On 14 June GeoNet reported that unrest at Ruapehu continued to decline. During the previous week volcanic tremor levels were weak, but remained above background levels recorded since 2013. Lake water temperatures had dropped to 23 degrees Celsius, which corresponded to the long-term average heat flow. Relatively high sulfur dioxide and carbon dioxide measurements were recorded in mid-May, but weather conditions prevented gas measurement flights after that time. The level of unrest was at moderate levels, though the volcanic processes that caused the unrest were unknown; 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