Geochemical Investigation of the Self-destruction of Gongensawa Geyser

Abstract

Noboribetsu thermal activity, originating in the candle end energy of Kuttara volcanism, had a typical geyser in Gongensawa valley outside of the explosion crater (Fig. 1), whose mechanism of intermitttent issuing could be explained best according to the cavityhypothesis after Mackenzie (Table 10 and Fig. 10-1).
This geyser decayed to disappear by alterations of the volcanism, which took place there between 1951 and 1953. The present paper describes the geochemical investigation on the full course of the typical geyser to disappearance (Fig 13).
In Noboribetsu thermal activity, chemical components including H₂O and heat energy were exhaled in three ways steam, molten sulfur and mineral water. These three phases were considered to originte in the same, hightemperature and high-pessure mineral water (Table 5, 6 and 8). Every phase had the specific chemical components delivered according to their solubility in each. That is, there existed the exo-magmatic hydrothermal differentiation of the original solution in this thermal activity. Adiabatic expansion of the original solution played the most important part in this differentiation.
The geyser originated in the secondary mineral water which had remained in a cavity in the liquid phase at the adiabatic expansion of the original solution and had a neutral brine property. But the amount of mineral water issuing through the geyser occupied only a minute quantity of the seconry water (Table 4). As shown in Table 3, the chemical property of geyser's water deviated from the mother's property on account of some evaporation of water and deposition of aragonite and calcite during the process of issuing of water up to the ground surface after the adiabatic expansion (Table 3 and 7). In other words, the content of chemical substances was most concentrated in geyser's water but a half of Ca²⁺ ion was lost in comparison with those of other springs. This deviation of the chemical property necessitated the presence of another cavity in which evaporation and deposition took place. This was the reason why the cavity-hypothethis after Mackenzie could be applied to Gongensawa geyser on the mechanism of intermittent issue.
The geyser occupied the geochmically important place where two branched activities, steam and water, were mixed each other again, and where all springs had much content of sulfuric acid. That is, there took place a neutralizing reaction between HCO₃^- ion bearing neutral solution and H⁺ ion bearing acid one, producing much CO₂ in this area. Only the geyser in this valley had no appreciable sulfuric acid content in its water and suffered from no effect of this neutralizing reaction to its intermittent issuing mechanism, when the volcanism was kept stable in 1949 (Table 10).
Noboribetsu thermal activity repeated alterations of its strength for last 100 years. At the alteration occurred between 1951 and 1953, the largest one in this volcanism, the intermittent issuing mechanism was destroyed by two causes. The one was the energydecrease of the mineral water issuing through only the geyser, controlling the mechanism before November of 1951 when Noboribetu thermal activity was kept violent and the original solution had the largest energy. Geyser's water was deprived of its energy by the increased activity of Showajigoku situated near itsetf. That is, the cause was only the disturbance of water way occurred just under the ground surface.