Teruki Oikawa

Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits

The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan, was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30–50xa0wt% of fine ash (silt–clay component). The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, and by applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of the PDC at the summit area were estimated as 2xa0×xa010−4–2xa0×xa010−3 and 24–28xa0m/s, respectively. The tephra thinning trend in the proximal area shows a steeper slope than in similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics.

Reconstruction of the 2014 eruption sequence of Ontake Volcano from recorded images and interviews

A phreatic eruption at Mount Ontake (3067xa0m) on September 27, 2014, led to 64 casualties, including missing people. In this paper, we clarify the eruption sequence of the 2014 eruption from recorded images (photographs and videos obtained by climbers) and interviews with mountain guides and workers in mountain huts. The onset of eruption was sudden, without any clear precursory surface phenomena (such as ground rumbling or strong smell of sulfide). Our data indicate that the eruption sequence can be divided into three phases. Phase 1: The eruption started with dry pyroclastic density currents (PDCs) caused by ash column collapse. The PDCs flowed down 2.5xa0km SW and 2xa0km NW from the craters. In addition, PDCs moved horizontally by approximately 1.5xa0km toward N and E beyond summit ridges. The temperature of PDCs at the summit area partially exceeded 100xa0°C, and an analysis of interview results suggested that the temperature of PDCs was mostly in the range of 30–100xa0°C. At the summit area, there were violent falling ballistic rocks. Phase 2: When the outflow of PDCs stopped, the altitude of the eruption column increased; tephra with muddy rain started to fall; and ambient air temperature decreased. Falling ballistic rocks were almost absent during this phase. Phase 3: Finally, muddy hot water flowed out from the craters. These models reconstructed from observations are consistent with the phreatic eruption models and typical eruption sequences recorded at similar volcanoes.

The spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at Miyakejima volcano, Japan

Basaltic feeder systems exposed in the caldera wall of Miyakejima volcano are classified into three groups: (1) effusive feeders, (2) moderately explosive feeders, and (3) highly explosive feeders. The surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them, ranging from non-explosive lava effusions to violent explosive eruptions, despite the apparent lack of influence of external water. Effusive feeders are filled with coherent (non-fragmented) intrusive rock, indicating no significant fragmentation in the feeder system. The other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part. Their uppermost parts show an upward-flaring shape. The transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma. The depth of the coherent–pyroclastic transition is deeper (20–150xa0m) in highly explosive feeders than in the moderately explosive feeders (<20xa0m), and coincides with the depth at which the system flares upwards. Presence of lithic fragments derived from the host rock in the products of the highly explosive feeder systems indicates the removal of the wall rock by explosive activity. This observation suggests that the fragmentation of rising magma promoted the enlargement of the feeder systems to form their upward-flaring shapes, by mechanical erosion and wall collapse.

Multiple lines of evidence for crustal magma storage beneath the Mesozoic crystalline Iide Mountains, northeast Japan

[1]xa0It has been recognized that the Iide Mountains, consisting of Mesozoic sedimentary rocks and Late Cretaceous to Paleogene granitic rocks, northeast Japan, are unique. Although they occur in a nonvolcanic region, hot springs in the Iide Mountains have anomalously high heat discharge values similar to those from hot springs in volcanic regions. In order to provide geochemical constraints on the heat source for the hydrothermal activity, the chemical and isotopic compositions of 10 gas and water samples were determined. The 3He/4He ratios determined range from 0.22 to 7.9 Ra, and the highest ratio is similar to MORB-type helium, indicating a significant contribution of primordial mantle helium. Considering the relationship between the magnitude of the 3He/4He ratios and the distance of the sample locations from the Kitamata-dake, the peak in the Iide Mountains where the largest ratio occurs, it is apparent that there is a sharp decrease in 3He/4He ratios laterally away from the peak. Furthermore, the peak is also the location where geophysical anomalies such as an anomalous conductive body and zones with low Vp and low Vs have been detected in the middle-lower crust. These high helium ratios are considered to indicate the likelihood of mantle-derived materials supplying MORB-type helium beneath the Kitamata-dake and the possibility that mantle-derived helium has been diluted by atmospheric and/or crustal components with lower helium ratios away from the peak. In order to examine whether or not ancient magma of Middle Miocene age is a possible source of the high 3He/4He ratios of the hot spring gases, we calculated the evolution in 3He/4He ratios of the ancient magma with time. As a result, it is concluded that the anomaly beneath the Iide Mountains is due to newly ascending magmas in the present-day subduction system rather than hydrothermal fluids related to late remnant magmatism of Middle Miocene age.

