GeoNet reported that a volcanic earthquake swarm beneath Ruapehu's summit Crater Lake was detected during 25-26 April. This type of swarm has been uncommon at the volcano in recent years, when volcanic tremor has dominated the seismicity. In addition, the crater lake water temperature had been rising since late 2015, from 25 degrees Celsius to 40 degrees in the last half of April. Similar temperature increases were detected in March 2011, April 2014, and February 2015.

In a 3 May notice GeoNet stated that the size and number of events in the swarm had decreased; a period of moderate volcanic tremor began at 1310 on 2 May and lasted about an hour. The lake temperature had peaked at 42 degrees, equal to the highest recorded temperature since 2011. At around 1400 on 7 May tourists on an overflight of the area observed vigorous steaming at the lake surface and upwelling bubbles. The lake temperature rose further, to 44 degrees, establishing a new high since temperatures started to be recorded in 2000. The earthquake swarm had stopped, though the seismic network continued to record volcanic tremor. On 11 May GeoNet stated that recent visits to the lake confirmed increased gas emissions from the crater lake. The lake temperature increased to 46 degrees and moderate levels of volcanic tremor continued. The Volcanic Alert Level was raised to 2 (moderate to heightened unrest) and the Aviation Colour Code was raised to 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