Plate tectonics-related aspects of volcanoes and volcanology are reviewed. They include : 1. the reason why a paper with the title of the present paper did not appear in the early 1970’s, when such papers were written in other disciplines of Earth Sciences, 2. aseismic slabs beneath arc volcanic zones, 3. tectonic significance of an independent group of monogenetic volcanoes (IGMV) as contrasted to polygenetic volcanoes which occur in the region with crustal stress field more compressional (or less tensional) than what is indicated by the IGMV. In the last chapter, a model is presented to explain the repeated life histories of Hawaiian volcanoes which consist of principal and post-erosional periods separated by an erosion interval of a few Ma. A polygenetic volcano is constructed during the principal period and an IGMV during the post-erosional period. According to the model, the two periods are interpreted as surface expressions of heating (compressional) and subsequent cooling (tensional) stages, respectively, mainly by a vertical line heat source of, created within a fast-moving plate by a fixed hot spot.

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Experimental melting studies on natural peridotite at high pressures are reviewed. Composition of partial melt formed along the dry solidus of the peridotite is quartz tholeiitic up to 0.5 GPa, olivine tholeiite similar to mid-ocean ridge basalt (MORB) at 0.8 to 1.0 GPa, alkali olivine basaltic between 1.5 and 2.5 GPa, picritic between 2.5 and 5 GPa, and komatiitic above 5 GPa. Thus, MgO content of the magma formed along the peridotite dry solidus increases as a function of increasing depth. Using partition coefficients K_D(Fe/Mg), K_D(Ni/Mg) and K_D(Mn/Mg) between olivine and silicate melts, a model is constructed in order to calculate fractional crystallization paths of primitive basaltic magmas. Ni and Mn contents of olivines of 250 spinel lherzolite xenoliths collected from the world were analyzed. Ni and Mn contents of the mantle olivines show narrow and coherent trends as a function of Fo mol%, and their compositional ranges are called as “mantle olivine array for ferro-magnesian elements”. Given the mantle array as a constraint for the fractional crystallization model, composition of primary magmas and Mg^*=100 Mg/(Mg+Fe) of residues are estimated. Estimated primary magma compositions for MORBs, Hawaiian tholeiites, Columbia river basalts and Quaternary basalts from Japanese volcanoes all fall in a range between MgO=9 and 14 wt%. Judging from ubiquitous occurrence of olivine xenocrysts showing deformation texture (kink banding), most picritic basalts (MgO＞15 wt%) erupted on the present day earth are considered to be mixtures of basaltic melts and olivine xenocrysts.

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Evidence for mixing of felsic and mafic magmas can be recognized from various disequilibrium or heterogeneous features in volcanic rocks. Some pyroclastic ejecta (e.g., those of the 1707 eruption of Fuji Volcano) change from initial felsic ejecta into mafic ejecta with a narrow transitional zone containing banded pumice. These ejecta are the examples showing the lowest degree of mixing. On the other hand, some mixed lava flows have nearly homogeneous groundmass with incompatible phenocryst assemblage (e.g., forsteritic olivine and quartz) or disequilibrium textures in phenocrysts (e.g., dissolved plagioclase). These are the examples showing the highest degree of mixing. There are various types of heterogeneous magmas between these extreme cases. The origin of diversity in heterogeneous magmas is discussed, based on the reviews of recent theoretical and experimental studies on the fluid dynamic aspects of magma mixing. Injection of a dense mafic magma into a felsic magma does not lead to the formation of a homogeneous mixed magma chamber, but results in vertically stratified one. A chemically stratified magma chamber can also be formed from a initially homogeneous magma chamber by liquid fractionation in which differentiated liquids formed along the wall segregate gravitationally. On the other hand, the stratification can turn over during the ascent through a conduit due to the contrasting mobilities of the less viscous mafic magma compared with the viscous felsic magma and mixed magma can be formed. The mode of magma mixing in a conduit greatly depends on whether the conduit is connected to the magma chamber or it is separated from the magma chamber (“a rootless conduit”). Two magmas with viscosity contrast can circulate in a chaotic way and efficiently mix with each other in the rootless conduit. The diversity of heterogeneous magmas may result from the diversity of ascending mechanisms through a conduit. Petrographic features indicate that magma mixing occurred shortly before the eruption. This fact also suggests the significance of magma mixing in a conduit. Previously, based on the contemporaneity of mixing and eruption, it was proposed that the injection of mafic magma into the felsic magma resulting in magma mixing could be a trigger of an eruption. However, the conduit mixing model gives another interpretation for the contemporaneity, that is, magma mixing is trig-gered by magma ascent accompanied with eruption.

