The strong volcanic tremor at Mt Ruapehu that has been recorded since early-March has been declining for the past two weeks. The heavy rainfall over the weekend caused a temporary 40 cm rise in lake level and a drop of about 2°C in water temperature. The lake level has returned to the normal pre-rainfall level. Aside from the short-term impact of this weather event, the crater lake temperature has decreased from a peak of 41 °C on 8 May 2022 to 37 °C today.

A gas measurement flight on 13 May 2022 confirmed high sulphur dioxide (SO2) and carbon dioxide (CO2) gas output, respectively of 179 and 1658 tonnes per day of gas. Both gases are released from relatively shallow magma, and although the gas output values are lower than the values measured two weeks ago, they remain elevated for Mt Ruapehu. Observations from the gas flight have also confirmed the upwelling in the northern vent areas of the lake continues.

Overall, even with the lower levels of volcanic tremor and decrease in lake temperature, gas emissions remain elevated, and the period of heightened volcanic unrest continues at Mt Ruapehu. Within the next four weeks, the most likely outcome of this unrest episode is minor eruptive activity that is confined to the lake basin, or no eruptions. This level of activity may generate lahars (dangerous volcanic mudflows) in the Whangaehu River.

The next most likely scenario is a larger eruption that impacts the summit plateau with volcanic surges and generates lahars in multiple catchments, like what was seen after the September 2007 eruption, or older events like those in 1975 and 1969. An eruption of this size would cause life-threatening hazards on the summit plateau and in valleys impacted by lahars.

The chance of a prolonged eruptive episode or a larger eruption, such as occurred in 1995-96 with wider ashfall impacts, is higher than it was two months ago, but within the next four weeks remains very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions.

Due to the heightened volcanic unrest, GNS Science staff are carrying out more frequent aerial observations and gas measurements when weather conditions are suitable.

Our interpretation of the observational data and activity is consistent with elevated volcanic unrest at the heightened level and therefore the Volcanic Alert Level remains at Level 2. The Aviation Colour Code remains at Yellow.

Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of elevated volcanic unrest.

The Volcanic Alert Level reflects the current level of elevated volcanic unrest. The Volcanic Alert Level should not be used to forecast future activity. However, at Volcanic Alert Level 2, eruptions are usually more likely than at Volcanic Alert Level 1.

Volcanic Alert Level 2 indicates the primary hazards are those expected during volcanic unrest; steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While Volcanic Alert Level 2 is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning. Volcanic Alert Levels 3, 4 and 5 are reserved for eruptions with varying impact distances.

For information on access to the Mt Ruapehu area, please visit the Department of Conservation’s websites on volcanic risk in Tongariro National Park in Tongariro National Park and follow the DOC Tongariro Facebook page for further updates.

For information about responding to volcanic activity there are guidelines from the National Emergency Management Agency (https://getready.govt.nz/emergency/volcanic-activity/).

GNS Science and its National Geohazards Monitoring Centre continue to closely monitor Mt Ruapehu for further changes.

Agnes Mazot

Duty Volcanologist

Media Contact: 021 574 541 or media@gns.cri.nz