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.
Matsumoto et al. (Fri,) studied this question.