Kenji Nogami

Reply to comment from Blanco et al. (2015) on “Evidence from acoustic imaging for submarine volcanic activity in 2012 off the west coast of El Hierro (Canary Islands, Spain) by Pérez et al. [Bull. Volcanol. (2014), 76:882–896]

We begin by noting our appreciation for the comment from Blanco et al. (2015) on BEvidence from acoustic imaging for submarine volcanic activity in June 2012 off the west coast of El Hierro (Canary Islands, Spain)^ by Pérez et al. (2014) because it provides the opportunity to maintain an open scientific debate on this issue within the right framework. This is especially important because one of the co-authors of the comment from Blanco et al. (2015) had previously made a suggestion to us that we should not send the acoustic imaging data taken on June 28, 2012, for publication. In our opinion, this recommendation was detrimental to open scientific debate, which is always tremendously beneficial for the development of science. Secondly, the comment from Blanco et al. (2015) suggests that readers may have been confused; we emphasize that the submarine volcanic activity in 2012 off the west coast of El Hierro described by Pérez et al. (2014) was not, as inferred by Blanco et al. (2015), a volcanic eruption. It has been well documented (e.g., Italiano and Nuccio 1991; Caracausi et al. 2005; García et al. 2006; Pérez and Hernández 2007) that new and/or sporadic volcanic activities, such as relatively weak or significant visible degassing processes during volcanic unrest, have commonly occurred both in subaerial and submarine environments of volcanic systems. Such activity includes things that are not a volcanic eruption, which implies release of juvenile volcanic material and not just the sudden release of steam/gas. Pérez et al. (2014) used only acoustic imaging data taken on June 28 as evidence for submarine volcanic activity in 2012 off the west coast of El Hierro. Without this data, it would have been impossible for us to submit our scientific contribution for publication.

Discharge rate of SO2 from Unzen volcano, Kyushu, Japan

The discharge rate of SO 2 from Unzen volcano, which started erupting in November 1990, has been measured with a correlation spectrometer (COSPEC) at a fixed ground-based station about 2.3 km SSW from the summit lava dome. The discharge rate of SO 2 increased greatly after the explosive eruptions on June 8th and 11th, 1991. High discharge rates of SO 2 (an average of 180 ton/day) were observed from July 1991 to January 1992, followed by a general decrease to an average of 110 ton/day during March 1992 and May 1994. The discharge rate of SO 2 correlates with the extrusion rate of magma. Judging from the sulfur inventory, i.e., 130-450 g of discharged SO 2 per ton of erupted magma. and the sulfur content of the extruded lava, the proportion of sulfur degassed from the magma is estimated to be ca. 80 %, suggesting an efficient volatile degassing. The discharge rate of the total volatiles is calculated to be 7,700 ton/day by combining the SO 2 discharge rate with the chemical composition of volcanic gases.

D/H and 18O/16O ratios of water in the crater lake at Kusatsu-Shirane volcano, Japan

Abstract The D/H and 18O/16O ratios of water in the active crater lake situated on the Kusatsu-Shirane volcano, Japan are about 20 and 6‰, respectively, higher than local meteoric water. The ratios show seasonal variations superimposed on a gradual change over nine years. The isotopic ratios started to increase in early 1990 and decrease in the spring of 1995. The seasonal variation which is high in winter and low in summer correlates with the temperature difference between lake water and ambient air. The large temperature difference in winter enhances the evaporation of lake water and produces the enriched isotopic ratios relative to the ratios in summer. The accumulation of snow and the decrease in the flux of meteoric water into the lake strengthens the winter-time isotopic enrichment. The enriched isotopic ratios of the lake water over a long time result from the addition of an end member with heavy isotopic ratios contained in a thermal fluid supplied to the lake. Considering the water balance in the lake, the isotopic ratios of the thermal fluid were found to be close to the lake water itself, suggesting the circulation of the lake water seeping through lake floor. Based on the correlation between Cl−concentration and the isotopic ratios, the contribution by the heavy end member was estimated to be 25–36% relative to the enrichment by evaporation. The heavy end member could be a liquid phase evolved from a parental fluid, which is a mixture of local meteoric water and a magmatic fluid as found in high-temperature volcanic gases.

