噴火の爆発力を蓄積する硬い“蓋”がなぜ短期間に修復されるのか？－火山の「繰り返し爆発」メカニズムを火口近傍の赤い岩石の成因から解明－

2026-04-27 産業技術総合研究所

産業技術総合研究所と九州大学の研究チームは、霧島山新燃岳の調査から、ブルカノ式噴火における「繰り返し爆発」の新たなメカニズムを解明した。従来不明だった溶岩の“蓋”の短時間再生について、火口近傍の赤色岩石やタフサイト脈の解析から、爆発で破壊された蓋が深部で溶結・癒着し、強度と気密性を回復することで再びガス圧が蓄積するモデルを提案。赤色火山灰はこの過程で大気による酸化で形成されることも示された。これにより、遠方で採取した火山灰の特徴から火口内部の状態や噴火推移を推定できる可能性が示され、火山活動の予測精度向上に貢献する成果とされた。

“蓋”の破壊と修復を短時間で繰り返す新しいブルカノ式噴火のモデル。
霧島山新燃岳の火口近傍の岩石は赤色を呈するのが特徴。
※原論文の図を参考に改変したものを使用しています。

＜関連情報＞

溶岩栓の破砕と修復を繰り返すことで、ブルカノ式噴火時に断続的な爆発が発生する
Repeated fracturing-healing behavior of lava plugs drives intermittent explosions during Vulcanian eruptions

Keiko Matsumoto;Nobuo Geshi
Geology Published:April 10, 2026
DOI:https://doi.org/10.1130/G54286.1

A repeated cycle of breaking and rebuilding of a lava plug is the key mechanism underlying intermittent Vulcanian explosions typically observed in andesitic volcanoes. Here, we investigated the mechanisms that enable repetitive explosions of the plug by analyzing the welding textures of tuffisite veins within proximal ejecta from the 2018 Shinmoedake (Japan) Vulcanian activity. Tuffisites were classified into six types based on welding intensity and oxidation. Welded tuffisite veins have fewer voids than nonwelded tuffisites, suggesting lower gas permeabilities. Strongly welded tuffisites showed greater tensile strength than their intact host rocks. The tuffisites showing red coloration indicate oxidation by atmospheric air at near-magmatic temperatures. We conclude that the tuffisites preferentially welded at elevated confining pressure at depth, whereas oxidation is caused by air permeation into tuffisite veins near Earth’s surface. Based on these observations, we propose a mechanism for explosion repetition: Following an explosion, fractures form within the plug, allowing deep-seated gas to escape. Subsequent tuffisite-vein welding clogs outgassing pathways, reforming the plug and recovering its strength. This process enables gas pressure buildup beneath the plug and air permeation through veins. When gas pressure exceeds the strength of the plug, the plug fractures during the next explosion. Thus, the interplay between fracturing and healing in the lava plug governs the repetition of Vulcanian explosions in a lava-filled conduit system.