On 11 and 12 January steam plumes were emitted from Ruapehu. This activity was believed to be associated with hydrothermal activity in the crater lake. Prior to the steam emissions, measurements taken on 8 December revealed that the lake water temperature was at 36-38°C and that it had been heating since late November 2001. Evidence of convection was observed at the S-central vent area including minor sulfur slicks, upwelling, and light steaming. Only minor volcanic tremor was recorded at Ruapehu. The volcano remained at Alert Level 1 ("Initial signs of possible volcano unrest.").

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.

Source: GeoNet