When geologists visited Ruapehu on 15 September, Crater Lake's temperature was 10°C, a decrease of 4° since August and the lowest temperature on record. Geologists attributed this decrease to continuing low heat flow aided by recent cold weather. No central convection was evident in the lake . One of at least three remaining convection cells in the N portion of the lake produced a strand of yellow sulfur. Since mid-l986, Mg/Cl ratios have remained near 0.06-0.07, suggesting heating of the lake by steam rather than by direct magma-water interaction.

Geologists noted that similar slow declines in activity over the past 8 years have usually been followed closely by rapid lake reheating.

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.