著者

安井 真也 高橋 正樹 金丸 龍夫 長井 雅史

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.293-325, 2021-12-31 (Released:2022-02-22)

参考文献数

36

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The Asama-Maekake volcano has been active during the last 10,000 years. Large-scale eruptions that occurred in the 18th and 12th centuries have been well studied, whereas little information is available for older eruptions. In this paper, we aim to reconstruct the history of this volcano in detail through a combination of extensive geological survey and 14C dating. The observation and description of twenty-one trench excavations, two drilling core samples, and many outcrops enabled us to build a stratigraphy of the eruptive products in much greater detail than ever before. The trench excavation sites cover an area of nearly 180 degrees around the volcano. These sites were selected mainly in the medial area at distances between 5 and 10 km from the summit crater. Many older deposits buried by thick younger deposits were found. The pyroclastic fall deposits of this volcano vary from a thick pumice layer to pumice grains scattered in the black soil, indicating a variation in the scale of sub-plinian eruptions. More than 120 samples for 14C dating were taken from the black soil immediately beneath the pyroclastic fall deposits. Some charcoals contained in the pyroclastic flow deposits were also subjected to dating. The 14C dating results were used for the correlation of deposits of different localities and distributions of some pyroclastic fall deposits older than 2000 years were revealed. The qualitative evaluation of the number and scale of eruptions throughout history might be possible using these data. Four classes of eruptive scales are recognized in the pyroclastic fall deposits in this study. Class 1: Defined by the isopach line for 128 cm thickness being able to be drawn on the map and the area enclosed by the 64 cm isopach line being more than 25 km2. The deposits are recognized at distant locations more than 50 km from the summit crater. Class 2: Defined by that the isopach line for 64 cm thickness being able to be drawn on the map and the area enclosed by the 16 cm isopach line being more than 15 km2. Class 3: The deposit of this class is recognized as a distinct layer that continues horizontally at each locality. In most cases, the observed thickness is less than several tens of centimeters and generally no structure can be observed. Class 4: This class comprises scattered pumice grains in the soil, for which the measurement of thickness is impossible. The deposits of classes 3 and 4 are seldom found at distances farther than 15 km from the crater. Most of the pyroclastic fall deposits of the period between 9400 and 3100 cal BP are of Classes 3 and 4. On the other hand, a large-scale eruption (Class 1) occurred about 2000 years ago, generating pyroclastic fall deposits in distant areas of more than 50 km from the crater. The recurrence interval of large-scale eruptions during the last 2000 years is estimated to be about 700 years. This is less frequent than in the period prior to 2000 years ago. Therefore, a change in eruption mode occurred about 2000 years ago when eruptions became infrequent but large in scale.

著者

安井 真也 金丸 龍夫 松田 ひかる

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.4, pp.479-488, 2022-12-31 (Released:2023-01-30)

参考文献数

9

著者

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.43, no.2, pp.43-59, 1998-04-30

被引用文献数

4

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A large amount of pyroclastic materials (ca. 1.7 km^3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO^*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.

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

---

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

安井 真也 高橋 正樹 島田 純 味喜 大介 石原 和弘

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.58, no.1, pp.59-76, 2013-03-29

被引用文献数

1

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桜島火山の歴史時代の大規模噴火である安永噴火(1779-1782年)と大正噴火(1914-1915年)の噴出物の岩相や層序,地形,噴火当時の記録を比較した.両噴火では山頂をはさんだ両測山腹で割れ目火口列が活動した.割れ目の推定の長さは大正噴火で約2.3km,安永噴火では5kmに及ぶ.噴火開始後数10時間の大正噴火と安永噴火の噴火様式は共通しており,プリニー式噴煙柱から火口近傍への大量の火砕物降下により斜面上に火砕丘を形成しながら火砕成溶岩をもたらした.引き続く数週間には両噴火とも溶岩流出が繰返されて溶岩原が形成された.その後は,大正噴火が陸上での溶岩流出を主としたのに対し,安永噴火では北東沖で海底噴火が起きて安永諸島を形成した点で大きく異なる.両噴火とも噴火初期に割れ目火口近傍へ著しい火砕物降下があることが特徴的である.これは火山体形成の観点からは,両噴火では山頂部の地形変化はほどんどないが,山腹斜面が成長したことを意味する.また桜島の大規模噴火の減災という観点では,居住地域近くまで到達しうる割れ目火口の活動への迅速な初期対応の重要性を示している.

