Production rates of extrusive and intrusive material are estimated on the basis of plate tectonic concept. For accreting and consuming plate boundaries, production rates of extrusive (volcanic) material are estimated at about 4 and 1 km³/yr, while the rates of intrusive igneous material for the two boundaries are about +9.5 and -14 km³/yr, respectively. Production rates of extrusive material for intra-oceanic and intra-continental plates are about 1 and 0.1 km³/yr, respectively. In the marginal seas which are thought to be another type of plate accretion, production rates of extrusive and intrusive material are estimated at about 0.1 and 1.2 km³/yr, respectively. Net addition to continental crust is about 0.9 km³/yr which is nearly equal to the classic estimates before the proposal of plate tectonics. However, the value of net addition to oceanic crust (1.8 km³/yr) contains large uncertainty because that the global production and consumption of intrusive material are one order of magnitude larger than the net production. Total heat flux from the earth’s surface (Q_t) is expressed by Q_t=Q_co+Q_cr+Q_cl+Q_m, where Q_co is average heat flux that possibly corresponds to radiogenic heat source, Q_cr and Q_cl are the “regional” and “local” excess conducted heat flux that may represent magmatic intrusion in the crust and circulation of hot water system, respectively. Q_m is heat flux due to mass transportation. Terrestrial heat flow values are reviewed in terms of major geological features. It is pointed out that the values higher than 1.0 HFU in oceanic areas may indicate Q_cr and at the ridge axis are considered to be Q_cl. On the other hand, conductive heat flux in geothermal areas and in the vicinity of volcanoes is excluded from the statistics. In geothermal areas, the global sum of heat flux Q_m is estimated at about 10¹⁷ cal/yr and that of extrusive volcanic material is about 5×10¹⁸ cal/yr, whereas the global conducted heat flux is about 2.3×10²⁰ cal/yr. It is emphasized, however, that Q_m by extrusive material is comparable to conducted heat flux near the plate boundary (say, along the zone of a few 100 km-wide).

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Volcanic eruption may be defined as a kind of volcanic phenomena in which volcanic materials are ejected from the earth’s interior to the earth’s surface in a relatively rapid fashion. One school of classifying the kind of eruptions is to name each type after the name of a volcano in which such a type of eruptive activity occurs characteristically. Examples are Strombolian, Vulcanian, Hawaiian, etc. Such nomenclature is not satisfactory due to inadequate and conflicting definitions, and the basic scheme of classification should be followed by parallel description of various factors which govern the nature of eruptions. Ascent of magma may be caused by bouyancy, tectonic squeezing, and vesiculation. Migration of H₂O with other components toward the top of magma column during the period of hundreds of years is apparent but the mechanism of transport is not clear. Complexity of governing factors of the vesiculation and fragmentation and difficulties with models of explosive eruptions are pointed out. Subaqueous eruptions are characterized by rapid quenching of magma by water, suppression of vesiculation due to high ambient pressure exerted by water column, generation of hydrovolcanic explosion in shallow water, and characteristic mode of transportation and emplacement in water. Subaqueous volcanic products are much more fragmentary than the equivalent subaerial volcanic products and grades into non-volcanic sediments.

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In this short paper, the meaning of volcano observation of physical parameters relating with volcanic phenomena is summerized with respect of change of elastic parameters, change of potentials such as gravity and geomagnetic and observation of fluid motion. The substantial problems on volcanic phenomena are enumerated and discussions are made, among them, on 1) observation for the study of the deep and shallow structures of volcanoes and 2) observation for the study of the mechanism of eruptions. Modernization of observation techniques is also described itemizing in seismic, ground deformation, gravity, geomagnetic, geothermic and others. In accordance with the prediction of volcanic eruptions, the national project of the prediction of volcanic eruptions in Japan which started in 1949 is described.

