Influence of Hydrothermal Recharge on the Evolution of Eruption Styles and Hazards During the 2018–2019 Activity at Kuchinoerabujima Volcano, Japan

doi:10.21203/rs.3.rs-956331/v1

Download PDF

Full paper

Influence of Hydrothermal Recharge on the Evolution of Eruption Styles and Hazards During the 2018–2019 Activity at Kuchinoerabujima Volcano, Japan

https://doi.org/10.21203/rs.3.rs-956331/v1

This work is licensed under a CC BY 4.0 License

Journal Publication

published 30 Jan, 2022

Read the published version in Earth, Planets and Space →

You are reading this latest preprint version

The activity of the 2018-2019 eruption of Kuchinoerabujima Volcano in Japan changed from continuous ejection of ash-laden plumes between October 21 and the middle of December, to intermittent explosive activity accompanied by several pyroclastic density currents until January 2019. To understand the behaviors of magma and hydrothermal fluid that controlled the eruptive sequence, we carried out component analysis, X-ray diffractometry, and leachate analysis for ash samples. The proportion of non-altered volcanic ash particles is ~15 % in the earlier phase, then it decreased to less than 10 % in the later explosive phase. Accordingly, the mineral assemblage of the volcanic ash samples changed from plagioclase-dominant to sulfate minerals-dominant. Concentration of SO₄^2- and Cl/SO₄ values of the ash-leachates decreased toward the later activity. These results indicate that the proportion of fresh volcanic rocks decreased and sulfuric acid fluid-derived sulfate minerals increased toward the later activities. Consequently, the 2018-2019 eruption at Kuchinoerabujima Volcano changed from magmatic activity to phreatomagmatic activity. Weak glowing of the crater was observed during the magmatic activity, indicating the volcanic conduit was hot enough to dry up the subvolcanic hydrothermal system. The following phreatomagmatic activity indicates that the hydrothermal fluid recharged after the magmatic eruption phase. Recharge of the hydrothermal fluid likely caused the variation of the eruption style, and is a process that may control the evolution of hazards during future eruption scenarios at similar active volcanoes in Japan and worldwide.

Planetary Geology

Eruptive sequence

phreatomagmatic eruption

sulfate minerals

ash leachate

hydrothermal alteration

volcanic hazards

Kuchinoerabujuma Volcano

Phreatic and phreatomagmatic eruptions at subaerial volcanoes are often small-scale and are frequently accompanied by explosions and pyroclastic density currents (PDCs). In particular, for volcanoes without a volcanic lake or meteoric water inflow, phreatic and phreatomagmatic eruptions are considered to be triggered by interactions between magma intrusion and hydrothermal fluids or overpressure of a hydrothermal system (Wohletz and Heiken, 1992; Schmincke, 2004; Stix and Moor, 2018). When a magmatic eruption occurs at a volcano with a hydrothermal system, eruption styles become more complicated because they are controlled by ascending magmas and interaction between the magma and hydrothermal fluids (Wohletz and Heiken, 1992; Schmincke, 2004; Stix and Moor, 2018). In such an eruption, volcanic ejecta usually contains hydrothermally altered material which is derived from sub-volcanic hydrothermal alteration zones (Minami et al. 2016). The character and behavior of the hydrothermal fluid and relationship with volcanic activity, however, are often not clear, because direct observation of the sub-volcanic hydrothermal system and/or the hydrothermal fluid during eruption is impossible. Additionally, precursory phenomena of a phreatic or phreatomagmatic eruption such as ground deformation and volcanic tremor are usually unclear until the time of eruption. From the viewpoint of preservation in geological records, the volumes of volcanic ejecta from some phreatic or phreatomagmatic eruptions are quite small (less than VEI <1), making it very difficult to examine the temporal variations of the hydrothermal system, character of the hydrothermal fluid, and relationship with the volcanic activity for past eruptions.

Therefore, to understand an eruptive sequence at such a volcano, it is necessary to understand behaviors both of magmas and hydrothermal fluids. In this study, we focused on the volcanic ash of the 2018–2019 eruption at Kuchinoerabujima Volcano in southwestern Japan, which hosts a hydrothermal system, to understand the interaction between ascending magma and hydrothermal fluids. Component analysis, mineral assemblage description, and ash-leachate analysis were carried out for ash samples collected continuously throughout the 3-month eruption sequence. As a result, this high-resolution investigation revealed that the eruption transitioned from magmatic to phreatomagmatic activity, driven by the interaction between magma and hydrothermal fluids. The eruptive sequence proposed in this study is important for the improvement of understanding the mechanisms of phreatomagmatic eruptions and for efforts to prevent volcanic disasters.

Background of Kuchinoerabujima Volcano

Kuchinoerabujima Volcano is an active volcano located on the northern part the Ryukyu arc, southwestern Japan (Fig. 1). Kuchinoerabujima Volcano has a WNW-ESE elongated conical edifice with ~1.2 km height from the surrounding sea floor. The subaerial part of the edifice forms a volcanic island named Kuchinoerabu-jima, with a length of 12 km and a maximum height of 657 m above the sea level. The population of the island is about 150 in 2010 AD. Most of the inhabitants live in the main village (Honmura), located ~3 km NW of the summit crater of Shindake.

The edifice of Kuchinoerabujima Volcano is a cluster of several volcanic sub-edifices (Geshi and Kobayashi 2007). The subaerial part of the volcanic edifice is primarily composed of andesitic lava and pyroclastic deposits, although small amounts of basaltic lavas are also present. Among them, Shindake and Furudake, situated at the central portion of the island, are the active volcanic centers which have erupted in the Holocene. Furudake is a composite volcanic cone that started to grow around 13 ka. Several sector collapses have divided the edifice of Furudake. Though no historical eruption from Furudake has been recorded, a block and ash flow deposit distributed on the eastern flank, one of the youngest deposits from Furudake, is dated at ~200 years ago (Geshi and Kobayashi 2007). Furudake has a circular crater with a diameter of ~400 m across at the summit.

Shindake is the youngest edifice on the island. The main part of the edifice is comprised of andesitic lava flows which erupted ~1000 years ago (Miki et al. 2002), covered by beds of the volcanic breccia produced by the recent phreatomagmatic–phreatic explosions. Shindake has a circular crater with a diameter of ~300 m across at its summit, and several N-S-trending eruption fissures. All known historic eruptions since the middle of nineteenth century occurred at Shindake (Geshi and Kobayashi, 2007; Geshi and Itoh, 2018).

Active geothermal areas and fumaroles at the summit of Shindake and Furudake indicate the presence of an active hydrothermal system within the volcanic edifice (Shinohara et al. 2011). Frequent phreatic–phreatomagmatic explosive activities have formed thick layers of explosion breccia with many hydrothermally altered components at the summit area of Shindake and Furudake.

