Mt Ruapehu could blow at any time - experts

Scientists fear build up of pressure beneath Crater Lake. Matt Bowen and Michelle Cooke report.

Climbers and trampers have been told to avoid Mt Ruapehu's summit zone after scientists warned the mountain could erupt without warning.

On the mountain though, it's business as usual.

Pressure was building up under the Crater Lake on Mt Ruapehu, increasing the likelihood of an eruption in the coming weeks, GNS Science said yesterday.

Measurements taken on the volcano over the past few weeks indicated eruptions were more likely "within weeks, not months".

A sudden release of pressure built up under the lake may lead to an eruption.

The Conservation Department issued a warning for trampers and climbers to avoid the summit hazard zone, a two kilometre area from the centre of the Crater Lake.

"We recommend that guiding companies do not take people into the zone. Any other climbers, trampers and walkers should not enter the zone," DOC Ruapehu area manager Jonathon Maxwell said. But Whakapapa ski field summer operations manager Callum Learmont said alert level fluctuations were simply part of life on an active volcano.

Their guided tours to view the crater lake don't kick in until December 15 but he said at the current alert level those would still go ahead.

"At this stage the alert level is one and we're pretty comfortable with that but obviously we monitor it and take it seriously," he said.

"It's something you take into account. You plan for it and it's part of your operation. It's something you have to work your way through."

If the situation worsened they'd take advice from GNS Science, who they're in close contact with, but at the moment things are running as per usual.

DOC warned that people in the area should be aware of an increased possibility of lahars on the tracks and especially when approaching the Whangaehu, Wahianoa, Mangaturuturu and Whakapapaiti streams.

"They should be conscious of potential lahar noise from upstream, and make their way across these streams quickly," DOC warned.

Mt Ruapehu, which last erupted in 2007, usually did not give any immediate warning of an eruption, GNS Science vulcanologist Steven Sherburn said.

Pressure build-up was thought to have caused the 2007 eruption and a smaller eruption in 2006.

The aviation colour code has increased from green to yellow as a warning for those flying over the region, but the volcanic alert level remained at level 1. Yellow indicates signs of elevated unrest, while level 1 indicates signs of volcanic unrest.

GNS Science believed the temperature a few hundred metres beneath the crater was about 800 degrees Celsius but the lake itself was only 20degC.

"This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake."

Small earthquakes have been occurring about 5km beneath the summit of Ruapehu since late October, but it was not known whether they were related to the increased temperature below the crater, GNS vulcanologist Brad Scott said.

"We perceive them to be completely unrelated at this stage," he said.

However, the quakes had the potential to build up processes and push magma further to the surface, which would further increase the likelihood of an eruption, he said.

GNS constantly monitors Ruapehu and had discovered over the last few weeks that the chemistry and gases within the Crater Lake were changing, Mr Scott said.

Ruapehu experienced a small scale eruption in 2007 and a series of large scale eruptions in 1995 and 1996.

All facilities, roads, walking tracks, alternative routes lower on Mt Ruapehu, and other activities on the mountain were still operating.