Mt Ruapehu alert level lifted after crater lake temperature rises 20C

Ruapehu's crater lake temperature has risen about 20C since mid-April. Photo / Maurice Costello

Mt Ruapehu's risk of eruption may have increased, but GNS volcanologists say nearby residents should not feel concerned.

GNS today announced the mountain's crater lake temperature had doubled in the past few weeks, rising from 25C to between 45C and 46C over the past couple of days.

Duty volcanologist Geoff Kilgour says scientists made two visits to Ruapehu yesterday, one flight to measure the gas output and other to sample the crater lake water and make additional ground-based gas measurements.

"Volcanic gas measurements indicate an increase in the amount of both carbon dioxide (CO2) and sulphur dioxide (SO2) output ... Seismic activity at Mt Ruapehu is usually dominated by volcanic tremor. Since the volcanic earthquakes in late April the seismicity has been dominated by volcanic tremor at varying levels. The level of tremor has increased but is not exceptional in terms of the last few years."

Volcanologist Brad Scott has quashed a few "urban myths" about why there is currently an increasing level of activity in the volcano.

The Ruapehu crater lake in 2013. Photo / Geonet

"It's got nothing to do with weather. And White Island, they're [volcanoes] all independent of each other."

Mr Scott says the activity is caused by molten lava getting trapped inside the volcano itself.

"When that new pulsar heat and hot rock comes into the volcano it's whether or not it can flow through the volcano and get out of it and if the holes in the volcano aren't big enough to let the gas through it just over pressurises and pops."

And for those in surrounding towns worried that a lahar will swamp them, Mr Scott says debris is unlikely to travel more than a few kilometres from the volcano itself.

"One of the biggest eruptions, in 1995, only a few blots got past the 3km or 4km mark and that was really rare. Being away from the volcano is very safe and even the standard places you can go. Different story if you go and climb the thing and you're camping at the crater lake or something."

As for where it travels, Mr Scott says the majority head out towards the Desert Rd but there have been some eruptions producing lahar on the northern side.

"If you're at the ski lodges, they're safe as. Ruapehu only affects within about 3km of the lake and the nearest part of the ski fields are about 4km or 5km away so it does make it a fairly safe environment and the rest, once off you're off the bottom of the volcano, nothing can touch you."

Mr Scott says Ruapehu Alpine Lifts has shifted all of its infrastructure - ski tows, towers, cafes - out of the valleys in case it did head in that direction.

As for how the volcanic unrest occurs? "That's the $64,000 science question of volcanologists all over the world."

The Department of Conservation also issued a warning to climbers and trampers on the mountain, to not enter the Summit Hazard Zone on Mt Ruapehu until further notice.

The Summit Hazard Zone is the area within 2km of the centre of the crater lake.

It encompasses all the peaks in the summit area, with Te Heuheu Peak at the north end of the summit area at the edge of the zone, and the upper Turoa skifield at the south.

Climbers and trekkers should refer to the Summit Hazard Zone map or use their map and GPS reading skills, to determine when they are approaching the zone.

"We recommend climbers, trampers and walkers do not enter the zone," said Paul Carr, DoC's operations manager for Tongariro.

"Guiding companies should also heed the advice and not take people into the zone."

No ski areas, other facilities or roads on Ruapehu or elsewhere in Tongariro National Park - including the Tongariro Alpine Crossing - are affected by this warning.