著者
山科 健一郎
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.44, no.2, pp.71-82, 1999
参考文献数
79
被引用文献数
1

Associated with the 1914 great eruption at Sakurajima volcano, southwestern Japan, the maximum height of volcanic cloud is discussed based on collected documents, sketches and photographs in those days. A series of photographs up to around 10 : 40 on January 12 (in Japanese Standard Time) represents that the volcanic cloud height attained to 7,000 to 8,000 m above sea level. After then, it proved that several documents reported the height to be 9,500-15,000 m, or even more than 18,000 m a.s.l, although it is difficult to obtain reliable evidences. Considering these reports and other observations from a distance, the height of 15,000 m is tentatively proposed here as a possible maximum value. According to an empirical relation, an eruption rate of small pyroclastic materials is suggested as, roughly speaking, 5,000 tons per second or 20 millions of tons per hour, if the volcanic cloud was 15,000 m in height.
著者
宮地 直道 富樫 茂子 千葉 達朗
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.49, no.5, pp.237-248, 2004
参考文献数
34
被引用文献数
4

A large-scale collapse occurred at the eastern slope of Fuji volcano about 2900 years ago, based on calibrated <sup>14</sup>C age of a wood sample collected in the resulting debris avalanche deposit. The collapsed slide deposit, called "Gotemba debris avalanche deposit" (Goda), is distributed on the eastern foot of the volcano covering an area of more than 53 km<sup>2</sup> The source amphitheater is not preserved because it became covered by younger tephra erupted from the summit crater. This avalanche deposit is overlain by the "Gotemba, mudflow deposits" (Gomf) emplaced repeatedly after the avalanche. Some now units of the Goda and Gomf entered pre-existing rivers and were finally emplaced as fluvial deposits. The Goda is composed of debris-avalanche blocks, showing jigsaw cracks, along with smaller blocks ranging from several tens of centimeters up to l m in diameter. The debris-avalanche matrix is a mixture of smaller nieces of blocks and ash-sized materials due to mainly shearing and fragmentation of large blocks. Igneous rocks include fresh and altered gray basaltic lava, weathered tephra including red scoria and white clay. Petrographical and geochemical data indicate that most blocks were derived from the Older Fuji volcano. The volumes of the Goda and Gomf are about l.05km^3 and 0.71km^3 respectively, based on presently available geological and borehole data. Since the blocks of Goda are composed mostly of the products of the Older Fuji volcano and the older stage lavas of Younger Fuji volcano do not extend to the eastern foot of Fuji volcano, a bulge of Older Fuji volcano must have existed in the eastern flank of Fuji volcano preventing the older stage lavas to now to the east. This bulge collapsed in the form of three blocks from the foot of the mountain. The abundance of hydrothermally altered deposits in the Goda and the absence of fresh volcanic products within the Goda suggest its origin as a rupture inside the altered deposits possibly triggered by a large earthquake or phreatic eruption.
著者
早川 由紀夫
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.40, no.3, pp.177-190, 1995
参考文献数
47
被引用文献数
24

Loam is an international scientific term, however, it has been used in a peculiar way in Japan. Japanese loam is a massive, brown, weathered rock unit composed of silt, clay, sand and occasional lapilli. It extensively covers coastal terraces, river terraces, ignimbrite plateaus and other uplands around volcanoes. Loam is not a product of soil forming process operated beneath the earth surface against rock bodies ; but it is a sediment accumulated slowly on the earth surface. Small-magnitude volcanic eruptions play a very minor role for the sedimentation. An eolian reworking process of pre-existing fine-grained deposits by the wind plays a major role. This is proved by following facts : 1) loam has accumulated even during the time when no ash-fall was observed ; 2) a volcano infrequently erupts explosively and the intensity of ash fallout is far lower than the sedimentation rate of loam ; it is about 0.1 mm/year ; 3) loam is hardly thickening toward a volcano. Very small particles carried from continental China by the westerlies at a high altitude are contained in loam, however, in the area around volcanoes their contribution is little for the formation of loam compared with eolian dust carried from nearby bare grounds by local winds at a low altitude. Loam does not accumulate all the year round. Just before and during fresh verdure, occasional strong winds pick up fine particles into the air from a bare ground which is dried up by a high-angle sunlight and high-temperatures. Eventually fine particles will settle down in vegetation. The most favorable season for loam deposition is April to May, in which more than half of an annual amount is achieved. It is convenient and practical to define a single eruption by a tephra layer which is not interbedded with loam. The thickness of loam can be used for the quantitative measurement of geologic time intervals, in years to thousands years, on certain conditions. Lithology of Japanese loam and the mechanism of sedimentation are identical to those of loess in other areas, such as China, northern Europe, northern America and New Zealand. There is no reason to hesitate to designate Japanese loam loess.
著者
小林 淳 萬年 一剛 奥野 充 中村 俊夫 袴田 和夫
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.51, no.4, pp.245-256, 2006
参考文献数
45
被引用文献数
1 4