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Partially molten layer, which is expected to exist in the Earth’s upper mantle, is a root zone for magmatic activity. Physical and chemical processes working in this layer set various constraints on this activity. Particularly melt-phase migration process in this layer is a key process which governs the accumulation and the segregation of magma. In Chap. 2, we briefly summarize melt migration process. In Chap. 3, various types of physical properties are reviewed in connection with the melt migration process. Permeability is the most important parameter in controlling the melt migration, although little is known. In Chap. 4, the current picture of partially molten layer is given through various observations. The point emphasized in this review is that the partially molten state is not a simple mixture of liquid and solid. This nature needs essentially new treatment.

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The following model is proposed to describe magmatic processes in relation to volcanic eruptions. An ascending mantle flow occurs for some reasons beneath the mid-ocean ridge, hot-spot and arc volcanoes. Magma is generated in the ascending mantle flow in which solidus temperature is lowered due to the pressure release. In the mantle flow, the magma is distributed along the edges and corners of mafic mineral grains and basaltic in composition. When melt channels are widely connected at higher degree of partial melting, the magma migrates through the mantle minerals with permeable flow. The ascending mantle flow finally passes the top of the mantle, and moves out aside and downward. This outgoing flow is gradually cooled so that the residual magma in it is partly solidified and has more abundant incompatible elements. Magma is segregated at the top of the mantle, forming separate magma bodies. The magma body, which may be identified with a moving magma chamber, migrates upward along a preexisting conduit in the crust. Fractional crystallization and chemical reactions with surrounding crustal rocks make magmas more felsic, especially when a small magma chamber ascends through a thick continental crust. Since the chemical differentiation is accompanied by significant increase in the magma viscosity, a more felsic magma becomes less mobile. Therefore an extremely felsic magma may be standing and crystallized in the interior of the crust as plutonic rocks.

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Volcanic earthquakes are the earthquakes originating from some magmatism. In this paper, the characteristics of these earthquakes and their source models are reviewed in relation to the volcanic eruptions. Volcanic earthquakes are classified in several ways. The classification of A-type earthquake, Btype earthquake, explosion earthquake and volcanic tremor follows the recorded features of earthquake-motions and their relation to volcanic eruption. Recently, people prefer to give more emphasis on the frequency components of the seismic wave, and to classify volcanic earthquakes into low frequency and high frequency earthquakes. The last decade development of advanced seismometric and other geophysical surveillance networks around several volcanoes in Japan has made it possible to infer source mechanisms of volcanic earthquakes. Such study has revealed the relation between the volcanic activities and the volcanic earthquakes little by little. Source models of explosion earthquakes so far proposed could be divided into two categories. The first is the model that the seismic wave is excited by the rapid increase of pressure in the volcanic conduit. The second model assumes that the wave is excited by a single force applied in the direction opposite to the blast direction. The second model was successfully applied to the explosion earthquakes at Mount St. Helens volcano and Asama volcano, and used to obtain their magnitudes and source time functions from long-period seismograms. Some new ideas on source models for volcanic tremor have been recently proposed such as the jerky extension of fluid-driven crack and the unsteady fluid flow in volcanic conduits. Since dominant frequencies of volcanic earthquakes are variable over wide frequency range, the broad-band and wide dynamic range seismographs installed near the volcanoes are desirable for more advanced interpretation of volcanic earthquakes.

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Useful expressions are collected and discussed for deformations due to (1) various types of magmatic pressure sources and (2) surface loading over a circular area. Subsequently (3) the stress concentration around spherical and cylindrical cavities in an infinite elastic medium is reviewed, providing a correlation between the regional stress field and the stress on the wall of the magma chamber and conduit. Taking three cases of Japanese active volcanoes, it is suggested that studies in mechanical state of the magmatic pressure sources and failure criteria of the surrounding rocks may be helpful to estimate long and intermediate-term potentials of large volcanic eruptions.

