東北久慈沖日本海溝の大陸側斜面壁(北緯40°06´)東経144゜12´より,biotiteを主要構成鉱物とするhornfelsがドレッジにより得られた.このhornfelsの全岩とそれより分離したbiotiteの⁴⁰Ar-³⁹Arプラトー年代は,それぞれ28.9±1.4 Ma (900-1200℃; 70.6% ³⁹Ar)および28.5±1.4 Ma (800-1100℃; 73.9% ³⁹Ar)である.この年代は,原岩が熱変成によりhornfels化したときの年代と考えられ,約2900万年前にこの地域に熱変成をもたらした大成活動があったことが推定される.この大成活動の要因としては,海洋底プレートが日本列島下にもぐり込むことによって引き起こされた可能性があげられる.しかし現在と同じ状態でのもぐり込みを考えた場合には,その大成活動の位置は現在の海溝軸にあまりにも接近しすぎているようにみえる.そこで,当時はもぐり込む海洋底プレートが現在よりも急角度でもぐり込んでいたか,あるいは海溝軸が日本列島に対してもっと東にあった可能性等を考える必要がある.後者の場合,海溝輔が東から西へ移動する際には,海溝壁のエロージョンが起こっていた可能性がある.

抄録全体を表示

Aso is a large caldera volcano in Kyushu, Japan. Several dikes, which cut the Pre-Aso volcanic rocks, are exposed in the southwestern region of the caldera (ONO and WATANABE, 1985). The dikes, trending NE-SW and N-S, are composed of pyroxene andesite and hornblende andesite. Systematic relation between the rocks type and direction of the dikes is not observed. In this study, K-Ar ages have been determined on these dike rocks. The ages of dike rocks range from ca. 0.8 to 0.5 Ma, being consistent with the ages of the Pre-Aso volcanic rocks at the caldera wall. There are no significant relations between the ages and the rock type. Except for one sample with weak alteration, the dikes trending NE-SW are older than those of N-S, suggesting a change of direction of the maximum pressure axis of regional stress in the volcanism.

抄録全体を表示

Three small volcanoes exist in the central part of the Japan Alps; i. e., Kumonotaira, Washibaike and Warimodake. They are collectively called Washiba-Kumonotaira volcano. The earliest volcanism of the Kumonotaira formed a small stratovolcano (1.0-0.9 Ma). Thereafter, a quantity of gravels of basement rocks were deposited around it. The volcanic activity restarted at approximately 0.3 Ma, and the main body of the Kumonotaira (lavas and pyroclastics) was formed overlying the gravel bed. The Washibaike consists of lavas and pyroclastics which erupted on the slope of the mountain of basement granitic rocks (approximately 0.1 Ma). The Warimodake is a minor mass of andesite lava and thought to be the remnant of a lava flow lying at the crest of the ridge. Rocks of these volcanoes are augite-bearing olivine basalt (SiO₂-51-52%), andesite (55-61%) with complex phenocryst assemblage and glassy dacite (63-64%) rich in hornblende, biotite and quartz phenocrysts. Among them, the andesite is dominant. The basalt constitutes the older stratovolcano of the Kumonotaira, and the dacite composes a part of the Kumonotaira and the Washibaike. On the FeO^*/MgO vs. SiO₂ diagram, these rocks are grouped into three. That is, 1. basalt with low SiO₂ and high FeO^*/MgO ratio 2. andesite with moderate SiO₂ and moderate FeO^*/MgO ratio 3. andesite and dacite with high SiO₂ and low FeO^*/MgO ratio Their total chemical variation cannot be explained by a single mechanism such as fractional crystallization. Complicated mechanisms including magma mixing would have caused the chemical variety of the volcanoes.

抄録全体を表示

Nine active volcanic plumes were observed during the Voyager encounter with the Jovian system. Enhanced tidal dissipation induced by Jupiter is a major energy source for it's active volcanism. The energy coming out from lo is estimated to be 1-2 W/m² which amounts to an order of magnitude larger than the terrestrial heat flow. Large part of this heat flow is transferred to the surface by localized hot spot volcanism. Internal structure and thermal evolution of lo is strongly influenced by tidal heating and advective heat transfer associated with volcanic activity. We review current understanding of tidal dissipation, heat flow measurements and heat flow mechanisms, surface morphology, color and spectral studies, and discuss the nature of volcanism and its role on the internal structure and thermal evolution of Io. A major question on the ionian volcanism is whether volcanic eruption material is silicate or sulfur compounds. According to the available data such as surface spectra and existence of high rugged mountains, layered crustal model consisting of massive silicate crust with superficial sulfur-rich outer layer is preferred. A wide variation of volcanic features is manifestations of complex interaction of silicate, sulfur and sulfur dioxide volcanism. Volcanic and tectonic styles of tidally heated body and unique volatile phases of sulfur and sulfur dioxide which control the ionian volcanic processes provide a significant insight for understanding geologic evolution of terrestrial planets.

抄録全体を表示