Volcanogical Society of Japan was first organized in 1932. Before that time, geological studies of many Japanese volcanoes were carried out as one of the projects of the Earthquake Investigation Committee. Great eruptions of Bandai-san (1888), Sakura-zima (1914), and Komaga-take (1929) were studied and descriced. These descriptions have been often referred to in literature. During the period from 1932 to 1940, the Society issued the periodical journal “Bulletin of Volcanological Society of Japan.” The papers published in this journal were largely concerned with geological and petrological subjects. Although many excellent observations on activity of some volcanoes were carried out by geophysicists during this period, little co-operation was made with geologists. The Society became dormant in iis activity in 1940. Having been stimulated by the eruptions of Miyake-zima (1940), Sy6wasin-zan of Usu (1943～45), and O-shima (1950～51), a small group of volcanologists organized “Study Group of Active Volcanoes” which was later re named as “Study Group of Volcano Physics”. The increase of activity of this group, coupled with the increasing need of cooperation with the International Association of Volcanology (IAV), made the volcanologists to decide to re-open the activity of the Volcanological Society of Japan in 1956.

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Historic backgrounds of the study of volcanic phenomena are largely concentrated to its geological features. Recent accumulation of the knowledge about the structure of the crust and the upper mantle of the earth and geophysical observations at volcanoes in manifold ways have made Volcanology in great advance. Four important and fundamental problems in Volcanology are discussed from geophysical point of view. They are 1) source of volcnic heat, 2) generation of primary magma, 3) roots of volcanoes and 4) prediction of volcanic eruption. Further research items to be pursued were listed. It is stressed that Volcanology should be cooperatively studied from geophysical, geological and geochemical points of view.

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Volcanology is the science dealt with a life of the volcano on the Earth. Genesis, movement and extinction of magmas in the earth which has been differentiating from its formation, volcanic phenomena observed on the earth such as movement of the ground surface, volcanic activity, volcanic eruption, formation of volcanoes, fumarolic activity and hot spring activity, volcanic products, and geochronology of volcano are the main subjects to be studied. Studies on prediction of volcanic eruption and utilization of volcanic energy and volcanic products are important in the applied volcanology.

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The modern scope of volcanology is described from the geological stand point. Indeed, the intensive cooperative researches are required to clarify the phenomena of volcanism, but more quantitative geological approachs will be required for the fundamental study of the volcanology which includes the problem of volcanic eruption, the problem of the original magma, and the problem of acidic magmas.

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The present state of volcanological research activity in many countries are briefly described and reviewed, together with the history and activity of the International Association of Volcanology.

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The writer intends here to describe the outline of the volcanic activities in Japan after the World War II, with special reference to the changes in the social conditions of the volcanic areas and to the development of our volcanological observation & research. During the period from 1945 to 1965, the volcanic activities such as eruptions & the so-called volcanic extra-ordinary phenomena were found out at 26 volcanoes of Japan as is shown in Table 1. The locations of those volcanoes are shown in Fig. 1. The destructive eruption has taken place once a year or so on the average, and 70 persons in total were killed by 10 eruptions of 6 volcanoes. But the volcanic activities in our country, as a whole, were rather calm during this period. Besides, it must be noticed that almost all the volcanic activities including very slight volcanic extra-ordinary phenomena are nowadays detected in Japan, and we shall scarcely have unexpected eruptions hereafter.

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This paper reviews the interrelationships between distribution of volcanoes and seismicity of the mantle in Japan, which suggest a common origin of primary basalt magmas with mantle earthquakes. The basic geographical information for Quaternary volcanoes in Japan is illustrated by a map and summarized in a table. These volcanoes are grouped into two volcanic belts: East Japan volcanic belt and West Japan volcanic belt. Near the border on the oceanic side of the volcanic belts the population of volucanoes is densest. The oceanic-side border of a volucanic belt is called volcanic front. The depth of mantle earthquake foci under the volcanic front is about 120 to 140km. Toward the continental side of the front, the volcanoes are distributed more sparsely and the mantle earthquake foci are deeper (Fig. 2). Another relationship between volcanoes and earthquakes is that the foci of the shallower shocks in the mantle are associated with the volcanoes derived from the more silicious and less alkaline basalt magma and those of deeper shocks with those from the less silicious and more alkaline basalt magma (Fig. 3). The third relationship is a complementary distribution of the volcanoes to the epicenters of the mantle earthquakes (Figs. 4 and 5). It is suggested that the generation of primary basalt magma and the occurrence of earthquakes in the upper mantle have a common source of energy in a common zone. This is probably responsible for all of island arcs.

