Shinmoedake volcano started a sub-plinian eruption after an interval of 300 years, since its latest magmatic eruption in 1716-1717. In 2003, the committee for volcanic disaster prevention consisting of local governments has been established. The committee published the Kirishima volcano disaster prevention handbook in March, 2008. Soon after, a series of precursory events of the 2011 eruption began. The media reported that pyroclastic flows were generated in all directions, but, actually, only one small pyroclastic flow occurred. Also an oblique eruption deposit was recognized on the upper slope of the volcano consisting of pumice-fall deposit showing slightly elliptic to concentric wrinkle pattern. The information provided by researchers were sometimes inaccurate and generated unnecessary concerns among the public. The main reasons for these are: 1) the disaster prevention map was not easy to understand, 2) the sense of impending crisis was unnecessarily increased by the announcements of a potential for sector collapse of the volcanic edifice, and 3) the eruptive history of the volcano was not reasonably understood among researchers, hence, it took a few days before researchers recognized that this is actually “an apparent magmatic eruption after 300 years interval”. These issues are some of the challenges that the research community should consider during future eruptions.

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A phreatic eruption occurred on August 22, 2008 from Shinmoedake Volcano, one of the members of Kirishima volcanic group, Kyushu, southwestern Japan. Some explosive craters and eruption fissures aligning in E-W direction for 800 meters were formed inside the summit crater and the western flank of Shinmoedake Volcano. These craters produced clay-rich tephra, consisting of non-juvenile lithic fragments with various degree of hydrothermal alteration. Ballistic blocks distribute in an area within 800 meters from the main crater. The total volume of the tephra produced this eruption is evaluated as 2×10⁸kg. Distribution of the tephra indicates that the main source of the tephra is S-17 crater, which is the largest crater located at the center of the crater chain. More than 70% of the tephra deposit inside the area within 1km from the craters, suggesting the low height of the eruption cloud. Absence of the juvenile materials suggests that this eruption was phreatic caused by a rapid release of steam from the hydrothermal system beneath Shinmoedake Volcano.

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Shinmoedake (Kyushu, Japan), which is one of the Kirishima Volcanoes, experienced several small eruptions in 2010, finally culminating in a sub-plinian eruption on January 26-27, 2011. After this sub-plinian phase, the eruption style shifts to the phase of vulcanian eruption or ash emission. This volcanic activity is still occurring. We here summarize the eruption history of Shinmoedake during the Edo period on the basis of historical records. The eruptions of Shinmoedake during the Edo period occurred in AD 1716-1717 (Kyoho eruption) and AD 1822 (the 4th year of Bunsei eruption). The Kyoho eruption, which was a large-scale (total amount of tephra: 2×10¹¹ kg) eruption, is divided into the following seven stages. Stage 1 (Apr. 10, 1716 to May 7, 1716): small eruptions occurred over two months; Stage 2 (Sep. 26, 1716): falling ash first observed at the foot of Shinmoedake; Stage 3 (Nov. 9 to 10, 1716): the first large eruption was observed, with pumice falling over a wide area; Stage 4 (Dec. 4 to 6, 1716): small eruptions; Stage 5 (Feb. 9 to 20, 1717): the second pumice fall eruption, with an intermittent ash fall eruption thereafter; Stage 6 (Mar. 3, Mar. 8, Mar 13, Apr. 8, 1717): ash fall eruptions; Stage 7 (Sep. 9, 1717): the last ash fall eruption. These eruptions, which continued intermittently over 17 months, were characterized by multiple repetitions of a large eruption. Based on the results of a comparison between the Kyoho eruption and the 2011 eruption, the eruptions from March 30, 2010 to January 26, 2011, were similar to Stages 1 to 3 of the Kyoho eruption; the eruptions after January 26, 2011, were similar to Stages 5 to 6 of the Kyoho eruption. In addition, the relatively large eruption events of Stages 3 and 5 of the Kyoho eruption and the January 26-27, 2011, eruption began without any noticeable precursors. The eruption in the 4th year of Bunsei (AD 1822) was a small eruption that lasted less than a day. The recent eruption sequences, which were also similar to the Edo period eruptions, are divided into a small-scale eruption (the 1959 eruption) and a large-scale eruption (the 2011 eruption). The eruption duration time of the small-scale (total amount of tephra: ＜ 10¹⁰ kg) eruption was less than a day. The eruption duration time of the large-scale (total amount of tephra: ＞ 10¹⁰ kg) eruption could be a few months or years. Both eruption sequences began with a small eruption. A large-scale eruption can occur a few months after the start of the eruption sequence. This is an important turning point in the eruption sequence of Shinmoedake.

