Few changes have been observed since the end of July. NZGS personnel visited Ruapehu on 19-20 and 24 August, 17-18 September, and 21 October.

On all the visits the gray Crater Lake had yellow sulfur patches on its surface and minor to moderate upwelling in the center and near the N shore. Snow lay within 0.5 m of the water's edge and showed no sign of water surge. Water temperatures were 24°C on 24 August, 29° on 17 September, and 25° on 21 October. Magnesium concentration in the water has remained unchanged since February, but chloride concentration has increased. The Mg/Cl ratio has gradually declined from 0.130 in February to 0.115 on 24 August and 0.113 on 17 September. Deformation surveys indicated no apparent summit inflation.

Volcanic tremor has been recorded for some time and peaked in early September. The NZGS interpreted the low [normal] tremor frequency and the unchanged magnesium concentration to indicate that magma is still deep beneath Crater Lake. The increased chloride concentration suggested that gas can freely vent into the bottom of the lake, and the slow rate of lake refilling suggested that no shallow magmatic intrusion had occurred.

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; I. Nairn, B. Scott, NZGS, Rotorua; A. Cody, F.R.I., Rotorua.