Shingo Takeuchi | Central Research Institute of Electric Powor Industry (original) (raw)

Papers by Shingo Takeuchi

Research paper thumbnail of Pre-eruptive Magma Viscosity and a Simplified Estimating Method

Magma viscosity is one of the most important factors controlling the activation of magmatic activ... more Magma viscosity is one of the most important factors controlling the activation of magmatic activity, because the timescale of magma motion is essentially controlled by the balance between viscous resistance and driving forces inside or outside magmas. Recent comprehensive petrological estimates of pre-eruptive magma viscosity and a new simplified method for estimating pre-eruptive melt viscosity are reviewed. Pre-eruptive magma viscosity means the viscosity of phenocryst-bearing magmas in pre-eruptive magma reservoirs. Recent comprehensive viscosity estimates show that basaltic to rhyolitic magmas have preeruptive viscosities over the range of 10 to 10 Pa·s. With increasing bulk SiO2 content, pre-eruptive magma viscosities roughly increase from 10 to 10 Pa·s. However, some andesitic to dacitic magmas have viscosities up to ca. 10 Pa·s. This is due to a large amount of phenocryst (ca. 50 vol %) in andesitic to dacitic magmas. Although bulk SiO2 content has often been used as a quali...

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Research paper thumbnail of Color measurements of volcanic ash deposits from three different styles of summit activity at Sakurajima volcano, Japan: Conduit processes recorded in color of volcanic ash

Journal of Volcanology and Geothermal Research, 2008

... Previous studies have shown that a variety of volcanic materials erupted during Vulcanian eru... more ... Previous studies have shown that a variety of volcanic materials erupted during Vulcanian eruption reflect different eruption histories of magma in conduits (eg, [Taddeucci et al., 2004], [Kennedy et al., 2005], [Clarke et al., 2007] and [Wright et al., 2007]). ...

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Research paper thumbnail of A melt viscosity scale for preeruptive magmas

A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groun... more A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groundmass SiO2 content) is proposed for sub-alkaline magmas. Melt viscosity is controlled by many magmatic properties (e.g., melt composition, melt water content, and temperature); however, these properties are linked by phase equilibrium in preeruptive magmas. In this study, the magmatic properties were investigated by compiling data of phase equilibria experiments performed under preeruptive conditions. Negative correlations are found between melt SiO2 contents and liquidus temperatures, and between liquidus temperatures and melt water contents. Both increasing melt SiO2 content and decreasing liquidus temperature have the effect of increasing the melt viscosity, producing a linear positive correlation between logarithmic values of melt viscosity and linear values of melt SiO2 content. For a specific melt SiO2 content, an increase in liquidus temperature causes a decrease in melt viscosity, whereas a decrease in water content causes an increase in melt viscosity. As a result of this opposing effect, the melt viscosity is strongly correlated with the melt SiO2 content (the correlation coefficient of ∼1). Based on this relationship, an empirical equation predicting logarithmic values of preeruptive melt viscosity is proposed as a linear function of melt SiO2 content, referred to as the melt viscosity scale. The equation reproduces melt viscosities for compiled experimental melts and natural melts with root-mean-square deviation of ∼0.4 and ∼0.5 log units, respectively. This method provides order-of-magnitude estimates for preeruptive melt viscosity. Its strength is in being applicable to examples for which a full dataset of preeruptive magmatic properties is lacking or has large uncertainties (e.g., slowly cooled lavas). The simplicity of this method enables us to easily and promptly estimate preeruptive melt viscosity. Combined with rheological models for multiphase magmas, the present method can be applied widely and thus greatly increase the number of case studies which include evidence-based estimates of preeruptive magma viscosity.

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Research paper thumbnail of Preeruptive magma viscosity: An important measure of magma eruptibility

Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst conten... more Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 10^1 to 10^8 Pa s. Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 10^2 to 10^7 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 10^5 to 10^6 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −0.1. In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼10^6 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >10^6 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <10^6 Pa s induced the following eruption of less eruptible, highly viscous magmas.

