著者

山元 孝広 高田 亮 石塚 吉浩 中野 俊

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.50, no.2, pp.53-70, 2005-05-20

被引用文献数

5

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The previous eruption history of Fuji volcano has been re-examined by new 100 radiometric carbon ages. The major unconformity between Ko-Fuji and Shin-Fuji volcanoes of Tsuya (1968, 1971) was caused by the edifice collapse resulting in the Tanukiko debris avalanche at about Cal BC 18,000. Voluminous effusion of basalt lava flows in the older ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971) had started at about Cal BC 15,000 and continued until about Cal BC 6,000. Deposition of black soil layer between the Older and Younger Fuji tephra layers of Machida (1964, 1977) started at Cal BC 8,000. After several thousands years quiescent time, basaltic eruptions in the middle ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971) had restarted at about Cal BC 3,600 and thin lava flows had piled up as the central volcanic cone, until about Cal BC 1,700. The eruption style of the volcano changed into explosive basaltic eruptions from the summit and the flank at about Cal BC 1,500; the S-10 to S-22 scoria fall deposits were generated in this first half period of the younger ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971). Also, basaltic pyroclastic flows cascaded down the western flank at about Cal BC 1,500, Cal BC 1,300, Cal BC 1,000 and Cal BC 770. The last summit explosive eruption (S-22) occurred at about Cal BC 300. Immediately after the S-22 eruption, basaltic fissure eruptions had repeated at the flanks until the 1707 Hoei eruption. New data suggest that the Fudosawa, Nissawa and Suyama-tainai lava flows in the southern flank are historical products at about Cal AD 1,000.

著者

伊藤 英之 脇山 勘治 三宅 康幸 林 信太郎 古川 治郎 井上 昭二

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.50, no.6, pp.427-440, 2005

参考文献数

27

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The Yakedake volcano is located in the southern part of the northern Japan Alps, central Japan. Yakedake volcanic hazard map was published in March 2002, and in June 2002, it was distributed to the inhabitants of Kamitakara village, Gifu prefecture, where is located 4-20km west from the volcano. In January 2003, the questionnaire survey was carried out on the inhabitants in order to know their attitudes to the volcanic hazard map and the level of their understanding of the contents of the hazard map. The Kamitakara village office distributed the questionnaires to 1,102 families through the headman of each ward, the headman collected 802 answers. The results of analysis were as follows. 89% of the respondents knew the existence of the hazard map and 35% read it well, but about 11% have not read the map at all. The elders have a tendency to have deeper understanding of the hazard map than younger ones, especially in elders who have experiences to meet some kinds of natural hazards. And the people who once attended the explanatory meeting of the hazard map, which was held for the residents living inside the disaster-prone area four times after the publication of the hazard map, also tend to have more proper understandings. The people who are engaged to the tourism give more attention to the volcanic hazard than others. The respondents have strong tendency to require more knowledge about the volcanic activities and hazards. We can say that the further activities by scientists, engineers and administrative officers are expected in order to establish an informed consent, that is, there should be a decision-making by inhabitants themselves and support by officers in charge with detailed explanations.

著者

西村祐一 宮地直道

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.4, pp.239-242, 1998-08-31

被引用文献数

1

著者

辻 智大 岸本 博志 藤田 浩司 中村 千怜 長田 朋大 木村 一成 古澤 明 大西 耕造 西坂 直樹 池田 倫治 太田 岳洋 福岡 仁至

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.129-160, 2023-09-30 (Released:2023-11-02)