We discovered a set of phreatic explosion deposits, herein referred to as the Owakidani tephra group, on the northern slope of Mt. Kamiyama and in the Owakidani fumarolic area of the Hakone Volcano. The tephra group is the product of the volcanic activities since the latest magmatic eruption of Hakone Volcano at around 2.9ka. It comprises five units named Hk-Ow1 to Hk-Ow5 in the ascending order. Both Hk-Ow1 and Hk-Ow2 comprise tephra fall deposits and secondary debris flow deposits. In addition to these deposits, Hk-Ow2 is also associated with surge deposits. Hk-Ow3, Hk-Ow4 and Hk-Ow5 consist of tephra fall deposits. The ash of these tephra fall deposits and the matrix of the secondary debris flows are principally composed of clay, altered lithics and secondary minerals supposed to be of fumarolic area origin. It is possible that Hk-Ow1 and Hk-Ow2 erupted from a fissure on the northeastern ridge of Mt. Kamiyama, while Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted at Owakidani. No juvenile material was found within the deposits of these eruptions except for Hk-Ow2, while the surge deposit of Hk-Ow2 contained trace amounts of volcanic glass fragment. Although it is considered that the principal nature of the eruptions of the Owakidani tephra group is phreatic, the deformation of the edifice around the source area implies the possibility of magma intrusion to shallow levels. Based on the calendar ages of the Owakidani tephra group and the stratigraphic position of the Kozushima-Tenjosan tephra, we estimated that Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted in relatively short intervals between the latter half of the 12th and 13th centuries. On the other hand, Hk-Ow1 and Hk-Ow2 erupted at around 3 kyr BP and 2kyr BP, respectively. The eruption ages of the Owakidani tephra group generally correspond to the seismic events that occurred in the Kozu-Matsuda Faults and the Tanna-Hirayama tectonic line. It is suggested that the activity of the Hakone Volcano may be closely related to the tectonic events in this region.
著者
萬年 一剛
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.66, no.2, pp.101-117, 2021-06-30 (Released:2021-07-27)
参考文献数
62

Tephra simulation codes were originally developed to estimate tephra fall hazards; however, existence of several unknown parameters inhibits accurate calculation without elaborate parameter tunings. One of the most sensitive unknown parameters is source magnitude distribution (SMD), which describes amount of particle release as a function of distance from the vent along the plume axis. SMD can be obtained using inversion technique from real eruption products. Inversion techniques are also required to obtain other parameters; plume height of the eruption and the wind system at the time of the eruption are the most important ones among them. Although plume height and wind system can be observable for the recent eruptions, they could have some uncertainties and require further refinements. Additionally, they are totally unknown for unobserved ancient eruptions; however, they are essential parameters to describe eruptions. There are two types of relationship between amounts of tephra deposition and eruptive parameters; linear and non-linear. Inversions for linear and non-linear parameters need different approaches and they are briefly reviewed here. Since SMD (linear) and other parameters (non-linear) are often needed to be obtained at a time, combined inversion approach, which is a hybrid of linear and non-linear inversions, was proposed and discussed in this review. The results of inversion can be evaluated using satellite data and other plume models including 3D simulations, and help to understand structure of eruption plumes. Since accurate inversions highly rely on granulometric data of the surface deposit, further developments of techniques to obtain granulometric data with lesser time and effort are also required. The SMDs obtained by recent studies show logarithmic decay and a classic theory of particle segregation from turbulent plume can be applicable; however, more case studies are needed, especially to evaluate effect of particle aggregation and in-situ observation of falling particle is critically important.
著者
高橋 正樹
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.40, no.1, pp.33-42, 1995
参考文献数
22
被引用文献数
6 7

