On 4 May GeoNet reported a cooling trend of the crater lake water at Ruapehu with a current temperature of 35 degrees Celsius. During February-April the water temperature had peaked at 42 degrees Celsius. The intensity of volcanic tremor had been weak during the past month after a peak in strength in early March; tremor had declined slowly almost in conjunction with the lake water cooling trend. During a visit to the lake the previous week volcanologists noted that the lake was a uniform gray color with a few surface slicks and no obvious upwelling. Since that visit the water level had risen and was flowing into the upper Whangaehu River. 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