After 3 years of quiescience, small hydrothermal eruptions began on or shortly before 21 May. On 16 May, major overflow of Crater Lake into the Whangaehu River began, as shown by pH and conductivity measurements downstream (at Tangiwai). Seismic activity, characterized by increased high-frequency tremor and local earthquakes, began [in early May, with some minor tremor on the 5th and small volcanic earthquakes on the 11th]. Eruptions were first observed in Crater Lake on 21 May and were also seen on 25 May.

On 28 May NZGS personnel noted 15 small hydrothermal eruptions during a 10-hour period. All the eruptions occurred in the lake center, where resumption of weak upwelling had first been observed on 26 April. The eruptions were characterized by updoming of the central lake surface, noisy ejection of water and mud jetting to 10 m above the lake surface, and waves radiating from the lake center. Clouds limited deformation measurements across the crater but a pair of horizontal angle observations indicated no major change in crater diameter.

The lake was battleship-gray in color, with clean snow around the water margin about 1 m above lake level. The lake temperature had increased to 45°C from the 20-25°C range of recent months. Interim analyses of the lake water show a progressive dilution throughout the summer months. Chlorine and magnesium contents have increased since activity began, but the Mg/Cl ratio has not changed significantly.

Geological Summary. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: I. Nairn and B. Scott, NZGS, Rotorua; P. Otway, NZGS, Wairakei.