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A climber who was "seconds from death" after being crushed in last night's Ruapehu eruption has had his right leg amputated below the knee but has had a "miracle" recovery.
Scores of people fled from the mountain last night after the eruption, which shook the volcano about 8.20pm and was witnessed by airline pilots.
The volcanic blast wiped out a monitoring hut near the crater lake, sending two muddy torrents down the skifields.
William Pike, a 22-year-old primary school teacher from Auckland, was in the Dome Vally Hut with three companions when a rock crashed through the roof and fell on him.
Mr Pike's friends were unable to move the rock and a Whakapapa Ski Field vehicle was called and freed him. The Taupo Lion Foundation rescue helicopter airlifted him to Taumarunui Hospital and then Waikato Hospital.
After four hours of surgery this afternoon, Mr Pike has regained consciousness. He is still suffering the affects of hypothermia and has damage to his left leg.
Dr John Bonning, the Waikato Hospital emergency department doctor who treated William at Taumarunui Hospital and then at Waikato said he was "amazed" at William's recovery.
"The fact that he's recovered is a miracle. He has recovered from the worst degree of metabolic derangement that I have ever seen. His body had pretty much shut down and he was so close to death ... literally seconds from death."
Dr Bonning said William's body temperature was 25 to 26 degrees Celsius and he was dangerously close to a cardiac arrest.
"For the majority of the night he was a hair breadth away from dying. It brought tears to my eyes to see him just now. He was so lucky it was just his legs.
"If there'd been any delays in any stage of the process, he would have been dead. Someone was with him every step of the way," said Dr Bonning.
"I congratulate everybody for what they did - to get him off the mountain, to Taumarunui, To Waikato. A young life with so much potential has not been wasted."
Mr Pike's parents Barry and Tracy said their son was a passionate climber.
"He lives to climb. He was in a place that he loved. He is a very conscientious climber and an incredibly active person," said Mrs Pike this morning.
Mt Ruapehu has slumbered today after last night's eruption but experts are wary of saying the volcanic activity is over.
A covering of ash could be seen around the volcanic crater at the summit of the mountain this morning.
The eruption at Mt Ruapehu last night threw debris as far as 1.5km from the crater's edge, GNS said.
"Impact craters" 1-2 metres wide had been caused by the debris landing after being hurled into the air, Brad Scott - a volcanologist at GNS - said.
A considerable amount of water had also pooled at the top of the volcano and then run off down the Whangaehu and Whakapapa glaciers.
The lahars - mud flows - created by the eruption had been minimal, reaching only a short distance down the mountain and not causing any damage on the skifields.
There was no lava in the eruption but the top of the mountain was blackened by mud and ash thrown out of the crater.
Department of Conservation scientist Harry Keys said there had been no more volcanic tremors today and he and his GNS colleagues were "getting more relaxed" as the day wore on.
However, he described Ruapehu as an "extremely unpredictable" volcano and said it was still possible that further eruptions could happen over the next two or three days.
The risk was high to extreme in the crater area.
"The only way we can manage that risk is by saying to people 'don't go up to the crater for a few days'.
"If people are in the summit area when the mountain erupts they would be very lucky to escape with their lives," Dr Keys said.
Skifield operator Ruapehu Alpine Lifts is keen to reopen tomorrow as they are in the middle of the busy and lucrative school holidays.
The company said it had taken the "prudent approach" in closing today.
Dr Keys said he was "relatively relaxed" about the skifields reopening tomorrow and the matter would be reviewed again in the morning.
Regardless of whether the skifields opened or not, Dr Keys said the advice authorities were giving the public was that they should not venture above the top of the skifield boundary.
There was minimal risk of a small lahar coming down the western boundary of the Whakapapa ski area, but that risk was managed by the alarm system in place on the mountain, he said.
"This is possibly the most heavily alarmed volcano in the world, but they can only tell us what the mountain tells us.
"Ruapehu is a difficult, unpredictable volcano. The risk (on the mountain) is never zero."
"Every five or 10 years there are eruptions which are hazardous if you are in the wrong place at the wrong time."
Dr Keys said scientists would not be heading up to the crater rim for a few days.
It was, however, important to get samples of the crater lake water to test for acidity and temperature.
"We do this kind of thing when it is necessary under strict management controls," he said.
