On 5 April, GeoNet reported that the temperature of Ruapehu's summit Crater Lake had been high for a sustained period and was currently between 38 and 39 degrees Celsius. The highest temperature since unrest began in October 2010 was 41 degrees Celsius, measured on 1 March. The report also noted that during the previous few weeks there was an increase in carbon dioxide gas emissions, increased seismicity, and changes in Crater Lake water chemistry. The unrest prompted GeoNet to raise the Aviation Colour Code although the Volcanic Alert Level remained at 1 (some 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