[S-VC28] International volcanology

[SVC28-P06] Blown out breccia explosive eruption: a type of Vulcanian explosion produced by the rheological structure of magma plugs

*松本 恵子¹、下司 信夫¹ (1.産業技術総合研究所地質調査総合センター)

キーワード：ブルカノ式噴火、新燃岳、レオロジー

Explosive eruptions of andesitic magma that typically produce angular and dense fragments are modeled as resulting from the breaking of a plug at the top of the magma column. However, the mechanism by which the densely solidified magma plug is broken has been unclear. Here, we explore this enigma by examining the textures of deposits from the 2018 explosive eruptions at Shinmoedake volcano, Japan, and use the results to assess brittle-ductile behavior of magma plugs. More than 90% of the proximal deposits in the area southeast of the summit crater were classified as dense and angular fragments with red surface rims, sometimes accompanied by non-welded to weakly-welded tuffisite breccia veins with red coloration. Similar facies were recognized within the wall rock of the 2018 vent in the summit crater. Rare but ubiquitous fragments with weakly to highly welded tuffisite veins that are black and have deformed shapes were recognized in the proximal area. The porosity was found to be higher for the interfilling ash with red coloration than those with black coloration, indicating a shallower origin for the former. The origin of the red color was revealed to be fine (<1 µm) particles of Fe oxide(s) that are present in the interstitial glass and mafic microlites that are located near the margins of each clast, irrespective of their size. Thus, fragments with red coloration indicate oxidation due to mixing with air after brittle fracturing. In contrast, fragments with black veins reflect ductile deformation of the plug at magmatic fO2 after brittle fracturing. These results support the following plug architecture: the upper part of the magma plug was brittle with air circulation, while the lower part was ductile with approximately magmatic fO2. We have integrated these observations with the 2018 eruption sequence to propose the new mechanism of “blown out breccia explosive eruption”, which is explained as follows. After the cessation of lava effusion around March 9, 2018, the vent became empty due to simultaneous explosive eruptions and was then buried by the surrounding ductile lava in the summit crater, which caused a local increase of strain rate at the top on the vent. As a result, the top of the vent plug became brittle whereas the lower and less deformed part remained ductile and choked the degassing pathway from the deeper part. When the strain rate of the lower part of the plug subsequently increased, it became brittle and the overpressure of the gas pocket was instantaneously released, resulting in the explosive ejection of the brittle fragments above. This “blown out breccia explosive eruption” can only occur in instances when hot lava effuses, cools and becomes solidified at a vent that has a supply of gas from deeper in the conduit. These new findings on magma plug dynamics will contribute to the understanding of the mechanisms of Vulcanian explosions and degassing in lava domes.