Recent eruptive activities of Shindake

The historical eruptions of Shindake, which are limited within the last ~200 years, can be grouped into four active periods: the middle of the 19th century, the 1930s, from 1966 to 1980, and the ongoing activities since 2014 (Geshi and Kobayashi, 2007; Geshi and Itoh, 2018). Because of the scarcity of written records, little is known about the eruptive activity in the 19th century. During the activity of the 1930s, intense explosions damaged facilities of sulfur mines in and around the summit crater of Shindake. An explosive eruption on December 24, 1933 destroyed a village 1.7 km east of the crater, causing 8 deaths and seriously injuring 27 others. The activities of the 1960s-70s eruptions are also characterized by intense phreatic-phreatomagmatic explosions. Ballistic blocks reached up to 3.5 km from the summit crater during the eruption on November 22, 1966. After the 1966 eruption, small eruptions occurred intermittently from the crater of Shindake through the 1970s. The series of eruptions was terminated by a phreatic eruption on September 28, 1980.

The eruptive activity at Shindake resumed with the eruption on August 3, 2014, following a 34-years hiatus. Prior to the resumption of eruptive activity, long-term inflation of the edifice, intermittent rise of seismic activity, and increased fumarolic activity were observed since the 2000s. Three major explosive eruptions and several minor ash emissions were recorded from 2014–2015. The first eruption occurred on August 3, 2014, from newly formed fissures in the eastern and western part of the summit crater of Shindake (Nakamichi et al. 2017). A minor PDC flowed along the western slope of the volcano and reached the coastline ~2.2 km from the crater. The second explosions occurred on May 29, 2015 from the summit crater. Low-temperature PDC flowed down all directions from the summit crater and reached ~3.5 km from the summit crater (Geshi and Itoh, 2018). The third eruption occurred on June 19, 2015 (Nakamichi et al. 2017). After the eruption in June 2015, no major eruption occurred until October 2018.

Sequence of the 2018–2019 activity

The 2018–2019 activity can be divided into four phases (Fig. 2; Sugai et al., 2019). Phase 1 (July 2018 to October 21) is characterized by inflation and increased degassing activity. The inflation was detected by GNSS during July–August, following a gradual deflation after the 2015 eruption (Japan Meteorological Agency, JMA, 2018). Flux of SO₂ increased from <500 ton/day in July to ~1500 ton/day in the middle of August, and then reached ~1700 ton/day in September. The seismic activity beneath the summit crater of Shindake increased since the second week of October. No ash emission was observed in Phase 1 (Fig. 3).

Phase 2 (October 21 to the early November) is characterized by intermittent emissions of ash plumes with intense degassing from the summit crater. The emission of ash-laden plumes from the summit crater started on October 21, followed by intermittent pulses of ash plume emissions with typical intervals between minutes and hours. Height of the ash-laden plumes reached ~1000 m above the crater rim in October–November (Fig. 3). The intensity of seismic activity increased with time. Weak glowing was observed at the summit crater by high-sensitivity monitoring camera from October to the middle of November. The SO₂ flux was 500–1000 ton/day during Phase 2.

Phase 3 (from middle November to December 18) is characterized by a decrease of the SO₂ flux and increase of the seismic and infrasonic activities with intermittent ash-emissions. The plume height increased up to 2100 m above the crater rim at the end of November, and then decreased to less than 1000 m. The SO₂ flux decreased to ~500 ton/day in December. Glowing of the crater was not observed since the middle of November. Seismic activity beneath the summit crater peaked at the middle of November and then decreased. Weak infrasonic waves with the emission of ash plume were detected from November to the first week of December. The inflation of the edifice observed from July ceased in November. The intermittent ash emission ceased on December 13.

Phase 4 (December 18, 2018–February, 2019) is characterized by the occurrence of explosive phreatomagmatic eruptions. Explosive eruptions accompanying ejection of ballistic blocks and strong infrasonic waves occurred at the summit crater on December 18, 28, January 17, 29, and February 2 (Figs. 2 and 3). The eruptions on December 18, January 17 and 29 produced pyroclastic flows (Fig 3). The eruption on January 17 was the largest event during this period. The eruption column rose up to 6 km above the crater, and ballistic blocks reached 1.8 km from the crater. The pyroclastic density currents flowed down to northwestern, southwestern, and eastern flanks and reached maximum 1.9 km from the crater (Fig. 3). Though sporadic minor wildfires were recognized in the area covered by the PDC on January 17, no major forest fires or carbonization of tree was observed in the areas covered by the PDCs, indicating that the temperature of the PDC deposits were lower than the ignition temperature of wood. The SO₂ flux was maintained at <500 ton/day though pulse-like increases of the emission were observed after explosions.

The volcanic ash samples of the volcanic activities during 22 October 2018 to 29 January 2019 were collected from Kuchinoerabujima island within a few hours to a day after the explosions without eluting to rains by Fukuoka Regional Headquarters, JMA and provided to Geological Survey of Japan (GSJ), AIST. The authors selected eleven samples for component analysis, X-ray diffractometry (XRD), and ash-leachate analysis (Table 1).

For characterization of mineral assemblage of ash containing water-soluble components, bulk ash samples were analyzed by the XRD (RINT2500; Rigaku Corp.) installed at GSJ, AIST. Ash samples for XRD were dried under 40 °C for twelve hours and powered by agate mill. A randomly oriented aliquot was prepared for each sample. Measurements were carried out at a rate of 2.0° per minute from 3° to 50° using a CuKα target X-ray tube with an acceleration voltage of 40 kV and filament current of 100 mA. The abundances of minerals in the ash were semi-quantitatively estimated based on peak intensities of the minerals. Mineral abundances in the ashes were estimated from the ratio of peak intensities of silica minerals of quartz, cristobalite, and tridymite, sulfate minerals of gypsum, anhydrite, and alunite, and sulfide mineral of pyrite versus rock-forming mineral of plagioclase.

To estimate the contribution of volcanic gases and hydrothermal fluid, ash-leachates were analyzed based the recommended method of Witham et al. (2005). Raw samples were extracted with Milli-Q grade deionized water at a ratio of 1:50 (water:ash in g). Then, the samples were shaken in an ultrasonic bath for 1 hour. After standing for about 2 hours, the supernatant was filtered through a 0.2 µm membrane filter. The filtrate was measured for F, Cl, SO₄, Na, NH₄, K, Ca, and Mg concentrations using ion chromatography.

Ash samples for the component analysis were washed in distilled water for a few minutes using an ultrasonic cleaner to remove adhering fine particles. After drying at 100°C, the particles were sieved into several classes of grain size using metal meshes. The particles 125 µm in diameter were observed by optical microscope. When a suitable amount of particles with sizes of 250 to 500 µm were obtained, they were used for observation instead. Ash particles were embedded in epoxy resin and polished to expose their interiors.