There is no positive correlation between the long-term eruption rate of large-scale felsic volcanism and its discharge volume of a single eruptive episode. This means that the storage of voluminous felsic magma at high-level in the crust is caused not by high magma production rate but by continuous accumulation of magma during a long repose time, if the long-term eruption rate reflects the averaged magma production rate. If the cruslal defomation is weak, the magma chamber could be stable in the crust; it is favorable for efficient accumulation of voluminous magma. In fact, the large-volume felsic volcanism occurs exclusively in the region of low crustal strain rate. The low crustal strain rate is considered to be essential for the formation of large-scale felsic volcanism. The large-volume felsic volcanic activity is present in the compressional tectonic stress field as well as in the extensional one; the difference in arrangement of principal stress axes is not related to the occurrence of voluminous felsic volcanism.
著者
三宅 康幸 小坂 丈予
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.43, no.3, pp.113-121, 1998-06-10 (Released:2017-03-20)
参考文献数
14
被引用文献数
2

A steam explosion occurred at about 14:30 JST, February 11th, 1995, in the hot-spring area near Yakedake volcano, central Japan. More than six workers were near the site of the explosion for the road construction, and four of them were buried by the ejected material and killed. A small initial explosion began at the bottom of a 4m deep moat dug by a backhoe and it was followed by the maximum explosion, which ejected about 6,000m3 of blocks (maximum length is more than 2m) and mud, with steam and volcanic gas. The ejecta contain gravels of welded tuff, granite and mesozoic sedimentary rocks, which are the components of a pyroclastic dike of Pliocene age, and pumiceous lapilli tuff derived from the terrace sediments covering the pyroclastic dike. The explosion caused a landslide from the western cliff and the vent was buried by the slid debris, most of which was blown away by the second explosion. All of these processes took place within a few minutes. A small depression (20×5m2) on the west of the mound of the ejecta may represent part of the vent; its depth is estimated to be about 60m or more. Gaseous S02(<30ppm) and H2S(<90ppm) were detected at the explosion site for three days after the explosion. The chemical composition of gas collected from the holes drilled after the explosion were nearly same as the gas from the summit crater of the Yakedake volcano. Because a wall-like Low-Q zone is suggested by seismologists beneath Yakedake volcano and the explosion site, it is most probable that there existed a magma beneath the explosion site and that the heat for the explosion was supplied by the magma and gas exsolved from the magma.
著者
井村 隆介 小林 哲夫
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.36, no.2, pp.135-148, 1991-07-15 (Released:2017-03-20)
被引用文献数
8

This paper presents results of geologic investigation of the eruptive activity in the last 300 years of Shinmoedake, an active volcano in the Kirishima Volcano Group. The recent activity of this volcano is divided into four eruptive episodes : the 1716-1717, 1771-1772, 1822 and 1959 episodes. The most important activity occurred in 1716-1717. During the 1716-1717 eruption, fallout deposits, pyroclastic flows and mudflows were widely dispersed around the volcano. The products of this episode show that the eruption progressed with time from phreatic to magmatic. These field data are in good agreement with historic records of eruptive activity. According to the historic records, the eruptive activity lasted from 11 March, 1716 to 19 September, 1717. The 1771-1772 and 1822 activities produced base surges, pyroclastic flows, fallout tephra and mudflows that were confined to the slope and eastern base of the volcano, but historic records do not reveal the details of these eruptions. The field evidence shows the same phreatic to magmatic sequence as the 1716-1717 activity. However, the eruptions of both episodes were on a smaller scale than the 1716-1717 eruption. The 1959 activity was well described. This episode produced minor gray silty to sandy lithic fallout tephra indicating that only phreatic activity occurred. The fallout was distributed northeast of the vent. In conclusion, the field evidence and historical records show that each eruptive episode of the current activity of Shinmoedake progressed from phreatic to magmatic. The eruptions are frequently accompanied by pyroclastic flows and mudflows.
著者
鈴木 隆介
出版者
特定非営利活動法人 日本火山学会
雑誌
火山.第2集 (ISSN:04534360)
巻号頁・発行日
vol.13, no.2, pp.95-108, 1968
被引用文献数
1

