Mt Tongariro - long considered a peaceful neighbour to the rowdy Ruapehu - is much more fiery than we ever realised, says a scientist sharing new findings today.
GNS Science volcanologist Brad Scott, among scientists giving presentations in a workshop at Whakapapa Village, has found evidence of eruptions at Mt Tongariro that weren't previously on the records.
Tongariro's dramatic eruptions in August and November of 2012 transformed our understanding of what had been considered a comparatively docile mountain, Mr Scott said.
When ash shot from the Te Maari Crater 7km into the sky shortly before midnight on August 6, it was thought at the time to have been the first blow there since 1896.
But Mr Scott has since found records indicating an eruption at the crater in 1928, with other events at Red Crater - a feature of the Tongariro Alpine Crossing - in 1909, 1926, 1927 and 1934. Further, there was volcanic unrest at the mountain right up until the 1970s, he said.
"We all knew Ruapehu and Ngauruhoe were pretty busy but, until now, nobody really perceived Tongariro as being as active as it has been."
The new information, with the 2012 eruptions, had led to a range of new protocols and emergency policies at the peak, including warning systems and more public information. "[And] we've upgraded monitoring equipment in the area and have put in place more seismographs."
Other research being presented would discuss findings around the ballistics of rocks that were ejected from the mountain in 2012, as well as the volcanic avalanches and debris flows that came with the eruption.
Mr Scott saw the event as a success story because authorities had been able to warn residents after monitoring picked up elevated activity.
Today's workshop will kick off a weekend reflecting on the 20th anniversary of the 1995-96 eruptions of Mt Ruapehu. This year also marks the 40th anniversary of the 1975 Ngauruhoe eruption and the 70th anniversary of the 1945 Ruapehu eruption.
Mt Ruapehu eruptions: 20 years on
• The 1995-96 eruptions ejected a total of 60 million cubic metres of acidic ash - blanketing districts up to 300km from the mountain, irritating eyes and throats in the central North Island, damaging car paintwork and machinery, contaminating rivers and water supplies, ruining crops, closing state highways, forcing airports to shut and killing livestock which ate ash-covered pastures.
• Electricity suppliers were hit with multimillion-dollar losses as ash shorted out power pylons and severely damaged turbines in the Rangipo power station. At times the ash plume reached as high at 10km, which represented a significant aviation hazard.
• The eruptions were similar in size to those in 1945, but their social and economic impacts were much greater. In 1945 there was just one ski area and no ski lifts on Ruapehu. By 1995, there were three ski areas and 36 ski lifts. By the mid-90s there were up to 10,000 people on the mountain on some days during winter.
Eruption gave snapper the shot of his career
"Bloody extraordinarily impressive," is how Craig Potton remembers the eruption that gave the Nelson photographer what became the picture of his career.
The image - showing a column of ash mushrooming into the twilight sky from a blown-out crater lake atop Mt Ruapehu - also now stands as one of the most iconic shots of the mountain's 1995-1996 eruptions, being commemorated with a 20-year anniversary at Whakapapa this weekend.
Mr Potton recalled how he had earlier been at Tongariro National Park collecting pictures and information for a book, and was back in the South Island when Ruapehu first blew in late September, 1995.
When he got another chance to photograph Mt Ruapehu erupting in June 1996, he didn't hesitate.
"I just thought, this is it, and I jumped on a helicopter down at Nelson Airport and just told the pilot, go for that mountain."
The pair arrived at Ruapehu just in time for sunset - and Mr Potton was able to capture the spectacle as the twilight covered the plume in a glorious pink hue.
Each eruptive sequence sent out pressure waves which shook the helicopter, which had to be kept upwind from the ash cloud.
"It was all there, just billowing away in front of us. I shot about 300 photographs while I was there, of which I printed 10 - but that picture really came out as the signature one."
It has since been displayed in galleries and exhibitions around the world.
Herald photographer Alan Gibson, who covered the 1995 eruption for the newspaper, also recalled taking the picture that won him the News Picture of the Year in the former Qantas Media Awards.
After waiting for days with a large press pack, the weather finally cleared, allowing him to capture the eruption against a brilliant blue sky, with the Chateau Tongariro Hotel in the foreground.
"It was absolutely spectacular - we flew up in a helicopter to the edge of the crater ... it really was a once-in-a-lifetime experience."