著者

早川 由紀夫 小山 真人

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.5, pp.403-407, 1998-10-30 (Released:2017-03-20)

参考文献数

21

被引用文献数

9

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

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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

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A large-scale collapse occurred at the eastern slope of Fuji volcano about 2900 years ago, based on calibrated ¹⁴C 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 km² 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

井村 隆介

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.5, pp.373-383, 1998-10-30 (Released:2017-03-20)

参考文献数

27

被引用文献数

5

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The eruptive sequence of the An-ei eruption of Sakurajima volcano (1779-1782) is revealed by historical records. From the evening of November 7, 1779 (the 29th day of the 9th month in the 8th year of An-ei), Kagoshima and its environs were shaken frequently. At 11 a.m. of the next day, the water in the wells in the island boiled up, spouting at several points and the color of sea became purple. On the noon of the same day, minor white plumes rose up from the Minamidake summit crater. At about 2 p.m., plinian eruption oecurred at the southern upper slope of Minamidake, and several tens of minutes later, at the northeastern flank of Kitadake. The height of eruption column reached about 12000 meters. It is estimated that a pyroclastic flow was generated at 5 p.m. The plinian eruption climaxed from the evening of November 8, to the morning of next day, and later was followed by emission of lava flows. The activity of the southern craters ceased within a few days, but lava emission from northeastern craters lasted for a long period. On November 11, the lava flow from northeastern craters entered into the sea. Since then, submarine explosions occurred repeatedly off the northeastern coast, and it continued to January 18, 1782. Nine small islands produced by this submarine volcanic activity during a year. Submarine explosions caused small tsunamis on August 6 and 15, September 9, October 3 1, November 9, 1780 and April 11, 1781.

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

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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

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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

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

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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

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

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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

長谷川 健 菊池 文太 柴田 翔平 井村 匠 伴 雅雄 常松 佳恵 山本 裕二 大場 司 鈴木 和馬 戸丸 淳晴 楠 稚枝 岡田 誠

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.68, no.3, pp.189-196, 2023-09-30 (Released:2023-11-02)

参考文献数

22

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Volcanic bomb is one of the most common eruption products around their source craters. Although paleomagnetic studies on volcanic bombs have a potential to provide high-resolution chronology of volcanic activity, particularly when compared with the known geomagnetic secular variation records, there are only a few such studies. In this contribution, we made an attempt to determine paleomagnetic directions from large (>1 m in diameter) volcanic bombs around “Tsubakuroswa craters”, located in Azuma volcano, for evaluating the potential use of volcanic bombs for paleomagnetic dating. Six oriented mini-cores were drilled from the central part of each large volcanic bomb, five in total, located on a gentle slope a few hundred meters south from the craters. All of the mini-cores were subjected to thermal demagnetization analysis, giving a well-determined characteristic remanent magnetization (ChRM) direction for each bomb as follows: site mean declination (Dm) of 350.6‒358.0º and inclination (Im) of 48.9‒50.8º with a 95 percent confidence limit (α95) smaller than 2.4º. The ChRM directions were consistent among the bombs, supporting the availability of volcanic bombs for further paleomagnetic dating research. Referring the geomagnetic secular variation record in this area, an all-site mean ChRM direction from the five bombs (Dec=355.5º, Inc=50.1º, α95=1.9º) most likely accounts for the derivation of the volcanic bombs by the Meiji Era (1893 CE) eruption. Historic pictures and descriptions are consistent with and support this interpretation. Previous reports suggested that the Meiji Era eruption did not eject magmatic materials and that the last magmatic eruption of this volcano was probably in 1331 CE. However, our results suggest that magmatic eruptions might have occurred here only ca. 130 years ago and may be largely affecting the current activity of this crater area. Our study suggests that volcanic bombs are potentially useful materials for paleomagnetic studies such as dating and establishing geomagnetic secular variation records.

著者

加藤 祐三

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:24330590)

巻号頁・発行日

vol.33, no.1, pp.21-30, 1988-04-01 (Released:2018-01-15)

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Since the late of May, 1986, many pumices have drifted to the Ryukyu Islands. These pumices are characterized by their size, color and components. The pumices vary in size: some are about 5 mm and the maximum size is about 20 cm. Most of these pumices are gray and a few of them are drak- or light-gray. The pumices contain several percent of essential black xenolithes which vary in size from 1 mm to 5 mm and rarely 2 cm. The rock name of pumices is olivine-augite-bearing trachyte and that of xenolithes is forsterite chromian diopside trachybasalt. These characteristics of the pumices correspond with those of pumices erupted from Fukutoku-oka-no-ba in the north of the Mariana Islands in January 18-21, 1986. Moreover, the chemical composition of host pumice (free from xenolith), bulk composition (including xenolith) and mineral chemistry of phenocryst also correspond with those of the pumices effused from Fukutoku-oka-no-ba. Based on these facts, it is evident that the pumices drifted to the Ryukyu Islands had their origins in Fukutoku-oka-no-ba. Judging from the data concerning the current and wind around the area through Fukutoku-oka-no-ba toward the Ryukyu Islands, it is confirmed that the pumices reached the Ryukyu Islands from Fukutoku-oka-no-ba after four months under the effect of current and wind which run both to the direction of west.