Component analysis of ash particles were conducted on the polished particles (approximately 80–200 particles for each sample) using elemental maps acquired by the micro X-ray fluorescence instrument (Bruker M4 TORNADO) at GSJ, AIST. Analytical conditions of the X-ray generator were set at 40–50 kV of acceleration voltage and 600 mA tube current for detection of the Kα lines of elements (Al, Si, S, K, Ca, Fe). Conditions for X-ray map acquisition were 2.00–10.00 s/pixel of dwell time, 1–5 cycles and 4 µm step of stage shifting. For detailed classification based on the ash appearance and microtextures, observations of ash particles were conducted using an optical binocular microscope, optical digital microscope (KEYENCE VHX-2000) and scanning electron microscope (SEM; JEOL JSM-6610LV) at GSJ. The SEM was operated with an acceleration voltage of 15 kV.

Minerals in the ashes

The XRD measurement identified minerals contained in the ashes such as silica minerals (quartz, cristobalite, and tridymite), sulfate minerals (gypsum, anhydrite, and alunite), sulfide mineral (pyrite), and plagioclase (Table 2). All ash samples show sharp peaks for cristobalite and plagioclase. Plagioclase and quartz are the only volcanic rock-forming minerals identified in this study. However, the quartz are regarded as hydrothermal in origin because it is not contained in the lava and pyroclastic deposits of the Shindake volcanic edifice where the 2018–2019 eruption vent opened (Geshi and Kobayashi, 2007).

Volcanic ashes of Phase 2 and 3 have similar mineral assemblages with each other, including abundant plagioclase, cristobalite, and anhydrite (Table 2). The Phase 2 ashes contain minor amounts of alunite, whereas the Phase 3 ashes contain minor amounts of gypsum. The ratios of Silica/Pl, Sulfate/Pl and Sulfide/Pl are almost constant in Phase 2 and 3 (Fig. 4).

The mineral assemblage of the ashes varied during the Phase 4 activities (Table 2). The Phase 4 ashes contain plagioclase, quartz, cristobalite, alunite, anhydrite, gypsum, and pyrite. X-ray peaks of plagioclase, anhydrite, and cristobalite are high and those of gypsum, quartz, tridymite, alunite, and pyrite are moderate to minor on average. The mineral assemblage of ashes of Phase 4 are characterized by the large abundance of anhydrite. In particular, the ashes of 18^th December and 17^th January show more intense X-ray peaks of anhydrite and sulfide than those of plagioclase. Therefore, the peak ratios of Sulfate/Pl and Sulfide/Pl are higher than the others (Fig. 4).

Ash-leachates

Compositions of ash-leachates varied throughout the eruption sequence (Table 3). Concentrations of SO₄ were low (< 2 wt.%) in Phase 2 and 3, and increased in Phase 4 with the maximum concentration of 8 wt. % (Fig. 5). In contrast, concentrations of Cl were almost constant during all Phases (Table 3), and the Cl/ SO₄ ratios shows inverse variation compared with the SO₄ concentration (Fig. 5). Concentrations of SO₄ and Cl of ash-leachates commonly agree with those of Ca and Na+K, respectively on a molal basis (Ossaka et al., 1998), indicating their occurrence in the form of CaSO₄ and (Na+K)/Cl on ashes. The Ca/SO₄ ratios are almost 1 in Phase 2 and 3, but decrease to 0.8 and 0.9 in Phase 4 (Fig. 5). In contrast, the ratio of (Na+K)/Cl are around 0.4 in Phase 2, and increased to almost 1 in Phase 4. These variations indicate that the origin of ash-leachate changed. The leachate composition of the ashes from the 2014 and 2015 eruptions of Kuchinoerabujuma Volcano are similar to those of Phase 4 ashes of 2018–2019 eruption (Table 3).

Component analysis of the ash grains

The collected ash samples are composed of non-altered rock fragments and variously altered particles (Table 4). To investigate temporal variation of ash particle components, the ash particles were classified into a non-altered type (Type 1), a weakly altered type (Type 2) and five types of intensely altered particles (Type 3-1 to Type 3-5), based on X-ray intensity maps, optical microscope observations and SEM backscattered electron (BSE) images (Fig. 6). Fragments of crystals were also found in the ashes.

Ash particles showing no traces of alteration were classified as non-altered particles (Type-1; Fig. 6a). These particles commonly display a glassy luster, exhibit translucent gray-color to transparent dark-color with fine patches of opaque minerals, and have angular to sub-angular shapes. Distinct signals of silicon, aluminum, and potassium were detected by X-ray map analysis. The particles have hyalopilitic or intersertal textures and consist of minor plagioclase phenocrysts and volcanic glass. The groundmass contains abundant microlites of plagioclase, pyroxenes, Fe-Ti oxides and silica-minerals. Vesicles are rarely recognized in the interstitial glass.

Partially altered particles (Type-2) are gray- to white-colored translucent particles with sub-angular to angular shapes (Fig. 6b). Features of this type are similar to those of the non-altered particles under the microscope and elemental maps (Fig. 6). The particles are characterized by groundmass textures consisting of fine aggregates of anhedral silica minerals with minor clay minerals, unclear crystal boundaries for microlites and tabular to irregular voids. Original volcanic rock textures are replaced by silica minerals and minor hydrothermal clay mineral(s) as pseudomorphs, although the area of the hydrothermal clay mineral(s) in each particle was much less than those of intensely altered particles. This silica mineral with dominant vugs is considered as the “residual vuggy silica” which is a typical texture of hydrothermally altered rocks (Stoffregen 1987; Sillitoe 2010).

Intensely altered particles (Type 3) show various visual features of white-, yellowish brown-, or dark-colored angular to rounded particles. Intensely altered particles can be subdivided into five subclasses based on their dominant elements as: silicon (Type 3-1), sulfur and calcium (Type 3-2), sulfur and iron (Type 3-3), sulfur (Type 3-4), and others (Type 3-5). When a particle showed heterogeneity in the composition, the particle was classified based on feature of the dominant area. Based on the combination of SEM-EDS and XRD results, these subclasses were identified as: silica minerals (quartz, cristobalite, and tridymite: Type 3-1). sulfate minerals (gypsum, anhydrite, and alunite: Type 3-2); sulfide (pyrite: Type 3-3), fine aggregates of sulfur-bearing minerals (Type 3-4); and others (Type 3-5).

