On 11 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. The lake water temperature continued to rise, peaking at 41 degrees Celsius. Steam plumes were visible rising from the lake; an overflight confirmed that they were caused by a combination of the warming lake and atmospheric conditions, with no eruptive activity. Tremor levels declined but remained high. A gas measurement flight on 4 May confirmed high levels of gas emissions with sulfur dioxide and carbon dioxide flux rates at 260 and 1,970 tonnes per day, respectively. Lake upwelling over the central and northern vents areas was also visible during multiple overflights. 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. 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