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Volcanic eruption is a discharge process of energies from the earth’s interior in the form of thermal energy of hot erupted products, kinetic energy of explosive movements, seismic energy, etc. Since the thermal energy constitutes a major part of the total energy discharged by eruptions, the studies on volume of volcanic products have called our attention, and lead the keynoting results on a spatial distribution of the volume of volcanic products (i.e. thermal energy discharge) and contribution of volcanic eruption to the global energy discharge rate, etc. It has not been well recognized, however, that volcanoes discharge significant amount of thermal energy even at a non-eruptive stage comparable with those energies in eruptions. In recent years, some remote measurement methods of heat discharge rate have been developed for inaccessible areas such as crater bottom or intense fumarolic zones, to which the earlier estimation methods were not applicable. Those methods were applied to the major active volcanoes mainly in Japan and heat discharge rates were estimated there. Non-eruptive heat discharge rates are compared with the eruptive energy discharge rates or with the volume of volcanoes to derive the eruptive energy discharge, and the result is summarized as follows. 1) Non-eruptive heat discharge rates around Northeast Japan Arc and Southwest Japan Arc are estimated to be 79 MW/100 km and 120 MW/100 km, respectively. These values are of the same order of magnitude as the thermal energy discharge rate at the eruptive stage estimated from the volume production rate of volcanoes. 2) Spatial distributions of non-eruptive heat discharge are similar to that of eruptive energy discharge, which has already been established. Namely the discharge is highest just behind the volcanic front, and rapidly decreases toward the inner side. 3) The ratio of eruptive and non-eruptive energy discharge reflects the regional stress. The heat source to maintain the non-eruptive heat discharge may be stored more easily under a tensile stress field but rather hard under a compressive stress field. Viscosity of magma is also expected to be another important factor, even though positive evidence is not presented in this review. Our knowledge about the heat source of non-eruptive thermal activities and the chronology of volcanic activities is insufficient to a better understanding on an energy discharge process from volcanoes, including the relations of intrusive and extrusive magmatism and their quantitative geological meanings.

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Quaternary arc volcanism in circum-Pacific arcs, especially the Japanese Islands, is reviewed and discussed from a tectonic viewpoint. The summary is as follows; (1) There is no correlation between the chemical and petrological characteristics of volcanic rocks and the depths of deep seismic zone. (2) The geographic position of “volcanic front” defined by SUGIMURA (1960) does not coincide with any contour of the hypocentral depth of deep seismic zone. (3) It is difficult to consider the inclined low velocity layer in the sinking slab as a partially molten part, on the basis of the fact that the upper one of the deep seismic planes coincides with the layer, for example in Northeast Japan. (4) There is a good correlation between the type of crystallization sequence in volcanic-front volcanoes determined by SAKUYAMA (1983) and the age of oceanic plate at trench in the circum-Pacific arcs. This suggests that the degree of partial melting in mantle wedge may be subject to a magnitude of slab pull. (5) There is a good correlation between the tectonic stress field and the variation of strontium isotope ratios for volcanic rocks. It is suggested that the degree of mixing between magma and crustal material may be subject to regional tectonic stress. (6) It is inferred that geographic position of volcanic front, degree of partial melting in mantle wedge, and tectonic stress field in the crust may be essentially subject to a secondary convection induced by a downgoing of slab.

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This review discusses the genesis of magmas in subduction zones on the basis of experimental and geochemical data mainly obtained in the 1980s. Across-arc compositional variations in arc basalt magmas are caused by the following factors; (1) depth of magma segregation, (2) degree of partial melting, (3) density of primary magma, and (4) refreshment of polluted mantle materials by depleted materials similar to the MORB source during the process of downgoing migration. The subducted lithosphere supplies elements with larger ionic radii with H₂O to form hydrated or polluted amphibole peridotite in the mantle wedge beneath the fore-arc region. The amphibole peridotite is transported downwards on the slab by the induced convection in the mantle wedge. Magmatism on the volcanic front is controlled by the dehydration of amphibole in the dragged mantle materials. High-magnesian andesite magmas, SiO₂-rich primary magmas in subduction zones, are produced by the partial melting of hydrated amphibole peridotite at shallower levels under higher geothermal conditions.