参考文献数

49

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Kuju volcano, located within Beppu-Shimabara graben central Kyushu, Southwest Japan, has been active in the recent 200,000 years. The 54 ka Handa eruption, as large as VEI 5 or 6 and the largest one of the volcano, released large-scale pyroclastic flow deposits (Handa pfd; Kj-Hd) and a wide-spread tephra (Kj-D ash and Kj-P1 pumice fall deposits) that has been reported at more than 500 km from the source. The stratigraphic relationships among the deposits from the Handa eruption are important for volcanology and disaster prevention, and have been studied in various studies, but there is no consensus on the stratigraphy. In this study, we examined the stratigraphic relationships and the eruption history based on the stratigraphic and petrographic studies around Kuju volcano, as well as on Shikoku and Honshu Islands. As the results, the stratigraphic relationships were revealed as follows. 1) The pumice fall deposit, that has been named Kj-Yu, was previously included in Kj-D ash layers, but is revealed to be a much older ejecta than Kj-D ash, along with the tephras newly named Kj-Tb1 and 2. 2) The clay-rich layer just below Kj-D was previously considered to be soil, but it contains a large number of volcanic ash particles so that it is defined as Kj-Y ash layer. 3) Three light brown fine ash layers, newly named Kj-D-U2, 4 and 6, sandwich between the blue grey sandy ash layers i.e. Kj-D-U1, 3, 5 and 7, are revealed to be the co-ignimbrite ash derived from Kj-Hd 1, 2 and 3 pfd, respectively. It suggests that the Kj-Hd1, 2 and 3 pfd are interbedded with Kj-D-U ash layers. 4) Kj-P1 overlies on Kj-D-U7 ash layer that mantled the reworked deposit of Kj-Hd3. 5) Kj-P1 is divided into lower and upper units based on the grain-size analysis, petrography, the chemical composition of glass shards and the isopach maps. Kj-S pfd was formed in the same time as the upper unit. Based on the results, the eruption history is assumed as follows. Pre-Handa eruption: the activity was low and the small-scale explosive eruptions that had released the pumice and volcanic fragments in loam (Kj-Y), followed by a relatively large explosive eruption that had formed Kj-AL. Early phase: the eruption started with phreatic eruption, sub-plinian eruption that deposited the lower unit of Kj-D ash. Subsequently, the eruption changed to vulcanian eruptions that ejected Kj-D-U. This eruption continued for a long period time. During the time, three large-scale pyroclastic flow eruptions happened and has formed Kj-Hd1, 2 and 3. Their co-ignimbrite ashes generated from the Kj-Hd pfds were deposited as Kj-D-U2, 4 and 6. Lahar were generated after Kj-Hd2 and 3 deposition. This phase was terminated by the deposition of Kj-D-U7 ash. Late phase: the plinian plumes occurred twice and deposited lower and upper lalyers of Kj-P1. The second one is the largest plinian eruption in the whole volcano history, with a large umbrella plume producing a wide-spread tephra at more than 500 km from the source and an intraplinian pyroclastic flow (Kj-S).

著者

及川 輝樹 谷 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.65, no.3, pp.83-87, 2020-09-30 (Released:2020-10-13)

参考文献数

18

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The activity of Toshima Volcano, Izu Islands in Japan, is divided into two stages: younger (the lava flows of parasitic craters) and older (the main stratovolcano) stages. The 14C age of “Kajiana” crater lava, the earliest lava of the younger stage, was approximately 11 ka (cal BP). Based on the results of radiocarbon dating and topographic analysis, we conclude that all three magmatic eruptions of the younger stage of Toshima Volcano occurred during the Holocene.

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, 2004-10-29 (Released:2017-03-20)

参考文献数

34

被引用文献数

5

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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

林 信太郎

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.43, no.4, pp.207-212, 1998-08-31 (Released:2017-03-20)

参考文献数

19

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Kampu volcano is a small stratovolcano situated at the central part of Oga Peninsula, Akita Prefecture. In 1810, an earthquake as large as M 6.5 occurred near this volcano. Yoshimasa Satake, the lord of the Akita clan, wrote two official reports to the Tokugawa shogunate. They included eruption records of Kampu volcano: “Yamayake” and “Yamayakekuzure”. These words were usually used for the eruption during Edo period and mean that the mountain was firing. Several reliable documents, which was written at Oga Peninsula included no eruption record. In addition, there is no eruption record in the note of Yoshimasa Satake, which is thought to have used for making the two official reports. It is concluded that the eruption descriptions of 1810 Kampu is false and created by Yoshimasa Satake at Akita clan office at Edo (Tokyo). The false eruption might have been created to make the exaggerated damage report of earthquake.

