Crater Lake temperature had increased to 23°C by 15 October ... . Upwelling over the main vent was first observed 15 October, while upwelling over the N vent was continuous since early September. Rapid oscillations of 2-7 cm in lake level were observed on 15 October, but there were no high surges, ash deposits, or other evidence of eruptions. Sulfur emissions were at background levels and lake color has remained battleship gray. The 15 October deformation data showed no significant changes since the July and August surveys.

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.