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Volcanoes are generally classified into monogenetic and polygenetic types. Monogenetic volcanoes erupt only once to form smaller volcanoes, such as maars, pyroclastic cones and lava domes. Polygenetic volcanoes erupt repeatedly from the same general vents (summit or main crater) for up to 10⁵ years to form larger volcanoes such as strato-volcanoes (composite volcanoes of Macdonald, 1972) and shield volcanoes of Hawaiian type. Monogenetic volcanoes tend to occur in clusters as flank and post-caldera cones. Some of the clusters are however, independent of polygenetic volcanoes and appear to be equivalent to them. The essential part of the conduit of a monogenetic volcano is inferred to be a simple dike, intruded into a newly formed crack, whereas a long endured pipe-shaped conduit may exist under a polygenetic volcano. The common occurrence of xenoliths in the eruptive products of monogenetic volcanoes may be related to this difference. Various lines of evidence, indicating the existence, depth, shape, volume and internal structure, of magma reservoirs are tabulated. A shallow magma reservoir appears to exist beneath polygenetic volcanoes with one to one correspondence, which is not the case for monogenetic volcanoes. Most flank volcanoes are monogenetic, thus indicating dikes within the polygenetic volcanic edifice. Dike formation is understood as a magma version of hydraulic fracturing. For the dike to intrude and propagate, would require either the increase of differential stress due to a decrease of minimum compression or increase of pore pressure over the sum of the minimum compression and the tensile strength of the rocks. Earthquakes are understood as the generation of elastic waves associated with an acute release of tectonic stress due to faulting. Accumulation of tectonic stress and strain prior to earthquakes is, then, a necessary part of earthquake phenomena in a broad sense, as well as their release after the event. Based on the above-stated understanding, possible mechanical correlations between volcanic eruptions and earthquake occurrences have been studied. Contractional strain around the magma reservoir can cause the squeezing up of magma within an open conduit causing a summit eruption on the one hand, and dike formation resulting in a flank eruption through the increase of pore pressure, on the other. Second boiling triggered by both the magmatic pressure decrease caused by dilatational strain and the dynamic excitation due to seismic waves might have the same effect as contraction. Decrease of minimum compression causing the increase of differential stress leading to dike formation will also contribute to the liklihood of flank eruptions. Both volcanic eruptions and earthquake occurrences can precede each other depending on geographical location in terms of faulting-related stress-strain changes which are calculated by the fault model of earthquakes. Actual possible examples of volcanic eruptions and earthquakes which are allegedly mechanically related are given. In order to demonstrate which mechanism is responsible for the correlation of the two phenomena, continuous strain measurement on and around volcanoes is necessary together with the observation of changes in the level of magma in crater bottoms.

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Recent advances and pending problems in the study of volcanic landforms during the last two decades are reviewed and discussed briefly. Particularly, the classification of volcanoes, the profiles of monogenetic- and strato-volcanoes, the volcano types and their global popuration percentages, and the developmental sequences of Japanese volcanoes are described in some detail.

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The significance to study the age of volcanism is stressed and discussed with some examples published. With the knowledge of age of volcanic activity, we can infer the duration of volcanism, eruption rate and so on. These factors give important constraints on the mechanism of volcanism. Ages of Hawaiian Islands have been much studied and the systematic increasing of ages from Kilauea to the northwestern islands has been explained by a moving plate over a “hot spot”. Rather short volcanic activity (mostly less than 1 m.y.) and larger eruption rate (more than 10⁴ km³/m.y.) for each island seem to be compatible with this model. Some exceptions like recent volcanic activity in the Oahu Island, however, may need further explanations. Canary Islands are characterized with longer volcanic activity, including many intermittent volcanic dormancies for several islands. Although hot spot has also been suggested under Canary Islands, it does not seem to explain the patttern of volcanic activity sufficiently for these islands. Oceanic islands have often maintained the volcanic activities for more than 10 m.y. (e.g. Mauritius Island). In such cases, plate model may request the existence of magma source in a moving plate or magma spots in the asthenosphere along the locus of the island. Seamounts in the Northwestrn Pacific seem to be rather younger than the surrounding ocean floor, which may also suggest the existence of some mechanism to form magma in a moving plate. According to the examples of the Deccan Traps and the Columbia River Plateau, the plateau formation seems to have occurred in rather short time (say, less than 5 m.y.) with great euruption rate (10⁵ km³/m.y.). Although some regions with assumed hot spots (e.g. Iceland, Hawaiian Islands) have a little less eruption rate (10⁴-10⁵ km³/m.y.) than that of a continental plateau, their volcanic activities continue for longer time, resulting often in producing greater volume of lavas in total. Isolated islands have an eruption rate of the order of 10² km³/m.y., which exceeds that of seamounts. Ridge system and/or “hot spot” may be among the rest most extensive volcanism on the earth and maintain the volcanic activity for rather long time. However the character of a hot spot is still not clear and it is imperative to study the mechamism to keep these systems which is also responsible for causing many volcanisms on the earth.