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Research paper thumbnail of Degassing conditions for permeable silicic magmas: Implications from decompression experiments with constant rates

The permeability in silicic magmas through connected bubble networks is an important physical pro... more The permeability in silicic magmas through connected bubble networks is an important physical property controlling magmatic degassing. In order to understand the condition of permeable degassing, we performed permeability measurements on vesicular rhyolitic glass products of decompression experiments with constant rates. The rhyolitic melts with ca. 4.7 wt.% water were decompressed at 900 °C from 180 MPa to a final pressure of 30, 16, 10, or 5 MPa with constant rates of 0.05, 0.005, or 0.002 MPa/s. Based on steady-state gas flow measurements for the quenched products, viscous and inertial permeabilities were calculated using the Forchheimer equation. The viscous permeabilities of products with > 80 vol.% total porosity were detectable: the permeability increased up to 10^− 11.7 m^2 with increasing total porosity. In contrast, products with < 80 vol.% were almost impermeable (< 10^− 15 m^2). The rapid permeability increase around 80 vol.% total porosity corresponds to the rapid increase of connected porosity to values similar to the total porosity. This correlation suggests that high permeability occurs in isotropically-vesiculated silicic magmas during decompression under the condition that almost all bubbles in the whole sample interconnect. Using a criterion based on the mass conservation of vertical gas and melt flows, we inferred the degassing conditions for the magmas with the permeability variation and magma ascent velocity simulated in the decompression experiments. The results suggest that silicic magma ascending at 0.08–2 m/s (decompression at 0.002–0.05 MPa/s) does not satisfy degassing conditions at pressures of ≥ 5 MPa. Extrapolating the experimental results to smaller ascent velocity suggests the possibility that degassing conditions can be achieved at a final pressure of 5–10 MPa with a magma ascent velocity of < 10^− 2 m/s (< 10^− 4 MPa/s for the decompression rate); this is consistent with geophysical and petrological estimates of ascent velocities for degassed magmas in non-explosive eruptions (10^− 5–10^− 2 m/s) and for non-degassed magmas in explosive eruptions (10^− 2–10^2 m/s). This suggests that a low magma ascent velocity of < 10^− 2 m/s is required for degassing at shallow levels in the conduits, causing the non-explosive eruption of degassed magmas.

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Research paper thumbnail of Permeability measurements of natural and experimental volcanic materials with a simple permeameter: Toward an understanding of magmatic degassing processes

Permeability measurement of quenched volcanic porous materials is an important approach to unders... more Permeability measurement of quenched volcanic porous materials is an important approach to understand permeability development and degassing of vesicular silicic magmas. In this study, we developed a gas permeameter to measure permeability of natural samples and experimental products. The permeameter has broad measurement ranges of pressure difference (101–105 Pa) and gas-flow rate (10^− 9–10^− 5 m^3/s). These ranges enable us to measure viscous permeability in the range of 10^− 17–10^− 9 m^2 for 1 centimeter-scale samples, using the Forchheimer equation, which includes the inertial effect of gas flow permeating through the samples. In addition, we improved the procedure for performing permeability measurements of mm-sized products of decompression experiments. Although a previous study reported the first permeability data for vesicular silicic glass products of decompression experiments, we found an overestimation in their permeability data due to problems in sample preparation, especially for very low permeability samples. Our improved measurements give lower permeability values than those of Takeuchi et al. (2005)(Takeuchi, S., Nakashima, S., Tomiya, A., Shinohara, H., 2005. Experimental constraints on the low gas permeability of vesicular magma during decompression. Geophys. Res. Lett., 32, L10312 doi:10.1029/2005GL022491).

We also evaluated the inertial effect on apparent permeability calculated using Darcy's law under the conditions of permeability measurement and magmatic degassing. The permeability measurements indicate that the apparent permeabilities of highly permeable samples (> 10^− 10 m^2 in viscous permeability) are affected considerably by the inertial effect under a high-pressure gradient (> 10^5 Pa/m) and reduced by more than 0.5 orders of magnitude from their viscous permeabilities. Also under magmatic conditions, the reduction of apparent permeability by more than 0.5–1.0 orders of magnitude may occur during the degassing of highly permeable magmas driven by a high-pressure gradient.