著者

東宮 昭彦

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.61, no.2, pp.281-294, 2016-06-30 (Released:2016-09-12)

参考文献数

105

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Recent developments in the understanding of magma chambers (reservoirs), pre-eruptive magma processes, and the conditions that lead to volcanic eruptions are reviewed mainly from a petrological point of view. A “magma reservoir” consists of inner “magma chamber (s)” filled with eruptible magma containing less than ca. 50% crystal, and outer “mush” region with more than ca. 50% crystal. Most of magma reservoirs are in a state of mush, so that “rejuvenation” or “remobilization” is necessary before eruption. Magma can erupt if its viscosity is less than ca. 106 Pa s. More viscous magma can erupt only after a precursory eruption of less viscous magma, such as a hybrid magma between the viscous magma and a less viscous mafic magma. In this context, pre-eruptive magma viscosity, i.e. magma viscosity at the magma reservoir, is an important measure to evaluate magma eruptibility. Dating for whole mineral or even its local point (e.g., zircon age) and diffusion analysis for various types of minerals (e.g., magnetite, olivine, pyroxene, and plagioclase) have revealed timescales of pre-eruptive magma processes. Eruption triggers, such as injection of high-temperature magma, are inferred to occur days to months before the eruption in many cases. Magma residence times, during which the magmas are in eruptible conditions, are years to decades for typical magma systems, but may reach hundreds of thousand years for large caldera systems.

著者

横山 光

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2016 (ISSN:24335320)

巻号頁・発行日

pp.51, 2016 (Released:2017-02-07)

著者

濁川 暁 石崎 泰男 吉本 充宏 寺田 暁彦 上木 賢太 中村 賢太郎

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2014 (ISSN:24335320)

巻号頁・発行日

pp.139, 2014-11-02 (Released:2017-02-10)

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

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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

杉森 玲子 前野 深

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.68, no.2, pp.59-73, 2023-06-30 (Released:2023-07-27)

参考文献数

41

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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.

著者

新村 太郎 三好 雅也 角野 浩史 上田 恭裕 森 康 長谷中 利昭 荒川 洋二 長尾 敬介

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.67, no.2, pp.135-147, 2022-06-30 (Released:2022-07-28)

参考文献数

36

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To clarify the age of the volcanic activity of Nekodake volcano, the authors conducted K-Ar dating on ten samples of volcanic materials, using the sensitivity method. Four samples yielded ages approximately 50 to 82 ka. These newly obtained K-Ar age data indicate that Nekodake volcano was active during the post-caldera stage of Aso volcano. Based on the obtained ages, petrological characteristics, and lithofacies, the volcanism of Nekodake can be summarized as follows. 1) Effusive eruption of basaltic andesite magma produced thick lava flows at about 82 ka, and also formed agglutinates over a wide area at about 66 ka. The main volcanic body was formed by these eruptions. 2) Andesitic magma intruded the central part of the volcano at about 64 ka (late stage or just after the formation of the main part) forming dykes in an ENE-SSW direction. 3) Basaltic andesite magma intruded the central part of the volcano at about 50 ka and formed dykes in a NW-SE direction.

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

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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

西川 空良 浜本 未希 本田 明紗海 図子田 和典 清藤 大河

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.68, no.2, pp.99-104, 2023-06-30 (Released:2023-07-27)

参考文献数

12

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

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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

及川 輝樹 池上 郁彦 渡部 将太

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.68, no.3, pp.171-187, 2023-09-30 (Released:2023-11-02)

参考文献数

73

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Submarine volcanic eruptions produce a large amount of drifting pumice around the globe at a frequency of once every several to ten years. However, there is little knowledge about what kind of eruptions produce them. Document records of large amounts of pumice washed ashore in the Nansei Islands in southwest Japan are summarized in this article, along with an assessment of their source volcanoes and eruption frequency and style. A large amount of drifting pumice has washed ashore on the Nansei Islands eight times (1778 or 1779, 1905, 1914, 1915, 1924, 1934, 1986, 2021) since the 18th century, seven of which were after the 20th century, at a frequency of several times every 100 years. This frequency is not remarkably low compared to other natural hazard events. The eruptions that provided the source of these drifting pumice were the Fukutoku-Oka-no-Ba 1904‒1905 and 1914 eruptions, the Myojin-Shou 1915 eruption, Submarine Volcano NNE of Iriomotejima 1924 eruption, the Showa Iwo-jima 1934 eruption and the Fukutoku-Oka-no-Ba 1986 and 2021 eruptions. In the 8 recorded volcanic eruptions, including the uncertain ones, 6 were due to submarine volcanic eruptions in the southern part of the Izu-Bonin Arc. It took 2 to 6 months (mostly 2 to 4 months) for drifting pumice to reach the shores of the Nansei Islands from the Izu-Bonin Arc. The eruption styles that generated a large amount of drifting pumice can be divided into three cases. (1) An eruption that ejects a large amount of pumice from the seafloor to the sea surface and causes a steam-based volcanic plume to rise at the center of the eruption. They often occur from vents with seafloor depths of several hundred meters (Submarine Volcano NNE of Iriomotejima 1924 and Showa Iwo-jima 1934), but they also occur in shallower waters (Myojin-Shou 1915). (2) An eruption that occurs in shallow water (<50 m), with repetitive Surtseyan eruption activity that forms islands composed of large amounts of pyroclastic material, while at the same time producing large amounts of drifting pumice (Fukutoku-Oka-no-Ba 1904‒1905, 1914 and 1986). (3) An eruption that occurs in shallow water (<50 m) and produces a volcanic plume reaching the stratosphere. This style of eruption forms an island and generates a large amount of pyroclastic material, including drifting pumice (Fukutoku-Oka-no-Ba 2021).