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First Mt Tongariro eruption in over 100 years.
A larger, molten eruption is possible if volcanic activity continues at Mt Tongariro.
The volcano rumbled into life at 11.50pm last night, sending ash and rock a kilometre into the air, prompting a potential threat warning for central North Island regions.
The "small scale" eruption was a total surprise, with the volcano last erupting in 1897, GNS Science said.
If the eruption develops, ash from Mt Tongariro could reach Auckland, disrupting domestic and international flights, University of Auckland vulcanologist Dr Jan Lindsay said.
"We know from looking at sediment cores from Auckland lakes that ash from Tongariro has reached Auckland many times over the last 80,000 years. Of course it only needs to be in the atmosphere to cause a problem," she said.
"The impact on flights in and out of Auckland airport could be significant."
GNS scientist Brad Scott said there was strong seismic activity for about 15 to 20 minutes after the eruption, however in the 24 hours prior and hours since, there had not been any significant activity.
The eruption was purely driven by the hydrothermal system, generating steam rather than molten lava, Scott said at a briefing this morning.
Hydrothermal eruptions tended to recur and could escalate into more larger and energetic molten eruptions which spewed out lava and rocks, he said.
Civil Defence has cancelled its "Volcanic Activity Potential Threat to New Zealand" alert.
GNS wasn't yet able to confirm exactly where the eruption came from but it was most likely near the Te Mari craters, near Ketetahi hot springs, on the northern side of the mountain.
The eruption was believed to have lasted only one or two minutes and was followed by a series of small earthquakes.
It generated an ash plume and ash fall, Scott said. Five to 15 millimetres of ash blanketed nearby properties.
Police urged residents to check water supplies to make sure they were not contaminated.
There was no immediate health risk to the community and there was no need to remain inside or keep doors and windows closed, police said.
Three people were evacuated from Mangatepopo Hut in Tongariro National Park.
Ski fields were open for business as usual.
'WE CAN'T REALLY PREDICT WHAT THIS WILL LEAD TO'
Eruptions had occurred on Tongariro intermittently from 1855 through to 1897, and it could not be ruled out that this was the start of a prolonged period of activity in the area, GNS vulcanologist Michael Rosenberg said.
"We have to expect the unexpected. We really can't predict what this will lead to."
Recent volcanic unrest at White Island was unrelated and coincidental to last night's eruption at Tongariro.
WITNESS ACCOUNTS
Truck driver Tama Coker was heading across the Desert Road while the eruption was happening and said the noise was like a train.
"There was a big flash," he said.
"I thought it was lightning and then it started raining sand. It was pretty thick. I heard it rumbling like a train."
Coker said that when he drove through the Desert Road he could not see the white lines on the road.
"I could just see the yellow glare on the mountain. I only had visibility of about 10 to 15 feet in front of me. It was a bit scary.
"It's something I'll probably never see again in my lifetime."
He said the sand-like ash had covered his truck, and the sign writing on the trailer was barely visible.
Local resident David Bennett who lives on the southern shores of Lake Rotoaira, about 6km away from the eruption, said he heard and saw the mountain erupt just before midnight last night.
He considered himself fortunate no rocks landed on his house last night.
"There were rocks being thrown out. It was like thunder and lightning and fireworks," Bennett said.
"It was spectacular. There were rumbling sounds and thunder and lightning coming out from the base of the eruption," Bennett said.
A few locals did drive to the Hirangi Marae in Turangi but most just stood and watched the spectacular show.
"It's a volcano. If it goes. It will go. We'll all be vapourised. Ruapehu and Ngauruhoe have erupted regularly over the years. Now it is Tongariro's turn."
Bennett's wife Robyn said she had not been able to sleep last night following the spectacular eruption.
"It looked like a huge mushroom cloud. There's a very strong sulphur smell in the air and it was very hard to breathe last night."
Robyn Bennett said she and her husband could still hear the mountain rumbling from their home this morning.
"The ash plume is rolling down the side of the mountain. I feel safe and I am not leaving."
Robyn Bennett said if the mountain did blow "our house will be in the middle of it. The lava flow will come down the valley towards us."
She could see three new vents from her home.
"They each look to be the size of the Ketetahi Springs."
Adventure HQ employee Kerry Wakelin said she took her dogs for a walk about 11.50pm last night.
"I saw flashes and lightning and a big black cloud. I thought it was a big storm," Wakelin said.
She had worked at the Whakapapa Ski Field during the major eruptions on Mt Ruapehu in the mid-1990s.
"When Ruapehu blew back then I had my bags packed and was freaking out. Last night I went to bed and had a good night's sleep," Wakelin said.
"The latest eruption is like a big old giant who has woken up, farted, rolled over and gone back to sleep," Wakelin said.
FLIGHTS AFFECTED
Flights to and from Gisborne, Taupo and Rotorua airports have resumed after earlier flights were cancelled this morning.
However, Hawke's Bay Airport appears to remain closed and flights in and out of Palmerston North continue to be disrupted.
Some flights to and from Gisborne, Rotorua, Taupo and Palmerston North were delayed or cancelled due to the eruption.
All flights in and out of Hawke's Bay Airport have been suspended. Check flight information for Napier airport here: http://www.hawkesbay-airport.co.nz/Flight_Information_14.aspx
- Michelle Cooke, Mike Watson and Danya Levy
- Fairfax NZ News
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