. . . Crater Lake temperature fell. . . . to 28.5°C on 16 October. The lake was steaming and dark yellow-green (sulfur) slicks were periodically observed near the lake center. Deformation measurements indicated a minor 8 mm (inflationary) increase in crater diameter. Seismicity since 8 September has been dominated by low-amplitude 2 Hz volcanic tremor. No eruptive activity has been reported. During a visit on 28 September a geologist observed a seiche with amplitude of 50 mm and period of ~35 seconds. A similar seiche was observed on 17 October 1986.

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; B. Scott, NZGS Rotorua.