GeoNet reported that the temperature of Ruapehu’s summit Crater Lake had been declining since May. During an overflight on 10 August scientists recorded gas emissions at typical low background levels. The next day the lake water temperature was 12.6ºC, one of the lowest temperatures since the 1995/1996 eruptions. The lake was a dark green, overflowing, and sulfur slicks outlined areas of upwelling. A strong sulfur odor was noted near the lake. The level of volcanic tremor which was at moderate levels during May-June had declined to typical background levels. The Volcanic Alert Level was lowered to 1 (minor volcanic unrest) and the Aviation Colour Code was lowered to Green.

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