著者

安井 真也

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.60, no.2, pp.269-274, 2015-06-30 (Released:2017-03-20)

著者

高橋 正樹 安井 真也 黒澤 貴之

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

巻号頁・発行日

vol.2010, 2010-10-09

著者

安井 真也 高橋 正樹 石原 和弘

出版者

日本大学文理学部自然科学研究所

雑誌

日本大学文理学部自然科学研究所研究紀要 地球システム科学 (ISSN:13432745)

巻号頁・発行日

no.41, pp.75-107, 2006

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 (Released:2017-03-20)

参考文献数

49

被引用文献数

4

---

The 1914-1915 Sakurajima eruption was the largest eruption in Japan in the 20th century and erupted andesitic magma was about 1.5km3 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

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.43, no.2, pp.43-59, 1998-04-30 (Released:2017-03-20)

参考文献数

22

---

A large amount of pyroclastic materials (ca. 1.7 km3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.

著者

安井 真也

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.3, pp.117-134, 2017-09-30 (Released:2017-10-11)

参考文献数

25

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The Asama-Maekake volcano has been active for about 10,000 years. Although the many pyroclastic fall deposits, which mainly consist of pumice fall layers, are indicative of past sub-Plinian eruptions, the stratigraphy of the deposits over the entire history of the Asama-Maekake Volcano has not yet been revealed. This is because the pyroclastic fall deposits are distributed in the various directions from the crater. Moreover, the deposits predating the 12th century have a similar occurrence and petrography, making it difficult to correlate deposits among different localities. Therefore, little information is available for the older eruptions. The pyroclastic fall deposit in the direction not leeward of the dominant wind is focused in this study. A pyroclastic fall deposit called Miyota pumice (referred to as As-My hereafter), which is characteristically distributed in the southern direction from the crater, was targeted. The distribution of As-My has barely been mapped owing to the difficulty resulting from its poor exposure. The 14C ages of the samples of black humus soil immediately beneath As-My at two localities were dated to 5720±30yBP and 5530±30yBP. These ages are almost the same as those of the pyroclastic fall deposits As-UB (Ubagahara) and As-Kn (Kuni) distributed on the northern flank. Since the stratigraphic relation among these deposits is difficult to determine, it is not clear whether As-My, As-UB, and As-Kn are products from a single eruption or different eruptive activities. At least, it can be considered that multiple sub-Plinian eruptions occurred around 6,000 years ago. In addition, black humus soil and intercalating four pumice fall deposits including As-My at a major outcrop on the SSE flank slope (Locality M72) provided information on the activity over the last 8,000 years, back to the earliest stage of the volcano. The 14C ages of samples of black humus soil taken from immediately beneath the pumice fall deposits were 3,830±30yBP, 4,710±30yBP, 5,530±30yBP (just beneath As-My), and 7,470±30yBP. This is the first time that such sequential data concerning eruptive ages on the flank slope has been obtained. Black soil contains angular lithic fragments, which are similar to the ash grains from Vulcanian eruptions (e.g., the 2004 eruption). This suggests that Vulcanian eruptions have occurred between sub-Plinian eruptions since the early stage of the Asama-Maekake volcano. Such information from outcrops on the flank slopes with various directions is required to reconstruct the detailed eruptive history.

著者

安井 真也

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.60, no.2, pp.269-274, 2015-06-30