Age of a tephra can be determined by simple stratigraphy, if adequate number of time-markers are provided. Eleven master tephras are chosen as the time-markers for the last one million years. They are Kikai-Akahoya (7.330 ka), Aira-Tanzawa (26.00 ka), Daisen-Kurayoshi (50.00 ka), Aso-4 (87.00 ka), Ata-Torihama (250.0 ka), Kakuto (340.0 ka), Suiendani-TE5 (420.0 ka), Kobayashi-Sakura (540.0 ka), Kaisyo-Toriitoge (650.0 ka), Shishimuta-Azuki (870.0 ka), and Shishimuta-Pink (1000 ka). The present earth surface and Bruhnes/Matuyama boundary (780.0 ka) play a same role as master tephras. Ages of some master tephras are assigned rather arbitrarily, however, it is productive to affix them once to a specific value. A tephra sandwiched between two master tephras is afforded its age by interpolating the thicknesses of loess between them. This technique, loess-chronometry, has the advantage of ability to measure an interval of tens to thousands years in the geologic past, over radiometric dating. More than 900 tephras are presently recorded and linked each other in a computer database including name, source volcano, age, magnitude, stratigraphy, and remarks. An updated version is listed in WWW at "http://www.la.gunma-u.ac.jp/〜hayakawa/English.html".

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Regional volcanology is defined as the field science to clarify the distributions and activities of volcanoes in time and space, which also gives the ground basis for all volcanological studies including physical volcanology and chemical volcanology. The detailed and trustful distribution of volcanoes in high quality can only be obtained by long-term, tedious field surveys that associate with fine laboratory analyses under feedback systems. The priority of field survey on sustaining high-quality volcanological study may not be reduced in future as well as at present, even if the laboratory techniques develop further more. One of the major purposes of regional volcanology is to visualize the subsurface structure of volcanoes and to clarify the past volcanic history which was in many cases obscured by the successive eruption deposits. Both of these two tasks require drill-hole data and geophysical exploration data in addition to adequate surface geology data. Intensive studies on a single volcano or a volcanic field throughout the synthesis of multidisciplinary data should be the key strategy in future as well as at present. Many hypotheses on the activities of volcanoes have been well examined in the cases where both main constituents of volcanoes and time-and-space distribution are clarified enough. Regional volcanology offers such test fields that contain high-quality geological data to the surrounding fields of science such as quaternary research, structural geology, and tectonics.

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K-Ar and ⁴⁰Ar/³⁹Ar geochronology is useful to solve various problems on the genesis and evolution of magmas and volcanoes. In this review paper, the usefulness of K-Ar and ⁴⁰Ar/³⁹Ar dating in volcanology is discussed, and three different approaches are described to further expand its significance in volcanology. They are 1) voluminous analyses, 2) precise analyses, and 3) accurate analyses in dating very young rocks. Obtaining considerable amount of ages is significantly important in clarifying the detailed growth history of volcanoes or volcanic fields. Systematic dating of regional volcanism should also contribute to construct tectonic evolution of the area. Well established conventional K-Ar dating by isotopic dilution method is to be expanded to significantly increase the age information on volcanoes and/or regional volcanism. Precise age information is particularly important to doccument the duration of volcanism and the detailed history of volcanoes, especially of older ages. The older the target volcanism is, the better resolution ages should be obtained in order to detect the small time difference between different volcanic episodes. Flood basalt volcanism, which fed enormous amount of magmas (10⁵-10⁶ km³) is now revealed to be of short duration (1-2 million years) from the advance of precise ⁴⁰Ar/³⁹Ar dating especially using a laser-fusion technique. This technique would be most promising for the detailed anatomy of volcanism in the future. Increasing demand is arising to date very young volcanic rocks accurately from the viewpoint of future hazard reductions. It is also useful to increase the knowledge how the magma plumbing systems are evolving beneath the active volcanoes. Due to the small accumulation of radiogenic ⁴⁰Ar in very young rocks, it is important to accurately know the initial ⁴⁰Ar/³⁹Ar ratios of samples. ⁴⁰Ar/³⁹Ar ratios in historic lavas are not equal to the atmospheric argon value, which is the basic assumption for the conventional method, but rather are on its mass-fractionation line. A better assumption of initial ⁴⁰Ar/³⁹Ar for age unknown samples is to calculate from analyses of stable ³⁸Ar/³⁶Ar ratios, and the method is called "mass-fractionation correction procedure". This method is not compatible to the addition of artificially concentrated ³⁸Ar spikes in the isotopic dilution method. Three different analytical techniques require three mass-spectrometers designated specially for each method, and are most appropriate for corresponding three geochronological approaches to volcanology. In the future, geochronological information from three different aspects should be tied up more closely with other physical and chemical approaches to integrate the model of magma generation, transport and eruption mechanisms.

