In recent years there has been a great increase in our knowledge of thermophysical properties of magma. The purpose of this paper is to review recent experimental developments on the properties of magma and discuss these properties at high pressures. First, the thermophysical properties of magma necessary to understand volcanological phenomena are surveyed. Secondly, the properties of magma at 1 atm are summarized with respect to two empirical laws, additivity (additive property of constituent oxides of magma) and the law of correspondihg state. Thirdly, the results of seismic wave velocities and electrical conductivity for partially molten peridotite are shown and some thermophysical properties of magma at high pressures are reviewed and discussed. In the final section, problems concerning the properties of magma that will be studied in the future are discussed.

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There are many geothermal fields and active volcanoes in Japan. Many of the active volcanoes have, on the slope or at the foot, the so-called geothermal areas where superficial manifestations such as fumaroles, boiling springs and thermal springs are included. Ten of active volcanoes in Japan have summit fumaroles but no geothermal areas and six of them have neither geothermal area nor summit fumarole. Most of these volcanoes are simply stratified volcanoes with beautiful scenery. Such morphological characteristics suggest that volcanoes are young and have never suffered from strong erosions nor crustal movements which may destroy the volcanic edifice. Therefore, it is supposed that the stratified structures in these volcanic edifices were left intact. Conversely, it is supposed that stratified structures of volcanoes with geothermal areas were destroyed by strong erosions or crustal movements. The meteoric water infiltrating into the underground from such a destroyed surface is heated by the deep heat source and rises up to the surface and makes thermal manifestations. Total heat discharge from geothermal areas in Japan is estimated to be 3.5×10⁸ cal/sec. Those from active volcanoes and hot springs are also estimated to be 10.1×10⁸ cal/sec and 12.8×10⁸ cal/sec, respectively. Total heat discharge from the Japan arc amounts to 1.2×10²⁵ ergs/year, about 60% of which by the so-called terrestrial heat flow and about 40% by mass transfer such as volcanic eruption, steam and thermal water. Representative underground temperatures below geothermal areas in Japan are 150-175℃ at 500 meters deep, 200-225℃ at 1000 meters deep and 225-250℃ at 1500 meters deep, from many bottom hole temperature data. These temperatures are much higher than those calculated from the background heat flow values in geothermal areas. The high underground temperatures suggest the existence of the upward flow of thermal fluid. Based on a simple model, the upward velocity of thermal fluid is estimated to be about 1.0×10^-7 cm/sec and the heat transferred by the flow about 25 to 30 HFU. As a reaction of the concentration of heat in a geothermal area, sometimes the heat fiow around the geothermal area would fall to less than the normal value. Therefore, if we estimate a thermal pattern of the earth on the supposition that a hydrothermal system exists, we have to take into account the heat transfer by a mass flow in addition to the conductive heat flow. A revised heat flow map was proposed, based on the above considerations. Thermal structures of the crust and upper mantle are discussed based on the heat flow and seismic wave attenuation data in the central Kyushu geothermal area, as an example. Seismic waves through the upper crust of central Kyushu attenuate more rapidly than those outside it. This fact may correspond to the existence of many water-filled fractures in the upper crust of central Kyushu. Perhaps the hot water bringing surface geothermal manifestations must circulate through such fractures. Uppermost mantle beneath central Kyushu has a very low Q value. Such a low Q zone agrees well with the high temperatures deduced from high heat flow values in central Kyushu.

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The global review and future prospect on the problems such as prediction, hazard and environment related to volcanic activities are made. It is difficult to draw clear borderlines among each problem such as prediction, hazard and environment, because they are so mutually connected. Researches on the prediction have been done in connetion with the subjects in the earth’s interior, Hazards take place on the earth’s surface including the hydrosphere. Environmental problems occur in the wide range from the earth’s surface to the stratosphere and global oceanic regions. Environment : Volcanic activities play an important role for the balance of the terrestrial environment. Among the volcanic products, volcanic gas and fine volcanic ash are most important as pollution substances for the environmental aspect. Problems of climate changes due to the large loading of aerosols from huge volcanic eruptions are also important items. However, for the research of pollution, data based on continuous observations for long time are essential. The tephrochronological studies are very effective to elucidate the paleo-environment, because they can identity volcanic ashes which influenced the climate in the past. Hazards : Direct hazards by volcanic eruptions mainly occur due to "flow" (pyroclastic flow, dry avalanche, base surge, debris flow, mud flow etc.). It is the most important and urgent problem to reduce and prevent these hazards. Determination of the effective magnitude scale to express the size of volcanic eruption is an important item to be worked out. Prediction : Prediction of volcanic eruption has been adopted as one of the nationally important items of research. Seismicity, crustal deformation, gravitative variation, geomagnetic variation, geothermal activity, chemical composition of volcanic gases are continuously observed to detect the behaviour of the magma. Among these methods, seismic observations around the volcanoes play the most important role, however, according to the seismic observations of the past twenty years, many felt earthquake swarms have frequently occurred near and beneath the volcanoes, but only a few of them indicated observable forerunning events for the volcanic eruptions. It is an important research program to find special symptoms to tell the volcanic eruptions among such seismic swarm activities.

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Volcanoes along island arcs and active continental margins around the Pacific area are generally considered to have been generated in relation to subduction at the island arctrench system. During the last decade, information on the distribution, size and structure, nature of activity, and petrography of these volcanoes has been considerably accumulated. On the basis of the above information together with the recent advancies in studies on structure of island arcs and origin of arc magmas, the distribution of these volcanoes in time and space is critically reviewed, especially on spatial relations, petrology, sizes, elevations of volcanoes, and sequence and frequency of volcanism.

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