GeoNet reported that water temperatures of Ruapehu's summit Crater Lake had been increasing since early December 2014, rising from 15 degrees Celsius to over 40 degrees in late January-early February. The temperatures declined to 31 degrees in mid-March, and then climbed again to 37-39 degrees. No other changes were detected at the lake. In addition, over the previous 2-3 weeks intermittent moderate-to-strong levels of volcanic tremor were detected, which had been some of the strongest recorded there over the past eight years. The report noted that historically there had not been a direct link between volcanic tremor and discrete volcanic eruptions or sequences of eruptions. 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