Over the past six weeks, Mt Ruapehu has exhibited the strongest volcanic tremor in two decades along with a rise in Crater Lake (Te Wai ā-moe) temperature. This period of heightened volcanic unrest continues. Over the last week, the level of volcanic tremor has varied, with bursts of strong tremor interspersed by short, periods of weaker tremor. This represents a change in character in the tremor, and the driving processes remain unclear.

The last three days has seen Crater Lake temperature rise to 38 °C following a four-week period at 36-37 °C. Our modelling suggests that to maintain the lake temperature and subtle rise requires ~200-300 MW.

Due to the heightened volcanic unrest, GNS Science staff are carrying out more frequent aerial gas measurements and Crater Lake sampling. A gas measurement flight on 28 April recorded the sixth highest sulphur dioxide (SO2) flux of 390 tonnes per day since 2003. Sulphur dioxide is a strong indicator gas and is derived from a relatively shallow magma body, which is perceived to currently exist a few kilometres beneath Crater Lake. Further gas flights will be conducted when weather conditions are suitable.

Sampling of Crater Lake was also conducted last week and during that visit, our scientists observed upwelling of Central Vent and reduced upwelling at the Northern vents area. During recent visits, active upwelling has only been observed at the Northern vents. It is important to note that Central Vent is the primary vent, whereas the Northern vents are a subsidiary vent system. We had surmised previously that Central Vent was sealed, blocking the main flow of fluids and gases into Crater Lake, however this vent now appears to be at least partially open.

We are still awaiting laboratory analysis of the latest Crater Lake fluid and gas samples. This will show us if magma is interacting with the hydrothermal system beneath the lake. We will report on those when available.

The high sulphur dioxide (SO2) flux, sustained carbon dioxide (CO2) and continuing strong volcanic tremor, continue to indicate that molten rock (magma) is driving this period of heightened unrest. Increasing Crater Lake temperature is reflecting the increasing upwelling of hot fluids and gases through Central Vent as well as the Northern vents.

Within the next four weeks, the most likely outcome of this unrest episode is no eruption, or a minor eruption that is confined to the lake basin. Small eruptions are still able to generate lahars, especially in the Whangaehu River. The next most likely scenario is an eruption that impacts the summit plateau and generates lahars in multiple catchments, similar to what was seen after the September 2007 eruption or older events like those in 1975 and 1969.

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 remains very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions.

The interpretation of this 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 Volcano Alert Level 2 is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning.

For information on access to the Mt Ruapehu area, please visit the Department of Conservation’s websites on volcanic risk 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's Get Ready website.

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

Geoff Kilgour Duty Volcanologist

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