Geomagnetic total intensity anomaly map at sea around the Nansei Syoto (Ryukyu Islands) and the Senkaku Syoto was compiled on the basis of the seven survey cruises carried out by the Hydro-graphic Department of M.S. A. from 1975 to 1982. Geomagnetic anomalies are divided into six zones in accordance with the geomagnetic features and topographic/geological characteristics of the region concerned. Some of the significant magnetic anomalies are analized by means of three-dimensional stacked prism model for better understanding of their tectonic implications. A complicated anomaly zone exists along the Paleo-Ryukyu volcanic Belt and the Ryukyu volcanic belt (KONISHI, 1965) and a magnetic smooth zone is recognized over the area of the Ryukyu geoanticline and the continental slope located to the east of the volcanic zones. It is usually recognized that lo-Tori Sima is a southern end of the Quaternaty volcanic front of the Ryukyu islands. However, the geomagnetic and topographic/geological features suggest that the volcanic chain should extend as for as the nothern part of south Ryukyu along the West Ryukyu knolls group (KATO et al., 1982). The geomagnetic anomaly pattern along the above volcanic chain shows systematic variation, i.e. short-wavelength and intermediate amplitude in the north area, relatively magnetic smooth zone in the central area, and the conspicuous dipole anomalies in the south area. This feature may reflect the difference of the volcanism along the volcanic front which might have resulted from the different tectonic situation between north Ryukyu and south Ryukyu as noted from recent study on tectonics of the Ryukyu Island Arc (KIZAKI, 1978). In addition to the above aspect, the followings are also concluded in this study. (1) The magnetic structure of Suwanose Sima shows the weakly magnetized part in the basement of the uplift. This kind of structure is seem to be important for understanding of the geology and volcanic history of the island. (2) Some of the uplift in the north Ryukyu volcanic belt, i.e. Daiiti-Amami Tai, Torisima Tai and Takara Tikai show the caldera like structures, although the further study should be made for the verification of caldera origin. (3) Some of the uplift of knolls and banks inside of the Tokara volcanic chain have lineaments in topography and are not accompanied by local magnetic anomalies. These characteristics may reveal that these uplifts are composed of the Pre-Miocene basement complex. (4) The lineated magnetic anomaly of positive sign, running NE to SW, is recognized from thebank named “Igyo Sone” to Kume Sima to the west of Okinawa Sima. This positive anomaly belt is identified as the volcanic region characterized by green tuff volcanism in Miocene Age. (5) The magnetic structure on Smt. 812 (24°37’N, 125°08’E) located in the Okinawa Trough and the uplift in the outer margine of the Tunghai Shelf show low magnetizations, suggesting the felsitic intrusive volcanic activity. These results are consistent with the geological/petrological features of them.

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During the DELP ’85 Cruise we observed following evidence for the presence of a Quaternary volcano in southeast margin of the Japan Basin. The station is located at 40°42’N latitude, and 139°03’E longitude at a depth of 3320 m, which is about 100 km northwest of Ichinomegata volcano of the northeast Japan arc. (1) Anomalous reflection was recorded up to about 75 m above sea bottom by a 3.5 kHz echo sounding system, which may represent either reflection from gas bubbles or solid fragments rising from a vent. A small (less than 200 m in diameter) basin structure was recorded just below the anomalous reflection. (2) High heat flow of ca. 250 mW/m^2 was obtained at the station. (3) We recovered rather fresh volcanic sand, consisting mostly of glassy fragments of basalt through rhyolite, and crystals of olivine, augite, hornblende, biotite, opaque mineral and plagioclase. These fragments conform a suite of island-arc alkalic rocks. The glasses have the highest alkali content among Quaternary volcanic rocks of the northeast Japan arc. Mineralogically and chemically, the volcanic sand is not correlated to heretofore known volcanic products around Japan. It is most probable that a Quaternary volcano is present near the station.

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Volcanic activities on the terrestrial planets tell us about fundamental processes of volcanism that operates under different initial and boundary conditions from those on the earth. Therefore it is very important to make comparative studies on volcanic phenomena among terrestrial planets for understanding essential mechanisms of volcanic processes in the earth. As the first step of geoscience surveys for terrestrial planets, a program of the Lunar Science Surveyor Mission is planned by a group of geophysicists, geologists and physicists in Japan. In this program, it is planned to investigate lunar internal structure by a seismological technique, and mineralogical and chemical compositions of lunar surface by remote sensing techniques. The collaboration with Japanese volcanologists, who have strong desire to study such a comparative planetary volcanology, is evidently essential. The Japanese Lunar Science Surveyor Mission will hopefully be made by the early 1990’s by using a modified version of the main rocket to be developped in the Institute of Space and Astronautical Science, Japan.

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Surface topographic features on terrestrial planets and satellites are mainly produced the cratering processes of impacting planetesimals and volcanisms due to the heat loss processes of the internalenergy. Basaltic and icy volcanisms are dominant on terrestrial planets and satellites of giant planets, respectively. Volcanic processes can be devided into three groups in terms of their topographic characteristics, i.e. plate recycling, hot spots, and flood (basaltic) flows. The multiplate tectonics is only dominant on the present earth, while a single-plate (or a few-plate‘) tectonics is dominant on other terrestrial planets. The plate recycling could be identified by the elongated or linear chain of volcanoes on Venus. The hot spot volcanism is characterized by a random or clustered distribution of shield volcanoes, and is dominant on Mars and lo. The spectacular sulfer-dioxide plumes on lo are energetically produced by the hot spot silicate volcanism beneath the S-SO₂ layer and silicate lithosphere. Flood (basaltic) flows are common for low lands or smoothed plains such as mare basalts on Moon (crater fill), smoothed plain basins on Mercury and so on. For icy volcanisms on satellites, similar classifications can also be made, although the lack of detailed geophysical observations makes some arguments about their origins still open.

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