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A Prospect of researches of volcanoes is given from the stand point of energetics of the earth’s interior. Volcanism is not an important mechanism for the energy transfer from the interior, while it is the major process of crustal formation, To test the hypothesis of mantle differentiation, the researches of continental plateau basalt may be crucial. Two types of tectonic activity, the growth of the Asian continent and the development of the Pacific Ocean, should be distinguished to clarify the volcanism in the Japanese Islands area. A research of lunar volucanism is proposed in relation to the terrestrial volcanism.

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Results of some recent studies concerning the age of volcanoes and the related problems are reviewed. Particulary, the results of the K-Ar dating of the Hawaiian volcanoes by McDougal and the Rb-Sr dating of ultramafic rocks by Roe are discussed in some detail. The method of paleomagnetic correlation of lava flows is also discussed.

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There are a large number of varieties of phenomena manifested in volcanic eruptions. They are mostly related with the characters of magmas by which respective volcanic activities are caused. In the cases where the magma comes in coetact with the underground water or some other abnormal conditions on its way ascending to the earth surface, however, volcanic eruptions may be manifested differently from those expected from the original character of the magma. Volcanic eruptions and their accompanying manifestations are listed as follows; Lava flow, lava lake formation, lava dome building, extrusion of volcanic spine, volcanic explosion, phreatic explosion, projection of fragmental ejecta, cinder cone building, projection of volcanic smoke, nuee ardente in the strict sense, pyroclastic flow of the intermediate type, pumice flow, directed volcanic blast, agglomerate flow, mud flow, formation of crater, explosion crater and fissure, fumarol formation, volcanic earthquake, explosion earthquake, volcanic rumbling, explosion sound, explosion blast wind, collapse of the volcanic edifice, melted sulphur flow, hot water flow, ground uplift, ground crack, upheaval and subsidence of the terrain, increase of the land, tilting, expansion and contraction of the ground, change of crater depth, depression and sinking around the crater (including caldera), formation of dammed lake, change in ground temperature, fumarole, hot spring, water spring and lake water, water column rising, warming and staining of sea water, tsunami, change in volcanic magnetism, lightning and thunder. The present paper deals only with principal phenomena of volcanic eruption, though there are not few accompanying manifestations inmportant for prediction of eruption.

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Various energies released in relation to a volcanic activity are classified and evaluated based chiefly on YOKOYAMA’s and SUGIMURA’s works. The energies are classified into four different categories, i.e. 1) heat transferred by solid and gaseous volcanic products (Eth), 2) energy spent in expansion of volcanic gas (mainly water) (Ee), 3) heat lost by underground conduction (Ec), and 4) work done against gravity (Ep). Eth is estimated at (1.4±0.3) × 10¹⁰・M (ergs), where M is the total mass of solid eruptive product in grammes. Ee is estimated at 1.5 × 10⁹・M (ergs) in which kinetic energy of explosion and of ground vibration is included. Ec is difficult to estimate in direct connection with M. Ec consists a part of excess value of terrestrial heat flow of volcanic belt over non volcanic one. Therefore, it can be put out of consideration in the discussion where heat flow values are calculated independently. Ep can be almost neglected when we consider the energy economy including the crust and upper mantle as a whole, because compensative work should be done for the gravity, below and around the volcano. Thus, the order of magnitude of the whole volcanic energies are expressed as 1.6±0.4 × 10¹⁰・M ergs (+Ec) in terms of the total mass of solid volcanic product, which is the only key to the magnitude of past volcanic activities.