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Magma, which buoyantly ascends from its source region to the upper crust, will be trapped at level of neutral buoyancy (LNB) in the upper crust, where density of magma and crust is equal. Since magma is gravitationally stable in the magma reservoir at LNB, decrease in magma density is necessary to restart magma ascent from the magma reservoir. Processes, which can cause density decrease of magma, may include vesiculation due to crystallization, bubble concentration at roof of a magma reservoir, and supply of bubbles from newly injected magmas. Once conduit is open to the surface, balance of load pressures of magma column and crust may control continuation of eruption. Since density of magma decreases effectively with ascent by vesiulation, driving force for eruption is quite dependent on volatile content in a magma. Eruption will terminate when volatile content in the magma decreases enough to lose the driving force, or when eruption conduit is closed by wall collapse. Since H₂O content in silicic magma is larger than mafic magma, it is likely that the former mechanism for termination of eruption is more important in mafic systems, but the latter is more important in silicic systems. Volatile content in an erupting magma is not only dependent on an original content, but also on volatile (including bubbles) accumulation and escape processes in a magma reservoir and conduit system. Volatile loss from ascending magma through a conduit has been proposed as a responsible mechanism to bifurcate an explosive plinian eruption and quiet lava-dome eruption from a volatile-rich magma. Separated two-phase flow of mixture of bubbles and magma can create intermittent emission of bubble-rich and bubble-poor magmas, such as strombolian eruption. Excess degassing is observed during eruption and quiescent stage of volcanic activity. The excess degassing from a quiescent volcano is characterized by continuous, intensive and high-temperature degassing. Those features of the degassing suggest that the degassing occurs from a convecting magma column. Excess degassing during plinian and lava-dome eruption suggests volatile oversaturation in a magma reservoir. Since observation of the excess degassing during eruption is so common, it is likely that bubble accumulation in a reservoir, in particular bubble supply from newly injected basaltic magma, provides driving force to start the eruption. Magma reservoir formed because magma is stable in the reservoir. Therefore, processes in a magma reservoir, such as vesiculation due to magma crystallization or magma mixing, is likely to be a key to understand initiation processes of volcanic eruption. Since magma inevitably loses heat during storage in a magma chamber emplaced in cool crust, crystallization is essential consequence of magma storage in a magma reservoir. However, crystallization process in a magma chamber is still poorly constrained and required further investigations. Although magma mixing could be a simple consequence of eruption from a stratified magma chamber, common observation of evidences of magma mixing not only in products of andesitic eruption but also in silicic ones suggests that magma mixing is a necessary process for the beginning of eruption. If basaltic magma is saturated with CO₂-rich bubbles, injection of basaltic magma beneath a silicic magma can cause bubble supply to the silicic magma to initiate eruption.

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The way to apply isotope geochemistry to reveal the dynamics of magmatic processes is discussed. One method is to reveal the isotopic disequilibrium in a magma. Most isotope studies have been applied to get the information of the magma sources, so samples which are less suffered by shallow magmatic processes, such as crustal contamination, assimilation, etc., are usually selected. However, such processes would be recorded as disequilibrium in a magma. Thus, geochemical disequilibrium between groundmass and phenocrysts, among phenocrysts and in a phenocryst, like zoning, would give the information of shallow magmatic processes. Data on major elements and some trace elements have been used for such purposes, but additional geochemical studies, especially noble gas isotopic studies, would be more powerful and reveal them clearer, because noble gases would be quite sensitive for magmatic processes. The other method is to apply geochemical data to constrain forward models. Geochemical data alone are far from establishing a physical image of the dynamic phenomena. However, they can be transferred through a model to model parameters which have more physical meanings ; melting degree, temperature of melt and size of magmatic bodies, for example. Then, geochemical study can be connected with the geophysical study, which would contribute the clarification of magma dynamics more.

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Seismic tomography has demonstrated three-dimensional images of seismic velocity perturbation in the Earth, which could be a snapshot of convection in the Earth assuming that seismic velocity depends on temperature and chemical compositions. Numerical studies together with tomographic images from seismic studies may make a picture of convection in the mantle clearer. Cold materials from subduction zones flow down to the core-mantle boundary with partial trapping at the phase boundary, and a broad hot counterflow uprises due to internal heat generation. The primitive component could survive due to the partial layering of the mantle convection system. The D″ layer is considered to be a reservoir of the fertile materials enriched in the slab component. Plumes generated by heat flow form the core uprise entraining primitive lower mantle materials, and reach the surface and generate hotspot magmas and LIPs (Large Igneous Provinces) magmas. These plumes have narrow conduit due to their strongly temperature dependent rheology, and are difficult to detect from geophysical observations. Dynamics of the circulation of the chemically distinct materials should be revealed based on realistic numerical models consistent with isotope data set including rare gases and heavy elements, and on secular variation of chemistry of the mantle magma sources in the history of the Earth.

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To understand the field that causes melting within the Earth, the relationship between styles of mantle convection and melting processes in subduction zones is discussed as an example, with special reference to the heat source of magmatism. Numerical experiments on mantle convection show that the subduction zone can be effectively cooled (e. g., down to 600 degree C under a steady state), if a closed convection cell in the upper mantle with standard heat sources is assumed. Additional heat sources such as radionuclides of high concentrations (40 times compared to a depleted mantle) or heat supply from the lower mantle are required to sustain a long-term (e. g., more than 100 m. y.) magmatism in subduction zones. In any case of these experiments, involving large variations in heat balance and mechanical boundary conditions, a downward flow along the subducting plate is dominant, which is compensated by a gently upward flow from the back arc side. In the presence of a small amount of H₂O (i. e., undersaturated), the absence of melting in a pressure range along the upward flow occurs owing to a negative dT/dP of the solid us with amphibole, whereas compression melting occurs in the same pressure range along the downward flow. This behaviour causes the melt to be produced in several separate regions at different depths, and may account for the existence of volcanic chains.

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Magmatism is the consequence of temporal and spatial localization of heat flux from the interior of the earth. Complex behaviors in volcanic activity are, therefore, accounted for by diversities in patterns of such localized energy flux. In general, the mechanisms for the energy flux to localize are governed by non-linear processes, such as Benard convection. Non-linear processes which operate in each elementary igneous process or in interactions between the processes should play an essential role in the complex volcanic phenomena. I expect that physics of those non-linear processes would give us some clues for understandings of the complex behaviors in volcanic systems.

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