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Great advances in studies on the genesis of magma have come during the last ten years from progress in the fields of experimental petrology, geochemistry, and plate tectonics. In experimental petrology, the effects of pressure and of volatile components such as H₂O and CO₂ on the composition of magmas formed in the upper mantle have been determined : with increase of pressure magmas formed by partial melting of upper mantle peridotite become more silica-undersaturated and more enriched in olivine component ; in the presence of H₂O magmas become more silica-rich, whereas in the presence of CO₂ they become more silicaundersaturated. Primary magmas thus formed may change their compositions by fractional crystallization or by partial zone melting during their ascent. Recent research has been concentrated on the origin of several possible primary magmas. Beneath the mid-ocean ridges abyssal tholeiite magmas may be formed by a relatively high degree of partial melting of diapirs of upper mantle materials, with or without subsequent fractional crystallization. In orogenic belts calc-alkaline andesite magmas may be formed by melting of hydrous upper mantle, with or without subsequent crystallization. Kimberlite magma and some very low-silica magmas erupted in the continental areas may be formed in the upper mantle in the presence of CO₂. Major problems, however, remain unsolved, such as a mechanism for generation of plateau basalts and mechanisms to explain compositional variations in time and space in both orogenic belts and mid-ocean ridges. Other research areas indispensable to further understanding of magma genesis are knowledge of the physical properties, such as density and viscosity, of magmas at high pressures and knowledge of the partitioning of trace elements between crystals and melts under known conditions.

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The physical properties of silicate liquids have been reviewed. These are viscosity, thermal conductivity, ultrasonic wave velocities, compressibility, attenuation, diffusion coefficient of Na-22 and 24, kinetics of crystal growth, density, surface tension and electrical conductivity. Some interpretaions of these data are placed on magmatic process.

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Phase and compositional variations in minerals have been investigated in the light of change of the physical conditions of magma during eruption. Significant role of water vapor pressure as well as temperature is demonstrated in the explosive activity of the “Futatsu-dake eruption” ; possible increase of the water vapor pressure before eruption is suggested by the reversed zoning of hypersthene and hornblende, and sudden decrease of it after the eruption is correlated with cease of hornblende crystallization and beginning of sector-zoned augite. Effect of undercooling of magma due to the volatile release is stressed. Temperature and oxygen pressure change and possible reduction of magma during eruption are well reflected by Fe-Ti oxide minerals.

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A classification of volcanic zones into six different categories is proposed based on the plate tectonic viewpoint (Table 1). They are volcanoes of oceanic ridge axis, island arcs and continental margins, marginal seas, rift valleys, seamounts and intra-oceanic islands, and plateau basalts. Studies are critically reviewed, putting emphasis on compositional characteristics of the volcanic rocks and mode of origin of the magma for each volcanic zones.