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The characteristics of earthquakes originating from volcanoes which have been obtained by many investigators were summarized. Those earthquakes are classified tentatively into explosion-earthquakes, volcanic-earthquakes and volanic micro-tremors. The explosion-earthquakes occur almost simultaneously with the outburst of the volcanoes of Vulkanian type. Therefore it is generally concluded that such earthquakes are generated from the mechanism of a single positive force. However, there are the examples having a mechanism of tension-cracking type. The volcanic-earthquakes are the small swarmed earthquakes occurring in a limited region in the volcanic district and closely related to the activity of volcanoes. There are different kinds. One is the A-type earthquake. Its focal depth is not extremely shallow (1～10km) and the wave form on the seismogram is similar to that of a shallow earthquake of tectonic origin. Another one is B-type occurring in the limited region beneath the active crater. The volcanic micro-tremors, customary called, include following various kinds of tremors. In the case of Strombolian or Hawaiian type eruptions, they follow the tremors originating from these frequent eruptions. On the other hand, there are non eruptive kind of tremors originating from the very small earthquakes beneath the active crater. And the tremors generating from the free vibration of magmatic chamber or column of lava and so on, are also found. The B-type earthquake or the volcanic micro-tremor are mainly composed of the seismic surface waves (Rayleigh or Love-type) which have been guided from the layered structure in the volcanic district, hence they have their own characteristic period. Such various kinds of volcanic earthquakes can be classified in the following three categories according their mechanisms of occurrence or origins of vibration, i.e. (1) The deeper earthquakes of volcanic origin (A-type), (2) Non eruptive earthquakes such as B-type one or some of the tremors originated from very small earthquakes, and explosion earthquakes occurring at shallow depth, (3) The tremors generated by the vibrations of magmatic chamber or column of lava and so on.

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Mean intensities of effective magnetization of volcanoes so far obtained by various investigators are listed in §2. It is also briefly discussed that the substantial part of the magnetization of volcanoes seem to be originated from the magnetization of volcanic mass above the ground surface rather than from the deep seated roots under the volcanoes. There are numbers of papers that report a fraction of magnetization of volcanoes has been changed at the time of eruption. A few of them are reviewed in §3. Results of aeromagnetic surveys which have been conducted extensively over the calderas in Japan are reported in §4.

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The volcanic crustal deformation is classified into two types: One is the parmanent deformation as the topographic change caused by the eruption, and another is the changeable deformation according to the change of the underground state. Generally, the magnitude of the parmanent deformation is remarkable and visible, although its area is limited. Otherwise, the magnitude of the changeable deformation is smaller than the former, but the dimension of its area sometimes amount to several ten km in diameter as shown in great Sakurajima eruption. Such changeable deformation gives us the useful informations for the volcanic activity. In this paper, some interpretations for the changeable deformation in Sakurajima volcano are introduced, and also some discussions for the abnormal tilt of the ground surface before the eruption in various volcanoes are tried.

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We can not have any concrete discussions on the origin of calderas without consideration for their present structures. Hitherto, many theories about their origin have been proposed mainly from the standpoints of surface geology and petrology. In this paper, first, the results of geophysical studies of the structures of several calderas in Japan are referred; The gravimetric method proves to be effective for that purpose because calderas were formed with the movements of a huge amount of volcanic ejecta. A discussion is tried on drawing a balance-sheet of caldera ejecta and it supports the conclusion that the deposits within the calderas consist of fall-backs of lithic fragments and also volcanic ejecta. In fact, this is verified by the core samples of a 1,000 meters deep boring dug at the middle of Kuttyaro Caldera, Hokkaido, which is the deepest boring within calderas at present as far as the writer knows. Next, the author refers the major volcano-tectonic depressions in New Zealand which are similar to calderas in their subterranean structures and ejecta, and suggests that they differ only in the configurations of the vents, central or linear and the magnitudes of eruptibilities. Volcanic activities would depend on the regional geologic structures. Judging from the structures of the calderas mentioned in this paper, we have no cogent reason to assume shallow "magma reservoir" immediately beneath the calderas of low gravity anomaly type. Location of magma reservoirs has an important bearing on origin and mechanism of caldera formation.

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Purposes of the volcanological observations undertaken by Universities or Institutions are the fundamental researches of volcanic phenomena including potential cause of volcanic action. Establishments of empirical formula for prediction of volcanic eruption should be also emphasized. For the comparison of volcanic activities of respective volcanoes, characteristics of seismographs and epicentral distances, for instances, are desirable to be the same. Main volcanological observatories attached to the universities and institutions and installed equipments were listed up for future adjustment of the above problem and also for future developments of the observatories.

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The volcanological work of the Japan Meteorological Agency (JMA) has been expanding rapidly after the 2nd World War. Especially, since 1962, the JMA has been making every effort to establish systematic observation of all the active volcanoes in our country. The JMA divides them into three classes (A, B and C), depending upon the natural and social conditions of the respective volcano. At present, the observations of the activities are performed permanently by the JMA at 17 volcanoes. The observatories at 4 A-class and 11 B-class active volcanoes are equipped with up-to-date instruments. The wire or wire-less tele-recording system is adopted to the seismometrical observations with succss at those active volcanoes. Besides, the JMA has 3 teams of volcanologists (emergency missions) at the headquarters in Tokyo, and they carry out periodical precise spot survey of the designated B- and C-class active volcanoes, too. Such being the case, the volcano observatories under the JMA are obliged to issue “Kazan Joho” (Volcano Informations) on the actual state of the volcanic activities whenever they detect any extra-ordinary volcanic phenomena at their own volcanoes. Moreover, A-class volcano observatories are bound to give out regular “Kazan Joho”, monthly. The results of the volcanological observations made by the JMA are published as the Volcanological Bulletin of the JMA (quarterly) in English.

