Special issue "Advancement of our knowledge on Aso volcano: current activity and background

n/

Dispersal and grain size characteristics of the May 14, 2018 Shinmoedake eruption deposit, Kirishima Volcano, Japan, based on post-eruption field survey and meteorological datasets

Abstract This study describes the dispersal and grain size characteristics of the May 14, 2018 Shinmoedake eruption deposits of Kirishima Volcano in southern Kyushu, southwestern Japan. We discuss the eruption sequence, including the temporal variations in the behavior of the plume, by combining field and meteorological datasets. Following a magmatic activity in 2011 characterized by a substantial change in the eruption style (from subplinian eruptions to lava effusion) and subsequent vulcanian explosions, the Shinmoedake crater experienced intermittent eruptions in 2018. The May 14, 2018 eruption began at 14:44 with a vulcanian eruption, with the eruption plume rising 4500 m above the crater rim. Thereafter, it transitioned to an ash eruption; the plume height decreased gradually until the eruption ceased at 16:10. The tephra fall deposits were distributed more than 27 km to the southeast of the source crater; the mass of the tephra fall deposit was approximately 2.1 × 107 kg, calculated based on an isomass map. The deposit incidence differed between the east and west sides of the major dispersal axis. The deposits found east of the main dispersal axis were primarily composed of coarse to medium sand-sized particles with no fine fraction (fine sand to silt in size). In contrast, the deposits west of the axis were finer-grained than those east of the axis. We analyzed photographs of the eruption plume, along with the regional meteorological data and the dispersal and grain-size characteristics of the deposits, and reached the following conclusion: during the May 14, 2018 eruption, the wind directions above the Shinmoedake crater fluctuated across altitudes. The westerly winds dispersed the eruption plume that rose to a higher altitude, containing coarser tephra associated with the initial vulcanian eruption, further to the east rather than along the main axis. In contrast, a lower-altitude ash eruption plume that was rich in fine materials was dispersed westward rather than along the main axis, which was influenced by northerly winds. The findings of this study can support the analysis of similar volcanic events. Graphical Abstrac

Temporal variations in discharge rate and component characteristics of tephra-fall deposits during the 2014–2015 eruption of Nakadake first crater, Aso Volcano, Japan

Abstract The 2014–2015 eruption of the Nakadake first crater at Aso Volcano in southwestern Japan was characterized by continuous ash emissions and intermittent strombolian eruptions. In this paper, we present the distribution, discharged mass, and components of tephra-fall deposits to examine the sequence of activities. We installed 21 ash samplers around the crater (SW crater rim to 9 km in all directions) and calculated the mass of ash-fall deposits based on 28 isomass maps. From November 25, 2014, to the end of January 2015, the cumulative erupted mass increased at a high discharge rate (2.2 × 104 tons/day). After February 2015, the cumulative erupted mass decreased to a low rate of 0.6 × 104 tons/day, although this rate rose slightly in March and late April 2015. The 2014–2015 tephra-fall deposits consisted of glass shards, crystal, and lithic grains. In the November 25–27, 2014 ash-fall deposits, lithic fragments, which are interpreted to be derived from lavas or pyroclasts of previous eruptions, were dominant (59–68%). Thereafter, the proportion of glass shards, which are probably juvenile materials of newly ascending magma, gradually increased with time, and the December 21–23, 2014 ash contained abundant glass grains (63%). The proportions of glass shards ranged from 29 to 50% until February 25, 2015. Subsequently, they decreased with time and reached 14% on March 17. Afterward, the proportions increased again prior to April 27 and ranged between 20 and 30% in May 2015. The total erupted tephra mass from the November 2014–May 2015 activity of Nakadake first crater was 2.1 × 106 tons (1.2 × 104 tons/day), which was less than the tephra deposits of previous activities that have occurred within the past few decades

タルマエヤマ ガリー デ ハッセイ シタ ユウセツ キ ゴウウ ニ ヨル ドシャ イドウ

This paper describes debris movement due to heavy rainfall during snowmelt season in gullies of Tarumae Volcano, Hokkaido, northern Japan. A large amount of debris derived from slope surface, which had been loosen by freeze-thaw cycles, resulting from the heavy rainfall (total 162mm) on April 22-23, 1990, deposited on valley bottom covered with snow. This debris was saturated by snowmelt water, thus turned into debris flow. The debris flows transported and deposited 340m^3 of fine materials (clay, silt, sand) with little coarse gravels. The sediment volume produced by this rainfall from slope surface was 400m^3. The volume is one order of magnitude greater in spring than in summer and in early winter. The return period of debris movement was estimated to be five years on the basis of the record of precipitation. The heavy rainfall during snowmelt season seems to play an important role on sediment production and transport processes in snowy subpolar zone