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We investigated geology of the Matsunodai debris avalanche deposit (MDA) and its adjacent area in Kuju Volcanic Group, Kyushu, Japan. Based on the eruptive history, the source of the MDA should be inferred to be the Mimata-Gairinzan lava. Volume of the collapsed part of the lava is calculated to be ca. 0.3 km³ . On the other hand, volume of the MDA is estimated as ca. 0.2 km³ , which is almost consistent with collapsed volume. The MDA can be divided into three terraces on the surface. Debris avalanche rushed down 6 km to the north, and spilled over the gentle slope at least 40 m above the lower land. We could not find any evidence for high-temperature deposition from previous reports on rock-magnetism of the MDA.

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The Kuju Volcanic Group consists of over twenty lava domes and small stratovolcanoes. It has effused 22.3 km³ of magma, consisting of lavas (15.5 km³), pyroclastic flows/surges and debris avalanches (4.4 km³ and fallout tephra (2.5 km³). The average magma discharge rate is 0.15 km³/ky. Most lavas formed hornblende andesitic and dacitic lava domes except for the Hiijidake which is a basaltic stratovolcano. Based on eruption types and rate, the eruptive history of the Kuju Volcanic Group can be divided into four stages: K1, K2, K3 and K4. The K1 stage (150 to 110 ka) is characterized by hornblende dacitic ignimbrite eruptions that formed the Miyagi and Shimosakata ignimbrite. The activity of K2 stage (110 ka to 60 ka) occurred in the western part of the group. More than eight lava domes of two-pyroxene hornblende andesite were formed, some with associated explosive activities that generated sub-plinian eruptions forming Hotokenoharu pumice-fall and Miyakono scoria-fall deposits. The K3 (ca. 54 kBP) is the most catastrophic and voluminous eruptive stage. The stage began with Kuju D ash-falls. Large-scale plinian eruptions formed the Kj-P1 pumice-fall deposit with a volume of 6.2 km³. During the plinian eruptions, the northern and southern portions of the eruption column collapsed, and generated the Handa ignimbrite. The Handa ignimbrite attained a volume of 5 km³. Some parts of the Kj-P1 pumice-fallout deposit were deposited atop the Handa ignimbrite, which makes this eruption a typical intraplinian type with a DRE volume of approximately 4.1 km³. A small-scale caldera might be formed during the K1 and K3 stages. The K4, a post-caldera stage can be divided into three sub-stages. During 49 cal kBP to 35 cal kBP, Hosshozan and Ogigahana hornblende-andesite domes emerged, and Kan’nojigoku and Shirani block-and-ash flow deposits had originated from them. Sub-plinian eruptions at Shirakuchidake volcano formed Itakiri lapilli fallout. The period during 32 to 7.3 cal kBP emplaced various magmas of basaltic to dacitic composition. A basaltic activity in this sub-stage formed Hiijidake stratovolcano (lava flows, Nagayu and Hiijidake scoria-fallouts and debris avalanches). During this period, many lava domes were also formed such as Hizengajo, Kujusan, Dainoyama, Mimatayama and Inaboshiyama volcanoes. Sector collapse of Kujusan and Dainoyama lava domes produced the Azamidai and Dainoyama block-andash flow deposits, respectively. Mimatayama eruption started with phreatomagmatic eruption forming Sugamorigoe pyroclastic surges. This is followed by the growth of the Older Mimatayama lava dome with the associated Sugamorigoe block-and-ash flows. The lava domes collapsed generating the Matsunodai debris avalanches. After the collapse, Younger Mimatayama lava domes grew inside the horseshoe shaped crater. After 7.3 cal kBP, the eruption centers moved to the east and the magma rate increased from 0.07 km³/ky to 0.51 km³/ky. The large andesite to dacitic lava domes of Yuzawayama, Tacchusan, Taisenzan, Kurodake were formed. Taisenzan is a complex andesitic stratovolcano. Later vulcanian eruptions formed Danbaru crater and Kj-A1 ash falls. The sub-plinian eruption deposited the Danbaru scoria-fallout. The vulcanian and sub-plinian eruptions generated the Komekubo ash and scoria fallout. The largest eruption at 1.6 cal kBP formed the Kurodake lava dome with a volume of 1.6 km³.

