Small hydrothermal eruptions were reported on 24 August as 1-m-high bubble-like updomings in the lake center. The upwelling generated waves and steam plumes. Steam plumes were also observed 29-30 August when several eruptions threw muddy water jets to >10 m above the lake surface. On 31 August lake temperature was measured at 40°C, up from 24.5°C on 17 August. During a 14 September inspection the lake was steaming strongly. The observed hydrothermal events did not appear to correlate with seismicity, which continued at generally low levels. The activity was regarded by NZGS personnel as part of the normal cycle of heatflow rather than precursory to a major eruption.

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, NZGS Rotorua; P. Otway, NZGS Wairakei.