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Recent progress of the study of volcanoes in oceanic areas is reviewed. Major advance of the study has been brought about through the development of the following lines of researches; (1) narrow-beam echo sounding investigation with the aid of precise navigation systems to determine detailed sea bottom topography. (2) deep sea drilling for the determination of stratigraphy of oceanic basement. (3) direct observation and sampling of oceanic basement by submersibles. (4) development of isotope systematics such as Sr-Nd-Pb and He-Ar. (5) increase of accuracy as well as production rate of routine analyses of volcanic rocks; i.e. petrography, mineralogy, major and trace element compositions. Precise high-pressure phase equilibrium studies also afforded important constraints on the modelling of magma genesis in oceanic areas. Geological, petrological and geochemical characteristics of the volcanoes in oceanic areas are described according to the following classification scheme; [A] Monogenetic volcano (a) Oceanic ridge volcano, (b) Back-arc basin volcano, [B] Polygenetic volcano (c) Seamount near oceanic ridge, (d) Seamount near spreading center of back-arc basin, (e) Hot spot seamount, (f) Abyssal plateau, (g) Oceanic island arc volcano. Genetic models of these volcanoes are discussed, and it is shown that the mantle diapir model of SAKUYAMA (1983) can be applied to polygenetic volcanoes in oceanic areas. Generation and uprise of mantle diapir (or blob), and intrusion of mantle diapir into lithosphere may determine the essential features of polygenetic volcanoes; i.e. volume, duration of activity, evolutional history, major and trace element compositions, and isotopic ratios.

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Usefulness of isotope studies in volcanology is discussed by showing some examples. Multiple isotope approach gives us significant information on the characteristics of source materials of volcanic products. Based on the reported data, isotopic characteristics of volcanic materials from the Japanese Islands are summarized as follows. (1) In the εNd-εSr diagram, Quaternary volcanic rock samples nearly lie on the “mantle array” or show slightly higher ⁸⁷Sr/⁸⁶Sr ratios. (2) In the NE (northeast) Japan and the Izu-Bonin region, ⁸⁷Sr/⁸⁶Sr ratios decrease across-arc from the volcanic front to the back-arc. (3) Generally, lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr ratios are observed for volcanic rocks from the SW (southwest) Japan than those from the NE Japan. (4) In the NE Japan, Sr and Nd isotopic ratios seem to have changed around 15 Ma for volcanic rocks. Quaternary volcanic rocks show higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr ratios than those before 15 Ma. (5) In the NE Japan, Pb isotopes in Quaternary volcanic rocks also indicate more radiogenic components for those from the volcanic front area than those from the back-arc area. (6) The highest values for the ³He/⁴He ratio in each region in the Japanese Islands are close to those of MORBs or slightly lower and they are observed around the volcanic front in the back-arc side. No systematic difference is observed in the distribution of the ³He/⁴He ratio between the NE Japan and the SW Japan. (7) No systematic distribution is observed for δ¹⁸O in Quaternary volcanic rocks in the Japanese Islands. On the basis of these data, the source materials of volcanic rocks in the Japanese Islands are discussed. The incorporation of crustal materials including pelagic sediments to the MORB-like materials is suggested in the amount of around a few percent. This interpretation is compatible with a recent discovery of cosmogenic ¹⁰Be enrichment in some volcanic rocks from the Japanese Islands. This indicates that we have an evidence for the subduction of a slab together with some amounts of sediments on it. As a typical example for a hot-spot type volcanism, Hawaiian volcanism is examined on the basis of radiogenic isotope studies. Sr, Nd, Pb and Hf isotopic ratios suggest that the magma sources for Hawaiian volcanism are related to the MORB-type source and the other types by mixing. High ³He/⁴He ratios observed in Hawaiian volcanic samples such as those from Loihi Seamount and Kilauea volcano suggest the occurrence of relatively primordial volatile components derived from the deeper part of the mantle than that of MORB. Further, systematic variations in the ³He/⁴He ratios with the volume of individual volcanoes together with other isotopic signatures suggest a model for the evolution of Hawaiian volcanism, where the rise of volatile-enriched diapirs from the Earth’s deep interior is essential However, the amount of noble gas (volatile) components would vary at each stage of a volcano evolution. Voluminous outpourings during the Hawaiian volcanism do not always reflect the direct material supply from the hot-spot source, but would reflect the combined effects of material transfer and heat supply.

