Possible mechanism of molten sulfur eruption: Implications from near-surface structures around of a crater on a flank of Mt. Shiretokoiozan, Hokkaido, Japan

Shiretokoiozan volcano in northern Japan is well known for its eruptions, which eject huge amounts of molten sulfur. Watanabe (1940) reported details of the 1936 eruption, but its mechanisms, and how and where the huge amount of sulfur is produced and pushed out remain unknown. The aim of this study is to elucidate the near-surface underground structure of this area and the mechanisms of the molten sulfur eruption. We implemented aerial photographic observations, geological surveys, hot spring analysis, Self-Potential survey and DC resistivity surveys at the western flank of Mt. Shiretokoiozan. The geology of this area is mostly composed of hydrothermally altered boulders, gravels, sand, and clay. Some areas of fumaroles are covered by sulfur cement. Chemical analyses revealed that SO42 − and Cl− are rich in hot water, which imply an area with upwelling hot water/gas below the surface. Results of DC resistivity surveys conducted at several sites show extremely low resistivity, suggesting an aquifer several meters below the surface. Compiling this evidence, we infer a possible mechanism of molten sulfur eruption: the sulfur has been produced and stored in an aquifer located at the eastern hill from Crater I for several decades by chemical reactions of volcanic gases; it gushes out when volcanic activity becomes high.