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Volcanic rocks have been studied with respect to oxygen and sulfur as light stable isotopes and to the isotope variations of strontium and lead due to radioactive decay, and variations of rare-earth elements (REE). This paper deals with studies on oxygen, strontium and lead isotopes and rare-earth elements which have provide their usefulness in the field of isotope geology. The ¹⁸O/¹⁶O ratios of a magma would vary during crystallization differentiation if the weighted average of the isotope ratio of the crystallizing minerals differs from the isotopic ratio of the remaining liquid phase. The oxpgen isotope data on volcanic rocks are in agreement with Sr isotope study in that they are compatible with the rocks having derived by fractional crystallization of magma with some contamination by sialic crust, or the initial enrichment of ¹⁸O in the source material in those samples from Kiso-ontake Volcano and Oki-Dogo Island in Japan. The ¹⁸O-enrichment in magma during crystallization differentiation is recognized in every rock series in Japanese Islands. Isotopic composition of strontium is not only a useful indicator of the ages of rocks and minerals, but it also contains information about the origin of igneous rocks and about the geologic processes that have affected their chemical compositions. No evidence has ever been presented to suggest that strontium isotopes are fractionated in nature, and it is therefore widely believed that fractionation effects are negligible. Rocks formed by melting, metasomatism, or assimilation of crustal materials will be labeled by having higher initial ⁸⁷Sr/⁸⁶Sr ratios than the ratios of uncontaminated rocks derived from the mantle. There is a positive correlation between Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios in some island arc tholeiitic suites and a decrease of ⁸⁷Sr/⁸⁶Sr ratios with increasing distance from the trench in some island arc. The ⁸⁷Sr/⁸⁶Sr ratios in volcanic rocks from Kurile and East Japan arcs in Japan, decrease from 0.7040-0.7055 closest to the trench to 0.7026-0.7034 furthest from it. Note that Kurasawa (1970b) resports the opposite trend in south-west Honshu, but this is due to high ratios in the volcanic rocks of West Japan. These volcanic rocks differ in such distinctive feature as K/Na ratios and TiO₂ content from other island arc rocks and may only indirectly reflect the typical island arc phenomenon. In either case, it is important to note that West Japan is situated in a region of a continental margin that do not correspond to typical island arc. These results are in agreement with Pb isotope studies on the volcanic rocks form Japanese Islands. In this report in situ magma differentiation of basaltic magmas are discussed by using Sr isotopes and variation of REE concentrations in the rocks.

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In this paper, it is intended to obtain information on the interior of the earth on the basis of chemical nature of volcanic gases. It seems impossible, however, to get any information on the interior of the earth without understanding volcahic gases themselves. To start with, analyses of volcanic gases are mentioned briefly followed by the presentation of a marked individuality of volcanic gases. On the basis of these knowledge, origin of volcanic gases is discussed in connection with the nature of magma. An emphasis was made on volcanic gases as degassing products from the earth. The possibility that N₂/Ar ratio of volcanic gases is a useful parameter which characterizes volcanoes in relation to global tectonics is discussed. Finally prediction of eruption through volcanic gases is mentioned.

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High temperature sodium chloride waters have been reported in many of active geothermal field developed in the central part of some Japanese Quarternary volcanoes such as Hakone, Onikobe, Kuttara, and Kuju. Many of micro-earthquakes occur in those active geothermal field, showing intimate relation between micro-seismic activity and strong geothermal activity. Provided the phase transformation of thermal waters to steam is the major cause for such types of volcanic micro-earthquakes, the chemistry of sodium chloride waters permits to estimate the temperature-pressure condition at depths where seismic activity takes place. The vapor-dominated system suggested by White et al. (1971) is an important concept to understand the distribution of acid sulfate waters at shallow depth in contrast with the development of hot water system characterized by neutral to alkaline sodium chloride waters at greater depth of the volcanic geothermal field.

