Mt Ruapehu has another tremor, crater lake temperature stabilises

Tremors had been weak to moderate for about three weeks, GNS duty volcanologist Geoff Kilgour said. Photo / Bevan Conley

There were strong volcanic tremors recorded at Mt Ruapehu over the weekend and GNS Science says the mountain's crater lake temperature has stabilised.

The Volcanic Alert Level remains at Level 2.

Tremors had been weak to moderate for about three weeks, GNS duty volcanologist Geoff Kilgour said.

On Friday evening volcanic tremors increased sharply and fluctuated between moderate and strong levels over the weekend.

But from Sunday those tremors returned to weak levels on the mountain.

Kilgour said the Crater Lake (Te Wai ā-moe) temperature had been decreasing at a rate of about 0.5C per day until it stabilised on the weekend and is now 22C.

"The return of moderate-to-strong volcanic tremor and stabilisation of Crater Lake temperature are consistent with a continued period of moderate volcanic unrest at Mt Ruapehu.

"We will carry out a gas measurement flight and Crater Lake sampling when the weather allows. As noted last week, the level of unrest is now considered moderate rather than heightened, however, the potential for eruptive activity remains."

Within the next three weeks, the most likely outcome of the ongoing unrest is for no eruption to occur, Kilgour said.

Minor eruptive activity, confined to the lake basin, was also possible, which could generate lahars (volcanic mudflows) in the Whangaehu River.

Kilgour said the next likely scenario was an eruption that impacted the summit plateau with volcanic surges.

"That event could generate lahars in multiple catchments, like what was seen after the September 2007 eruption.

"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, with wider ashfall such as occurred in 1995-96, was higher than it was before the start of elevated unrest in March 2022, Kilgour said.

" ...but within the next three weeks, this scenario remained very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions."

The increased tremor highlighted "significant uncertainty" about the state of the current period of unrest in the volcanic activity, Kilgour said.

"Our interpretation of the observational data and activity is, therefore, still consistent with moderate volcanic unrest and therefore the Volcanic Alert Level remains at Level 2. The Aviation Colour Code remains at Yellow."

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