Estimation of total discharged mass from the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014

The total mass discharged by the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, was estimated using several methods. The estimated discharged mass was 1.2xa0×xa0106xa0t (segment integration method), 8.9xa0×xa0105xa0t (Pyle’s exponential method), and varied from 8.6xa0×xa0103 to 2.5xa0×xa0106xa0t (Hayakawa’s single isopach method). The segment integration and Pyle’s exponential methods gave similar values. The single isopach method, however, gave a wide range of results depending on which contour was used. Therefore, the total discharged mass of the 2014 eruption is estimated at between 8.9xa0×xa0105 and 1.2xa0×xa0106xa0t. More than 90xa0% of the total mass accumulated within the proximal area. This shows how important it is to include a proximal area field survey for the total mass estimation of phreatic eruptions. A detailed isopleth mass distribution map was prepared covering as far as 85xa0km from the source. The main ash-fall dispersal was ENE in the proximal and medial areas and E in the distal area. The secondary distribution lobes also extended to the S and NW proximally, reflecting the effects of elutriation ash and surge deposits from pyroclastic density currents during the phreatic eruption. The total discharged mass of the 1979 phreatic eruption was also calculated for comparison. The resulting volume of 1.9xa0×xa0106xa0t (using the segment integration method) indicates that it was about 1.6–2.1 times larger than the 2014 eruption. The estimated average discharged mass flux rate of the 2014 eruption was 1.7xa0×xa0108xa0kg/h and for the 1979 eruption was 1.0xa0×xa0108xa0kg/h. One of the possible reasons for the higher flux rate of the 2014 eruption is the occurrence of pyroclastic density currents at the summit area.

Distribution and mass of tephra-fall deposits from volcanic eruptions of Sakurajima Volcano based on posteruption surveys

We estimate the total mass of ash fall deposits for individual eruptions of Sakurajima Volcano, southwest Japan based on distribution maps of the tephra fallout. Five ash-sampling campaigns were performed between 2011 and 2015, during which time Sakurajima continued to emit ash from frequent Vulcanian explosions. During each survey, between 29 and 53 ash samplers were installed in a zone 2.2–43xa0km downwind of the source crater. Total masses of erupted tephra were estimated using several empirical methods based on the relationship between the area surrounded by a given isopleth and the thickness of ash fall within each isopleth. We obtained 70–40,520xa0t (4.7u2009×u200910−8–2.7u2009×u200910−5-km3 DRE) as the minimum estimated mass of erupted materials for each eruption period. The minimum erupted mass of tephra produced during the recorded events was calculated as being 890–5140xa0t (5.9u2009×u200910−7–3.6u2009×u200910−6-km3 DRE). This calculation was based on the total mass of tephra collected during any one eruptive period and the number of eruptions during that period. These values may thus also include the contribution of continuous weak ash emissions before and after prominent eruptions. We analyzed the meteorological effects on ash fall distribution patterns and concluded that the width of distribution area of an ash fall is strongly controlled by the near-ground wind speed. The direction of the isopleth axis for larger masses is affected by the local wind direction at ground level. Furthermore, the wind direction influences the direction of the isopleth axes more at higher altitude. While a second maximum of ash fall can appear, the influence of rain might only affect the finer particles in distal areas.

Orientation of the Eruption Fissures Controlled by a Shallow Magma Chamber in Miyakejima

Orientation of the eruption fissures and composition of the lavas of the Miyakejima volcano indicate tectonic influence of a shallow magma chamber on the distribution of eruption fissures. We examined the distributions and magmatic compositions of 23 fissures that formed within the last 2800 years, based on a field survey and a new dataset of 14C ages. The dominant orientation of the eruption fissures in the central portion of the volcano was found to be NE-SW, which is perpendicular to the direction of regional maximum horizontal compressive stress (σHmax). Magmas that show evidences of magma mixing between basaltic and andesitic magmas erupted mainly from the eruption fissures with a higher offset angle from the regional σHmax direction. The presence of a shallow dike-shaped magma chamber controls the distribution of the eruption fissures. The injection of basaltic magma into the shallow andesitic magma chamber caused the temporal rise of internal magmatic pressure in the shallow magma chamber. Dikes extending from the andesitic magma chamber intrude along the local compressive stress field which is generated by the internal excess pressure of the andesitic magma chamber. As the result, the eruption fissures trend parallel to the elongation direction of the shallow magma chamber. Injection of basaltic magma into the shallow andesitic magma chamber caused the magma mixing. Some basaltic dikes from the deep-seated magma chamber reach the ground surface without intersection with the andesitic magma chamber. The patterns of the eruption fissures can be modified in the future as was observed in the case of the destruction of the shallow magma chamber during the 2000 AD eruption.