Just like Mt Tongariro's surprise bang on Wednesday afternoon, what happened a few kilometres away at 8.20pm on September 25, 2007, came suddenly and violently.

Shortly before airline pilots noticed a black plume rising above Mt Ruapehu, a volcanic blast threw ash, rocks and water across the summit area, sending two muddy torrents down the skifields.

Inside a hut on the edge of the crater lake, William Pike and James Christie heard a "massive boom" before the building's door was blown from its hinges and mud and rock poured inside. Mr Pike's crushed leg later had to be amputated.

The warning signs Mt Ruapehu gave in the days before that explosive moment are being seen again now - worrying scientists that the mountain could be about to produce a similar-sized eruption.


GNS volcanologist Michael Rosenberg said Mt Ruapehu has been showing two forms of unrest, which are considered unrelated.

There have been 45 earthquakes about 5km beneath the mountain since early August, but 35 of those have come in the past month.

This is being considered the product of "rock-breaking" - tectonic activity separate from the magmatic behaviour inside the volcano above.

It's the other red flag - a bottling up of volcanic gas - that is most concerning scientists, who last week raised the mountain's Aviation Colour Code from green to yellow, warning that an eruption could come within the next few weeks.

A tell-tale sign was that the lake's temperature had remained relatively stable since March, hovering between 20C and 25C, while gas temperatures below were 800C.

The lake's comparatively docile warmth was out of character with the temperatures of 40C it had reached in previous years, Mr Rosenberg said.

"What that suggests is that the heat and fluids coming off the magma aren't getting right through to the surface."

Mt Ruapehu houses three "conduit" pipes, but only the central vent, which runs directly up to the base of the crater lake, has been active over the past few thousand years.

Scientists believed volcanic gases and fluid in this pipe was dissolving old lavas, precipitating different hydrothermal minerals that were sealing up cracks in the rock above.

The seal it created was not perfect - but enough to make scientists sit up and take notice, especially because of the similarity with the causes of the 2007 eruption.

"Volcanic gases are good at transporting heat and travel up the conduit pipe and through the broken rock all the time, presumably leaking into the bottom of the lake," said Associate Professor Phil Shane, from the School of Environment at the University of Auckland.

"If you go deeper into the volcano - let's say kilometres down - you might find new magma that doesn't even have to be rising to create gas, and that gas and heat that comes with it needs to go up and escape.

"If the conduit is filled with old rocks, these gases can build up and then you get to a threshold where it just goes pop," he said.

"What makes it hazardous is not so much that they're bigger eruptions or that they happen all the time ... it's just that they can happen with minimal warning."

Preceded by a seven-minute quake measuring 2.8 on the Richter scale, Ruapehu's 2007 eruption sent skiers fleeing down the mountain.

"It blasted out a jet of rock and mud across the summit plateau of Ruapehu, but it didn't really produce a widespread ashfall - mostly it was close by the mountain," he said.

"But certainly the blast and fallout of the large rocks posed quite a significant hazard for people in the mountain within a few kilometres."

It was similar to other bursts in 1969 and 1975, but smaller than the big blows that occurred 50 years apart in 1895, 1945 and 1995.

Mt Ruapehu - which translates to either a "pit of noise" or "exploding pit" - presents the full arsenal of an active volcano.

It could spit tephra bombs kilometres into the air and ashfall over large areas, especially to the east in prevailing westerly winds.

The chances of a lava flow were relatively low and fearsome pyroclastic flows were yet to be seen on Ruapehu.

More likely are devastating lahars created by the crater lake emptying and splashing over its sides.

Powerful enough to smash through bridges by collected mud and debris, lahars could gush down the Whakapapaiti stream toward the skifield, or pour into the Whangaehu River to the east.

In 1953, a lahar damaged a rail bridge at Tangiwai and caused New Zealand's worst rail disaster, killing 151 people. Mr Rosenberg said such a lahar was another real possibility.

Scientists visited the mountain yesterday to take samples which would indicate whether the temperature and chemistry of the gases and lake water had changed.

"At this stage, we think perhaps Ruapehu has changed into that situation where there's a part-seal - a barrier to stop the heat, fluid and gas getting out, causing a pressure build-up beneath," Mr Rosenberg said.

A Ruapehu eruption would come as the fourth volcanic eruption New Zealand has seen this year - and after yesterday's encore at Mt Tongariro, the third within the explosive Taupo Volcanic Zone.

While our volcanoes straddled the tectonic plate boundary known as the Pacific Ring of Fire, their individual eruptions were not connected and did not influence each other.

"All of the volcanoes in the North Island are related to one another only in that they are on top of the plate boundary," Mr Rosenberg said.

"They have a separate magma system which means a separate body of magma underneath the crust and have a separate plumbing system."