Seismicity returned to normal levels on 25 November after a moderate-to-large volcanic earthquake occurred at Ruapehu on 21 November. Observations on 25 November revealed no signs of eruptive activity. Scientists found that upwelling sediment in Ruapehu's crater lake caused the lake to change from its normal blue-green color to dark gray. In addition, the temperature of the lake was relatively low (22°C in comparison to 21°C in September), which further supported the theory that no eruptive activity occurred after the earthquake. The volcano remained at Alert Level 1 (on a scale of 0-5).

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