─ ─ 97 ( ) 永噴火で大量に形成された溶岩について検討し,島弧玄 武岩質溶岩の表面形態についての記載とその成因につい ての議論を行うとともに,安永大規模噴火における溶岩 流出のプロセスについても明らかにすることをめざし た。 2 安永噴火の推移 歴史時代噴火の推移は古記録からある程度推定が可能 である。ここでは村山(1979)によってまとめられた「武 江年表」,「大島山火記」,「日本噴火誌」,「震災予防調査 会報告第八十一号」などに書かれた古記録および古記録 にもとづいた記録をもとに,安永噴火の推移を推定し た。 噴火は安永 6 年(1777 年) 7 月 29 日(旧暦)に始まった。 1 はじめに 日本列島の第四紀火山には玄武岩質のものは少なく, 伊豆大島,三宅島,富士など特定の火山に限られている。 そのため,玄武岩質溶岩の表面形態に関する記載や議論 は,富士火山の青木ヶ原溶岩など一部の例(小幡・海野, 1999;高橋・他,2004・2005 など)を除きあまりなされ てこなかった。伊豆大島火山最後の大規模噴火は江戸時 代後期の安永噴火(1777 -1778 年)であるが(Nakamura, 1964),安永噴火では,現在の三原山火砕丘が形成され るとともに,大量の玄武岩質溶岩が流出し,カルデラの 北東部を埋め尽くして,海岸から海へと流入している (一色,1984 など)。安永噴火の噴出物の総量は約 6.5 億 トンと推定されている(Nakamura, 1964)。本論では安 御園生 裕介 * ・高橋 正樹 ** ・安井 真也 ** The An-ei eruption of Izu-Oshima volcano began in June (the lunar calendar) 1777 A.D. and continued to January 1778 A.D., constructing the Miharayama pyroclastic cone by violent strombolian to sub-plinian eruptions. From March to November 1778 A.D., the mode of eruption changed and nearly aphyric basaltic lavas poured out from the vent situated in the northern and southern foot of the Miharayama pyroclastic cone. The basaltic lava flow erupted from the northern vent is more voluminous than that of the southern one. Firstly, pahoehoe-type lava issued from the northern vent in March and flowed down eastward, which reached to the Nakanosawa valley. In September, the eruption came to be cli- mactic and aa-type lava poured out from the southern vent, which breached the caldera wall and flowed down along the Akasawa valley. Concurrently, voluminous aa-type lavas consisting of many lava lobes effused from the northern vent and spread the caldera floor eastward, which flowed down along the Gomisawa valley and reached to the sea. The issuing lava changed to transitional-type and then pahoehoe-type lava effused. At last, a lava cone consisting of pahoehoe-type was formed around the northern vent. The lava cone comprises more than three lava lobes, where develop various inflation structures such as bulbous toe and tumulus. The change of surface morphology of lava was probably caused by the change of eruption rate; the high rate brought about aa-type lava and low rate produced pahoehoe-type lava. The effusion of lavas from the northern vent continued to at least November 1778A.D.. Keywords : lava, pahoehoe, aa, eruption, Izu-Oshima volcano 伊豆大島火山安永玄武岩質溶岩の表面形態 Surface Morphology of the An-ei Basaltic Lava Flow in the Izu-Oshima Volcano Yusuke MISONOU * , Masaki TAKAHASHI ** and Maya YASUI ** (Received September 30, 2006) * Graduate School of Integrated Basic Sciences, Nihon University: 3 -25 - 40 Sakurajosui Setagaya -ku, Tokyo, 156 -8550 Japan ** Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University: 3 -25 -40 Sakurajosui Setagaya -ku, Tokyo, 156 -8550 Japan * 日本大学大学院総合基礎科学研究科: 〒156 -8550 東京都世田谷区桜上水3 -25 -40 ** 日本大学文理学部地球システム科学科: 〒156 -8550 東京都世田谷区桜上水3 -25 -40 日本大学文理学部自然科学研究所研究紀要 No.42 (2007)pp.97 -116 53
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─ ─97 ( )
永噴火で大量に形成された溶岩について検討し,島弧玄
武岩質溶岩の表面形態についての記載とその成因につい
ての議論を行うとともに,安永大規模噴火における溶岩
流出のプロセスについても明らかにすることをめざし
た。
2 安永噴火の推移
歴史時代噴火の推移は古記録からある程度推定が可能
である。ここでは村山(1979)によってまとめられた「武
江年表」,「大島山火記」,「日本噴火誌」,「震災予防調査
会報告第八十一号」などに書かれた古記録および古記録
にもとづいた記録をもとに,安永噴火の推移を推定し
た。
噴火は安永6年(1777年)7月29日(旧暦)に始まった。
1 はじめに
日本列島の第四紀火山には玄武岩質のものは少なく,
伊豆大島,三宅島,富士など特定の火山に限られている。
そのため,玄武岩質溶岩の表面形態に関する記載や議論
は,富士火山の青木ヶ原溶岩など一部の例(小幡・海野,
1999;高橋・他,2004・2005など)を除きあまりなされ
てこなかった。伊豆大島火山最後の大規模噴火は江戸時
代後期の安永噴火(1777-1778年)であるが(Nakamura,
1964),安永噴火では,現在の三原山火砕丘が形成され
るとともに,大量の玄武岩質溶岩が流出し,カルデラの
北東部を埋め尽くして,海岸から海へと流入している
(一色,1984など)。安永噴火の噴出物の総量は約6.5億
トンと推定されている(Nakamura, 1964)。本論では安
御園生 裕介* ・高橋 正樹** ・安井 真也**
The An-ei eruption of Izu-Oshima volcano began in June (the lunar calendar) 1777 A.D. and continued to January 1778 A.D., constructing the Miharayama pyroclastic cone by violent strombolian to sub-plinian eruptions. From March to November 1778 A.D., the mode of eruption changed and nearly aphyric basaltic lavas poured out from the vent situated in the northern and southern foot of the Miharayama pyroclastic cone. The basaltic lava flow erupted from the northern vent is more voluminous than that of the southern one. Firstly, pahoehoe-type lava issued from the northern vent in March and flowed down eastward, which reached to the Nakanosawa valley. In September, the eruption came to be cli-mactic and aa-type lava poured out from the southern vent, which breached the caldera wall and flowed down along the Akasawa valley. Concurrently, voluminous aa-type lavas consisting of many lava lobes effused from the northern vent and spread the caldera floor eastward, which flowed down along the Gomisawa valley and reached to the sea. The issuing lava changed to transitional-type and then pahoehoe-type lava effused. At last, a lava cone consisting of pahoehoe-type was formed around the northern vent. The lava cone comprises more than three lava lobes, where develop various inflation structures such as bulbous toe and tumulus. The change of surface morphology of lava was probably caused by the change of eruption rate; the high rate brought about aa-type lava and low rate produced pahoehoe-type lava. The effusion of lavas from the northern vent continued to at least November 1778A.D..
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