This would be undertaken by helicopter but Dr Keys was unsure whether it would happen tomorrow or Friday.
The eruption was "small to moderate" - smaller than the eruptions of 1969 and 1975 but bigger than last October's.
"Typically you'll have two or three smaller events over the next few days if this plays out like 1975," Mr Scott told Radio New Zealand.
He described the eruption as steam-driven, rather than being caused by new lava coming into the volcano.
Police last night checked ski lodges crammed with school holiday makers near the Chateau as a precaution.
Roads, including State Highways 47 and 48, were closed following concerns that ash could cause slips onto them. The Main Trunk railway line was closed while the damage was assessed.
Road and rail links were later re-opened.
The Ruapehu District Council, the Manawatu Wanganui Civil Defence Emergency Management Group and GNS Science were last night assessing the extent of the volcanic activity.
Ministry of Civil Defence & Emergency Management director John Hamilton said there were no immediate reports of other injuries.
"As a precaution, the ski fields have been evacuated and people should not go on to the mountain until the authorities consider it to be safe.
"Scientific data is now being studied and the mountain will be inspected at day break."
This morning Ministry of Civil Defence spokesman Vince Cholewa said people would be moving onto the mountain at first light to check the damage.
"It looks like it's smaller than initially anticipated," he said. "Police have decided it's not necessary to continue the evacuation of the mountain".
GNS duty volcanologist Craig Miller said an eruption from the crater lake about 8.20pm sent plumes of black ash into the air which were witnessed by pilots flying overhead.
"There's been an ash cloud go up which is what the pilots have seen. They have also seen it from the skifields - basically a big black cloud. The groomer drivers have seen it."
Lahars flowed down the Whakapapa and Whangaehu catchments and black ash fell on Turoa ski field.
Mr Miller said the eruption set off the Eruption Detection System, which is how GNS was notified, and is believed to have destroyed the Dome Shelter, a shed with monitoring equipment which lost signal shortly after the eruptive.
Mr Scott, the volcano surveillance co-ordinator at the Institute of Geological and Nuclear Sciences at the Wairakei Research Centre, said the eruption was moderate - Level 2 on a scale of one to five.
Staff at the Whakapapa ski field alerted the institute about 8.23pm, he said. The eruption caused a 7-minute earthquake which measured 2.8 on the Richter scale.
He said the jolt was not large but would have been felt by people on and around the mountain. "It was what we call a blue sky eruption. It came from nowhere. We had an eruption, then it was over and done with."
Mr Miller compared last night's eruption to similar ones in 1969 and 1975 but said it was probably smaller than the one exactly 12 years ago that caused widespread damage.
He said there had been just one eruption but it was difficult to know if others would follow.
"At the moment it looks like there has just been one big burst but something else could happen. The burst pretty much came out of the blue, which is what Ruapehu tends to do. It gives you zero warning, so there's nothing to say that won't happen again."
Staff were last night interpreting seismic activity and taking reports from people in the area but needed to wait until today to assess damage.
Holidaymaker Sharon Sweeney Lauder, who was staying on the Ruapehu Rd, said she and family felt a tremor about 8.30pm.
"The three of us were sitting in the lounge and felt an earthquake. I said 'Oh I wonder if the Mountain erupted, I wonder if there will be any skiing tomorrow."
Mrs Sweeney Lauder said the tremor wasn't followed by sirens or any further tremors so she and her family didn't think much more of it.
Ruapehu District Council communications manager Paul Wheatcroft said he had spoken to one person in a ski hut who said he was outside and hadn't noticed a thing on the mountain.
Mr Wheatcroft said he had heard that the ski huts were all being evacuated. While he was unsure how many people were in the huts it was school holidays so they were likely to be crowded.
Both northern police and fire communication centres had been notified about "black puffs of smoke" coming from the mountain.
A fire spokesperson said the report had been passed onto the Department of Conservation, who were investigating.
MetService ambassador Bob McDavitt said they had notified the Civil Aviation Authority, the Airways Corporation and airports to avoid the area so as not to strike ash.
They issued a Sigmet, or Significant Meteorological Information, a weather advisory containing meteorological information concerning the safety of all aircraft.
The sigmet area last night, which contained ash blowing in gusts of 25 knots, was a triangle from Mt Ruapehu to Whakatane and across to Hamilton but could change, Mr McDavitt said.
- with NZPA