Four weeks of volcanic unrest at Mt Ruapehu, the longest run in two decades

Mt Rupaehu has now had four weeks of volcanic unrest. Photo / Supplied

There have now been four weeks of volcanic unrest at Mt Ruapehu.

Despite the continued unrest, a GeoNet scientist says the chance of the volcano erupting remains low.

The Volcanic Alert Level remains at Level 2 as tremors continue and the temperature of Crater Lake stabilised at 37 degrees Celcius for the past week.

The level reflects the current level of elevated volcanic unrest, and while it is not used to forecast future activity, eruptions are usually more likely than at Volcanic Alert Level 1, Geonet says.

GeoNet duty volcanologist Agnes Mazot said it has now been the longest period of tremor recorded over the past 20 years.

"While Volcano Alert Level 2 is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning."

An observation flight and sampling of the crater lake were performed last week.

"Our scientists observed some upwelling in the lake, indicating that hot gas and liquid are still making their way into the lake.

"The Crater Lake remains a battleship grey colour with upwellings only at the northern vents area and no overflow."

Sulphur slicks were also present on the lake surface. All these characteristics are usual for Ruapehu Crater Lake during a heating phase, she said.

Modelling suggested that about ~230 MW of heat was required to sustain this high lake temperature.

"Over the past 30 days, the lake temperature increased by 7°C, which is like previous heating phases."

But the heating had slowed down and temperatures have now been steady around 37 °C for the last week, she said.

"This slowing of the heating, in spite of the ongoing strong level of volcanic tremor continues to indicate heightened volcanic unrest as pressure could be building up at depth."

She explained the sustained nature of the volcanic unrest, coupled with the volcanic tremor signals, elevated gas output and the hot lake remained indicative of processes being driven by molten rock (magma) interacting with the geothermal system within the volcano.

"The most likely outcome of this unrest episode within the next four weeks is still that no eruptive activity occurs, as no eruptions have followed unrest in the past 15 years."

However, there is also a possibility of a single or multiple eruptions that could impact the summit area and generate lahars into some catchments draining off the volcano, especially the Whangaehu Valley.

"The size of these eruptions, if they did occur, would probably be like that of September 2007, or the slightly larger April 1975 or June 1969 events."

The chances of a prolonged and larger eruption is higher than it was two months ago, but remains very unlikely, she said.

"Such an eruption would most likely only follow a sequence of smaller eruptions."

Information on access to the Mt Ruapehu area can be found on the Department of Conservation's websites on volcanic risk in Tongariro National Park.

The Aviation Colour Code remains at Yellow.