著者

中道 治久

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.207-220, 2022-06-30 (Released:2022-07-28)

参考文献数

57

著者

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.349-371, 1998

参考文献数

53

被引用文献数

3

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Reevaluation of places, type, magnitude, and influences of the 800-802 A.D. eruption (Enryaku eruption) of Fuji Volcano, Japan, was made through tephrochronology and analyses of historical records. The Nishi Kofuji fissure on the northeastern slope is newly recognized as a crater of the 802 A.D. flank eruption. The Nishi Kofuji fissure ejected fallout scoria toward ENE and lava flows, which can be correlated with Takamarubi and Hinokimarubi 11 Lavas on the northeastern foot. The Tenjinyama-lgatonoyama fissure on the northwestern slope probably erupted during the Enryaku eruption and ejected fallout scoria and lava fiows. A series of historical documents and paintings (Miyashita documents), which are unauthorized, personal records and are regarded to be unreliable by many historians, includes many detailed descriptions of paleogeogra-phy around Fuji Volcano and of the Enryaku eruption. Although some of the descriptions were exaggerated and conflict with geological observations, some of them are concordant with geologic data. The Enryaku eruption probably gave serious damages to ancient traffic routes particularly on the northwestern-northeastern foot of Fuji Volcano. The Gotenba area, which is located on the eastern foot, was also damaged by thin ash-fall and probably by lahars. This caused a temporal, southward relocation of the offical trafiic route, which had passed through the Gotenba area.

著者

安井 真也 高橋 正樹 石原 和弘 味喜 大介

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.52, no.3, pp.161-186, 2007-06-29

被引用文献数

3

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The 1914-1915 Sakurajima eruption was the largest eruption in Japan in the 20th century and erupted andesitic magma was about 1.5km^3 DRE (Dense Rock Equivalent) in volume. Pumice fall and lava flows were generated from the fissure vents on the western and the eastern flanks of the volcano and pyroclastic cones were formed around the vents. Eruptive style changed with time. It is divided into three stages. After the initial, vigorous, Plinian eruption of about 36 hours (Stage 1), extrusion of lava associated with intermittent ash-emitting eruptions with or without detonations lasted for about 20 days on both sides (Stage 2), followed by an outflow of lava for more than 1.5 years on the eastern side (Stage 3). Consequently, the vast lava fields, which consist of a number of flow units formed on both sides of the volcano. Some units of lava show evidence of welded pyroclastic origin, suggesting clastogenic lava. In the western lava field, surface blocks characteristically consist of pyroclastic materials which show variable degrees of welding even within a single block. Typical eutaxitic textures and abundant broken crystals are also recognized under the microscope. Some flow units can be traced upstream to a pyroclastic cone. These features indicate that many flow units of lava on the western flank are clastogenic, which were generated by the initial, Plinian eruption of Stage 1. In the eastern lava field, evidence of pyroclastic origin is rarely discernable. However, the content of broken crystals varies widely from 20% to 80% in volume. Most lava flows, which were erupted in Stage 2 associated with frequent ash-emitting eruptions, contain broken crystals more or less than 50%. This fact indicates that magma in the conduit experienced repetitive fragmentation and coalescence due to intermittent explosions prior to outflow. Lava flows of Stage 3 contain much smaller amounts of broken crystals indicating gentle outflow of coherent lava. Relatively large-scale lava deltas developed toward the sea in the eastern lava field. Eyewitness account at that time reports that ocean entry of lava from several points started several months after the beginning of Stage 3. Although small-scale breakouts formed at the flow fronts of some lava on both sides, a large volume of the deltas can not be accounted for by secondary breakouts of ponded lava within the precedent flow lobes. It is considered that lava tube system fed lava to form the lava deltas.

著者

杉下 七海 常松 佳恵 伴 雅雄 佐々木 寿

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.1, pp.77-89, 2022-03-31 (Released:2022-04-26)

参考文献数

27

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Ballistic projectiles are large pyroclasts (>0.1 m in diameter) traveling through the air without being affected by the flow of gas. This phenomenon is harmful (and potentially fatal) when a volcanic eruption suddenly occurs as the ballistic velocity is quite high, sometimes reaching several hundred meters per second. Therefore, it is important to simulate the trajectory of ballistic projectiles in an affected region. We have estimated the ejection conditions of the 1895 Zao eruption by visually comparing simulated results using a numerical model called “Ballista” to actual block distributions obtained from field observations and aerial photographs. Interestingly, around Goshikidake (northeast of the Okama crater) the farther blocks were from the crater, the larger the block size was. The ejection direction was estimated to be 120° from the north (southeast direction), because the deposit blocks are spatially dense in this direction. The ejection angle was estimated to be 10°, and the ejection velocity was estimated to be 110-120 m/s. The estimated eruption velocity of the 1895 Zao eruption was similar to that of the 2014 Ontake eruption and within the range of small vulcanian eruptions. Although we often worry that a magmatic eruption will occur after a phreatic eruption, it is also possible that a vigorous block emission will occur with a considerably high ejection velocity during a phreatic eruption.

