Eruption dynamics and plumbing system of Shikabe Geyser in southern Hokkaido, Japan, revealed by field observation inside and outside the conduit

The development of observation and data analysis techniques has advanced the understanding of the plumbing systems that contribute to the eruption dynamics of geysers. However, the interrelations between the plumbing system, dynamic and thermal processes inside the conduit, and surface phenomena remain unclear. We recorded video images, pressure, and temperature inside and outside the conduit of the Shikabe Geyser, a geysering well in southern Hokkaido, Japan. Video observations revealed that numerous bubbles are generated in the deep part of the conduit immediately after the eruption begins. However, the water temperature at depth is lower than the boiling point throughout the eruption cycle. The directly measured volume of discharged water indicate that the water supply rate during the eruption phase is significantly higher than that during the recharge phase. At the Shikabe Geyser, there may be void spaces composed of cracks and porous media around the deep part of the conduit, and high-temperature water containing dissolved CO2 gas is likely supplied from the reservoir. Dissolved CO2 plays a critical role in decreasing the boiling point of water. When an eruption occurs, steam with CO2 gas nucleation and expansion owing to decompression drives the vigorous discharge of water at the vent. Simultaneously, bubbles are generated inside the cracks in the aquifer owing to decompression, which may contribute to oversupply during the eruption.