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There are scores of active volcanoes in Japan, and we have been suffering from volcanic disasters since the dawn of history. In recent years, due to the rapid changes in the social conditions of the volcanic areas, the danger to human lives and properties in case of volcanic eruptions has been enlarged year after year. The writer intends here to sketch in broad outline the progress of the observation and research required for the prediction of volcanic eruptions. The scientific study of our volcanoes commenced soon after the Meiji Restoration of 1868, and the first volcano observatory in our country was established on the flank of Asamayama in 1911. After the 2nd World War, owing partly to the development of electronic techniques, various kinds of volcanological observations have made progress remarkably. We have been making every effort to carry out geophysical and geochemical observations of the so-called forerunning phenomena of volcanic eruptions and to make clear the relationship between the eruptions and their premonitory phenomena, Nowadays, "unexpected eruptions" are very rare in Japan, because appropriate informations on the observed state of activities of many volcanoes are issued frequently and the public can learn that some volcanoes are uneasy. However, an exact prediction of the time, place, type and scale of an eruption remains still to be impossible. Our present volcanological observations and research have too major aspects; the practical and the purely scientific. But it is difficult to deal with them separately, because a phase that belongs to fundamental science today may become of great practical importance within a few years.

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The recent high-pressure experiments suggest that tholeiite magma is generated at shallower parts of the upper mantle whereas alkali basalt magma is generated at deeper parts either by fractional crystallization or by partial melting of the upper mantle materials. There is a possibility, however, that alkali basalt magma is produced from tholeiite magma by differentiation involving alkali enrichment near the earth’s surface. The field and experimental studies suggest that the differences in the course of differentiation between the tholeiitic and alkali rock series would depend on the silica saturation or silica undersaturation of the magma, and those between the tholeiitic and calc alkali rock series would depend on the oxygen partial perssure in the magma.

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Reasoning about the molten parts in the earth’s crust and upper mantle and various phenomena in magma reservoir requires a knowledge of the way in which the physical properties of molten rocks vary under combined high temperature and high pressure. This paper reports the review of measurements of the velocity and attenuation of longitudinal wave, viscosity, electrical conductivity and the growth velocity of crystals in molten rocks. A conclusion derivable from these measurements is that there is some possibility of the detecting of molten parts in the earth’s crust or mantle.

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Some informations on the water pressures in magma reservoir have become available by using some experimentally determined equilibrium diagram (Fig. 1, 2, 3). But in the case of the acidic magma, the system of quartz-orthoclase-albite-anorthite will give more precise informations than quartz-orthoclase-albite system (Fig.4). In Fig. 5, idealized quartz-orthoclase-albite-anothite system is shown.

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Some aspects of the physico-chemistry of the silicate melts are discussed, with special reference to the melts, basaltic or rhyolitic in composition. Characteristic features of some ternary or quaternary systems pertinent to the basalt problems are first explained. Nearly simultaneous crystallization of olivine, monoclinic pyroxene, and plagioclase in various basaltic rocks in nature as determined by thermal experiments shows close agreement with the crystallization in these systems. An important role of oxygen partial pressure in the basaltic magma is clarified by the different crystallization trends in the system MgO-FeO-Fe₂O₃-SiO₂. Enrichment in either iron or silica can be explained by the difference in the PO₂ value. The so called “granite system” of Or-Ab-Q-H₂O affords a basis for the petrochemistry of the rhyolitic rocks. The remarkable difference in the tie lines of alkali feldspar and liquid in the rhyolitic or pantelleritic rocks can be ascribed to the presence in the pantelleritic liquid of normative Ns and Ac molecules. Origin of the felsic volcanic rocks of the Taupo district, New Zealand is discussed, in relation to the shift of the minimum melting points with the PH₂O in the granite system. A new hypothesis of explosive volcanism is proposed by Yoder based on the study of the An-Di-H₂O system. He regards the sudden relief of pressure on the hydrous magma as the prime cause for such explosive activity.

