NZGS personnel observed reduced activity during a 1.5-hour visit on 30 May. Upwelling in Crater Lake had been continuous and fairly vigorous when visited from mid-February through mid-April, and intermittent on 7 May. However, upwelling was not evident on 30 May, although faint yellow sulfur slicks were visible near the center of the lake. The water temperature near the lake outlet was 31°C, 7.5° lower than 22 days earlier. The small explosions through the lake that had occurred intermittently in previous months were not observed on 30 May, and there was no ash on snow surrounding the lake. Tiltmeters recorded 4 µrad of deflation since 13 April, in contrast to <1 µrad of change 12 February-13 April.

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: P. Otway, NZGS, Wairakei.