Crater Lake temperature had increased from 11.5 to 24.5°C between visits by geologists on 3 July and 17 August. Convection of lake water was occurring at both the central and N vents on 17 August. Outflow had increased from 30 l/s to >200 l/s, and total heat flow from the lake had increased by a factor of 4 since the July visit, reaching ~180 MW.

No recent eruptions appear to have occurred and no significant deformation or seismicity has accompanied the lake heating. Lake temperature had been relatively stable after dropping rapidly between November and December 1986, but reached a low in July (figure 7).

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.