著者

田島 靖久 及川 純 小林 哲夫 安田 敦

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.1, pp.45-68, 2022-03-31 (Released:2022-04-26)

参考文献数

105

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Shinmoedake is the compound volcano in Kirishima Volcano and the most active volcano in Japan, having recorded frequent magmatic eruptions during 1716-1717, 2011, and 2018. The three geological active periods of Shinmoedake in the last 8 ka were recorded by a geological survey (Tajima et al., 2013a). The geological eruptive time category of Shinmoedake is divided into long-term, middle-term, and short-term activities. Short-term activity is captured by monitoring and covers a period of several years or more. The magma eruption rates during middle-term activities were estimated to be several times higher than the long-term magma eruption rate. Moreover, the centers of magma eruptions within each middle-term period had stabilized in terms of location. Additionally, the magma eruption rates during each period of middle-term activity were not constant. Therefore, knowledge regarding the variation in the magma production of Shinmoedake during geologically short-term, middle-term, and long-term activities is required to understand its development and plumbing system. In this paper, we compile recent geological investigation results of Shinmoedake and propose a rational conceptual model of its current state supported by petrological and geophysical data. A well-known conceptual plumbing model of Kirishima Volcano was proposed by Kagiyama et al. (1997). The seismic attenuation spot (reservoir A) is located at a depth of 4-5 km below Karakunidake (Oikawa et al., 1994), and a wide P-wave velocity anomaly area (reservoir W) is situated at a depth of 10-15 km below Kirishima Volcano (Yamamoto and Ida, 1994). Recently, geophysical observations have indicated that magma was supplied from a depth of 8-10 km (reservoir B) to the western area of Shinmoedake during the 2011 magmatic eruption (Nakao et al., 2013). In addition, petrological analysis suggested two different sources of silicic magma from a level of reservoir A and mafic magma from a level of reservoir B (Suzuki et al., 2013a). Therefore, reservoir B might have been connected to reservoir A, where magma mixing occurred during the 2011 eruption. Furthermore, analysis of the deep low-frequency (DLF) earthquake of the 2011 eruption of Shinmoedake revealed that the DLF activities at a depth of 20-27 km (reservoir L1) in the eastern part of Kirishima Volcano were involved (Kurihara et al., 2019). Reservoirs L1 and B may also be connected. These results support the increasing activities of Kirishima Volcano revealed by the geological survey (Tajima et al., 2013a). It is concluded that the complex magma plumbing system of Shinmoedake may cause different magma eruption rates during periods of middle- and long-term activities.

著者

和田 穣隆 南川 実咲

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.4, pp.281-291, 2021-12-31 (Released:2022-02-22)

参考文献数

23

---

Surface structures of magmatic dikes reflect dike emplacement processes. Excellent exposures of the Miocene Hashigui-iwa dike (Wakayama, SW Japan) exhibit well-preserved surface structures including drag folds, scour marks, extension fractures, and cusps. Scour marks are evident on the drag-folded surface, and are in turn cut by extension fractures. Inside the cusps, which are clefts formed by adjacent convex margin irregularities, the drag-folded margins enclose mudstone lenses. Based on our field observations, we infer an emplacement mechanism whereby magma fingers ascended through moderately consolidated host sediments, forming scour marks on chilled margins and causing repeated drag folding. Continued magma flux allowed the fingers to expand, generating extension fractures on the chilled margins. Ultimately, the fingers coalesced, forming the now-preserved cusp structures.

著者

野上 健治

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.4, pp.385-393, 2021-12-31 (Released:2022-02-22)

参考文献数

30

著者

横山 勝三

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.45, no.4, pp.209-216, 2000-08-28 (Released:2017-03-20)

参考文献数

10

被引用文献数

1

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The Ito ignimbrite, the product of a big eruption at Aira caldera about 24,500 y B. P., is distributed very extensively around the caldera in south Kyushu. The region within about 70 km from the center of the caldera was the previously known extent of distribution of the ignimbrite. Recent field research revealed, however, Iocal but extensively-scattered distribution of the ignimbrite in many places beyond the previously known extent of distribution northwest to northeast of the caldera. The farthest site of distribution of the ignimbrite is located about 90 km north of the caldera, indicating that the Ito pyroclastic flow originally spread at least 20 km farther than the previously known extent. The ignimbrite in the remote region is characteristically fine-grained compared with the one near the source. Both pumice and lithic fragments in the ignimbrite decrease, as a whole, in size with distance from source. However, the size of lithic fragments increases in the mountainous area beyond 70 km from source. This is because lithic fragments were incorporated into the pyroclastic flow from local land surface probably due to increased turbulence of pyroclastic flow during the passage on the irregular basal relief. The most remote ignimbrite, at a site 90 km from source, attains to about 35 m in thickness and contains abundant lithics of 5-15 cm in diameter, suggesting that the Ito pyroclastic flow spread farther beyond.

