火山
Online ISSN : 2189-7182
Print ISSN : 0453-4360
ISSN-L : 0453-4360
論説
多量の漂流軽石を発生させる噴火—南西諸島における軽石の漂着記録とその給源火山の活動から
及川 輝樹池上 郁彦渡部 将太
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ジャーナル フリー

2023 年 68 巻 3 号 p. 171-187

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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).

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© 2023 特定非営利活動法人日本火山学会
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