Monitoring of volcanic eruptions at Yugama crater lake by aqueous sulfur oxyanions

Variations of polythionates (sulfane disulfonates) and sulfate in the Yugama crater lake, Japan, have been monitored for more than 25 years. Just before the 1982 eruption at the crater lake, polythionate ions decreased to zero from the normal level of about 2000 ppm and sulfate ions increased from 2500 to 5000 ppm. During the 1982 eruption polythionate and sulfate ions varied inversely in concentration and the variations exactly coincided with the frequency of volcanic earthquakes and subsequent explosions. These observations are interpreted in terms of aqueous reactions of fumarolic SO₂-H₂S gases, resulting in precipitation of alunite. The behavior of polythionate and sulfate ions strongly suggests that they are useful indicator for prediction of impending volcanic hazards from active crater lakes.