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Research paper thumbnail of Experimental constraints on the low gas permeability of vesicular magma during decompression

Change in permeability during vesiculation has been measured for vesiculated products from decomp... more Change in permeability during vesiculation has been measured for vesiculated products from decompression experiments. The permeabilities of the experimental products are less than 10^−15.5 m^2 at vesicularity less than 45 vol %, and increase by about 2 orders of magnitude with the vesicularity increase from 45 to 80 vol %. The permeability-vesicularity relationship of the experimental products is quite different from those obtained for natural eruptive materials by previous measurements, which reported a dramatic increase in permeability from 10^−15.5 to 10^−13 m^2 with vesicularity increase from 2 to 40 vol %. The quenched experimental products can be considered as snapshots in vesiculating magma during decompression. Therefore, the permeability variation in vesiculating magma is close to that obtained from the experiments than that obtained from natural eruptive materials, because the latter has experienced complex deformation in the late stages of eruption.

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Research paper thumbnail of Precursory dike propagation control of viscous magma eruptions

Plinian eruptions of phenocryst-rich, viscous silicic magmas have been reported, and in several c... more Plinian eruptions of phenocryst-rich, viscous silicic magmas have been reported, and in several cases the climactic eruptions were preceded by a small eruption of the magmas formed by mixing with more mafic magmas. The eruptive sequence suggests that the less viscous mixed magmas formed a dike prior to the climactic eruption of the viscous silicic magmas. The viscosities of seven sets of precursory and climactic magmas were investigated. All the climactic silicic magmas had viscosities >∼10^6 Pa·s, in contrast to the precursory mixed magmas, which had viscosities one to five orders of magnitude smaller. These viscosity contrasts can be interpreted by adopting a hypothesis of “dike-propagation limit” for magma viscosity, above which the magmas are too viscous to form dikes without freezing. The dike-propagation limit estimated by the dike propagation model of Rubin and the maximum excess pressure retained by the tensile strength of a chamber's wall rock are consistent with the observation of the threshold viscosity of 10^5– 10^7 Pa·s between the precursory and climactic magmas. The consistency of the seven examples with the physical model suggests that precursory dike formation by less viscous mixed magma is one of the effective mechanisms for tapping silicic viscous magma chambers.

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Research paper thumbnail of Role of precursory less-viscous mixed magma in the eruption of phenocryst-rich magma: evidence from the Hokkaido-Komagatake 1929 eruption

During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-ric... more During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing. A similar eruption sequence has been reported for some other eruptions (Pallister et al. 1996; Venezky and Rutherford 1997), suggesting that eruption of a mixed magma is a precursor of phenocryst-rich magmas. For the purpose of understanding the tapping processes of the phenocryst-rich magma chamber, we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography. For the precursory mixed magma we estimate 33±4 vol.% phenocrysts, andesitic–dacitic melt composition, 3 wt.% H2O content, and temperature of 1040°C. In comparison, for the climactic, silicic end-member magma we estimate 48±3 vol.% phenocryst, high-silica rhyolitic melt, 3 wt.% H2O, and temperature of 950°C, respectively. The mafic end-member magma, which was not erupted, is thought to be an almost aphyric basaltic–andesitic magma, based on mass balance calculation of the phenocryst content. The proportion of the mafic end-member magma component in the mixed magma was calculated to be 20–40 wt.%. On the basis of these data, we estimate magma viscosities of 10^3.9, 10^6.9, and 10^2.0 Pa s for the mixed, silicic end-member, and mafic end-member magmas, respectively. The calculated density differences among these magmas are inconsequential when possible errors are considered. We calculate the minimum excess pressure required for dike propagation to be 31 MPa for the silicic end-member magma and 8 MPa for the mixed magma, using the estimated viscosity and dike propagation model of Rubin (1995). If we assume that excess pressure is limited by the wall rock strength of the magma chamber, excess pressure retainable in the magma chamber is less than ca. 20 MPa. This suggests that the mixed magma was able to ascend to the surface without freezing, whereas the viscous silicic end-member magma could not. The formation and precursory eruption of the mixed magma are, therefore, effective and necessary initiation processes for the phenocryst-rich, viscous magma eruption.