Type 3-1 particles are fine-grained silica minerals with minor plagioclase and clay minerals (Fig. 6c). They are characterized by pseudomorphs with complete replacement of the original phenocrysts of volcanic rock-forming minerals by silica minerals, and by groundmass textures consisting of fine aggregates of anhedral silica minerals, unclear crystal boundaries for microlites, and tabular to irregular voids. These features are considered to represent the typical residual vuggy texture of hydrothermally altered rocks (Stoffregen 1987; Sillitoe 2010). Some Type 3-1 particles are coated by fine aggregates of sulfate and/or sulfide minerals on the surface. Vugs in those particles are often filled by the same materials. The sulfate mineral particles (Type 3-2) typically appear as an aggregate of coarse euhedral sulfate crystals (>100 µm: Type 3-2C; Fig. 6d and Fig. 7c and d) or fine euhedral sulfate crystals (<50 µm: Type 3-2F; Fig. 7g and f). Type 3-2F particles are accompanied by aggregates of fine silica and clay minerals. The sulfide particles (Type 3-3) have a matrix consisting mainly of fine pyrite (>~10 µm) and sulfate minerals with minor silica and clay minerals (Fig. 6e). Fine aggregates of sulfur-bearing particles (Type 3-4) commonly contain minor amounts of gypsum and/or fine anhydrite crystals (Fig. 6f). Type 3-5 particles contain calcium, potassium and aluminum and show variable features under microscopic observation, and SEM-EDS analysis (Fig. 6g, h and i). The elemental association possibly indicates they contain zeolite group minerals and/or clay minerals such as Kaoline group minerals, pyrophyllite, and muscovite, but these minerals were not detected by XRD likely due to their low contents. In addition to these observations, many Type 2 and Type 3 particles from Phase 4 show red coloration. Approximately 5 to 15% of particles from Phase 4 show red coloration, in contrast to the red particles in Phase 2 and 3 which were less than 1%. The color was recognized on particle surfaces and on opaque minerals.

Temporal variations of volcanic ejecta

The presence of non-altered particles (Type 1) is a critical characteristic of the volcanic ashes of Phase 2 and 3 activities (Fig. 8). The volcanic ashes mainly contain non-altered particles and partially altered particles (Type 2). Proportions of the aggregates of euhedral sulfate particles (Type 3-2) and silicic alteration particles (Type 3-1) are less than 10 % and 30%, respectively. All the Type 3-2 particles were the aggregates of coarse euhedral sulfate (Type 3-2C; Fig.7 a–d). The Phase 2 and 3 ashes are abundant in plagioclase with moderate to minor cristobalite, anhydrite, gypsum, and alunite (Table 2). The abundance of plagioclase indicates that the Phase 2 and 3 ashes mainly consist of fresh volcanic rock in accordance with the non-altered and partially altered particles-dominant mineral assemblages (Fig. 4 and Table 2). High values of Cl/SO₄ (1.9–2.0) for the ash-leachates were measured for the ashes from the earliest eruption and these results agree with the low proportion of water-soluble sulfate minerals (gypsum and anhydrite; Table 2 and Fig. 4).

Proportions of the particle types changed markedly in Phase 4 (Fig. 8). Proportions of non-altered particles (Type-1) decreased to less than 2 % with the exception of the volcanic ash of 28 December. In contrast, the total proportion of sulfur-bearing intensely altered particles (Type 3-2, 3-3 and 3-4) increased to 20–50 %. The proportion of silicic altered particles (Type 3-1) are less than 20 % which did not change from Phase 2 and 3. The proportion of weakly altered particles (Type 2) decreased to 30–50 %. The intensely altered particles of Type 3-2 Type 3-3 and Type 3-4 are dominant particle types in Phase 4. In contrast to those in Phase 2 and 3, almost all the Type 3-2 particles in this phase were the aggregates of fine euhedral sulfate (Type 3-2F; Fig. 7e–h). Sulfide dominant particles (Type 3-3) and sulfur-bearing particles (Type 3-4) also increased from late Phase 3 (6^th Dec. 2018) to Phase 4 (Fig. 8). The increase of these particles indicates that sulfide (pyrite) crystallization became dominant. During the same period, fine sulfate crystals (Type 3-2) also increased in abundance. The volcanic ashes of Phase 4 have sulfate and silica dominant mineral assemblages and high concentrations of SO₄ and low levels of Cl/SO₄ for the ash-leachates, indicating that these ashes contain abundant water-soluble sulfate minerals (Figs. 4 and 5). These results agree with the high proportion of intensely altered particles and low proportion of non-altered particles (Fig. 8). The volcanic ash of the eruption on 28 December is an exception of Phase 4 activity. They contain nearly 10% of non-altered particles and have a plagioclase dominant mineral assemblage (Figs. 4 and 8).

Origin of ash particles

Volcanic ash from the 2018–2019 eruption at Kuchinoerabujima Volcano consists of various particle types, which vary in abundance throughout the eruptive phases, suggesting changes in the particle origins. In this section we discuss the origins of each particle type and their relationship to the eruptive phases.

The non-altered ash particles (Type-1) are characterized by abundant plagioclase, pyroxene, Fe-Ti oxides with minor plagioclase phenocrysts. Groundmass textures are commonly hyalopilitic or intersertal (Fig. 6). The vitric, blocky, and poorly vesiculated features of the non-altered particles agree with the general features of vulcanian eruption products (Morrissey and Mastin, 2000) and are similar to juvenile materials of the 2014–2015 eruption at Kuchinoerabujima Volcano (Geshi et al., 2016). These features suggest that the non-altered ash particles originated from a newly intruded magma during the 2018–2019 eruption. The blocky shape and luck of obvious pumice and particles with fluidal form suggest that the non-altered particles were derived from a partially solidified magma.

Partially altered particles (Type-2) whose alteration textures were recognized only in groundmass, were possibly derived from the volcanic edifice or the newly intruded magma head, because they also exhibit hyalopilitic or intersertal textures which are similar to the non-altered particles (Fig. 6). The alterations characterized by vuggy silica and crystallization of silica minerals in the groundmass indicate selective alteration of low resistance groundmass. These selective alteration textures suggest that reaction between original volcanic rock and hydrothermal fluids was short enough to leave microlites and phenocrysts unaltered. Observed alteration minerals were mostly silica minerals while clay minerals were minor. This mineral assemblage corresponds to the “residual silica zone” which indicates alteration by hydrothermal fluids with a strong acidity (Stoffregen, 1987; Nogami and Yoshida, 1993; Hedenquist et al., 2000 Sillitoe, 2010). Therefore, the alteration of the partially altered particles was mainly caused by cation leaching with strongly acidic hydrothermal fluids for a short period.

Silica minerals-dominant intensely altered particles (Type 3-1) have a similar groundmass texture with the partly altered particles. Type 3-1 particles are characterized by pseudomorphic silica minerals and abundant vugs. These features indicate a sustained leaching by strongly acidic hydrothermal fluids, and correspond to features of the “residual silica zone”, which is a common alteration type in a volcanic-hydrothermal system (Sillitoe, 2010). This evidence indicates that Type 3-1 particles are derived from a hydrothermally altered zone (residual silica zone) formed prior to the present eruptive activity.

