Mount Ruapehu’s crater lake has recorded its lowest temperature in 54 years as minor volcanic unrest continues, volcanologists say.
An eruption at the 2797m stratovolcano remains “unlikely” as the lake’s temperature declined from 11C in August down to 8C.
This is the lowest measured temperature since 1970, with a temperature of 9C recorded in 1988.
GNS Science duty volcanologist Steve Sherburn said the temperature fluctuates in response to changes in volcanic heat input.
“Since mid-February there has been little heat coming in, so the temperature has been declining.”
He said history has shown eruptions at Mount Ruapehu are more common when the lake is warmer than average but also has a slightly higher than normal probability of erupting during cool periods.
Since 1970, 115 eruptions large enough to impact the summit area have occurred, with only three taking place when the lake is cooler than 15C.
Sherburn said the reduced heat flow suggests some mineral sealing of the vent areas.
“Although the amount of sulphur dioxide (SO₂) volcanic gas passing through the lake has also declined during 2024, the fact that we are still measuring gas indicates the vent is not completely blocked.
“This interpretation is supported by observations we made on September 30 of upwellings in the crater lake with slicks of sulphur floating on the lake surface.”
The cooling of the crater lake, alongside declining SO₂ gas flux, is consistent with the cooling of the hydrothermal system beneath the lake, he said.
“However, an unheralded eruption might occur if there is a substantial and sudden release of gas that builds up pressure beneath a partly sealed vent below the lake.”
Sherburn said recent observations are consistent with “ongoing, low-level” volcanic unrest.
“Low lake temperatures and low gas emissions indicate that the vent system beneath the lake is partially sealed, limiting heat and gas flow to the surface.
“We believe the possibility of an eruption is still low.”
The Volcanic Alert Level remains at 1 and the Aviation Colour Code remains green.