Activity from Crater Lake was limited to thin, dark slicks that occasionally appeared over the central vent, and weak to moderate upwelling from the N vents during fieldwork on 31 August and 13 September. No steaming was evident and the lake remained a clear blue-green color. Little deformation was detected, but heavy snow-cover prevented measurements at most of the network.

Lake temperature was only 10.2°C on 13 September, 7° cooler than on 17 July. Diminished heat flow into the lake has lowered its temperature to the 10°C range only eight times since 1980, and phreatic eruptions occurred within 1-2 months of half of those cooling episodes. One was a moderately explosive vent-clearing event on 8 December 1988, but the others were relatively small.

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, IGNS Wairakei [formerly DSIR].