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Research paper thumbnail of Pre-eruptive Magma Viscosity and a Simplified Estimating Method

Magma viscosity is one of the most important factors controlling the activation of magmatic activ... more Magma viscosity is one of the most important factors controlling the activation of magmatic activity, because the timescale of magma motion is essentially controlled by the balance between viscous resistance and driving forces inside or outside magmas. Recent comprehensive petrological estimates of pre-eruptive magma viscosity and a new simplified method for estimating pre-eruptive melt viscosity are reviewed. Pre-eruptive magma viscosity means the viscosity of phenocryst-bearing magmas in pre-eruptive magma reservoirs. Recent comprehensive viscosity estimates show that basaltic to rhyolitic magmas have preeruptive viscosities over the range of 10 to 10 Pa·s. With increasing bulk SiO2 content, pre-eruptive magma viscosities roughly increase from 10 to 10 Pa·s. However, some andesitic to dacitic magmas have viscosities up to ca. 10 Pa·s. This is due to a large amount of phenocryst (ca. 50 vol %) in andesitic to dacitic magmas. Although bulk SiO2 content has often been used as a quali...

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Research paper thumbnail of Color measurements of volcanic ash deposits from three different styles of summit activity at Sakurajima volcano, Japan: Conduit processes recorded in color of volcanic ash

Journal of Volcanology and Geothermal Research, 2008

... Previous studies have shown that a variety of volcanic materials erupted during Vulcanian eru... more ... Previous studies have shown that a variety of volcanic materials erupted during Vulcanian eruption reflect different eruption histories of magma in conduits (eg, [Taddeucci et al., 2004], [Kennedy et al., 2005], [Clarke et al., 2007] and [Wright et al., 2007]). ...

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Research paper thumbnail of A melt viscosity scale for preeruptive magmas

A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groun... more A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groundmass SiO2 content) is proposed for sub-alkaline magmas. Melt viscosity is controlled by many magmatic properties (e.g., melt composition, melt water content, and temperature); however, these properties are linked by phase equilibrium in preeruptive magmas. In this study, the magmatic properties were investigated by compiling data of phase equilibria experiments performed under preeruptive conditions. Negative correlations are found between melt SiO2 contents and liquidus temperatures, and between liquidus temperatures and melt water contents. Both increasing melt SiO2 content and decreasing liquidus temperature have the effect of increasing the melt viscosity, producing a linear positive correlation between logarithmic values of melt viscosity and linear values of melt SiO2 content. For a specific melt SiO2 content, an increase in liquidus temperature causes a decrease in melt viscosity, whereas a decrease in water content causes an increase in melt viscosity. As a result of this opposing effect, the melt viscosity is strongly correlated with the melt SiO2 content (the correlation coefficient of ∼1). Based on this relationship, an empirical equation predicting logarithmic values of preeruptive melt viscosity is proposed as a linear function of melt SiO2 content, referred to as the melt viscosity scale. The equation reproduces melt viscosities for compiled experimental melts and natural melts with root-mean-square deviation of ∼0.4 and ∼0.5 log units, respectively. This method provides order-of-magnitude estimates for preeruptive melt viscosity. Its strength is in being applicable to examples for which a full dataset of preeruptive magmatic properties is lacking or has large uncertainties (e.g., slowly cooled lavas). The simplicity of this method enables us to easily and promptly estimate preeruptive melt viscosity. Combined with rheological models for multiphase magmas, the present method can be applied widely and thus greatly increase the number of case studies which include evidence-based estimates of preeruptive magma viscosity.

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Research paper thumbnail of Preeruptive magma viscosity: An important measure of magma eruptibility

Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst conten... more Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 10^1 to 10^8 Pa s. Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 10^2 to 10^7 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 10^5 to 10^6 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −0.1. In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼10^6 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >10^6 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <10^6 Pa s induced the following eruption of less eruptible, highly viscous magmas.

