On 5 November, GeoNet reported that Ruapehu's summit Crater Lake was hot during field visits in December 2011 and January 2012, exhibiting temperatures over 35 degrees Celsius. The lake cooled afterwards, reaching 16 degrees in May and fluctuating between 18-24 degrees during June-October. Scientists visited the lake in late October and reported that the water temperature was 19.5 degrees. During that visit they observed weak convection near the center of the blue-green lake.

GeoNet noted that Ruapehu is often seismically active; during the last month weak volcanic tremor was recorded and more recently several small earthquakes under the volcano had been detected. The largest earthquake was an M 2.

An overflight on 26 October to measure gas flux revealed that sulfur dioxide was 63 tonnes per day and carbon dioxide was 908 tonnes per day. The Aviation Colour Code remained at Green and the Volcanic Alert Level remained at 1 (signs of 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