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

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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

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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.

著者

及川 輝樹 原山 智 梅田 浩司

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.1, pp.21-25, 2001-02-27 (Released:2017-03-20)

参考文献数

23

被引用文献数

3

著者

西来 邦章 石毛 康介 島田 駿二郎 中川 光弘

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.62, no.2, pp.83-94, 2017-06-30 (Released:2017-07-25)

参考文献数

39

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Zircon fission-track (FT) and uranium-lead (U-Pb) dating were carried out to determine the ages of the Biei, Tokachi, and Sounkyo pyroclastic flow deposits in the Biei and Kamikawa areas of central Hokkaido, northern Japan. We collected pumiceous tuff samples of the Biei pyroclastic flow deposits from two sites in the middle and lower reaches of the Biei River, and Tokachi pyroclastic flow deposits from one site to the west of Tokachi caldera. A sample of welded tuff from the Sounkyo pyroclastic flow deposits was obtained from one site in the lower reaches of the Antaroma River. The FT ages of the Biei pyroclastic flow deposits are 0.81±0.08 Ma and 0.72±0.08 Ma, identical to each other within 1σ error. However, they differ from an age of 1.91±0.06 Ma reported previously from the upper reaches of the Biei River. Based on the present data and previous results on the ages and petrographical characteristics of the deposits, they can be divided at least two geological units with different eruption ages. A FT age of 0.058±0.018 Ma (1σ) was obtained from the Sounkyo pyroclastic flow deposits. On the basis of previous studies concerning the distribution and petrographical characteristics of the deposits, this age was obtained from Hb-type pyroclastic flow deposit among the Hb- and Py-type flows of the Sounkyo pyroclastic flow deposits. The Tokachi pyroclastic flow deposits yielded a U-Pb age of 1.24±0.02 Ma (2σ), which falls within the wide range of ages reported in previous studies. Because the Tokachi pyroclastic flow deposits have a wide distribution and a wide range of ages, they can be divided into several geological units with different eruption ages, as with the Biei pyroclastic flow deposits.

著者

上澤 真平

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.53, no.6, pp.171-191, 2008-12-29 (Released:2017-03-20)

参考文献数

48

被引用文献数

1

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On May 24th 1926, the eruption of Tokachidake volcano, in central Hokkaido, efficiently melted the snow pack on the hill slope, triggering the Taisho lahar which killed 144 people in the towns of Kamifurano and Biei. A geological survey and paleomagnetic and granumetric studies were conducted on the northwestern slope of Tokachidake volcano to reconstruct the sequence of the 1926 eruption and decipher the triggering mechanism for the Taisho lahar. The Taisho lahar deposits in the proximal area of the volcano are divided into five distinct units (unit L1, L2, and A through C, from oldest to youngest). Unit L1 is an older lahar deposit that underlies the 1926 deposits. The 1926 sequence consists of debris avalanche deposits (unit A and C), a laminated sandy debris flow deposit (unit B), and a lahar deposit including scoria clasts (unit L2). Each unit contains hydrothermally altered rocks and clay material with more than 5 wt.% fragments smaller than 2mm in diameter. The progressive thermal demagnetization experiments show that the natural remanent magnetization (NRM) of all samples in unit A, B and C have a stable single or multi-component magnetization. The emplacement temperatures are estimated to be normal temperatures to 620℃ for unit A, 300 to 450℃ for unit B, and normal temperature to 500℃ for unit C. On the basis of geological and paleomagnetic data and old documents, a sequence for the eruption and the mechanism of formation and emplacement of the Taisho lahar can be reconstructed. The first eruption at 12: 11 May 24th triggered a small lahar (unit L2). Collapse of central crater at 16:17 May 24th 1926 then resulted in a debris avalanche containing highly altered hydrothermal rocks with hot temperatures ranging from 300 to 620℃ (unit A). The debris avalanche flowed down the slope of the volcano, bulldozing and trapping snow. Immediately following the collapse, a hot (approximately 400℃) hydrothermal surge (unit B) melted snow and transformed into a lahar causing significant damage and deaths in the towns downstream. Just after the generation of the lahar, another collapse occurred at the crater causing another debris avalanche (unit C).

