GeoNet reported that, at least for the last 10 years of continuous lake-temperature monitoring, heating cycles at Ruapehu’s summit Crater Lake have occurred over periods of about 12 months with temperatures ranging from 15 to 45 degrees Celsius. However, the temperature remained elevated at 30 degrees for a period of six months starting in September 2018. GeoNet noted that during the previous two weeks the lake temperature increased further, at a rate of around a half of a degree per day, to 42 degrees on 9 April. Tremor levels also increased. The Volcanic Alert Level remained at 1 (minor volcanic unrest) and the Aviation Color Code remained at 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