Minor phreatic eruptions were reported from late January through February, but only steam emission has been reported since then. Crater Lake's temperature dropped from 42.5°C on 26 February to 32° on 22 March, and 31.3° on 6 April, suggesting to geophysicists that heat flow through the lake had dropped by roughly an order of magnitude. A continuous temperature monitor was installed near the Crater Lake outlet on 21 March. Minor inflation that had accumulated across the N crater rim between 26 February and 22 March had nearly disappeared by 5 April. Seismic records in March and early April showed little or no tremor or volcanic earthquakes.

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.