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Tables 1 and 2 represent classification of pyroclastic fragments and pyroclastic rocks originally proposed by Wentworth and Williams. The classification is solely based on the nature of the fragments. It is advisable to use classification of pyroclastic deposits based on the mode of emplacement and deposition of the material. Table 3 is a proposed scheme of such classification. Diagnostic characters of each type of deposits are described. Attention is drawn to the importance of recognition of subaqueous pyroclastic deposits which are supposedly very common in geologic formations. Autobrecciated lava, which is a subaqueous lava of viscous magma such as andesite, is also common among the Japanese Tertiary formations and are apt to be misinterpreted as mudflow or as pyroclastic flow deposit. Autobrecciated lava grades to pillow lava with decrease of viscosity of magma.

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The distribution of minor elements in volcanic rocks is reviewed especially on basaltic rocks. The log-normal distribution schemes are summarized based on the data by Ahrens. As an example, bi-lognormal distribution of chlorine is emphasized on the Hawaiian lava. Minor elements are classified into two groups, one is volatile, and the other is non-volatile. The distridution of volatile elements is interpretated by solubility into magma, which is related to the partial pressure of each volatile component in magmatic gases. The distribution of non-volatile elements is based on the Goldschmidt’s rule, and this rule is expanded to the distribution law defined by Nernst. The mean values of minor elements in crust, basalts, and the special basaltic rock, W-1, are listed.

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Near future developement of the utilization of natural steam for generating electric power is estimated by using Dr. D. E. White’s table, and the relation between this geothermal utilization and some fundamental problems for the behaviour of magma is considered.

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The term “volcanic sublimates” is defined and the purpose of this investigation and the results of the preceding investigations are described. The mineralogical descriptions of the volcanic sublimaties which have been reported recently in Japan are shortly reviewed. Some genetic considerations of the volcanic sublimates are shown.

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Recent advances in the studies of hot spring phenomena (volcanic hot springs) as volcanic activity are reviewed. Especially characteristics of the volcanic hot springs are described by the examples of the Tamagawa Hot Springs and hot springs in Iwo-zima. The main subjects to be studied in future are also described.

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Most of chemical compositions of volcanic gases reported are not complete on account of inadequate analytical methods. It is very important to establish the proper analytical methods and sampling methods for each volcanic gas. Recent advances in the studies of volcanic gases were reviewed as follows: Analytical method; chemical composition (main and minor components) of volcanic gases from lava lakes, fumaroles, and hot springs; radioactivity of volcanic gases; variation in chemical composition and radioactivity; classification of volcanic gases; relations among chemical components, calculation of chemical composition of magmatic gases.

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Volcanic emanations have been studied with respect to H, He, B, C, N, O, S and Ar isotopes. This paper deals with studies on H, C, O and S isotopes which have proved their usefulness in the field of chemical volcanology. 1. Carbon isotopic exchange equilibrium between methane and carbon dioxide enables to calculate an equilibrium temperature deep underground. 2. δC¹³ values of total carbon from volcanic areas fall in the narrow range 0～-5%, suggesting a significant contribution of limestone carbon dioxide to volcanic gases. 3. δS³⁴ values for volcanic sulfur compounds are explained by sulfur isotopic exchange equilibria among them. 4. δS³⁴ values for total sulfur discharged from volcanic areas seem to approach to the meteoritic troilite value. 5. Waters from volcanic areas can not be distinguished from surface waters in terms of hydrogen isotopes, except that some of them are marked with high O¹⁸ values, which are close to silicate oxygen values of igneous rocks. Magmatic water can be distinguished by O¹⁸ concentration. This, however, can not tell whether it is vadose or juvenile in origin.

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The magmatic emanations separated from the mother magmas or igneous bodies differentiate into solid, liquid and gaseous phases on account of coolng, depression of pressure, and reactions depending on the physical, chemical and geological environments. On the ground surface at the atmospheric temperature and pressure, these three phases formed are called the volcanic sublimates (solid), the volcanic waters (volcanic hot springs) (liquid) and the volcanic gases (gas), respectively. Variation in the nature and properties of these volcanic products at each stage of differentiation is explaind by the cooling of the magmatic emanation and the model experiments. Genetical relations among many volcanic emanations can be explained by the standpoint of physical chemistry of the magmatic emanations. For further details, see "Differentiation of magmatic emanation." (Iwasaki et al. 1966).

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