One of the fundamental differences between volcanic cone and non-volcanic mountains such as folded mountains is that the latter itself is a part of the earth's crust, while the former is taken as a heavy load which is laid upon the pre-existing earth's crust within a short geological time and is durable for several tens of thousands of years. In this respect, a volcanic cone resembles an ice sheet, a huge building and a large dam. It is, therefore, postulated that volcanic cone settles down by its own weight. From this point of view, characteristics of the subsidence of some strato-volcanic cones in Japan and Indonesia (Table 1) are comprehensively discussed in this paper. The results are summarized as follows. The settlement of volcanic cone causes various deformations at the foot of volcanic cone such as ring fault, thrust and the circular anticlinally uplifted ridge, all of which tend to encircle the volcanic cone settled. Based on the modes of these deformations at the foot, the settlement of volcanic cone is classified into three types ; 1) fault type, 2) fold type, and 3) mixed type. They are schematically shown in Fig. 5. Which type among the three takes place seems to depend on the nature of the basal rocks beneath the volcanic cone (Table 1 and Fig. 5). The fault type occurs in the case where Pliocene sedimentary rocks are thinner than about two hundreds meters in thickness and also most of the basal rocks are composed of Tertiary sedimentary rocks older than Pliocene. On the contrary, in the case where Pliocene sedimentary rocks are thicker, generally several hundreds to thousands meters, the fold type or the mixed type results. Magnitude of settlement is of order of one to two hundreds of meters in the depth settled. Rate of settlement of Iizuna volcano, which belongs to the fold type, is estimated to be of order of about four millimeters per year. Distance from the center of volcanic cone to the circular deformed feature (D), which is thought to show the magnitude of deformation originated by the settlement, is proportional to the relative height of volcanic cone (H), which can be taken as the substitute for the weight of volcanic cone (Fig. 6). Such relationship between D and H is also found in the case of guyot, which is surrounded by circular moat or ridge (Fig. 5), but not found in the case of collapse calderas as shown in Fig. 6.
著者
小野 晃司 渡辺 一徳 星住 英夫 高田 英樹 池辺 伸一郎
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.40, no.3, pp.133-151, 1995
参考文献数
41
被引用文献数
21

We describe in this paper the character of ash eruption of Nakadake volcano presently going on and maintain that the products of the volcano during the recent geologic past are the main constituents of ashy soils distributed around, especially to the east of, the volcano. Nakadake volcano, the only active central cone of Aso caldera, mainly discharges black, sandy essential ash of basaltic andesite during its active period. The ash is the most voluminous constituent of the products of Nakadake through its activity of recent more than 20,000 years. We call ash eruption for the activity which discharges dominantly ash. The ash is divided into three groups : block-type ash, splash-type ash, and free crystals. The block-type ash, most common, is polyhedral surrounded by a few flat planes. It is formed by brittle fracturing of semi-solid top part of the magma column. While, the splash-type ash, discharged only during the very active phase in active periods, is derived from liquid magma which underlies the semi-solid top of the column. The ash is transported by gas stream from the magma in a quasi-steady state or intermittently, and is distributed around Nakadake volcano in a near-circular pattern by a low eruption column usually less than 1,000 m in height. Long-term thickness contours of the ashy soil from Nakadake volcano in three time intervals, separated by the present earth surface and three ash or pumice layers, of recent more than 20,000 years also show near-circular pattern but a little elongated to the east. 'Loam beds', mainly composed of decomposed and argillized volcanic ash, are said to be formed by accumulation of aeolian dusts during periods of no eruption in volcanic districts. But, loam beds (Akaboku) and black humic soils (Kuroboku) distributed around Aso volcano are composed mainly of primary fall-out deposits of ash or pumice along with aeolian dusts. Ash eruption of Nakadake mostly produces sandy ash rather continuously without long time break but in small rate of discharge. A correlation of detailed columnar sections eastwards of Nakadake crater shows most single strata of ashy soils, light brown- to black-colored, thin according to the increase of distance from the source crater. The ash, especially very fine ash distributed in the distal area, easily decompose and lose primary stratification to form a massive layer which is hardly discernible from aeolian loam beds by their close resemblance. Not only fine ash layers but Plinian pumice fall layers form 'loam' beds which are not distinguishable with adjacent aeolian beds by further decomposition. The thickness contours of the Kanto loam elongate and thin eastward from Fuji volcano over the Kanto plain. The thickness of loam beds changes regionally, thicker in volcanic areas and thinner in non-volcanic areas. These facts suggest contributions of primary pyroclastic falls for thickening of 'loam beds'
著者
井村 隆介 小林 哲夫
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.36, no.2, pp.135-148, 1991
被引用文献数
16