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The Kirishima Volcano Group is composed of many Quaternary volcanoes occupying an area about 600km². Ohachi Volcano is a small, active stratovolcano situated at the southeastern part of the Kirishima Volcano Group. It has a considerably large crater compared to the size of its volcanic edifice, with thick agglutinates that crop out on the wall of the crater. We studied the eruptive history of Ohachi Volcano by tephrochronological method, and recognized 15 tephra deposits around the volcano. Geological and archeological data and documentary records of eruption indicate that the oldest tephra from the Ohachi Volcano is the Araso tephra (AsT) erupted in ca. AD 700. All other tephra deposits were erupted in historic time such as Katazoe tephra (KzT) in AD 788, Miyasugi tephra (MsT) in ca. AD 1000, Takaharu tephra (ThT) in AD 1235, and Takachihogawara tephra 1-11 (TgT-1 to TgT-11) in the period of AD 1250-1700. Thus, the Ohachi is a very young volcano at 1300 years old. We recognized two types of magmatic eruption : sub-plinian and vulcanian. Lava emission took place at least four times in association with sub-plinian eruptions. Among these eruptions, ThT is the most voluminous, and a pyroclastic flow was also generated. The total volume of erupted magma (volcanic edifice included) was calculated to be about 2.5×10⁸m³ (DRE). Eruption rate was not constant throughout the eruptive history of Ohachi volcano, that is, the eruptive history is conveniently divided into three stages by the eruption rate : the early stage from AD 700 to 1235 (ca. 540 years), the middle stage from AD 1236 to 1717 (480 years), and the late stage from AD 1718 up to present (290 years). This indicates that more than 80% of the total amount of magma was produced in the early stage, while the remaining 20% of magma was erupted in the middle stage. Although there are many recorded eruptions during the late stage, it is difficult to find any tephra deposits around the Ohachi Volcano, which suggests that the scale of volcanic eruption is progressively diminishing through time. The volcanic edifice of Ohachi is composed mainly of succession of thick tephra layers, some of them changed to agglutinates. Correlation of the proximal deposits to the distal tephra suggests that the volcanic edifice had grown mainly during the two eruptive stages of KzT and ThT, which are comparatively large-scale sub-plinian eruptions (order of 10⁷m³ to 10⁶m³). Hence, the most important process in generating agglutinates is rapid accumulation of voluminous tephra around the crater which are generated during sub-plinian eruptions.

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A new fumarolic activity at Ohachi Volcano of Kirishima Volcano Group started on December 13, 2003 after about 80 years of inactivity. Two small fumarolic vents, T8 and T9, were formed on the slope within the Ohachi crater. Ejected materials were mainly fine-grained mud (altered ash), that were distributed within the southwestern sector of the crater. Small accessory lapilli, which were coated by mud, were scattered within 20 meters of T8 vent. No essential material was found. Alunite, kaolinite, 10 Å-halloysite were present as clay minerals. The volume of ejected material was very small, probably less than 10m³. At the time of this writing, the fumarolic activity at Ohachi crater is still going on. To understand the present activity, we investigated old documents of historic eruptions of this volcano, in particular those that occurred during the Meiji and Taisho eras (1880-1923), to determine the mode of eruptions during that time. The first stage of the eruption started with a fumarolic activity, which became more active. Then magmatic activities continued for about 40 years. The early phase of magmatic activities was characterized by vulcanian eruptions generating a large volume of ash, while the latter phase was characterized by sporadic but strong vulcanian explosions accompanied by ejection of volcanic bombs and blocks. At present, it would be difficult to judge whether the fumarolic activity will become more active, and proceed to magmatic stage. However, the results of our investigation of historical eruptions would still be useful to predict the nature of future eruption of this volcano.

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