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An accretional energy of a growing terrestrial planet produces at least partial melting in the upper silicate layer of a few hundred kilometers, which is called the magma ocean, when the mass of the growing body exceeds that of the Moon (about 8×10²²kg). Various topographic characteristics among terrestrial planets can be understood by the relative importance of factors governing the cratering, volcanic, tectonic, and erosion/sedimentation processes. Modes of volcanisms that produce differences of the surface topography are (1) plate recycling, (2) hot spots and (3) flood basalt flows, depending upon various cooling processes of the magma ocean. Relative amounts of the heat loss from a planet through each mode of volcanisms are estimated mainly from topographic features of terrestrial planets by referring modes of the heat loss from the earth. It is suggested that the flood basalt flow would play an important role at some stage in the heat loss budget of Venus, Earth, Mars and probably Moon. Varieties of modes of the heat loss among terrestrial planets can be interpreted by difference in the thermal history that may reflect to volcanic features together with the lithospheric thickness, surface topography and gravity, and other characteristics.

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Recent progresses in researches on large-scale pyroclastic flows and their source calderas are reviewed. Subsurface structures for the Valles type caldera at Long Valley and Yellowstone are confirmed by means of seismic profiling, gravity, deformation measurements and drilling. Pyroclastic flows, lake deposits, moat rhyolites and slumped megabreccias fill the caldera floor. Basement formations are subsided up to 1-2 km tilting block-wise up to 10 degrees. No major rupture is confirmed on the basement formations. Existence of ring fracture is clearly proven based on geochronological and petrological data of moat rhyolite and component analysis of lithic clasts within pyroclastic flow deposit. Head of magma reservoir beneath Long Valley Caldera is located 4.5-7 km below surface and the reservoir size is comparable to the surface caldera depression. On the contrary, subsurface structure for Japanese funnel-shaped caldera is not yet confirmed clearly. Major interests on recent researches for large-scale pyroclastic flow are concentrated on grain size analysis and numerical or model experiments of fluidization to reveal flow and emplacement mechanisms. Ground surge, ground layer, fines-depleted ignimbrite and co-ignimbrite ash fall are defined based on grain size analysis. Co-ignimbrite lag breccia deposit is recognized as proximal facies. Low-aspect ratio ignimbrite is considered to be more turbulent than regular pyroclastic flows. Gravitational column collapse model is proposed. Fluidization experiments show that the pyroclastic material is not always completely fluidized compared with the case of well-sorted industrial material suggesting the semi-fluidized behavior of pyroclastic flows.

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Significant progresses in recent studies on tephra are reviewed. In relation to volcanological aspects of tephra, recent works on mechanism of vesiculations and disruption of magma, formation of eruption column, transportation of tephra, recognition of types and magnitude on eruptions, and roles of interaction between external water and magma are discussed. The progresses are mainly based on the intensive researches on the recent eruptions observed by volcanologists such eruptions as the 1980 St. Helens, the 1977 Usu and so on. Attempts on quantitative understanding of eruption phenomena such as total eruption volume, initial population of grain-size, and dispersal/fragmentation values of tephra are discussed. Recent studies on phreatomagmatic eruptions on the basis of the experimental researches of magma/water interactions and field observations are remarkable progresses in volcanological studies. In relation to tephrochronological aspects, recent works on identification techniques of tephra layers, discovery of wide-spread tephras, dating methods of tephras, and applications of tephrochronology are summarized. Identification techniques of tephra layers based on determinations of refractive indices and major, minor and trace element compositions of glass and/or minerals were highly advanced in the last 10 to 20 years. Those are effective tools for identification of wide-spread tephras. Wide-spread tephras such as AT ash from Aira Caldera, K-Ah ash from Kikai Caldera, B-Tm ash from Baegdusan Volcano and others were recognized in Japanese Islands and also in deep-sea sediments around Japan. They were dated by radiometric and stratigraphic dating methods. Radiometric dating methods applicable to the late Quaternary tephras are ¹⁴C, Fisson Track, Ionium, K-Ar, Thermoluminescence, and Electron Spin Resonance methods. Because the dates of tephras were mostly obtained by ¹⁴C dating, the radiometric ages of tephras over 40000 years are lacking so far. The accurate radiometric ages of tephras for this range are strongly required for further developments of tephra studies. It is stressed that tephra studies will play great roles on establishment of eruption histories of poligenetic volcanoes, activity aspects of one cycle eruptions, life time of magma and zoning of magma chamber.

