A brief period of strong heating in Crater Lake was accompanied by small volcanic earthquakes and possibly by minor eruptions. Continuously recorded lake temperature data showed a gradual decline to 16°C by mid-June, then little change until a sharp increase began about 1 July. Temperatures reached 24.4°C on the 18th before declining again to 13° by late August. A series of small volcanic earthquakes occurred 5-14 July, none exceeding M 1.8.

Severe winter weather limited observations near the time of the increased activity, although the lake appeared normal on 11 July. When briefly observed on 12 August, evidence of 1-2 m of surging was visible under fresh (about 10 August) snow around the lake margin. More detailed observations during fieldwork 27 and 29 August revealed dirty, ash-covered ice under fresh snow 1-2 m above lake level, and widening of the lake's outlet channel by previous strong outflow or surging. No clear patterns were evident in summit-area deformation data.

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, DSIR Wairakei.