Plume visible over Mt Ruapehu

A steam plume was visible over Mount Ruapehu on Tuesday morning, GNS Science says.

Duty volcanologist Geoff Kilgour said in a bulletin on the Geonet website that the first sign of the plume above the mountain's Crater Lake (Te Wai ā-moe) occurred at about 9.45am.

It ascended slowly to about 1 to 1.5km above the summit area, remaining visible until around 10.30am.

GeoNet image of Ruapehu from earlier this morning. Photo / GNS Science

Kilgour said images of the plume indicate that atmospheric conditions favoured cloud formation, as cloud was also forming elsewhere around the volcano. The plume was seen widely around the volcano.

No seismic or acoustic activity accompanied the steam plume, indicating it was not generated by strong activity in the lake.

"We cannot rule out that very minor activity has occurred, which is why our scientists conducted an observation flight early [on Tuesday] afternoon. Observations from that visit indicate that central vent continues to be actively upwelling, and the lake level is marginally higher."

Image taken of Crater Lake (Te Wai ā-moe) from the observation flight this afternoon. Photo / GNS Science

Kilgour said the current lake temperature was about 39ºC.

"In the past, lake temperatures this high, combined with cool and calm atmospheric conditions, have generated visible steam plumes above the lake. We may expect more plumes to be seen over the coming weeks/months under suitable weather conditions."

Laboratory analysis of the latest Crater Lake fluid samples showed results were very similar to past analyses and showed no clear indication that magma is interacting with the hydrothermal system beneath the lake, Kilgour said.

The interpretation of this activity was consistent with elevated volcanic unrest at the heightened level and therefore the Volcanic Alert Level remains at Level 2. The Aviation Colour Code remained at Yellow.

Geonet would conduct further gas measurement and observation flights as weather conditions allow.

GNS Science and its National Geohazards Monitoring Centre were closely monitoring Mt Ruapehu for further changes.

On Monday, Kilgour said the volcano was in its sixth week of raised volcanic activity and had exhibited the "strongest volcanic tremor in two decades". However, it was most likely there would be no eruption, or a minor eruption confined to the lake basin, within the next four weeks.

"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."

An April 28 gas measurement flight recorded the sixth-highest sulphur dioxide flux of 390 tonnes per day since 2003. Lake monitoring had also noted changes to vents, including one previously sealed vent appearing partially open.

The period of heightened unrest is driven by molten rock (magma), Kilgour said.

He said that within the next four weeks it was most likely there would be no eruption of Mt Ruapehu, or a minor eruption confined to the lake basin - though as an active volcano, it has the potential to erupt with little or no warning.

"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."

Volcanic alerts and more information

• 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, 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.