Temporal variations in the constituents of volcanic ash and adherent water-soluble components in the Unzen Fugendake eruption during 1990-1991

A change in the chemical compositions of volcanic gases is one of the noticeable phenomena that frequently occurs prior to an eruption. Analysis of the water-soluble components adhering to volcanic ash is available for remote monitoring of volcanic gases from inaccessible volcanoes. It is a secure method for monitoring volcanic activity without using particular devices. Prolonged volcanic eruption at the Unzen Fugendake volcano from 1990 to 1995 started with a phreatic eruption after 198 years of dormancy. Volcanic activity changed from a phreatic and phreatomagmatic eruption to a magmatic eruption with pyroclastic flows in May 1991. The relationship between the chemical composition of volcanic ash and the contents of the water-soluble components adhering to it are discussed in relation to the early stage of the long-term eruption. Volcanic ash ejected by phreatic and phreatomagmatic eruption before dome formation was the product of the alteration in the volcanoclastic materials beneath the surface. The ash had a high content of water-soluble components, which was caused by the absorption of hydrogen chloride and sulfur dioxide gases from magma into wet debris before dome formation. Volcanic ashes which were generated by pyroclastic flows after dome formation were fresh lava fragments. While the contents of water-soluble sulfate adhering to the ash noticeably decreased, those of water-soluble chloride adhering to the ash hardly decreased. The considerable decrease in the contents of water-soluble sulfate was caused by the reaction of volcanic gases with dry lava fragments. Contrary to this, the concentration of hydrogen chloride gas in ash clouds was extremely high, which obstructed the decrease in the water-soluble chloride content in the ash. Volatility of chlorine and sulfur from volcanic rock suggests that the inner temperature of pyroclastic flows was higher than 600∼700°C at least.

Behavior of fluorine and chlorine in volcanic ash of Sakurajima volcano, Japan in the sequence of its eruptive activity

At Sakurajima volcano, strombolian eruptions forerun vulcanian explosions, and volcanian explosions are often followed by continuous ash eruptions. Change in the mode of its eruptive activity is drastic and release of volatiles from magma corresponding to its eruptive activity is examined by determination of fluorine and chlorine in volcanic ash. The F contents of all the ash samples are markedly higher than the arithmetic mean value of Japanese volcanic rocks (ca. 300 μg/g). Further, the water-soluble Cl contents of almost all the ash samples are also extremely higher than those of Japanese volcanic rocks (less than 50 μg/g). This is attributed to fixation of F and Cl in hot gas onto the volcanic ash particles in the crater and/or eruption plumes. The differences in those contents among the “strombolian eruption”-, “vulcanian explosion”- and “continuous ash eruption” groups are significant, which implies that hot gas rich in F and Cl are issued during strombolian eruptions, while those halogens are less concentrated in hot gas after strombolian eruptions. The water-insoluble Cl content of the “strombolian eruption” group is significantly higher than those of the other groups. This result implies that Cl and the other volatiles are intensely released from magma during the doming period before vulcanian eruptions.Yet, it is inferred that continuous ash eruption is the final phase of vulcanian one.

The 1997 phreatic eruption of Akita-Yakeyama volcano, northeast Japan: Insight into the hydrothermal processes

A small-scale steam explosion occurred on Karanuma crater on the summit of Akita-Yakeyama volcano on August 16, 1997 after a dormancy of 46 years. Chemical compositions of the fumarolic gases at the summit and hot spring waters around the volcano were monitored before the eruption.Obvious changes in the composition and outlet temperatures of the fumarolic gases were not detected, neither before nor after the 1997 eruption. Hydrogen and oxygen isotopic ratios of the gas condensates and hot-spring waters at the Yunuma crater indicated that a hydrothermal reservoir, where the fumarolic gases separated from the hot-spring waters at 150°C, existed in a shallow place beneath the crater.Smectite, kaolinite and pyrophyllite were identified in the clay fraction of the volcanic ejecta. Although pyrophyllite should have been formed at about 1 km beneath the summit, it was not directly derived from the deep zone during the 1997 eruption but had been ejected by previous eruptions. The Cl/S values of the water leachates of the ejecta were about 0.7, which indicated that the volcanic gas which caused the eruption was rich in HCl. However, the fumarolic gases and the water samples collected from the summit area contained little chloride. The source of the water-soluble chloride might be high-temperature magmatic gases that have been estimated as the source of Cl-SO4 type thermal water. Such magmatic gases might have caused the 1997 eruption.

