 AD. Since then, eruptions have repeated about every 20 years (1962 and 1983 AD). These recent three eruptions are characterized by effusion of lavas and scoria from fissures at flank of the volcano, and only 1940 one accompanied the summit eruption. Although most of the products are nearly aphyric augite basaltic andesites and basalts, ejecta erupted from northern fissure during the 1940 eruption are characterized by the presence of plagioclase and olivine megacrysts. These megacrysts should not be comagmatic but accidental, because the olivine megacrysts often show deformed texture (kink-bands) and also because plutonic xenoliths consisting of olivine and plagioclase with the same compositions as the megacrysts are found. Except for the megacrysts, phenocrystic minerals in all of the 1940 ejecta are divided into two types, B (basalt type) and A (andesite type). The B type consists mainly of calcic plagioclase, magnesian olivine and augite, each showing normal zonation. By contrast, the A type is characterized by the presence of Ti-magnetite and by more evolved composition of phenocrysts than those in the B type. Phenocrysts in the A type are usually reversely zoned. These suggest that the 1940 ejecta are produced by magma mixing between basaltic and andesitic magmas which were derived from different magma storage systems. On the other hand, the 1962 and 1983 products contain only A type phenocrysts with minor evidence of magma mixing. In contrast to the 1940 eruption, both the 1962 and 1983 eruption events effused nearly homogeneous magmas. The whole-rock chemistry and mineral chemistry shows that magma has become systematically more basic from 1940 to 1983, indicating that the 1962 and 1983 ejecta are not products of simple fractional crystallization of the 1940 andesitic magmas. In each eruption episode, the products from different fissures can be distinguished mineralogically and chemically from each other. This fact requires independent magma transport systems, which connected the magma storage systems with respective fissures.

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The edifice named “Kama yama” on the summit of Asama Volcano is a well-proportioned cone, truncated by an U-shaped, small depression on its northern outer-slope. Thick welded pyroclastic fall deposits are exposed on the crater-wall of Kama yama and the top surface of the welded deposits is topographically traced to the top surface of the lava flow of the “Tenmei” (1783 A. D.) eruption (“he Onioshidashi lava flow”). Welded pyroclastic materials correlative with the welded deposits of Kama yama crater-wall also locally crop out on the outer surface of Kama yama just below the deposits derived by the recent vulcanian-type explosions. These pyroclastic materials are petrologically identical to the juvenile materials of the 1783 eruption, and the welded stratigraphic succession of Kama yama is remarkably similar to that of the 1783 pumice fall deposits in the distal area. Kama yama was thus most likely built-up during the 1783 eruption, which was mainly plinian-type. Coarse pyroclastic materials likely accumulated near the vent to form Kama yama cone during the climactic phase of the plinian eruptions. The depression of the northern part of the cone and topographical continuity between the cone and the Onioshidashi lava flow indicate that the effusion of the Onioshidashi lava flow was accompanied with partial collapse of the northern steep slope of Kama yama. Old documents and old pictures also support this scenario of the growth of Kama yama during the climactic phase of the 1783 eruption. Formation of a pyroclastic cone being accompanied with plinian activity is one of the common features of explosive eruptions in many andesitic volcanoes. Geology of such cones may provide key information for understanding of the eruption dynamics of intermediate magma compositions.

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It has been regarded that the eruption activity had been generated at the Iwate volcano in AD 1683, 1686- 89, 1719, 1730 and 1732. In this study, the eruption history of Iwate volcano in the Edo period is reexamined by judging the credibility of historical records of eruption. The following conclusions were obtained from my investigation, a) The mistake is found in the description of the age in the historical document which has been regarded as the evidence of the 1683 eruption, b) According to a letter of the previous lord of Morioka clan and the reports of government authorities, a violent eruption began with eruptive-column forming above central crater of the stratovolcano on March 26 AD 1686, and became quiet in the early morning of 27th. The pyroclastic deposit discharged by this eruption is the Kariya scoria with the distribution axis from the central crater in northeast and the southeast foot. The volcanic hazard by this event was caused due to the lahar which occurred because of the melt of the snow of the mountain. Human damage did not occur though several houses were swept away by lahars. c) The credibility of the historical documents assumed for the 1686 eruption which continued even in AD 1687 and AD 1689 is weak. In fact, the eruption started in AD 1686 was ended within the year, d) The credibility of historical documents which describe the 1719 and 1730 eruption is also extremely weak, f) The 1732 eruption was recorded in the administrative report of Morioka clan. This historical documents describe the effusion of lava-flow from the northeastern foot of the Iwate volcano. It is assumed that this report corresponds to Yakehashiri lava flow. The lava effusing started at midnight of January 21 and continued for 7-10 days. The inhabitant who resided in the the foot of the mountain was evacuated from the village due to frequent volcanic earthquake. The human and material damage did not occur in this eruption.

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