Our understanding on volcanic eruptions has been greatly developed in these 15 years by innovative researches beyond the conventional petrology and mineralogy. This special issue contains the reviews of some recent studies on the mechanisms controlling mode of volcanic eruption, especially on dehydration and degassing of hydrous silicic magmas. The reviews have been edited to stimulate the interdisciplinary researches that combine theoretical, experimental and filed approaches. The editor hopes that this issue would help readers to gain a new perspective on the application of the Earth material sciences to the studies on volcanic eruption, although only a part of the related fields is included.

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Water diffusion in silicate melts is a controlling factor of magma ascent process in volcanic eruption, because it dominates the rates of bubble nucleation and growth in magmas and the decrease of water contents by water diffusion causes drastic changes in the physical properties of silicate melts (viscosity and density). This paper reviews experimental studies for water diffusion in rhyolitic to basaltic melts and glasses. In rhyolitic and dacitic melts (polymerized silicate melts), water mainly dissolves as hydroxyl group (OH) and molecular water (H₂O), but only H₂O is considered as the mobile species. The H₂O diffusivities in rhyolite and dacite increase with water contents and temperature. The mechanism of water diffusion in andesite to basalt (depolymerized silicate melts) is still unclear, but some studies presented the data of total water (OH + H₂O) diffusivities in andesite and basalt by assuming the diffusion of one component water. Those data show that the diffusivity of total water increases with melt depolymerization (from rhyolite to basalt) at high temperatures (e.g., > 1400 °C at 0.7 wt% water), but the magnitude relation inverts at the lower temperatures due to the difference of temperature dependences of total water diffusivity in rhyolite to basalt. Recently, a few studies explored the compositional dependence of total water diffusivity and expressed the total water diffusivity as a function of the SiO₂ content or the NBO/T value. To test and develop those models, additional experimental data are needed in the future studies, and the developed-comprehensive model for water diffusion in silicate melts and glasses will make a significant contribution to understanding of volcanic eruptions.

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This paper reviews recent studies on textural analysis of pyroclasts, especially those on degassing in the volcanic conduit. Potential importance of geophysical observations on eruption events inspired by progresses in magma rheology is suggested, and links between quantitative textural analysis and forward approaches from both theory and experiments are discussed.
    Permeability of the fragmented materials may provide useful information to consider permeable flow degassing in the volcanic conduit. Permeability data of pyroclasts reported so far, however, show somewhat dispersed values which can not be fitted by a simple relation with porosity based on a percolation theory. This indicates that geometrical diversities originated form bubble coalescence, deformation and collapse are important to control degassing efficiency. Geological observations of “fossil” conduit have been made, in which brittle behavior of viscous magmas under high shear stresses near the volcanic conduit has been underscored.

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The concept of vesiculation and gas loss of magma during ascent in a conduit, and the results of analytical studies of natural eruptive materials are reviewed in comparison with theoretical and experimental studies. Vesiculation of magma is a complex interplay of nucleation, growth, and coalescence of bubbles. In the recent two decades, theoretical and experimental studies on nucleation and growth of bubbles brought us a lot of information about the vesiculation mechanism by applying simplified models to natural samples. However, there are few works on coalescence and gas loss, which is believed to be the most dominant effect on the diversity in eruption style. In fact, most natural pyroclasts and some experimental products seem to be suffered from bubble coalescence. Thus, we should bear in mind when applying theoretical models to natural samples that estimations for bubble nucleation rate and growth rate without considering bubble coalescence may lead to misunderstanding. However, if we analyze natural samples systematically in terms of volatile content, some physical properties and texture, as well as geological information, it would be possible to reconstruct many processes recorded in the eruptive materials.

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Vulcanian eruptions are relatively frequent eruptive phenomena in island arc volcanism. The high frequency provides us many opportunities to observe the eruptive behavior and to sample the erupted materials. Vulcanian activity of the Sakurajima volcano located at southern Kyushu in Japan has continued since 1955 and been precisely monitored by Sakurajima Volcano Observatory. Therefore, the vulcanian eruptions of Sakurajima volcano can be a common, interesting target to study the eruptive behavior from the various viewpoints of material analysis, geophysical observation and numerical modeling. Recently, many researchers have focused on pressurization processes of magmas in shallow conduits to cause vulcanian eruptions. In this paper, we review several studies on observations and models of generation of excess pressure in shallow conduit magmas just before vulcanian eruptions. We evaluate the possibility of pressurization processes by dehydration-induced crystallization and buoyancy of vesicular conduit magmas. The results of this study indicate that the dehydration-induced crystallization is not effective for excess pressure generation, if crystallizing magma in shallow conduit contains a significant amount of exsolved gas phase. Instead, excess pressure can be generated in a shallow conduit by buoyancy of vesicular conduit magma, when partially solidified magma at the vent of conduit prevents magma ascent and the conduit is filled with vesicular magmas from magma chamber located at ca. 3 km depth to the shallow conduit.

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Permeability data for natural and experimental products of magma that have been reported by several studies are reanalyzed on the basis of percolation theory. I use a power-law relationship between gas permeability k and porosity φ with a critical porosity φ_c and a critical exponent e in the form, k ∝ (φ-φ_c)^e, after reviewing some dependences of φ_c and e on characteristics of porous media. As a result, the k-φ data of silicic magmas from non-explosive eruptions are found in a whole region containing two models of continuum percolation: Swiss-cheese model (φ_c ∼ 0%, e ∼ 4.5) and inverted Swiss-cheese model (φ_c ∼ 30%, e ∼ 2), and no data clearly denotes φ_c much higher than 30%. I also point out that it is necessary to consider careful textural characteristics of porous samples in analyzing the k-φ data in order to assess the process of formation of the texture adequately. Important textural characteristics are i) cross-sectional area of gas channel used for normalizing k because of estimating appropriate values of φ_c and e, ii) elongation of pore and alignment of the direction, and iii) sample scale defined as a ratio of the sample length to a characteristic length of the major axes of elongated pores, because φ_c depends on the degree of the elongation, the alignment and the sample scale.

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This paper reviews theoretical studies on the 1-dimensional steady flow in volcanic conduits. Modeling of 1-dimensional steady flow enables us to understand the relationship between the dynamics of magma ascent and diverse features of volcanic eruptions. The numerical results of the conduit flow model largely change by incorporating various effects such as vertical change of conduit radius, relative velocity between gas and liquid phases, gas escape from magma, magma viscosity change due to vesiculation and crystallization, and magma fragmentation due to gas overpressure in bubbles. The multiple steady solutions in some models explain some observed diversities in the eruption styles for given magma properties and geological conditions. Analytical solutions are important to understand relative importance of the parameters.

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