著者

宮縁 育夫 飯塚 義之 遠入 楓大 大倉 敬宏

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.3, pp.157-169, 2021-09-30 (Released:2021-10-29)

参考文献数

19

---

Prior to the onset of magmatic activity at the Nakadake first crater, Aso Volcano (SW Japan) from July 2019 to June 2020, multiple small eruptions occurred between April and May 2019. The May 3-5 eruption was one of the largest events during the pre-magmatic activity period. An ash-fall deposit from the early stage of that eruption (15 : 00-18 : 00 in JST on May 3) was distributed to the south of the source crater, whereas the ash erupted after 20 : 00 on May 3 was dispersed southwestwards. The May 3 15 : 00-18 : 00 ash was composed mainly of fine particles (<0.25 mm in diameter) and fell as accretionary lapilli (<0.8 mm). In contrast, ash after 20 : 00 on May 3 consisted mainly of 0.5 mm grains but lacked silt and clay content. Based on an isomass map, the total discharged mass of the May 3-5, 2019 eruption was about 700 tons. Although lithic (50 %) and altered glass (30-40 %) grains were dominant in both ash-fall deposits, they also included small amounts of black to pale-brown fresh glass shards (2-4 %) inferred to be juvenile material originating from newly ascending magma. After the May 3-5 event, small ash emissions occurred intermittently until July 2019. The proportions of fresh glass shards included in the May-July 2019 ash-fall deposits gradually increased; ash erupted in early July contained 7 % fresh glass grains. Small-scale magmatic activity began on July 26, 2019, and continued to mid-June 2020. The April to early July 2019 ash emissions at Nakadake first crater are inferred to be precursor phenomenon of the late July 2019 to mid-June 2020 magmatic eruptions. It is very important to clarify temporal variations in the mass and component characteristics of erupted materials for understanding the sequence of events and predicting future eruptive activity.

著者

穴井 千里 宮縁 育夫 宇津木 充 吉川 慎 望月 伸竜 渋谷 秀敏 大倉 敬宏

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.3, pp.171-186, 2021-09-30 (Released:2021-10-29)

参考文献数

29

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Nakadake volcano, the current active center of the Aso central cones (Kyushu), is one of the most active volcanoes in Japan. It has been active since ca. 22-21 cal ka, and has formed the old edifice (22-21 cal ka), the young edifice (around 5 cal ka) and the youngest pyroclastic cone (until present). The lava flows from the young edifice spread on the flank of the volcano several times around 5 cal ka. These lavas are supposed to give stratigraphic markers for constructing the eruptive history of Nakadake volcano, but the similarity in chemical composition and lithology hampers distinguishing and correlating them. We have conducted a paleomagnetic study to distinguish and correlate the lavas since the paleomagnetic secular variation (PSV) provides a high-resolution age information. If lava units have a temporal difference of more than 50 years, they could be distinguished by their paleomagnetic directions. The samples were collected from 9 lava flows and 8 agglutinate layers (welded scoria-fall deposits) and were subjected to the paleomagnetic and rock-magnetic measurements. These samples, from visual inspection, appear to be influenced by chemical alteration in the surface of the outcrop by sulfides of volcanic gases. To check a rock-magnetic effect of the chemical alteration of the lavas and agglutinates, thermomagnetic analyses were made on chip samples from the top (surface of rock) and bottom (inside of rock) of the collected paleomagnetic cores. The thermomagnetic analyses indicate that the core top and bottom samples show the same behaviors, in spite of the difference in color, and the carriers of magnetization of each core are titanium rich (titanium content, x, is about 0.6) and poor (x is about 0.1-0.2) titanomagnetites. The natural remanent magnetization of each sample shows a simple, single vector component in alternating field demagnetization experiments, which well defines the primary component. Site mean directions can be categorized into three different direction groups. These data suggest that the eruption producing lava flows and/or agglutinates occurred at three different ages. Furthermore, the paleomagnetic directions of one group is not consistent with the directions of the eruptive ages of Nakadake young edifice assigned from the previous stratigraphic studies. Comparing these directions with the paleomagnetic secular variation curve which has been drawn from basaltic volcanoes in the northwestern part of Aso central cones, the ages of the direction groups can be assigned to around 6.0-4.3 cal ka and 3.5 cal ka, respectively. This result demonstrates that paleomagnetic studies can greatly contribute for establishing the eruptive histories of volcanos.