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Behavior of volatiles such as H₂O, CO₂, SO₂, and halogens during degassing of basaltic magmas has been reviewed based on their abundances in volcanic rocks and high temperature volcanic gases. Degassing is controlled mainly by the solubilities of gases in magmas. Analyses of the volatiles in glass inclusions in phenocrysts of erupted rocks can afford important information on the role of volatiles in the pre-eruptive magmas. However, the volatiles are partially or even almost completely degassed from the magmas during their ascent and shallow emplacement, depending on the solubilities. Thus care must be taken to interpret the chemical and isotopic data on the volatiles in subaerially erupted rocks. On the contrary, volatile materials trapped under high pressure in glassy parts of submarine basalts can give a clue to estimate the chemical and isotopic compositions of volatiles in the mantle. Budgets of volatiles between the crust and mantle in the current platetectonic regime suggest an important view that water is now being carried into the mantle from the crust, if the present production rates of volcanic rocks at ridges, hot spots and island arcs are balanced with the subduction rates of the oceanic crust, because of the increased water content of the oceanic crust due to hydration during submarine hydrothermal alteration. Volcanic gases from island-arc andesitic volcanoes, especially those from Japan, are generally rich in H₂O, indicating secondary incorporation of meteoric groundwater during magmatic and post-magmatic stages. It is essential to remove the secondary modification in order to deduce the primary information from the volcanic gas studies. Stable isotopic studies of the volcanic gases and associated thermal waters are powerful tool in this respect. High temperature end-members of the andesitic exhalations have been characterized by the discharge of gases with the isotopic compositions (δD, δ¹³C, δ³⁴S) that are significantly higher than those from the primary basaltic magmas. The δD values of water in high temperature volcanic gases from many andesitic and dacitic volcanoes fall in a narrow range bewteen -15 and -35‰ irrespective of their localities. These characteristics may be related to genesis of andesitic magmas.

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Outlines of the studies on the fundamental unit landforms (of lava flows, lava domes, pyroclastic flows, pyroclastic cones, debris avalanches and debris flows), the evolutions of individual volcanoes and temporal and regional changes of the volcano types, are briefiy described. Some of them are summarized as follows : (1) Some stratovolcanoes such as Shasta volcano are different from those of Japan in morphology and evolution, accompanied with extrusion of rhyolite domes and effusion of basalt lava flows almost simultaneously. (2) It should be reinvestigated whether co-magmatic small to medium scaled volcanisms prior to catastrophic pyroclastic erupttions associated with caldera formations existed in Japanese caldera volcanoes such as Kutcharo, Aso etc. (3) Vulsini volcano near Rome, Italy, seems to be an intermediate type between shield and caldera volcanoes. The gentle slopes outside the caldera are composed of pyroclastic flows and thin fluidal lava flows. (4) Between shield and stratovolcanoes there are intermediate type ones surrounded by very gentle and extensive lower slopes composed of basalt lava flows such as Newberry, Medicine Lake Highland, Klyuchevskoy volcanoes, etc. (5) The evolutions of the most Quaternary stratovolcanoes in Japan can be volcanostratigraphically explained by a hypothesis that they have been probably provided from individual magma reservoirs composed of parental basalt magma rising as diapirs from the upper mantle. (6) The volcanism in Japanese Islands since 2-3 Ma has changed from acidic to basic in nine regions such as Tokachi, Sengan, Aizu-Shirakawa, etc., suggesting a process different from that of rising diapirs of basalt magmas, e.g., that partial melting of crust by heating of basaltic diapirs from upper mantle occurred firstly, forming subsequently the caldera volcanoes such as Onikobe, Hakkoda, etc. and that basalt magmas as the diapirs rose secondly up, resulting in the formation of the stratovolcanoes such as Taisetsu, Iwate, Nasu volcanoes, etc.

