Hiromitsu Taniguchi

Relationship between crater size and ejecta volume of recent magmatic and phreato‐magmatic eruptions: Implications for energy partitioning

Relationships between crater diameter (D in meters) and ejecta volume (V in cubic meters) of recent magmatic and phreato-magmatic eruptions are expressed as D=0.11 V0.42, and D=0.97 V0.36, respectively. Crater diameters of phreato-magmatic eruptions are ca. 2–5 times larger than those of magmatic eruptions of similar ejecta volume, suggesting that magma-water interaction in phreato-magmatic eruptions generated mechanical energies of crater formation 1–2 orders of magnitude larger than equivalent-volume magmatic eruptions. The conversion ratio of thermal energy to kinetic energy in phreato-magmatic eruptions is estimated to be 0.7–10 percent, which is in accord with the presently available data of laboratory experiments on melt-water interaction [Wohletz and McQueen, 1984].

Entropy dependence of viscosity and the glass-transition temperature of melts in the system diopside-anorthite

Viscosities of diopside-anorthite melts were measured over the wide range of temperature (near the glass-transition temperature−1580°C/1bar) and pressure (5–20 kb/above the liquidus temperature). The measurements were carried out by the fibre-elongation method for low temperature and the counter-balanced sphere method for high temperature at 1 bar, and the sinking and floating spheres method for high temperature at high pressure. Some of the values obtained deviated slightly from those in the literature. The data on viscosity and the glasstransition temperature have been interpreted on the basis of the configurational entropy theory, by which temperature and compositional effects on viscosity were explained well. The configurational entropies at the glasstransition temperature of magmatic silicate melts are almost constant if we use an average molecular weight (amw) or “bead” as a unit; 8.0±1.2 J/K·amw, 1.1 ±0.2cal/K·bead. The latter value coincides well with the value from the literature for organic polymers. The negative deviation from linearity of the glass-transition temperature of intermediate melts may be interpreted as the effect of the mixing entropy. The calculated glasstransition temperature-composition curve using the mixing entropy agreed well with the experimental values.

Some physical properties and melt structures in the system diopside-anorthite

Infrared spectra, thermal expansion, viscosity, and other physical properties in the melt system diopside-anorthite were measured in air at 1 atmosphere. The constituent silicate units in the melts are mainly a sheet-silicate and sorosilicates including a single-chain silicate with a low degree of polymerization, and a three-dimensional silicate is present as a minor component. Small amounts of ortho-silicate are detectable in melts rich in diopside. Generally speaking, the degree of polymerization of the constituent units, the proportion of units of low NBO/T, increases with an increase of anorthite. With the increase of anorthite, the linear thermal expansion coefficients for the glassy state decrease from 9.12 × 10−6 K−1 (diopside) to 5.67 × 10−6 K−1 (anorthite), and for the super-cooled liquid state near the glass transition temperatures also decreases from 80 × 10−6 K−1 (Di) to 26 × 10−6 K−1 (An). With the increase of anorthite, the glass transition temperatures (Tg) increase from 993 K (Di) to 1115 K (An), and the gradient dTg/dN is almost constant on the Di-rich side (An < 30), but it increases sharply on the An-rich side. Free volumes at Tg decrease from 0.210 (vol. frac., Di) to 0.067 (An) with the increase of anorthite. Viscosities in the present measurements are in good agreement with those by Scarfe et al. (1983). The apparent activation energy for viscous flow calculated from the Arrhenius equation clearly showed a maximum or break at the eutectic composition, and the gradients dEη/dN on the Di-rich side are steeper than that on An-rich side. It can be assumed that there are breakpoints near Tg also in composition-thermal expansion coefficients and free volume at Tg relations, respectively. These anomalous changes are believed to be related to the change of melt structure and its free volume.

Densities of melts in the system CaMgSi2O6-CaAl2Si2O8 at low and high pressures, and their structural significance

Density measurements have been carried out on the melt system diopside-anorthite from room temperature to 1600° C at 1 atm, and from 1400° C to 1800° C at pressures up to 20 Kb. The densities were determined based on the dilatometric curve and density at 22° C for lowtemperatures, the double-bob Archimedean method for high-temperatures at 1 atm, and on the sinking and floating spheres method for high-pressure conditions.The results at 1 atm indicate that the thermal expansion coefficient of the glassy state is almost constant, while that of the liquid state decreases with increasing temperature. Density decreased with increasing anorthite content for both glassy and liquid states. Melts in the liquid-state mix ideally with respect to volume, while the glassy state exhibits a maximum excess volume at Di30An70. Density-pressure relations clearly show a density reversion between diopsiderich and anorthite-rich melts; the anorthite-rich melt becomes denser than diopside-rich melt at pressures above 8 kb.The free volumes of both the liquid and glassy states decreased with increasing anorthite content.Isothermal compressibilities and the hard-sphere diameter have been calculated based on the hard-sphere liquid model using thermal expansion coefficients and surface tension data. Calculated compressibilities for diopside-rich melt (Di:>Di60) agreed well with the experimental data, while calculated and observed compressibilities for anorthite-rich melt did not. This evidence indicates that diopside melt may be regarded as a discrete-melt composed of small constituent units (about 10 Å in average diameter) and much interstitial space, while anorthite melt is a three-dimensional network melt with little interstitial space. The critical composition Di60An40 is similar to that of the eutectic and corresponds to breaks between composition and other physical properties. It is proposed that the composition may reflect a kind of “critical state” in the substitution of the “continuous structure” of anorthite melt for the “discrete structure” of diopside melt. The critical state may be interpreted based on the site-percolation theory.