Type 3-2 particles consist of abundant sulfate minerals (Fig. 6d), sometimes with minor amounts of clay minerals (Fig. 7). The dominance of sulfate minerals indicates alteration by sulfate-acid hydrothermal fluids (Reed, 1997) such as under the conditions of an “advanced argillic alteration zone” which commonly contains clay minerals of kaolinite and pyrophyllite (Sillitoes, 2010). Minor amounts of clay minerals in Type 3-2, however, do not agree with its feature. The Type 3-2C particles are commonly observed during Phase 2 and 3, when alunite and anhydrite are dominant sulfate minerals. The coarse euhedral shapes indicate that these sulfate crystals freely grew in sulfate saturated hydrothermal fluids. On the other hand, the fine sulfate crystals (Type 3-2F) suggest a high nucleation rate indicating that Type 3-2F particles formed under a higher supersaturation degree than that for Type 3-2C (Otálora and García-Ruiz, 2014). The high degree of supersaturation for Type 3-2F was likely achieved by a rapid evaporation of sulfate-acid hydrothermal fluids. Therefore, Type 3-2F particles were directly derived from sulfate saturated hydrothermal fluid before the development of an advanced argillic alteration zone.

The matrix of Type 3-3 particles consists of fine pyrite and sulfate minerals with minor silica and clay minerals (Fig. 6). This assemblage is similar to Type 3-1 particles, which are dominated by silica minerals and accompanying sulfide minerals in vugs, on the rims of particles, and on crystal boundaries between silica and clay minerals in groundmass. These cutting relations indicate that sulfide crystallization followed silicification alteration, which suggests that Type 3-3 particles are formed by accretion of Type 3-1 particles with sulfide precipitation.

Type-3-4 particles commonly appear as silica minerals with abundant vuggy altered materials coated with fine aggregates of sulfur-bearing minerals (Fig. 6f). The texture of Type 3-4 particles is similar to Type 3-1 particles. The precipitation of sulfate and sulfide minerals in vugs and on the surface of the particles indicates that the sulfate and sulfide mineral crystallization followed the silicic alteration.

Type 3-5 particles possibly contain clay minerals (Fig. 6g–i), albeit in notably low amounts (Table 2 and 4), when compared with the products of phreatic or phreatomagmatic eruptions from sub-volcanic hydrothermal systems that commonly contain abundant clay minerals (e.g., Ohba 2011, Minami et al., 2016). The low proportion of these clay minerals may indicate the absence of a mature hydrothermal alteration zone around the conduit and that the volcanic ashes were mainly derived from an immature altered zone such as a partly altered magma head.

Eruptive sequence of Kuchinoerabujima 2018–2019 eruption

In the following section, we discuss the correlations between volcanic ash componentry and the monitoring data throughout the eruption sequence (Fig. 2). From July to the middle of October 2018 (Phase 1), SO₂ flux increased, and edifice inflation had gradually progressed. These monitoring data may imply that magma was ascending to a shallow level at that time. Phase 1 can be interpreted as a precursory phase of the 2018–2019 eruption (Fig. 9).

Absence of a hydrothermal system beneath the crater during Phase 2 is suggested based on the following reasons. Regular but low volume ash emission during Phase 2 implies minimal or no interaction between magma and a hydrothermal system. Observation of the glowing crater in Phase 2 indicates a high-temperature for the vent. The low proportion of ash particles derived from hydrothermal fluids (Type 3-2F), and the high proportion of ash particles derived from the intruding magma head (Type 1 and 2) and the pre-existing hydrothermally altered zone (Type 3-1 and 3-2C) imply that there was minimal or no interaction with a hydrothermal system (Fig .9). Lesser abundance of water-soluble sulfate in Phase 2 indicates the absence of a dense fluid phase and implies the lack of a hydrothermal system (Figs. 4, 5, and 9). Phase 2 activity was characterized by almost continuous ash emission (Fig. 2). The dominance of non-altered and partially altered particles (Type 1and Type 2; Fig. 6),high flux of SO₂ gas (>500 t/day; Fig. 2), and the presence of a volcanic glow indicate that ascending magma drove the eruptive behavior during Phase 2 (Fig. 9). Altered particles collected during Phase 2 are primarily Type 3-1 and Type 3-2C, suggesting that most of the altered materials were derived from the shallow part of a pre-existing hydrothermal alteration zone. During the continuous ash emission and volcanic glow, a stable hydrothermal system cannot exist and the coarse euhedral sulfate needs to be derived from a pre-existing hydrothermal altered zone. Therefore, the volcanic activities of Phase 2 can be interpreted as magmatic (Vulcanian) or phreatomagmatic explosions which were probably induced by a shallow intrusive magma head.

Although the intermittent ash emission continued in a similar style, some changes in monitoring data were observed during Phase 3 (Fig. 9). The inflation of the volcanic edifice paused, SO₂ emission decreased (~500 ton/day: Fig. 2), and glowing of the crater was not observed during Phase 3. These changes may indicate that the amount of the ascending magma decreased, leading to a decrease of the non-altered particles after the last day of the intermittent ash emission on December 13 (Phase 4: Fig. 8). The decrease of non-altered and partially altered particles and increase of highly altered particles (Type 3) in Phase 4 indicates that hydrothermal alteration occurred during the interval between the last ash emission of Phase 3 (December 13) and the first explosion of Phase 4 (December 18; five days).

Phase 4 activities were characterized by explosive eruptions accompanied by pyroclastic density currents and infrasound. These explosive activities were likely caused by the interaction between hot magma and a liquid phase hydrothermal fluid, as per the traditional explanation of phreatomagmatic eruptions (Schmincke, 2004). The hydrothermal fluids did not exist during Phase 2 and recharged during Phase 3. Although a mechanism for recovery of the hydrothermal fluids is not clear, the edifice deflation during Phase 3, which was likely caused by the pressure decrease around the conduit, may have resulted in infiltration of groundwater near the conduit to form a hydrothermal system (Fig. 9).

The recovered hydrothermal fluid may be a sulfate-rich brine as indicated by the mineral assemblage and leachate composition, which is characterized by low Cl/SO₄ values (< 0.3) and high SO₄ concentrations (8 wt. % in maximum; Fig. 5 and Table 3). Large volume ratios of sulfate minerals to plagioclase up to 9 were observed during Phase 4 (Fig. 4). These sulfate minerals are mostly gypsum and anhydrite (Fig. 4 and Table 2) and appeared as fine aggregates of sulfate minerals (Type 3-2F; Fig. 7g and h). This occurrence suggests that the fine sulfate and sulfide minerals precipitated directly from a saline hydrothermal fluid (Type 3-2F, Type 3-3 and Type 3-4; e.g. Zimbelman et al., 2005) and indicate that the hydrothermal fluid become sulfate- and sulfide-saturated after late Phase 3 (6 December 2018). The low percentage of non-altered particles may indicate the amount of new magma intrusion decreased and the eruptive activity changed from magmatic to more hydrothermal. Precipitation of the abundant sulfate minerals may have causeed choking of the volcanic conduit (e.g., Mick et al., 2021), resulting in a change of eruption style from continuous ash emission (Phase 2 and 3) to intermittent explosive eruptions with repeated sealing and breaking of the seals (Phase 4: Fig 9).