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Prediction of volcanic eruptions has been one of the major goals of scientific researches for a mankind. Short review is made from the view points of historical background and the present status. Modern scientific researches in volcanology were initiated in Japan, similar to those in seismology, in the late 19th century by the foreign scientists, who were invited by the government to help the quick modernization of Japan. Studies by Nauman and Milne are the examples. Instrumental observation of volcanic activities was initiated by Sekiya, the first Japanese professor in seismology, at the time of the cataclysmic eruption of Bandaisan in 1888. He brought a seismometer to the summit area and made a daily seismometrical observation. Omori’s studies in the early 20th century were the greatest milestone in volcanology. At the time of Torishima eruption in 1902, where all the 125 inhabitants were lost, Omori insisted firmly that we need a volcano observatory, which should conduct a continuous surveillance at the active volcanoes of Japan. After the successful prediction and well organized evacuation prior to the eruption of Usu in 1910, Omori was quite confident that 《the problem of prediction of great volcanic eruptions is, “in some cases”, not very difficult》. Omori also pointed out in 1911 that the volcanic energy is manifested essentially by pushing upwards the underground lava masses in a certain zone. The idea is again confirmed at the 1914 Sakurajima eruption through the seismic and geodetic data, and we now know that the idea is equivalent to the first attempt for the inflation-deflation model, the basic theoretical model today. Continuous monitoring of the semi-continuous eruptive activity at Asama in 1935-1961 gave a chance to Minakami for accumulating a knowledge for the eruption prediction in detail. Minakami proposed a prediction technique which involves A-type to B-type variation of volcanic earthquakes and the practical estimation of the eruption prediction probability based on B-type earthquakes. This technique coupled with the inflation modelling led us to the new era in early 1960’s for the eruption prediction study. The era, when many active volcanoes in Japan were started to be continuously monitored by Japan Meteorological Agency and hence, Japan was a leading country in volcano observations. The National Project for Prediction of Volcanic Eruptions in Japan started in 1974 for establishing a practical application of the prediction. Because of the extensive use of a new technique such as telemeters and CPU-aided observation and analysis systems, a few volcanoes are now well monitored by university observatories. The best example is the Sakurajima, where the newly installed tiltmeters and extensometers in a deep underground vault near to the crater show a consistent inflation before each summit eruption. Similar success in the prediction of volcanic eruptions was reported during the dome building eruptions at Mt. St. Helens during 1980-1982. Both are the special cases in which the eruptive activity continued quasi-continuously with similar repeated pattern. Regularly repeated earthquake swarms prior to the all seven historical eruptions of Usu give another patterns of pre-eruption process. The difference from the above cases may be only in the time scaling problems. Inflational deformation was luckily observed before 1977 eruption of Usu which was closely related to the occurrence of low-frequency earthquakes. Those regularities are believed to be caused by the highly viscous nature of dacitic magma. Even in the case of Miyakejima eruption, where basaltic magma erupted after a relatively long repose time, the minor seismic swarm was repeatedly monitored twice in 1962 and 1983, suggesting a high possibility of the successful prediction in the future if more adequate observations are available. Now we know a little better than

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This paper consists of six parts : introduction, disasters caused by volcanic gases, precursory chemical phenomena related to eruptions, post-eruptive activity changes and related geochemical changes, continuous chemical monitoring for eruption prediction, and what we have to do for the chemical prediction of eruption. Observed facts which are sometimes inconsistent with each other, and several proposals for the chemical prediction of eruption, especially continuous data acquisition are critically reviewed and discussed. It is emphasized thar the accumulation of the results on continuous observations, laboratory experiments and theoretical considerations should be combined with harmony. Cooperation of geochemistry with seismology, geomagnetic studies. geodesy and experimental petrology is urged.

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