アソ カザン ニ オケル ゴ カルデラ フンカ カツドウ ノ データベース カ : テフラ ケンキュウ ニ ヒツヨウ ナ データベース ゾウ

わが国ではこれまで多くのテフロクロノロジーによる火山噴火史研究が行われてきたが、フィールド調査だけでなく、データのとりまとめに関しても解決すべき問題は多い。本論では、阿蘇火山の後カルデラ噴火活動史に関する調査を例に、テフラ研究を効率的に進めるために構築すべきデータベース像について述べる

Luminescence dating of volcanogenic outburst flood sediments from Aso volcano and tephric loess deposits, southwest Japan

Luminescence dating has been applied to volcanogenic outburst flood sediments (Takuma gravel bed) from Aso volcano, Japan, and tephric loess deposits overlying the gravel bed. The poly-mineral fine grains (4-11 μm) from loess deposits were measured with pulsed optically stimulated luminescence (pulsed OSL) and post-IR infrared stimulated luminescence (pIRIR) methods, whereas the Takuma gravel bed containing no quartz, was measured with IRSL and pIRIR methods using sand sized (150-200 μm) plagioclase. The loess deposits date back at least to ∼50 ka by consistent IRSL, pIRIR and pulsed OSL ages from the lowermost part of the loess deposits from one section. The ages obtained from the bottom part of the other loess section are not consistent each other. However, we consider that the pIRIR age (72±6 ka) which showed negligible anomalous fading is most reliable, and regard as a preliminary minimum age of the Takuma gravel bed. The equivalent doses (De) for the plagioclase from the Takuma gravel bed have a narrow distribution and the weighted mean of the three samples yield an age of 89±3 ka. This age is in agreement with the last caldera-forming eruption of Aso volcano (∼87 ka) and it is likely that the pIRIR signal has not been bleached before the deposi-tion. IRSL dating without applying pIRIR using small aliquots was also conducted, however, the IRSL signal shows no clear evidence of an additional bleaching during the event of outburst flood from the caldera lake. © 2013 Silesian University of Technology, Gliwice, Poland. All rights reserved

カザンバイ フンシュツ オ シュタイ ト スル カザン シュウヘン イキ ニ オケル マイボツ ドジョウ ノ ニンテイ : アソ カザン デ ノ ジレイ

It is very important for reconstructing volcano eruptive history to identify paleosols interbedded between tephra layers because they indicate dormant or gentle periods of volcanoes. However, it is difficult to recognize paleosols around volcanoes dominating long-term small ash-emitting activity for a long time. Gain-size, total carbon content and phytolith analyses and measurement of soil hardness of paleosols and fine-grained tephra layers (ash-fall deposits) were undertaken at a proximal (4km) and a distal (11km) sites of active Nakadake crater, Aso Volcano (southwest Japan) whose activity is characterized by small ash eruption, to discuss effective discrimination between paleosols and tephras using their physical and chemical properties. Paleosols were finer grained than tephra layers at the proximal site whereas there was no distinct difference in grain size between them at the distal site. Since Holocene tephras tended to be more consolidated than the paleosols, hardness may be an effective indicator to distinguish paleosols from tephras only in the Holocene. Although Holocene paleosols had higher total carbon contents than the tephras, both paleosols and tephras in the late Pleistocene contained extremely low carbon. Phytolith concentrations of Paleosols were significantly higher than those of tephras both in the Holocene and late Pleistocene. Therefore, phytolith analysis is a useful method to divide into paleosols and tephras event at distal sites although the analysis is needed a practiced technique

アソ カザン コメヅカ ノ フンカ ネンダイ

Komezuka Volcano, located in the northwestern part of the post-caldera central cones of Aso Volcano, SW Japan, is a basaltic monogenetic volcano comprising a scoria cone (370-380m in basal diameter; 80m in height) and lava flows (10.5km^2; 5×10^7m^3). We obtained ^(14)C ages of 3,070±40 years BP from a buried soil below silty ash underlying Komezuka lava, which corresponds to 3,370-3,210cal years BP, and 2,760±40 years BP (2,950-2,770cal years BP) from a soil above silty ash overlying Komezuka lava. The age of soil below the lava suggests that the eruption age of Komezuka Volcano is about 3,300 cal years BP. The eruption age is consistent with the age of Ojodake Volcano (3,600 cal years BP) whose lava underlies Komezuka lava. In the northwestern part of the post-caldera central cones, Late Holocene monogenetic volcanic activity commenced with sub-plinian eruptions and lava extrusion from Kishimadake Volcano at approximately 4,000 cal years BP, followed by sub-plinian eruptions and lava extrusion from Ojodake Volcano at 3,600 cal years BP, and ceased with strombolian eruptions and lava extrusion from Komezuka Volcano at 3,300 cal years BP