著者

横尾 亮彦 石井 杏佳

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.3, pp.229-240, 2021-09-30 (Released:2021-10-29)

参考文献数

40

著者

南 裕介 中川 光弘 佐藤 鋭一 和田 恵治 石塚 吉浩

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.3, pp.211-227, 2021-09-30 (Released:2021-10-29)

参考文献数

30

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Meakandake Volcano is a post-caldera active stratovolcano located on the south-eastern rim of Akan Caldera, eastern Hokkaido, Japan. Recent eruptive activity has occurred in 1955-1960, 1988, 1996, 1998, 2006, and 2008 at Ponmachineshiri, which is one of several volcanic bodies that form the stratovolcano. These events indicate that Ponmachineshiri has a high potential for future eruptions. In order to better understand the hazards posed by Meakandake Volcano, this study focused on the modern eruptive activity of Ponmachineshiri during the last 1,000 years. The authors conducted field observations at outcrops in the summit area, excavation surveys on the volcanic flanks, component analysis for pyroclastic deposits, and radiocarbon dating for intercalated soil layers. As a result, at least four layers of pyroclastic fall deposits derived from Ponmachineshiri during the last 1,000 years were recognized, ranging from Volcanic Explosivity Index (VEI) levels of 1 to 2. In chronological order, the major pyroclastic fall deposits consist of Pon-1 (10th to 12th century; VEI 2), Pon-2 (13th to 14th century; VEI 2), Pon-3 (15th to 17th century; VEI 1), and Pon-4 (after AD 1739; VEI 1), with small-scale (VEI<1) phreatic and phreatomagmatic eruption deposits intercalated within Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits. The presence of scoria and minor pumice in the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits suggests that these eruptions were phreatomagmatic events. On the other hand, the absence of juvenile materials in the Pon-4 pyroclastic fall deposits suggests that the activity was a phreatic eruption. The decreasing proportion of juvenile materials in eruptive deposits over the last 1,000 years is consistent with a reduced magma contribution and indicates that the development of the hydrothermal system is likely to play an important role in future eruption scenarios for Meakandake Volcano.

著者

奥村 聡 三輪 学央

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.1, pp.35-43, 2021-03-31 (Released:2021-03-25)

参考文献数

65

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A phreatomagmatic explosion is a type of eruption observed on the Earth’s surface. This explosion is common because Earth is a water planet and its surface is extensively covered with water. The mechanism of this explosion can be explained as follows: magma and water mix, following which efficient heat conduction occurs and the water evaporates, finally causing the explosion. However, the mechanism of mixing of the high viscosity magma with water during its ascent remains elusive, although it often causes a phreatomagmatic explosion. In this paper, we review the previously proposed mechanisms of phreatomagmatic explosion and then, based on the petrological and geophysical observations for the 2015 eruption of Kuchinoerabujima Volcano, evaluate whether the recognized mechanisms can explain the mixing of high viscosity magma with water. To explain such an explosion, we consider a new model based on the rheological view of the magma. The laboratory experiments have revealed that the high viscosity magma exhibits shear-induced brittle fracturing, resulting in dilatancy and increased permeability. In addition, the fracturing is a common process observed in high viscosity silicic magmas intruded into shallow parts of the upper crust. Based on these observations, we propose that the shear-induced brittle failure of the high viscosity magma in a volcanic conduit causes the decompression of fluid in the magma and water in the crust diffuses into the magma, resulting in heating and pressurisation of water and additional fracturing (magma fracturing and water diffusion model). The feedback between pressurisation, fracturing, and additional thermal interactions results in an explosion. This hypothesis is attractive because of the efficient mixing of high viscosity magma and water, thus facilitating a spontaneous interaction during magma ascent. To strengthen this hypothesis, additional laboratory experiments and field-based observations will be necessary in future studies.