From June to August 2015, Mt. Raung located at the east end of Java, Indonesia, erupted effusively with Strombolian activity at the intra-caldera cinder cone. The effused lava from the base of the ...cone entirely buried the caldera floor with a lava bed and ultimately formed a temporary lava lake. In this study, details of the eruption sequence and formation process of the temporary lava lake are analyzed based on a series of high-resolution images combined with the high-frequency thermal observation results of Himawari-8. The volume of effused lava was also estimated using a digital elevation model (ASTER GDEM) based on the lava bed bathymetry in the caldera and lava bed distribution at several points over the course of the eruption. The total effusion volume was estimated to be 7.5 × 107 m3, and the average effusion rate was 1.4 × 106 m3/day. The activity consisted of four stages. The Precursory Stage was characterized by low-level thermal anomalies, which are thought to result from high-temperature spots at the bottom of several pits formed in the buried vent, probably due to magma head partial exposure or gas emission. Pulses 1 and 2 were effusive stages that formed the lava bed. The pattern recognized in the time-series variation of thermal anomalies was confirmed to reflect the lava effusion rate using the volumes estimated in this study. The lava bed is divided into the Lower and Upper lava groups, which were generated in Pulses 1 and 2, respectively, except for the fine lobes formed around the margin of the Lower lava group. The Terminal Stage also showed low-level thermal anomalies, most likely caused by gas emissions. The intra-caldera cone grew to approximately 100 m higher than its initial height by mid-August and then collapsed into the conduit at the end of the Terminal Stage. The lava bed increased in thickness through lobe multi-layering and later through the addition of lava directly from the vent into the lava bed interior. Over the course of the lava bed growth, the bed outline nearly matched the caldera contours. Uplift of the lava bed surface caused lateral fluidization of the bed, and molten lava was extruded at the bed margins, forming fine lobes. This process enlarged the zone of fine lobes around the lava bed. Thus, the lava bed growth resulted in the appearance of a lava lake. In the middle of Pulse 1, a small amount of hot material was thrown onto the slope beyond the caldera rim. In the future, the caldera floor uplift caused by this activity (approximately 49 m) will increase the risk of erupted material ejection outside the caldera. In a comparison with the 2013–15 Nishinoshima eruption, which was a similar effusive eruption, Mt. Raung was shown to have predominantly large lobes over most of the lava bed, while in Nishinoshima, narrow medium-scale lobes occupied the majority of the surface. This may be attributed to the high effusion rate of lava for Mt. Raung, which was approximately seven times higher on average than the lava effusion rate for Nishinoshima.
•The total effusion volume was 7.5 × 107 m3.•The average effusion rate was 1.4 × 106 m3/day.•The lava bed increased in thickness through multi-layering and the addition of lava.•The lava bed outline nearly matched the caldera contours.•Uplift of the lava bed caused lateral fluidization of the bed.
Historical materials have revealed that the Hokkaido-Komagatake Volcano erupted in 1640. In this study, we reviewed in detail the historical materials from a period closer to the eruption, which had ...yet to be investigated. We then evaluated the reliability of the historical materials and tried to interpret their descriptions from a volcanological point of view. As a result, we found descriptions that support the previous understanding of the number of deaths, tsunamis, and volcanic edifice collapses, or provide more detailed information. In contrast, we also found descriptions of the duration of the 1640 eruption, which lasted about one day and night, of the fallout tephra containing charred wood chips suggesting the occurrence of high-temperature phenomena and subsequent buoyant plumes, and of the volcanic activity that continued for a long time after the eruption. Examination of these historical materials revealed a picture of the eruption that could not be understood from the historical materials used in the past. This study demonstrates that investigating the characteristics of historical materials and the reliability of their descriptions and comparing the information obtained from them with volcanological knowledge can be useful in clarifying the phenomena and processes of past volcanic eruptions.
