On 2 May GeoNet reported that elevated unrest at Ruapehu continued during the previous week, consisting of lake water heating, volcanic gas output, and strong volcanic tremor. Variable tremor levels were characterized by bursts of strong tremor and short periods of weaker tremor. The lake water temperature had risen to 38 degrees Celsius. During an overflight scientists observed reduced upwelling in the lake over the Northern vents and additional upwelling over the main Central Vent, indicating that it was at least partially unblocked. A sulfur dioxide flux of 390 tonnes per day was measured during a flight on 28 April, the sixth highest amount since 2003. The sustained carbon dioxide and sulfur gas emissions, along with high tremor levels, continue to indicate that magma is driving this period of heightened unrest. At around 0945 on 3 May a steam plume was visible slowly rising as high as 1.5 km above the crater rim; it dissipated by 1030. The plume was not accompanied by seismic or acoustic signals, and weather conditions favored cloud formation. Scientists on an overflight that afternoon observed active upwelling and a slightly higher lake level, similar to observations the previous day. The lake temperature had risen to 39 degrees Celsius. Results from lake water chemistry analysis showed no indication that magma was interacting with the hydrothermal system beneath the lake. 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