Surface tension of melts in the system CaMgSi2O6-CaAl2Si2O8 and its structural significance

The surface tension between silicate melts and air has been measured for melt compositions lying on the diopside-anorthite (Di-An) join from 1300° C to 1580° C. It ranges from 300 dyne/cm to 400 dyne/cm, and decreases with increasing temperature, except for a pure diopside composition. At relatively high temperatures, the surface tension decreases as the anorthite content increases, whereas at lower temperature it is almost constant. These results suggest that diopside melt has a more discrete structure at higher temperatures, whereas, anorthite-bearing melts do not dissociate in the temperature range studied. They also suggest that the structure of both the surface and interior parts of the melt are almost identical at lower temperatures, but at higher temperatures, the surface part has a more polymerized structure with Al2O3 enrichment. The surface energy, obtained from the relationship between surface tension and temperature, increases from 294 erg/cm2 (Di composition) to 1013 erg/cm2 (Di40An60) with increasing anorthite content.

Universal viscosity-equation for silicate melts over wide temperature and pressure ranges

Abstract The viscous behavior of melts in the system diopside-anorthite has been studied to find a universal viscosity-equation independent of pressure and composition. The present results suggest that the WLF-equation is appropriate: all viscosity-temperature data of six melts along the diopsideanorthite join over a wide temperature range at 0.1 MPa pressure are plotted on a unified curve in accordance with the WLF-equation. For diopside melts, the equation also works well at high pressures up to 2 GPa; it includes the success of estimation of pressure dependence of the glass-transition temperature ( T g ), d T g /d P , using viscosity data both at 0.1 MPa and high pressures. If one has only three constants; T g at 0.1 MPa, the viscosity at T g and the d T / g d P value, it is possible to estimate not only the viscosity-temperature relation but also the pressure dependence for these melts. The configurational entropy theory provides a theoretical background for the WLF-equation in interpreting aspects of the viscous behavior of these melts.

On the volume dependence of viscosity of some magmatic silicate melts.

SummaryDensity and viscosity measurements of three melts of volcanic rock composition (basalt and andesite) at low temperatures were carried out to understand the role of free volume in the viscous behavior of a magma and to estimate the flow unit in the melts. The data combined with literature data suggest the following conclusion: free-volume theory is not applicable to these silicate melts; the relation between viscosity and the inverse of free volume does not yield a straight line in a wide temperature range from the glass-transion temperature to 1550°C. However, two depolymerized melts, diopside and Oki-Dozen alkali basalt (OAB), yield almost linear relationships. Thus, the free-volume theory should hold to a fairly good approximation for these two melts. Based on this approximation, the radius of flow unit for diopside melt was calculated to be about 4.7 Å, and that for Oki-Dozen alkali basalt to be about 4.2 Å. The three-dimensional silicate anions which may correspond to the flow unit are Si14O3514− and Si16O4016− for diopside melt, and Si10O2510− and Si12O3012− for OAB melt. The temperature effect on the initial slope of the viscosity-pressure relation has also been examined in the frame of free-volume theory. It was concluded that the relative increase of the initial slope of the relation with increasing temperature might be caused by the increase of free volume.

Direct measurement of over pressure of a volcanic blast on the June 1991 eruption at Unzen Volcano, Japan

Over pressure of a volcanic blast has been measured for the first time by a lead-plate blastmeter at Unzen Volcano, Kyushu, Japan. The over pressure of the volcanic blast on June 8 was 0.28 bar at 2700 m distance from the crater, and that was less than 0.06 bar at 4400 m distance. The estimated explosive yield of the explosion accompanied by the blast was 1.1 × 107 kg TNT (5 × 1020 erg) calculated based on the over pressure and distribution of the blast effect. The explosion energy is as much as 1/600 of that of the blast at Mount St. Helens on 18 May 1980. The lead-plate of the blastmeter at 2700 m has also recorded the collision with some accretionary lapillis. The maximum particle velocity of the blast was estimated to be about 75 m/sec based on the trace of the collision. The significance of over pressure for the prediction of volcanic hazards is also discussed briefly.

Melting relations of hydrous pyrolite in CaO‐MgO‐Al2O3‐SiO2‐H2O System at the transition zone pressures

Phase relations and melt compositions in CaO-MgO-Al2O3-SiO2-pyrolite under hydrous (+2% of H2O) and anhydrous conditions have been determined at 13–20 GPa and 1600–2220°C. Liquidus and solidus temperatures for the hydrous system are about 50–100°C and 180–240°C lower than those for the dry system, respectively. Majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 GPa. Olivine is a liquidus phase at 13 GPa and both periclase and majorite are the liquidus phases at 20 GPa in the dry pyrolite. We observed expansion of the stability field of anhydrous phase B in hydrous experiments. Compositions of partial melts at 13–20 GPa are generally similar in dry and hydrous systems, but hydrous melts contain more SiO2 at 13–17 GPa. The melts formed by low degree of melting have Al2O3-depleted and CaO-rich compositions. Trends of hydrous melt compositions are generally consistent with those of aluminum-depleted komatiite magmas.

Infrared spectroscopic study of the silicate anionic structures of some magmatic silicate melts

The relation between the species of silicate anions in a silicate melt and their infrared characteristic frequency is discussed. A simple relation approximated with a quadratic equation is established between the ratio of non-bridging oxygens to silicon atoms of a silicate anion and the characteristic frequency. Based on this relation, the silicate anions in some magmatic silicate melts were estimated. The results obtained in the present study agreed well qualitatively with those estimated by some researchers on the basis of other spectroscopic methods.