Inspections by NZGS personnel revealed no significant changes. On 15 and 21 July, and 16 August, Crater Lake appeared its usual battleship gray color. Moderate upwelling was observed over the vents at the N end of the lake, none over the central vent. Water temperature measured at the outlet was 18.7°C in July (2.2° higher than on 15 June), and 18.3° in August. Concentrations of magnesium (778 and 744 ppm) and chlorine (7,310 and 7,470 ppm) were virtually unchanged for the two months. Horizontal deformation surveys showed that by August the distance between two stations on opposite sides of the 600 m-wide crater had increased 16 mm from a 2-year minimum value measured in June.

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; B. Scott, A. Cody, NZGS, Rotorua.