著者

上野 寛 森 博一 碓井 勇二 宮村 淳一 吉川 一光 浜田 信生

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.47, no.5, pp.689-694, 2002-11-29 (Released:2017-03-20)

参考文献数

14

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We studied high-frequency earthquake swarm associated with the eruption of the Usu volcano in 2000 using the data observed by a national seismic network in southern Hokkaido. To get a precise hypocenter location, we applied the double-difference method and station correction to hypocenter determination. Systematic shift of epicenters possibly caused by heterogeneous velocity structure of the upper crust is needed to be consistent with the initial motions of the seismograms at the nearest station. Concentration of hypocenters under the northern flank of the volcano in the initial stage suggests that the magma started its activity at about 5 km in depth at the region. Concentric expansion of swarm area occurred before the eruption and formed doughnut pattern of which center is located near the summit of the volcano. Doughnut pattern may represent relaxation of stress under the volcano which is caused by magma movement and pore pressure change under the volcano.

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

被引用文献数

6

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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

上木 賢太 寺田 暁彦

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.57, no.4, pp.235-251, 2012-12-28 (Released:2017-03-20)

参考文献数

32

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

被引用文献数

4

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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 6,000m^3 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 (20×5m^2) 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 S0_2(<30ppm) and H_2S(<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

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.38, no.6, pp.223-226, 1993-12-20 (Released:2017-03-20)

参考文献数

7

被引用文献数

1

著者

宮縁 育夫

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.62, no.1, pp.1-12, 2017-03-31 (Released:2017-03-28)

参考文献数

27

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Janoo Volcano (550-750 m in basal diameter;150 m in height) is located in the northwestern part of the post-caldera central cones of Aso Volcano, central Kyushu, southwestern Japan. The volcano had been thought to be a cinder cone composed entirely of scoria-fall deposit and older than 7.3 ka. Fieldwork in and around the volcano has re-examined the detailed tephra stratigraphy and eruption age of Janoo Volcano. A black humic paleosol divides an upper pumice-fall deposit from a lower scoria-fall deposit. The upper pumice-fall deposit shows only two pure pumice bed sections with pumice clasts scattered in a brown massive ash elsewhere in the deposit. The deposit is composed mainly of light gray well-vesiculated dacitic (SiO2=65.4-67.7 wt.%) pumiceous clasts containing biotite phenocrysts, and abundant banded pumices, suggesting a mixture of silicic and mafic magmas. Based on the phenocryst assemblage and age, the pumice-fall deposit is correlated to the Aso central cone pumice 1 (ACP1;4.1 ka), which is the only pumice-fall deposit erupted from Aso Volcano during Holocene time. The lower scoria-fall deposit is more than 30 m thick and constitutes most of the Janoo cinder cone. It includes brownish black to brown well-vesiculated basaltic andesite (SiO2=54.7-55.5 wt.%) scoriaceous clasts and cauliflower bombs with radially arranged cooling joints. The Akamizu lava (SiO2=57-59 wt.%) distributed west of the Janoo cinder cone, whose source was previously unknown, is attributed to Janoo Volcano based on the lava’s petrographic characteristics. A 14C age of 3830±30 years BP, which corresponds to 4.2-4.1 ka, was obtained from the humic paleosol interbedded between the ACP1 and Janoo scoria. The stratigraphy and characteristics of the tephra deposits suggest the following eruption sequence. The initial eruption at Janoo Volcano occurred at 4.9-4.3 ka and was strombolian in style forming the Janoo cinder cone. After lying in repose for a few hundred years, Janoo Volcano erupted again, and produced the ACP1 tephra containing abundant banded pumices and Akamizu lava at 4.1 ka. The southern half of the Janoo cinder cone was destroyed probably by the effusion of Akamizu lava. Volcanic activity forming Kishimadake, Ojodake, Komezuka and Kamikomezuka volcanoes in the northwestern part of post-caldera central cones at 4-3.3 ka was derived from basaltic to basaltic andesite magmas, whereas the eruption products of Janoo Volcano have a wide range in chemistry from basaltic andesite to dacite. Activity of Janoo Volcano is characterized by the presence of a dormant period (a few hundred years), allowing a paleosol to develop on the scoria-fall deposit, before ejection of both mafic and silicic magmas in the late eruption.

