SURVEILLANCE OF RUAPEHU CRATER LAKE, NEW-ZEALAND, BY AQUEOUS POLYTHIONATES

Aqueous sulfur speciation of samples of crater lake water from Crater Lake collected at Mt. Ruapehu, New Zealand since 1968 has been determined using high-performance liquid chromatography. Polythionates (SxO62-) are produced in the lake through reactions between sulfur dioxide and hydrogen sulfide gases from the subaqueous fumaroles. Their concentrations vary from 0 to a few hundred mg/l, corresponding to the activity of the volcano. The concentration of the total polythionates (SIGMASxO62- = S4O62- + S5O62- + S6O62-) is found to be a good indicator of changes in the subaqueous fumarolic activity at Crater Lake. The molar SxO62-/Cl- ratios of the lake water can be used to divide the volcanic activity of Crater Lake into four stages: Stage I Period of weak volcanic activity with no hydrothermal explosions. This stage is characterized by low total SxO62- with a distribution order of S5O62- > S4O62- > S6O62-. H2S is predominant among dissolved gases in the lake water. Stage IIa: Quiescent period with minor hydrothermal explosions. The lake water contains high SIGMASxO62- with a distribution order of S5O62- > S4O62-. Dissolved H2S and SO2 gases are very low. Stage IIb: Increased fumarolic activity with frequent hydrothermal explosions. Low SIGMASxO62- with a distribution order of S4O62- > S5O62- > S6O62-. Sulfur dioxide but no H2S is detected in the water. Stage III: Period of phreatomagmatic eruptions with lahars and intensive seismic activity. No SxO62- exists but dissolved SO2 is high in the lake water. The moderate-sized explosion of December 8, 1988, which was not preceded by any changes in lake water polythionate chemistry or seismic activity, is hypothesized to be a vent-cleaning eruption caused by blockage of ascending magmatic gases that was caused by solidification of a molten sulfur pool at the bottom of Crater Lake.