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Research paper thumbnail of Degassing conditions for permeable silicic magmas: Implications from decompression experiments with constant rates

The permeability in silicic magmas through connected bubble networks is an important physical pro... more The permeability in silicic magmas through connected bubble networks is an important physical property controlling magmatic degassing. In order to understand the condition of permeable degassing, we performed permeability measurements on vesicular rhyolitic glass products of decompression experiments with constant rates. The rhyolitic melts with ca. 4.7 wt.% water were decompressed at 900 °C from 180 MPa to a final pressure of 30, 16, 10, or 5 MPa with constant rates of 0.05, 0.005, or 0.002 MPa/s. Based on steady-state gas flow measurements for the quenched products, viscous and inertial permeabilities were calculated using the Forchheimer equation. The viscous permeabilities of products with > 80 vol.% total porosity were detectable: the permeability increased up to 10^− 11.7 m^2 with increasing total porosity. In contrast, products with < 80 vol.% were almost impermeable (< 10^− 15 m^2). The rapid permeability increase around 80 vol.% total porosity corresponds to the rapid increase of connected porosity to values similar to the total porosity. This correlation suggests that high permeability occurs in isotropically-vesiculated silicic magmas during decompression under the condition that almost all bubbles in the whole sample interconnect. Using a criterion based on the mass conservation of vertical gas and melt flows, we inferred the degassing conditions for the magmas with the permeability variation and magma ascent velocity simulated in the decompression experiments. The results suggest that silicic magma ascending at 0.08–2 m/s (decompression at 0.002–0.05 MPa/s) does not satisfy degassing conditions at pressures of ≥ 5 MPa. Extrapolating the experimental results to smaller ascent velocity suggests the possibility that degassing conditions can be achieved at a final pressure of 5–10 MPa with a magma ascent velocity of < 10^− 2 m/s (< 10^− 4 MPa/s for the decompression rate); this is consistent with geophysical and petrological estimates of ascent velocities for degassed magmas in non-explosive eruptions (10^− 5–10^− 2 m/s) and for non-degassed magmas in explosive eruptions (10^− 2–10^2 m/s). This suggests that a low magma ascent velocity of < 10^− 2 m/s is required for degassing at shallow levels in the conduits, causing the non-explosive eruption of degassed magmas.

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Research paper thumbnail of Permeability measurements of natural and experimental volcanic materials with a simple permeameter: Toward an understanding of magmatic degassing processes

Permeability measurement of quenched volcanic porous materials is an important approach to unders... more Permeability measurement of quenched volcanic porous materials is an important approach to understand permeability development and degassing of vesicular silicic magmas. In this study, we developed a gas permeameter to measure permeability of natural samples and experimental products. The permeameter has broad measurement ranges of pressure difference (101–105 Pa) and gas-flow rate (10^− 9–10^− 5 m^3/s). These ranges enable us to measure viscous permeability in the range of 10^− 17–10^− 9 m^2 for 1 centimeter-scale samples, using the Forchheimer equation, which includes the inertial effect of gas flow permeating through the samples. In addition, we improved the procedure for performing permeability measurements of mm-sized products of decompression experiments. Although a previous study reported the first permeability data for vesicular silicic glass products of decompression experiments, we found an overestimation in their permeability data due to problems in sample preparation, especially for very low permeability samples. Our improved measurements give lower permeability values than those of Takeuchi et al. (2005)(Takeuchi, S., Nakashima, S., Tomiya, A., Shinohara, H., 2005. Experimental constraints on the low gas permeability of vesicular magma during decompression. Geophys. Res. Lett., 32, L10312 doi:10.1029/2005GL022491).

We also evaluated the inertial effect on apparent permeability calculated using Darcy's law under the conditions of permeability measurement and magmatic degassing. The permeability measurements indicate that the apparent permeabilities of highly permeable samples (> 10^− 10 m^2 in viscous permeability) are affected considerably by the inertial effect under a high-pressure gradient (> 10^5 Pa/m) and reduced by more than 0.5 orders of magnitude from their viscous permeabilities. Also under magmatic conditions, the reduction of apparent permeability by more than 0.5–1.0 orders of magnitude may occur during the degassing of highly permeable magmas driven by a high-pressure gradient.