The volcanic ash of the eruption on 28 December is an exception of Phase 4 activity (Fig. 2). They contain nearly 10% non-altered particles with plagioclase as the dominant mineral assemblage (Figs. 4 and 8). This implies that a new magma intruded again around 28 December.

Effect of hydrothermal fluid for eruptive sequence and hazard implication

As we discuss above, the 2018–19 eruption of Kuchinoerabujima Volcano underwent an evolution in style from activity dominated by magma ascending at the beginning (Phases 1, 2 and 3) to eruptions dominated by interaction between magma and hydrothermal fluids (Phase 4: Fig. 9). This change can be explained by a reduction in the magma supply from Phase 2 to Phase 4. The hydrothermal system recovered after Phase 3 when the magma head descended, and the interaction between magma and hydrothermal fluid resulted in intense explosive activities during Phase 4. The Phase 4 activities were accompanied by several pyroclastic density currents, the largest of which occurred on 17 January 2019 with a runout distance of 1.9 km from the crater and 500 m short of the densely populated area of Mukaehama. It is an important finding from the viewpoint of volcanic disaster prevention that such violent volcanic phenomena can occur in the later phase of an eruption sequence. This study highlights the role that interaction between magma and hydrothermal fluids plays in controlling the eruption style. Therefore, undertaking detailed observation of the alteration products in volcanic ash is important for monitoring variations in the degree of interaction between shallow magma and a hydrothermal system, and therefore for forecasting changes in eruption style during eruption scenarios.

Hydrothermal system recovery has also been observed in the 1931–1945 and 1966–1980 eruptions of Kuchinoerabujima Volcano (Geshi et al., 2016) and some other volcanoes (e.g., the 2000 eruption of Usu Volcano: Ui et al., 2002; Tomiya and Miyagi, 2002; the 2016 eruption of Aso Volcano: Morita 2018). Transitions of eruption styles from magmatic to phreatic have also been interpreted from geological records at many volcanoes (e.g., Chokai Volcano 1801 eruption: Minami et al., 2019; Tokachidake Volcano 1962 eruption: Uesawa 2014; Okmok Volcano, Alaska: Wong and Larsen, 2010; Cape Verde archipelago: Tarff and Day, 2013). These examples suggest that recovery of hydrothermal fluids during an eruption sequence is a common phenomenon at active volcanoes.

Temporal changes to the componentry of volcanic ashes emitted during the 2018–2019 eruption at Kuchinoerabujima Volcano in Japan were investigated to understand the behaviors of magmas and hydrothermal fluids within the shallow conduit of the edifice. The eruption produced continuous ash-laden plume emission during Phase 2 and 3, then the eruptive activity varied to intermittent explosive activity with several pyroclastic density currents in Phase 4. The proportion of non-altered and partially altered ash particles indicative of magmatic materials was relatively high in Phase 2 and 3, then it decreased in the later explosive Phase 4. Accordingly, mineral assemblages of the volcanic ashes changed from plagioclase dominant to sulfate minerals dominant. The concentration of SO₄^2- and Cl/SO₄ values of the ash-leachate samples decreased toward the later activity. These observations show that the 2018–19 eruption of Kuchinoerabujima Volcano shifted from magmatic activities dominated by ascending juvenile magma at the beginning to phreatomagmatic activities driven by the interaction between magma and hydrothermal fluids towards the end of the sequence. Recovery of hydrothermal fluid in the upper conduit region during the eruption sequence caused the later phreatomagmatic activities. This case study serves as a pertinent recommendation that real-time description and interpretation of volcanic ash samples collected during an eruption scenario is a critical task for understanding and forecasting changes to magma-hydrothermal fluid interaction dynamics that can prevent volcanic disasters.

AIST: National Institute of Advanced Industrial Science and Technology; JMA: Japan Meteorological Agency; PDC: Pyroclastic density current; GSJ: Geological Survey of Japan; SEM-EDS: scanning electron microscope with energy-dispersive X-ray spectroscopy; XRD: X-ray diffractometer; GNSS: Global Navigation Satellite System

Ethics approval and consent to participate

Not applicable

Consent for publication

Not applicable

Availability of data and materials

The all of datasets used in this study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests.

Funding

This study received no specific grant from any funding agency.

Authors' contributions

YM conceived the study and carried out the XRD analysis, the SEM–EDS analysis, and drafted the manuscript. KM described volcanic ash particles using optical microscope, micro X-ray fluorescence instrument, and SEM-EDS. NG drafted geology of Kuchinoerabujima Volcano and arranged JMA negotiation. HS carried out chemical analysis of water-soluble species of volcanic ashes. All authors read and approved the final manuscript.

Acknowledgements

We gratefully acknowledge the staff of Fukuoka Regional Headquarters, JMA who collected and provided ash samples. We would like to thank Dr. Chris Conway of Geological Survey of Japan for English language editing. The discussion with Dr. Junichi Itoh and Dr. Teruki Oikawa of Geological Survey of Japan were helpful for drafting.

Geshi N, Kobayashi T, (2007) Geological Map of Kuchinoerabujima Volcano (1:25,000). Geological Map of Volcanoes 14, Geological Survey of Japan, AIST. (in Japanese with English abstract)

Geshi N, Iguchi M, Shinohara H, (2016) Phreatomagmatic eruptions of 2014 and 2015 in Kuchinoerabujima Volcano triggered by a shallow intrusion of magma. Journal of Natural Disaster Science 37:67-78

Geshi N, Itoh J, (2018) Pyroclastic density currents associated with the 2015 phreatomagmatic eruption of Kuchinoerabujima volcano. Earth Planet Space 70:119. doi: https://doi.org/10.1186/s40623-018-0881-x

Hedenquist JW, Arribas A, Gonzalez-Urien E, (2000) Exploration for epithermal gold deposits. Rev. Econ. Geol. 13:245-277

Japan Meteorological Agency, (2018) Major eruptive activities in Japan in 2018, in “Monthly report on earthquakes and volcanoes in Japan, November 2018” 3pp.