Interaction between magmatic fluid and meteoric water, inferred from 18O/16O and 36Ar/H2O ratios of fumarolic gases at the Kusatsu Shirane volcano, Japan

Fumarolic gases from the Kusatsu Shirane volcano, Japan were analyzed for the content and isotopic compositions of H2O, the abundances and isotopic compositions of noble gas. The observed δ18O and 36Ar/H2O ratios could not be explained by an existing hydrothermal model in which a mixing between magmatic vapor (MAV) and meteoric water (ASW), a single-step separation to vapor and liquid phases and a subsequent partial H2O vapor removal from the vapor phase have been considered. We constructed a revised model in which two-step mixing-separation and a subsequent partial H2O vapor removal by condensation were considered. In the model, the first step was the mixing between MAV and ASW, and a subsequent separation to a primary vapor and liquid phases. In the secondary step, the primary vapor mixed with another fluid which has a δ18O lower than the primary vapor phase and a low 36Ar/H2O ratio relative to ASW. The mixture separates to secondary vapor and liquid phases. The fluid with low 36Ar/H2O ratio could be ASW partially vaporized in the geothermal area. The secondary vapor ascended and discharged as fumarolic gas. In the ascending process, the secondary vapor suffers a partial H2O vapor removal by condensation in various degrees.

Nature and origin of volcanic ash in the 2000 eruption of Usu volcano, southwestern Hokkaido, Japan

Usu volcano, one of the most active volcanoes in Japan, erupted at regular intervals of ca. 30 years during the 1900s. The 2000 eruption took place on March 31 following 23 years of dormancy. An increase in seismic activity and remarkable ground deformation preceded the eruption. Many small-scale craters were successively formed in two areas at the foot of the volcano and ash was ejected intermittently from them. Ten samples out of the 2000 ash collected around the volcano during half a year were analyzed. They were depleted in SiO2 but enriched in FeO compared with the essential ejecta of the five major historic eruptions and all were highly altered as a whole. XRD analysis revealed smectite as a major clay mineral in the ash and kaolin as a minor one. The clay mineral assemblage and the trend of change in chemical composition of the 2000 ash are quite similar to those of the 1977–1978 ash. No temporal changes in chemical composition and clay mineral assemblage of the 2000 ash were detected. The percentages of the LOI and the relative proportions of SiO2 in the ashes and the essential ejecta showed a linear correlation. These results indicate that the constituents of the 2000 ash were alteration products of the volcaniclastics of the historic eruptions in the subsurface.

Leaching rates of rock-forming components through acidic alteration

Abstract A series of experiments on the interactions of a glassy basaltic andesite with acidic solution in a flowing system were carried out. Leaching indices of seven components (Na, K, Ca, Mg, Fe, Al and Si) were defined to compare the relative leaching behaviour of these components at all reaction stages. At 160 °C and using 0.24 N H 2 SO 4 solution, Na, Ca and Al were leached easily, compared with K, Mg, Fe and Si. Although Si concentrated in the final residual rock, it was not always the hardest component to leach. The leaching behaviour of these seven components in a glassy rock agreed with that in crystalline rock. Results of three experiments at 80, 120 and 160 °C using 0.12 N HCl solution showed that the rise of temperature accelerated the leaching rate of each component, whereas the leaching index for each component was not affected by temperature. This indicated that the leaching processes for these seven components were independent of temperature. On the other hand, in a series of the experiments at 160 °C using 0.24, 0.12 and 0.0024 N HCl solutions to examine the effect of acidity, the change in acidity of the reacting solution affected not only the reaction rate but also the reaction processes of the components. Na, Ca, Fe and Al became hard to leach as acidity diluted and Si became easy to leach and comparable to these components.

Relationship in chemical composition between mother solution and allophane-like aluminosilicate precipitate through neutralization of acid hydrothermal water by seawater

A series of experiments on the formation of allophane-like aluminosilicate precipitates were carried out through neutralization of acid solutions in variety of relative proportions of Si, Fe and Al by seawater. The retrieval and decomposition procedures were available for precise determination of those components in the precipitates. Iron in the mother solutions was completely precipitated and Al was mostly precipitated, however, Si was partly co-precipitated with iron and aluminum. Iron and/or Al coexisting with Si caused a remarkable rise of the precipitated proportion of Si. This was ascribed to co-precipitation of these components on the formation of the precipitates. Difference in effect on the precipitated proportion of Si between Fe and Al was not noticeable. This result indicated equivalence of Fe to Al on the formation of the precipitates through neutralization. The correlations in the (Fe + Al)/Si and the Fe/Al molar ratios between the mother solution and the precipitates strongly designated that the relative proportions of Si, Fe and Al in hydrothermal water discharged from submarine volcanoes were able to be determined uniquely through chemical analysis of the precipitates causing discolored seawater.