This paper presents results of geologic investigation of the eruptive activity in the last 300 years of Shinmoedake, an active volcano in the Kirishima Volcano Group. The recent activity of this volcano is divided into four eruptive episodes : the 1716-1717, 1771-1772, 1822 and 1959 episodes. The most important activity occurred in 1716-1717. During the 1716-1717 eruption, fallout deposits, pyroclastic flows and mudflows were widely dispersed around the volcano. The products of this episode show that the eruption progressed with time from phreatic to magmatic. These field data are in good agreement with historic records of eruptive activity. According to the historic records, the eruptive activity lasted from 11 March, 1716 to 19 September, 1717. The 1771-1772 and 1822 activities produced base surges, pyroclastic flows, fallout tephra and mudflows that were confined to the slope and eastern base of the volcano, but historic records do not reveal the details of these eruptions. The field evidence shows the same phreatic to magmatic sequence as the 1716-1717 activity. However, the eruptions of both episodes were on a smaller scale than the 1716-1717 eruption. The 1959 activity was well described. This episode produced minor gray silty to sandy lithic fallout tephra indicating that only phreatic activity occurred. The fallout was distributed northeast of the vent. In conclusion, the field evidence and historical records show that each eruptive episode of the current activity of Shinmoedake progressed from phreatic to magmatic. The eruptions are frequently accompanied by pyroclastic flows and mudflows.
著者
石橋 秀巳 岩橋 くるみ 安田 敦 諏訪 由起子 長崎 志保 外西 奈津美
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.66, no.2, pp.119-129, 2021-06-30 (Released:2021-07-27)
参考文献数
51

Recent studies proposed empirical equations describing the relations between amphibole single-phase chemistry and the pressure-temperature-compositional conditions of the coexisting melt. These methods are called amphibole single-phase thermometer, barometer, and melt-chemometer, and have been used in the previous ten years to investigate magma reservoir processes of subduction-related volcanoes. Here, the three methods are briefly introduced with their reliabilities. Then, we review the applications of these methods to clarify magma reservoir processes, chiefly using as examples three volcanoes of Kyushu, i.e. the Tsurumi-dake, Aso and Unzen volcanoes. The pressure-temperature-SiO2 content conditions of the melts estimated from amphiboles enable us to determine physicochemical conditions of the end-member melts of magma mixing, even for cases in which the mixed melt is perfectly homogenized and/or the end-member melt is chemically similar to the mixed melt. We could further identify a phenocryst mineral-melt disequilibrium in a magma, which is usually difficult to recognize from petrography and is a potential factor of misinterpretation for magma reservoir processes, based on the results. Furthermore, the estimated pressures constrain the depth conditions of magma plumbing systems, which can be cross-checked by the results of geophysical observations. These results demonstrate the usefulness of the methods for investigating magma reservoir processes.
著者
西村 太志
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.64, no.2, pp.53-61, 2019-06-30 (Released:2019-07-06)
参考文献数
14

Eruptive activities of volcanoes are examined by using a global data catalog of volcanoes provided from Smithsonian Institution. Yearly numbers of volcanic eruptions with a volcanic explosivity index (VEI) larger than or equal to 2 are almost constant from 1900, and the frequency distribution of the magnitude of volcanic eruptions obeys a power law in the range of VEI≥2. About 10 to 30% of volcanic eruptions with VEI≤2 end within one day, and 8 to 15% continue for more than 1 year. On the other hand, a few percentages of large eruptions with VEI≥3 end for a few days, and 25-30% continue for more than 1 year. Once an eruption occurs, the occurrence possibility of new eruptions at volcanoes locating within a 100km distance increases about 30% for 0.2 year. When the volcanoes locating at a region with a radius of 200km are examined together, yearly numbers of eruptions per volcano fluctuate within a factor of three for a few tens of years at more than about 90% of the regions. Frequency distribution of the yearly number of eruptions follows an exponential decay, which suggests an existence of a characteristic frequency of the eruptive activity on the globe. These averaged features of eruptive activities at volcanoes around the world can be used to give some basic characteristics of the occurrence of new eruptions.
著者
池辺 伸一郎 藤岡 美寿夫
出版者
特定非営利活動法人 日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.46, no.4, pp.147-163, 2001
参考文献数
28
被引用文献数
1 2

Yunotani spa is located to the west part of the Central cones of Aso Volcano. Since the 14th Century, many people have been visiting this hot spring. A few explosions were referred in an old document "Nagano-ke-nikki; Diary of Nagano family". However, the details of these events are not yet clear. We have found a set of historical records on the explosion called "Yunotani Catastrophe" that took place at the solfatara of Yunotani spa in July 6, 1816. These records, two drawings and five reports on the explosion, describe the events and damages in detail. A lot of "hot-mud" with many blocks were ejected from two vents, and destroyed 12 bathrooms and injured one person in the area about 100 m from the vents. Judging from the descriptions and occurrences of the hydrothermally altered ejecta around the vents, it can be said that this event was caused by steam explosions from the solfatara. By the revelation of these records, it became clear that an explosion occurred not only at Naka-dake but also in the area outside of Naka-dake in the historic age. This fact is important for the prevention of volcanic disaster in Aso Volcano.