LATEST: Further eruptions of Mt Tongariro could continue for months, if not years, scientists say.
An eruption at Te Maari crater about 1.30pm yesterday sent a two-kilometre-high ash plume shooting into the sky, though the mountain subsided later in the day.
It remained quiet overnight but GNS vulcanologists were not expecting it to stay that way for long.
Yesterday's eruption was the second this year after Tongariro erupted in August - the first time in more than 100 years.
Vulcanologists only had those historical events to compare the recent activity to, and said that a series of eruptions in the late 1800s indicated that more eruptions were to follow.
"We cannot say what will happen next at Tongariro but the scenario considered most likely, based on the August 2012 eruption and the description of late 1890s eruptions, is that we could expect another eruption of similar size at any time during the next few weeks," GNS vulcanologist Brad Scott said.
Known eruptions at Tongariro occurred in 1869 and intermittently in the years between 1886 and 1897.
Also, research based on volcanic events around the world showed that eruptions usually came in a series, Scott said.
"When you get an eruption it can stop, but most of the time there's further eruptions over months or years."
Any further eruptions at Tongariro, however, were not expected to escalate in size.
Among the estimated 400 people walking the Tongariro Crossing yesterday was a party of 100 children, teachers and parents from Tamatea Intermediate, in Napier, who were near Ketetahi hot springs when the crater erupted.
"We were right up there next to it," teacher Lomi Schaumkel said. "It was just amazing".
"It was pretty scary from where we were and it looked absolutely spectacular, the ash that came out. It really did look like one of those atom bomb explosions, and it made a rumbling sound. Some panicked, some didn't. Everyone came down safely."
English tourists Peter and Angie Glanvill reckoned they were only about 200 metres from the eruption.
"We were coming down the track and turned the bend to see it erupting," Glanvill said. "It was so incredibly slow-moving, like everything was in slow motion. We stood there for a moment and watched it, with a lot of others on the track."
His wife said: "It was the icing on the cake for us, we've never been close to anything like this."
A Kiwi film crew were in the middle of interviewing a geologist and a vulcanologist on the mountain when the crater erupted behind them.
The three crew from web-video production company 90 Seconds TV were making a film to encourage Australians to travel to the Taupo area when "she blew her top", chief executive Tim Norton said.
"There was a big boom, which lasted quite a while. There was quite a lot of euphoria, lots of screaming and yelling. There wasn't a lot of fear."
The fear came later, when the smoke and ash began to pour into the sky.
"When that happened, when they felt the rumble deep in the ground, that was a little bit frightening."
More eruptions are predicted over the coming week, and not just at Tongariro. GNS Science last week warned of a possible eruption at Mt Ruapehu after pressure began building up under the Crater Lake.
And scientists say yesterday's eruption has not lessened the chance of that.The volcanoes are close to each other but scientists did not believe that the activity at both was related.
However, they couldn't exclude the possibility, GNS vulcanologist Nico Fournier said earlier this week, and were studying it.
They also were looking at the relationship between recent earthquakes and volcanic activity.
The warning of future eruptions had put organisers of the Round Taupo Challenge cycle race on tenterhooks, but at this stage Saturday's event was still going ahead.
Director Kay Brake said organisers were monitoring incoming information and had several contingency measures in place should there be further eruptions.
Businesses such as Adventure HQ in National Park were likely to be closed until further notice. Manager Paul Ratlidge said his was one of 26 firms that provided shuttle transport to walkers on the crossing.
"We basically haven't got a business now. We've just got to suck it up. It's the nature of the business here, like bad weather - we can't operate."
At the same time, it was an exciting time to be involved. "It's a humbling experience. It's nowhere as big as the Ruapehu eruptions, but it brings home what it is to be close to an active volcano."
GNS vulcanologist Tony Hurst said all eyes had been on Ruapehu and yesterday's eruption took them by surprise, despite the larger eruption at Te Maari crater in August.
There were no earthquakes of note in the leadup to the eruption, which would have given scientists a clue.
"It was unexpected," Hurst said.
Taupo District Council emergency manager Phil Parker said it was unusual to have to monitor two mountains at a time. It was now simply a waiting game to see how and when Mt Ruapehu erupted.
- © Fairfax NZ News
Should ratepayers fork out for increased security to keep vandals at bay in Pukekura Park?Related story: Cameras set to catch vandals