A New Zealand volcanologist says the rising temperature at Mt Ruapehu's crater lake is a regular occurrence, but there is still a heightened probability of an eruption.
GNS Science on Monday increased the volcano's alert level to 2 after its crater lake hit 31C, up from about 27C in late February, and "strong levels of volcanic tremor" were recorded. It's the result of the volcano beginning a new heating cycle, which occurs regularly at Ruapehu.
"Mt Ruapehu is basically doing what Mt Ruapehu does," volcanologist Brad Scott told AM on Tuesday.
"The crater lake heats up, reaches a temperature of 40C-45C and then it cools down to somewhere below 30C, to 20C. It does this over six to nine months periods."
"The volcano is more active than it was. In particular, the temperature is rising. We've been seeing volcanic earthquake activity in the style of volcanic tremor. That tremor has been relatively strong compared to what we have seen the last four or five years."
While alert level 0 means there is no unrest and level 1 means the volcano is awake, Scott said level 2 is "critical" as it means "we are detecting things happening in the volcano with an increased probability of eruption".
"Level three is the volcano is in eruption. So the alert level system, I feel quite strongly, reflects the status of the art of prediction," Scott said.
"'Hey, we can see this volcano has changed. It's got a little bit more active, there is a slightly bigger probability of an eruption. Hence, we're sharing that information."
According to the GeoNet website, 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". These hazards include explosions, flying rocks, lava flows, landslides, earthquakes and steam eruptions.
Scott noted that Ruapehu hasn't erupted since 2007, so many heating cycles have passed in that time without issue. It's not a matter of how imminent an eruption is, he said, but if one will occur at all.
What would be cause for concern was if the unrest escalated, he told AM.
"If we were to see chemical changes in the crater lake, changes in seismicity, more heat flow into the crater lake. To maintain the lake at its current temperature requires about 300 megawatts of heat flow. If that was to increase, then we would be more concerned."
The increased temperature is thought to be a result of molten material being thrown up into shallow chambers in the volcano.
"The model and our understanding of how the volcano works from looking at the rocks it throws out, there is molten material that is sort of five or six kilometres deep, a body of molten material there," Scott said.
"Every so often, some of that moves to a shallower depth, the sort of one to two kilometre distance below the crater. What we're seeing at the moment is probably some material moving from deep to shallow storage in the volcano and therefore it's just transferring the heat associated with it to the surface."
He said the Department of Conservation has different management regimes in response to the alert levels, so anyone visiting Tongariro National Park, particularly Ruapehu, should check in with DoC about any restrictions.
Scott noted the alert level increase was only for Ruapehu and not for other volcanos in the area. The only other volcano in New Zealand at level 2 is Whakaari/White Island. All others are at level 0.