Mick E, Stix J, de Moor M, Avard G, (2021) Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering. Journal of Volcanology and Geothermal Research. 416:107297

Miki D, Iguchi M, Eto T, Solihin A, (2002) Paleomagnetic ages of the lava flows of Shindake, Kuchinoerabujima. DPRI, Kyoto Univ. 159-168 (in Japanese)

Minami Y, Imura T, Hayashi S, Ohba T, (2016) Mineralogical study on volcanic ash of the eruption on September 27, 2014 at Ontake volcano, central Japan: correlation with porphyry copper systems. Earth Planet Sp 68, 67 (2016). https://doi.org/10.1186/s40623-016-0440-2

Minami Y, Ohba T, Hayashi S, Saito-Kokubu Y, Kataoka KS, (2019) Lahar record during the last 2500 years, Chokai Volcano, northeast Japan: Flow behavior, sourced volcanic activity, and hazard implications. J Volcanol Geotherm Res 387:106661 doi: https://doi.org/10.1016/j.jvolgeores.2019.106661

Morita M (2019) Temporal variations of plume activites before the 8 October 2016 eruption of Aso volcano, Japan, detected by ground-based and satellite measurements. Earth Planets Space 71:7 doi: https://doi.org/10.1186/s40623-019-0986-x

Morrissey MM, Mastin LG, (2000) Vulcanian eruptions. In: Sigurdsson H. (ed.) Encyclopedia of Volcanoes. Academic Press. San Diego, 463-476.

Nakamichi H, Iguchi M, Taneguri T, Sonoda T, (2017) Quantification of seismic and acoustic waves to characterize the 2014 and 2015 eruptions of Kuchinoerabujima Volcano, Japan. J Natural Disaster Sci 38: 65-83

Nogami K, Yoshida M, (1993) Leaching process of rock forming components through acidic alteration. Jour. Chem. Soc. Japan. 3:251-258

Ohba T, (2011) hydrothermal mineral-bearing volcanic products: Relationship with subvolcanic hydrothermal systems, and styles and patterns of their formation. Jour. Geol. Soc. Japan. (117)344-356 (in Japanese with English abstract)

Ossaka J, Nogami K, Hirabayashi J, (1998) Water-soluble components on volcanic ash from Tokachi-dake Volcano, Japan during the 1988-1989 eruption. Bull Volcanol Soc Japan 43: 25-31 (in Japanese with English abstract)

Otálora F, García-Ruiz JM, (2014) Nucleation and growth of the Naica giant gypsum crystals. Chem. Soc. Rev. 43:2013-2026

Sillitoe RH (2010) Porphyry Copper Systems. Economic Geology 105:3-41. doi: 10.2113/gsecongeo.105.1.3

Stoffregen R (1987) Genesis of acid-sulfate alteration and Au-Cu-Ag mineralization at Summitville, Colorado. Economic Geology 82:1575-1591. doi: 10.2113/gsecongeo.82.6.1575

Stix J, de Moor JM (2018) Understanding and forecasting phreatic eruptions driven by magmatic degassing. Earth Planets Space 70, 83. https://doi.org/10.1186/s40623-018-0855-z

Sugai A, Shinohara E, Nakamura M, Mori T, (2019) Eruptive activities of Kuchinoerabujuma volcano after 2018. In Abstracts of the Volcanological Society of Japan the 2019 fall meeting, Kobe University, Kobe, 25-27 September. 2019 [in Japanese]

Tomiya A, Miyagi I, (2002) The Eruptive Products and Magma Process of March 31, 2000, eruption of Usu volcano. Bull. Volcanol. Soc. Japan 47:663-673. (in Japanese with English abstract)

Ui T, Nakagawa M, Inaba C, Yoshimoto M, Geological party, Joint Research Group for the Usu 2000 Eruption, (2002) Sequence of the 2000 Eruption, Usu Volcano. Bull. Volcanol. Soc. Japan 47:105-117. (in Japanese with English abstract)

Tarff RW, Day SJ, (2013) Chilled margin fragmentation as a trigger for transition from Strombolian to phreatomagmatic explosive activity at Cova de Paul Crater, Santo Antao, Cape Verde Islands. Bull Volcanol 75:735 doi: 10.1007/s00445-013-0735-0

Uesawa S (2014) A study of the Taisho lahar generated by the 1926 eruption of Tokachidake Volcano, central Hokkaido, Japan, and implications for the generation of cohesive lahars. J Volcanol Geotherm Res 270:23-34

Witham CS, Oppenheimer C, Horwell CJ, (2005) Volcanic ash-leachates: a review and recommendations for sampling methods. J Volcanol Geotherm Res 141:299-326

Wohletz K, Heiken G, (1992) Volcanology and geothermal energy. University of California Press

Wong LJ, Larsen JF, (2010) The Middle Scoria sequence: A Hoocene violent strombolian, subplinian and phreatomagmatic eruption of Okmok volcano, Alaska. Bull Volcanol 72:17-31 doi: 10.1007/s00445-009-0301-y

Zimbelman DR, Rye RO, Breit GN, (2005) Origin of secondary sulfate minerals on active andesitic stratovolcanoes. J Volcanol Geotherm Res 215:37-60

Table 1 Sample collection information.

Sample ID	Date of eruption	Time of eruption onset	Comment	Sampling time		Locality	Direction from the crater
				From	To		Direction from the crater
18-Keb02-4	Oct. 21, 2018	18:31		n.d.	23rd Oct. 17:30	Nemachi	NE, 2 km
18-Keb03-1	Oct. 22, 2018 Oct. 25, 2018	from 15:19, Oct. 22 to 15:21, Oct. 25; from 16:21, Oct. 25 to 17:30, Dec. 13, 2018		Oct. 23^rd 16:00	26^th Oct. 13:16	Tashiro	NNW, 3 km
18-Keb07-2	Nov. 5, 2018	14:20		n.d.	5^th Nov. 15:20	Maeda	NW, 2.5 km
18-Keb09	Nov. 12, 2018	8:30–9:30 (two explosions)		11^th Nov.	12^th Nov. 9:40	Tashiro	NNW, 3 km
18-Keb12-1	Nov. 20 and 21, 2018			20^th Nov. 10:00	21^st Nov. 14:10	Honmura	NW, 3 km
18-Keb15	Dec. 6, 2018			6^th Dec. 10:00	6^th Dec. 16:00	Yumugi	ENE, 3 km
18-Keb17	Dec. 18, 2018	16:37	PDC	18^th Dec. 13:30	20^th Dec. 8:30	Tashiro	NNW, 3 km
18-Keb19-1	Dec. 28, 2018	22:09		n.d.	29^th Dec. 11:30	Nakama, Yakushima-cho	SW, 30 km
19-Keb03-3	Jan. 17, 2019	9:19	PDC	n.d.	18^th Jan. 9:00	Furudake	SSE, 2 km
19-Keb06	Jan. 20, 2019	20:51		18^th Jan. ca. 8:00	22^nd Jan. 14:30	Nanakama	SE, 1.5 km
19-Keb09-2	Jan. 29, 2019	17:13	PDC	n.d.	30^th Jan. 14:00	Nanakama	SE, 1.5 km

Table 2 Minerals in the volcanic ash of the 2018–2019 eruption at Kuchinoerabujima Volcano.

Sample ID

Date of eruption

Qtz

Cri

Tri

Alu

Gyp

Anh

Pyr

Silica/Pl

sulfate/Pl

sulfide/Pl

18-Keb02-4

Oct. 21, 2018

***

**^†

n.p.