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The Pacific plate began to subduct from the Japan trench beneath the Northeast Japan arc 42 m.y. ago. No volcanic activity, nevertheless, has been observed until 28 m.y. B.P. It is assumed that the time between 42 and 28 m.y. B.P. was spent to accumulate volatile elements from the subducted plate in the low-velocity layer. Then after, it was ready for the generation of magma. It is designated the Cryptal stage of late Cenozoic Mizuho orogeny. The orogeny in the ordinary sense started with explosive volcanism 28 m.y. ago. The Initial stage is from 28 to 17 m.y. B.P. Andesitic magma is characteristic of the Initial stage. The volcanic production rate is very high because of the tensional tectonics which was resulted from the opening of the Sea of Japan. The definite magmatic zoning did not exist probably as in the present tensional volcanic area such as the mid-oceanic ridge and marginal sea. Hydrothermal solution well circulated through the tensional crust which is very high in porosity. It was probably very high in the heavy metal contents. The model proposed harmonizes such a matter that the present metalliferous brines are mainly described in the tensional volcanic area. The period between 17 and 13 m.y. B.P. is designated the Transitional stage roughly correlated with the Nishikurozawa stage. Tectonics in the Northeast Japan arc became gradually compressional since about 17 m.y. B.P. The hydrothermal solution was forced to ascend with the advance of compressional environment, because the porosity of crust decreased gradually. Most of Kuroko-type and probably vein-type deposits were formed in the final stage of a single eruptive cycle in the Transitional stage. It is a reason why the most of mineral deposits in the Mizuho orogenic belt were formed about 13 m.y. ago. It seems likely that mineral deposits in the other orogenic belts were also formed in the transitional period from the tensional tectonics to the compressional one. It should be commented, however, that there are some exceptional cases. For example, the deposits in the Red Sea were formed under the tensional environment. The writer's hypothesis is applicable only to the ore formation in the common crust. The different metallogeny should be considered, if the orogenic belt is intercalated with a lot of soluble beds such as evaporite and limestone. Furthermore, the writer mentioned the metallogeny of mineral deposits related genetically to volcanism, but not to plutonism. After 13 m.y. B.P. the Northeast Japan arc has entered the Stationary stage and has become a typical island arc. The compressional tectonics, as a whole, has prevailed through the Northeast Japan arc. The magmatic zoning from tholeiite through high-alumina basalt to alkali olivine basalt has been established. The volcanic production rate declined markedly.

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Relations between volcanic activity and environmental problem, global or regional, were discussed generally. It was emphasized that, among the volcanic products, volcanic gas and fine volcanic ash are most important as pollution substance, as shown as follows : 1st. Both in these volcanic products and usual smokes from factories, common chemical constituents are contained. For instance, they are HF, HCl, SO₂, H₂S, heavy metals and derived constituents of them. 2nd. Amounts of volcanic gas or fine volcanic ash discharged from the crater in active stage are not so small as usually considered. In spring of 1975, the season in which the activity of the volcano was rather high, mass flow of SO₂ discharged from the crater of Sakurajima Volcano reached to the level of about 1100 tons per day. So the total amounts of volcanic gas might be tremendous. Furthermore, it was estimated that for twenty years, about 1×10⁸ tons of fine volcanic ash was discharged from the volcano. 3rd. The residence time of volcanic gas and fine volcanic ash in the atmosphere is rather long as compared with other volcanic products. And these volcanic products, volcnaic gas and fine volcanic ash, may be carried and travel very far from the effusing point. Effects caused by them are long and wide. Especially in the near places of the volcano, effects and influences are often serious as Kogoshima City near Sakurajima. In future, the total amounts of these volcanic products discharged from whole active volcanos in the world must be estimated more accurately. If so, the evaluation of influences of volcanic activity for the global environment may be possible more quantitatively. And, suitable air pollution monitoring system set around the active volcano is conisdered to be quite effective for this purpose.

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Large volcanic eruptions inject massive amounts of particulate material into the stratosphere ; in fact, much or most of the “Junge layer” of stratospheric aerosols in the 20 to 25km region is produced by volcanic eruptions. Several authors have attempted to quantify the climatic effects following volcanic eruptions, mainly the changes in mean temperature, and have shown that volcanic injections do lead to cooling on a hemispheric or global average scale. In this article we review this problem. Physical basis of the problem is also adressed placing primary emphasis on the radiation aspects of the effects of increased aerosol loading in the atmosphere.

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