Nishinoshima volcano suddenly resumed eruptive activity in April 2017 after about 1.5 years of dormancy since its previous activity in 2013–2015. Nishinoshima is an uninhabited isolated island. We ...analyzed the eruption sequence and the eruptive process of the 2017 eruption (17 April–10 August: 116 days) by combining high-temporal-resolution images from Himawari-8 and high-spatial-resolution images from the ALOS-2, Landsat-8, and Pleiades satellites. We used these data to discuss how temporal variations in the lava effusion rate affected the flow formations and topographical features of the effused lava. The total effused volume was estimated to be 1.6 × 10
7
m
3
, and the average effusion rate was 1.5 × 10
5
m
3
/day (1.7 m
3
/s). Based on variations in the thermal anomalies in the 1.6-μm band of Himawari-8, which roughly coincided with that of the lava effusion rate estimated by ALOS-2, the activity was segmented into five stages. In Stage 1 (17–30 April: 14 days), the lava effusion rate was the highest, and lava flowed to the west and southwest. Stage 2 (1 May–5 June: 36 days) showed a uniform decrease in flow, and lava flowed to the southwest and formed the southwestern lava delta. During Stage 3 (6–15 June: 10 days), the lava effusion rate increased in a pulsed manner, the flow direction changed from southwestward to westward, and a narrow lava flow effused from the southern slope of the cone. In Stage 4 (16 June–31 July: 46 days), the lava effusion rate decreased and lava flowed westward through lava tubes, enlarging the western lava delta. Around the end of July, lava effusion mostly stopped. Finally, in Stage 5 (1–10 August: 10 days), explosive eruptions occurred sporadically. The variation in lava effusion rate seemed to play an important role in forming different flow patterns of lava on Nishinoshima. In Stages 1 and 3, lava flowed in multiple directions, while in Stages 2 and 4, it flowed in single direction, probably because the effusion rate was lower. A pulsed increase in the lava effusion rate during Stage 3 caused new breaks and disturbances of the lava passages near the vents, which resulted in changes in flow directions. Differences in the size of lava lobes between the southwestern and western deltas are also considered to result from differences in the lava effusion rate.
Quantitative analysis of bubble textures in a large number of volcanic pyroclasts is critical to investigating the eruption dynamics in a volcanic conduit. Here, we used a digital stereo microscope ...with low-angled ring illumination (DSM-LaRI) to measure bubble textures on unpolished cutting surfaces of pumice clasts. As the DSM-LaRI enhances brightness contrast between the bubbles (pores) and the matrix, we easily obtained the two-dimensional data on the size and shape of bubbles by image analysis. The DSM-LaRI imaging provided the distributions of size and shape of bubbles at least 50 µm across. We applied the DSM-LaRI to analyze more than 1000 pumice clasts from the 232 AD Taupo eruption and measured the mean bubble radius (
R
¯
) and the mean deformation degree (
D
¯
) in the individual clasts. The distribution of
R
¯
and
D
¯
in each layer showed a distinctive difference between the fallout and the flow deposits. These quantitative data are consistent with a qualitative classification in a previous study. Although the new DSM-LaRI method has the disadvantage of the low spatial resolution, it allows for the analysis of a large number of pumice clasts in a short time, which can address larger scale heterogeneity, by efficiently generating a large representative suite of bubble size and shape data to link bubble textures to conduit processes. This provides vital information for quantitatively modeling eruption dynamics.
The sudden eruption of Mount Ontake on September 27, 2014, led to a tragedy that caused more than 60 fatalities including missing persons. In order to mitigate the potential risks posed by similar ...volcano-related disasters, it is vital to have a clear understanding of the activity status and progression of eruptions. Because the erupted material was largely disturbed while access was strictly prohibited for a month, we analyzed the aerial photographs taken on September 28. The results showed that there were three large vents in the bottom of the Jigokudani valley on September 28. The vent in the center was considered to have been the main vent involved in the eruption, and the vents on either side were considered to have been formed by non-explosive processes. The pyroclastic flows extended approximately 2.5 km along the valley at an average speed of 32 km/h. The absence of burned or fallen trees in this area indicated that the temperatures and destructive forces associated with the pyroclastic flow were both low. The distribution of ballistics was categorized into four zones based on the number of impact craters per unit area, and the furthest impact crater was located 950 m from the vents. Based on ballistic models, the maximum initial velocity of the ejecta was estimated to be 111 m/s. Just after the beginning of the eruption, very few ballistic ejecta had arrived at the summit, even though the eruption plume had risen above the summit, which suggested that a large amount of ballistic ejecta was expelled from the volcano several tens-of-seconds after the beginning of the eruption. This initial period was characterized by the escape of a vapor phase from the vents, which then caused the explosive eruption phase that generated large amounts of ballistic ejecta via sudden decompression of a hydrothermal reservoir.
Previous research indicates that Yakushima Island, southwestern Japan, may have been struck by a huge tsunami before or soon after the arrival of the Koya pyroclastic flow during the 7.3 ka ...caldera‐forming Kikai eruption, but this has not yet been confirmed. This paper describes sedimentological and chronostratigraphic evidence showing that Unit TG, one of three gravel beds exposed on the Koseda coast of northeast Yakushima Island and investigated here, is a tsunami deposit. Unit TG is a poorly sorted, 30 cm thick gravel bed overlying a wave‐cut bench and underlying a Koya pyroclastic flow deposit. Sparse wood fragments in Unit TG were dated at 7 416–7 167 cal year BP. The constituent gravel clasts of Unit TG are similar in composition to those of modern beach and river deposits along the Koseda coast. Unit TG also contains pumice clasts whose chemistry is identical to that of pumice derived from the 7.3 ka eruption at Kikai caldera. The long‐axis orientations and composition of gravel clasts in Unit TG suggest that they were transported by a landward‐travelling high‐particle‐concentration flow, which suggests that Unit TG was deposited by a tsunami run‐up flow during the 7.3 ka Kikai caldera eruption, just before the arrival of the major Koya pyroclastic flow at the Koseda coast. Whether the 7.3 ka tsunami was caused by a volcanic eruption or an earthquake remains unclear, but Unit TG demonstrates that a tsunami arrived immediately before emplacement of a Koya pyroclastic flow.