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Research paper thumbnail of Experimental constraints on the low gas permeability of vesicular magma during decompression

Change in permeability during vesiculation has been measured for vesiculated products from decomp... more Change in permeability during vesiculation has been measured for vesiculated products from decompression experiments. The permeabilities of the experimental products are less than 10^−15.5 m^2 at vesicularity less than 45 vol %, and increase by about 2 orders of magnitude with the vesicularity increase from 45 to 80 vol %. The permeability-vesicularity relationship of the experimental products is quite different from those obtained for natural eruptive materials by previous measurements, which reported a dramatic increase in permeability from 10^−15.5 to 10^−13 m^2 with vesicularity increase from 2 to 40 vol %. The quenched experimental products can be considered as snapshots in vesiculating magma during decompression. Therefore, the permeability variation in vesiculating magma is close to that obtained from the experiments than that obtained from natural eruptive materials, because the latter has experienced complex deformation in the late stages of eruption.

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Research paper thumbnail of Precursory dike propagation control of viscous magma eruptions

Plinian eruptions of phenocryst-rich, viscous silicic magmas have been reported, and in several c... more Plinian eruptions of phenocryst-rich, viscous silicic magmas have been reported, and in several cases the climactic eruptions were preceded by a small eruption of the magmas formed by mixing with more mafic magmas. The eruptive sequence suggests that the less viscous mixed magmas formed a dike prior to the climactic eruption of the viscous silicic magmas. The viscosities of seven sets of precursory and climactic magmas were investigated. All the climactic silicic magmas had viscosities >∼10^6 Pa·s, in contrast to the precursory mixed magmas, which had viscosities one to five orders of magnitude smaller. These viscosity contrasts can be interpreted by adopting a hypothesis of “dike-propagation limit” for magma viscosity, above which the magmas are too viscous to form dikes without freezing. The dike-propagation limit estimated by the dike propagation model of Rubin and the maximum excess pressure retained by the tensile strength of a chamber's wall rock are consistent with the observation of the threshold viscosity of 10^5– 10^7 Pa·s between the precursory and climactic magmas. The consistency of the seven examples with the physical model suggests that precursory dike formation by less viscous mixed magma is one of the effective mechanisms for tapping silicic viscous magma chambers.

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Research paper thumbnail of Role of precursory less-viscous mixed magma in the eruption of phenocryst-rich magma: evidence from the Hokkaido-Komagatake 1929 eruption

During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-ric... more During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing. A similar eruption sequence has been reported for some other eruptions (Pallister et al. 1996; Venezky and Rutherford 1997), suggesting that eruption of a mixed magma is a precursor of phenocryst-rich magmas. For the purpose of understanding the tapping processes of the phenocryst-rich magma chamber, we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography. For the precursory mixed magma we estimate 33±4 vol.% phenocrysts, andesitic–dacitic melt composition, 3 wt.% H2O content, and temperature of 1040°C. In comparison, for the climactic, silicic end-member magma we estimate 48±3 vol.% phenocryst, high-silica rhyolitic melt, 3 wt.% H2O, and temperature of 950°C, respectively. The mafic end-member magma, which was not erupted, is thought to be an almost aphyric basaltic–andesitic magma, based on mass balance calculation of the phenocryst content. The proportion of the mafic end-member magma component in the mixed magma was calculated to be 20–40 wt.%. On the basis of these data, we estimate magma viscosities of 10^3.9, 10^6.9, and 10^2.0 Pa s for the mixed, silicic end-member, and mafic end-member magmas, respectively. The calculated density differences among these magmas are inconsequential when possible errors are considered. We calculate the minimum excess pressure required for dike propagation to be 31 MPa for the silicic end-member magma and 8 MPa for the mixed magma, using the estimated viscosity and dike propagation model of Rubin (1995). If we assume that excess pressure is limited by the wall rock strength of the magma chamber, excess pressure retainable in the magma chamber is less than ca. 20 MPa. This suggests that the mixed magma was able to ascend to the surface without freezing, whereas the viscous silicic end-member magma could not. The formation and precursory eruption of the mixed magma are, therefore, effective and necessary initiation processes for the phenocryst-rich, viscous magma eruption.

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