The volcanic alert level at Mt Ruapehu has been raised to reflect "moderate to heightened volcanic unrest" and the "potential for eruption hazards".
The move by GNS Science comes amid a new heating phase at the mountain, with the temperature registered at the crater lake hitting 31C and "strong levels of volcanic tremor" also being recorded.
Mt Ruapehu now sits at alert level 2, the same as Whakaari/White Island. All other volcanoes in New Zealand are at level 0.
"Typically, over periods of months, Ruapehu’s crater lake undergoes heating and cooling cycles. A new heating cycle has started and has been accompanied by strong levels of volcanic tremor (volcanic earthquakes)," GNS Science duty volcanologist Craig Miller said in a post on the GeoNet website on Monday.
"We consider there is an increased likelihood of eruptive activity as strong tremor is indicating increased gas flux through the system. Despite an increase in gas flow, the lake temperature is only responding slowly, suggesting a partial blockage may exist in the vent beneath the lake. This could allow pressure to build up within the volcano."
The GeoNet website says alert level 2 reflects "moderate to heightened volcanic unrest" and the most likely hazards listed are "volcanic unrest hazards" and the "potential for eruption hazards".
"An eruption may occur at any level, and levels may not move in sequence as activity can change rapidly," the website says.
"Eruption hazards depend on the volcano and eruption style, and may include explosions, ballistics (flying rocks), pyroclastic density currents (fast moving hot ash clouds), lava flows, lava domes, landslides, ash, volcanic gases, lightning, lahars (mudflows), tsunami, and/or earthquakes.
"Volcanic unrest hazards occur on and near the volcano, and may include steam eruptions, volcanic gases, earthquakes, landslides, uplift, subsidence, changes to hot springs, and/or lahars (mudflows)."
While the lake temperature has risen recently, it's below the January 2022 peak of 32C. It declined to around 27C-28C in late February, but has been heating since March 13. GNS Science says volcanic tremor activity can occur during these heating phases.
"The modelled heat flow into the lake is currently estimated at about 330 MW (megawatt), up from 100 MW in February 2022," Miller wrote.
"The lake is currently overflowing into the Whangaehu river, but the flow level is believed to be minor. The lake colour is expected to change to a darker grey colour as sediments on the lake floor are disturbed during the influx of hot fluids heating the lake."
While Miller said the volcanic alert level "should not be used to forecast future activity", he said eruptions are "usually more likely" at alert level 2 than alert level 1.
The aviation colour code remains at green.