著者

黒墨 秀行 土井 宣夫

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.48, no.3, pp.259-274, 2003-07-10

被引用文献数

5

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The Nigorikawa Caldera in southwest Hokkaido, Japan, is 3 km in diameter at the outer rim. Drilling data from 42 geothermal wells of up to -3,000 masl (m above sea level) has been used to study the internal structure of the caldera. Interpretation of the data shows an angular funnel shape, with a wide upper region (3×2.5 km) tapering to a narrower lower region (0.7×0.5 km). The shear zone is the same shape as the caldera, that is, rectangular with a NE-SW elongation. The caldera is infilled with vent-fill material, lake and alluvial deposits, landslide deposits, and post-caldera intrusions. The vent-fill material is a gray, non-welded lapilli tuff and tuff breccia, which homogeneously includes accidental lithics and shattered fragments, which were sheared during pyroclastic eruption, as well as accretionary lapilli occurring up to -824 masl. The vent-fill is intercalated with many lithic bands or lithic dominant zones that dip toward the caldera center. No large fault displacement can be recognized around the caldera wall. The Nigorikawa Caldera was formed ca 12,000 years ago by violent pyroclastic flow eruption, fall-back, and the following subsidence by compaction with degassing.

著者

前田 美紀 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.57, no.1, pp.19-35, 2012-03-30 (Released:2017-03-20)

参考文献数

42

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Formation mechanism of basaltic pyroclastic flows has not been sufficiently clarified yet because basaltic pyroclastic flows do not occur as frequently as felsic ones. We studied the Osawa pyroclastic flow 3 deposit (OsPfl-3), which took place on the western flank of the Fuji volcano between 2.9 and 3.0 ka. OsPfl-3 has two flow units and one cooling unit, which have a combined volume of 6.2 × 106m3. The flow overlies another unit composed of two scoria fallout deposits (YokSfa-2a and 2b) which sandwich a pyroclastic flow deposit (OtPfl). OsPfl-3 mainly consists of welded blocks and dense blocks with composition and petrographical characteristics of basaltic andesite. Some of the dense blocks have cracks on their surfaces and look like “cauliflower-shaped bomb”. They have a flat surface on one side with concentration of vesicles near the surface. The matrix of OsPfl-3 has dense fragments that are thought to have originated from dense lava blocks and poorly vesiculated scoria. The emplacement temperature of the blocks is estimated to be higher than 580℃ from thermoremanent magnetization measurements. These observations indicate that the blocks in the OsPfl-3 originated from welded pyroclasts, lava flow or lava lake at the summit crater. The sequence of the eruptions that formed OsPfl-3 and underlying deposits are summarized as follows: Stage 1: Deposition of fallout tephras (YokSfa-2a and 2b) and an intercalated pyroclastic flow (OtPfl) which are composed of fairly vesiculated scoria; Stage 2: Formation of lava flow or lava lake at the summit crater, and deposition of pyroclastics on the lava; Stage 3: Occurrence of the pyroclastic flow (OsPfl-3) caused by collapse of lava and pyroclastics. OsPfl-3 is prominently distributed on the western flank. This observation implies that the westward flow from the source lava that filled the summit crater could cross the lower part of the crater rim.