7.3 ka鬼界カルデラ噴火時において, 南九州一帯が巨大津波に襲われた可能性が先行研究によって度々指摘されてきたが, 未だ十分な検証がなされているとは言いがたい. 今回の我々の検討の結果, 屋久島北東部の小瀬田海岸に露出した完新世礫層の1つである Unit TGが, この時生じた津波堆積物であると判断した. Unit TGは地震によって隆起した完新世波食ベンチを覆い, 幸屋火砕流堆積物に覆われ, 本層に含まれる材片の14C年代から7.3 kaに発生したイベント堆積物であると理解される. 礫の原岩組成は近接する現世の礫浜および河川礫と類似し, 基質部には鬼界カルデラ噴火に由来する軽石が多量に含まれている. 礫のファブリックは, それらが海側からの高密度流によって輸送されたことを示している. 以上の事実から, Unit TGが鬼界カルデラ噴火中に発生した津波の遡上流によって堆積したと解釈される. この津波のトリッガーは特定できていない.
Location maps of the Kikai caldera and the Koseda coast on northeastern Yakushima Island.
Kirishima volcano consists of more than 20 eruptive centers. Among them, Shinmoe-dake had magmatic eruptions in October 2017 and March 2018. Subsequently, another active cone, Iwo-yama, had phreatic ...eruptions in April 2018. These events were unique in that the 2018 eruption was the first effusion-dominated eruption of Shinmoe-dake and the first simultaneous activity of two cones of the Kirishima volcanic group ever documented. We report the detailed sequence of the events by combining areal photos, satellite images, and seismo-acoustic data analyses with the other published information. The seismo-acoustic data clarify the eruption onset and the transitions of the behaviors in three stages for each of the 2017 and 2018 eruptions. For both eruptions, we present regularly repeated tremors or ’drumbeat’ earthquakes in the second stage, which interpret as gas separation from magma, leading to the ash-poor plume in the 2017 eruption or the effusive eruption in the 2018 event. We also propose that the 2017 and 2018 eruptions of Shinmoe-dake and the 2018 eruption of Iwo-yama are sequential events linked by the degassing of magma beneath Shinmoe-dake. An eruption like the 2017–2018 eruptions of Shinmoe-dake would leave few geological records and could be captured only by modern techniques. Although Shinmoe-dake has been believed to be an example of less-frequent eruptions, effusive eruptions like the 2018 case might have occurred more frequently in the past , but the following eruptions had obscured their records. The timelines summarized in this study will be useful in future studies of Kirishima volcanoes and world equivalences.
Graphical Abstract
Basaltic eruptions sometimes show an explosive and complex nature; thus, clarifying the sequence and controlling parameters is essential for understanding their causes. The An’ei eruption of the ...Izu-Oshima volcano during 1777–1792 was a complex basaltic eruption producing lava flows, pyroclastic falls, and ash plumes. We reconstructed the transition of the eruption style based on geological data combined with comparisons with data from historical documents and used chemical analyses to develop a magma plumbing model. The An’ei eruption started in August 1777 with scoria ejection. The scoria deposit was classified into Units A–C. Unit A scoria was produced by early weak explosions and more intense subsequent explosions. Unit B scoria marked a return to weak plumes before the summit eruption reached its climactic phase in November 1778 and explosively ejected Unit C scoria. Several lava flows were also effused from the foot of the scoria cone during these periods of scoria ejection. After a 5-year hiatus, the eruption ultimately shifted to persistent, weak ash ejection and pyroclastic surges. The tephra volumes of Units A, B, and C were estimated at 1.9–4.3 × 10
7
, 0.6–4.5 × 10
6
, and 1.3–3.2 × 10
7
m
3
, respectively. Associated column heights of 8–11, 3–10, and 9–12 km were obtained for Units A, B, and C, respectively, resulting in sub-Plinian classification. Chemical analyses have shown that the plagioclase phenocryst content increased as the eruption progressed. The transition from relatively weak activity with Strombolian and sub-Plinian explosions, caused by aphyric magma, to short-period activity with more intense sub-Plinian explosions, caused by porphyritic magma, can be explained by evacuation of magma from multiple reservoirs with different contents of plagioclase phenocrysts. Simultaneous lava flows that have different petrological features from those of the scoria eruptions also suggest multiple magma reservoirs and pathways. This view of the temporal change in eruptive style, corresponding to change in magma type, is essential for understanding the eruptive processes of large-scale basaltic eruptions of the Izu-Oshima volcano and contributes to clarifying the nature and hazards of basaltic eruptions which turn into explosive activities in general.
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