On 26 April GeoNet reported that elevated unrest at Ruapehu had been ongoing for the past five weeks, characterized by lake water heating, volcanic gas output, and strong volcanic tremor. Tremor levels fluctuated the past week but represented a record for the longest and strongest tremor episode ever recorded at the volcano. The lake water temperature remained at 37 degrees Celsius, indicating a substantial amount of heat from magma at a shallow depth (0.5-2 km), though water chemistry showed no change based on sampling during 31 March-15 April. The Volcanic Alert Level remained at 2 (on a scale from 0-5) and the Aviation Color Code remained at Yellow.

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