GeoNet reported a warming trend of the crater lake water at Ruapehu, with a high temperature of 43 degrees Celsius. During an overflight to measure gas emissions the previous week, scientists observed that the lake was a uniform gray color (suggesting it is well mixed) and some water overflow at the lake’s outlet. Gas output had increased in response to the heating cycle; the amounts of carbon dioxide (CO2) and sulfur gases (SO2 and H2S) in the plume were the largest measured in the past two decades. Short-lived pulses of volcanic tremor were coincident with gas emissions. The Volcanic Alert Level was raised to 2 and the Aviation Color Code was raised to Yellow on 21 December.

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