0.5

0.2

0.0

18-Keb03-1

Oct. 22, 2018
Oct. 25, 2019

***

*^†

n.p.

***

n.p.

1.0

0.7

0.1

18-Keb07-2

Nov. 5, 2018

***

n.p.

***

**^†

n.p.

0.6

0.3

0.0

18-Keb09

Nov. 12, 2018

***

n.p.

***

**^†

n.p.

1.0

0.5

0.1

18-Keb12-1

Nov. 20 and 21, 2018

***

*^†

n.p.

0.9

0.4

0.1

18-Keb15

Dec. 6, 2018

***

n.p.

1.1

0.6

0.1

18-Keb17

Dec. 18, 2018

***

3.3

8.1

0.5

18-Keb19-1

Dec. 28, 2018

***

n.p.

1.4

0.7

0.1

19-Keb03-3

Jan. 17, 2019

***

n.p.

***

2.0

6.1

0.4

19-Keb06

Jan. 20, 2019

***

1.9

9.0

0.5

19-Keb09-2

Jan. 29, 2019

***

n.p.

***

n.p.

0.9

1.1

0.1

The intensities of X-ray peaks in the XRD analysis are shown as: ***: intense; **: weak; *: minor; and n.p.: no peaks were determined.

Mineral abbreviations; Pl: plagioclase including anorthite and albite; Trd: tridymite; Crs: cristobalite; Qtz: quartz; Gp: gypsum; Anh: anhydrite; Alu: alunite and natro-alunite (minamiite); and Py: pyrite.

The peak intensities of alunite basically used K-alunite and those with † are natro-alunite (minamiite).

Ratio of peak value refers to the total peak intensity of maximum peaks of silica minerals of tridymite, cristobalite, and quartz (silica / Pl); sulfate of gypsum, anhydrite, and alunite (sulfate / Pl); and sulfide of pyrite (sulfide / Pl), divided by maximum peak intensity of plagioclase (Pl), respectively.

Table 3 Water-soluble species on volcanic ash samples of the 2018–2019 eruption at Kuchinoerabujima Volcano.

Sample ID	Date of eruption	F*	Cl*	SO₄*	Na*	NH₄*	K*	Mg*	Ca*	Cl/SO₄**	(Na+K)/Cl**	Ca/SO₄**
18-Keb02-4	21^st Oct. 2018	354.2	2484.0	3566.5	584.3	7.1	80.3	98.0	2631.3	1.89	0.39	1.77
18-Keb03-1	26^th Oct. 2018	354.9	1719.3	19684.7	437.0	0.0	56.9	58.9	8529.4	0.24	0.42	1.04
18-Keb07-2	5^th Nov. 2018	325.7	1128.1	12273.8	315.4	0.0	43.6	45.0	5429.1	0.25	0.47	1.06
18-Keb09	12^th Nov. 2018	387.7	2021.8	8658.6	576.8	0.0	85.7	79.1	4469.4	0.63	0.48	1.24
18-Keb12-1	20^th Nov. 2018	244.8	570.4	7846.1	272.5	0.0	17.5	27.1	3309.1	0.20	0.76	1.01
18-Keb15	6^th Dec. 2018	479.8	2572.0	22610.9	479.3	0.0	81.4	90.9	10036.3	0.31	0.32	1.06
18-Keb17	18^th Dec. 2018	466.6	2315.0	79124.1	2135.3	16.5	120.6	479.6	29034.2	0.08	1.47	0.88
18-Keb19-1	28^th Dec. 2018	127.5	1815.9	29738.1	1099.9	0.0	183.7	416.7	10845.4	0.17	1.03	0.87
19-Keb03-3	17^th Jan. 2019	235.1	724.5	82760.6	434.7	0.0	85.4	381.3	29152.4	0.02	1.03	0.84
19-Keb06	20^th Jan. 2019	134.8	4725.5	55724.3	684.0	0.0	57.2	940.2	21172.4	0.23	0.23	0.91
19-Keb09-2	29^th Jan. 2019	57.8	1195.3	23466.3	850.1	0.0	93.5	352.9	7851.9	0.14	1.17	0.80

*: in mg/kg. **: in molar ratio

Table 4 Type classifications of volcanic ashes from the 2018–2019 eruption at Kuchinoerabujima Volcano.

Type	Type-1	Type-2	Type-3
	Non-altered	Partially altered	Intensely altered
Sub-type			Type 3-1	Type 3-2	Type 3-3		Type 3-4		Type 3-5
Name of particle			silica	sulfate	sulfide		sulfur-bearing		others
Optical discription	-glassy luster, translucent gray-color to transparent dark-color particles -angular to sub angular shape	-translucent gray- to white-color particles -angular to sub-angular shapes	-white-, yellowish brown-, or dark-colored particles -angular to rounded shapes
X-ray map (intense elements)	Si, Al, K	Si, Al, K	Si	S, Ca	S, Fe	S		Al, K, Ca
Minerals contained	…	…	silica mineral(s)	sulfate mineral(s)	sulfide mineral	sulfur-bearing minerals		zeolite group minerals? clay minerals?
Minerals contained	…	…	Qz, Cri, Tri	Gyp, Anh, Alu	Pyr	Gyp, Anh		zeolite group minerals? clay minerals?
Textural description	-no traces of alteration	-groundmass: consisting of fine aggregate of anhedral silica minerals, unclear crystal boundary of microlite and tabular to irregular voids -Original volcanic rock textures are replaced to silica minerals and minor hydrothermal clay mineral(s)	…	[Type 3-2C] -an aggregate of coarse euhedral sulfate crystals [Type 3-2F] -euhedral sulfate crystals included in the aggregate of fine silica and clay minerals	-mainly pyrite and sulfate minerals with minor silica and clay minerals	-minor but commonly contain gypsum and/or anhydrite crystals		…

Graphicalabstract.jpg

Download PDF

Journal Publication

published 30 Jan, 2022

Read the published version in Earth, Planets and Space →

Editorial decision: Minor Revision
06 Nov, 2021
Review #2 received at journal
05 Nov, 2021
Review #1 received at journal
01 Nov, 2021
Reviews received at journal
15 Oct, 2021
Reviewers invited by journal
15 Oct, 2021
Reviewer #2 agreed at journal
14 Oct, 2021
Reviewer #1 agreed at journal
14 Oct, 2021
Editor assigned by journal
06 Oct, 2021
Submission checks completed at journal
06 Oct, 2021
Editor invited by journal
06 Oct, 2021
First submitted to journal
04 Oct, 2021

You are reading this latest preprint version

Influence of Hydrothermal Recharge on the Evolution of Eruption Styles and Hazards During the 2018–2019 Activity at Kuchinoerabujima Volcano, Japan

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Sampling And Methods

Results

Discussion

Conclusions

Abbreviations

Declarations

References

Tables

Supplementary Files

Status:

Journal Publication

Version 1