Mt Ruapehu erupting in 1995. Photo / File
Devastating earthquakes and tsunamis have been more visible in headlines than eruptions in the past few years – but as yesterday proved, New Zealand's ever-present volcanic threat is just as real.
"It's a beautiful environment - but things can change very quickly," University of Auckland volcanologist Professor Shane Cronin said of our famously active landscape.
"That's why when there are warnings and rapid changes, people do need to be prepared to act fast."
Spread across two plates and on the Pacific Ocean's explosive Ring of Fire, our country is home to a dozen volcanic sites under regular watch.
GNS Science is New Zealand's principal watchdog on volcanic activity, and has the authority to raise alert levels at any moment, setting off a series of actions which could include mass evacuations and diversion of flights.
But the timing of quick-fire eruptions such as White Island's on Monday is notoriously hard to pick – even after alert levels were raised.
While scientists might be able to forecast a 20 per cent chance of an eruption happening tomorrow, they can't narrow that down to a time like 2pm on a Tuesday.
Eruptions within the volatile Taupo volcanic zone - as also seen at Mt Ruapehu, Mt Tongariro and now White Island - happen with little warning each decade.
But the biggest bangs we could face - rare and extreme caldera eruptions - would be unlikely without months, or years, of notice.
One of the difficulties, however, is none of these huge eruptions have been monitored in historic times, so scientists can only really speculate about what they might see.
Our best-known example, the Hatepe or Taupo eruption around 1800 years ago, would have created effects visible in China and Rome and fired a devastating 1.5km-high pyroclastic flow that covered the landscape with ash and pumice for 80km around.
A Bay of Plenty scenario described in a 2005 Civil Defence report, serious enough to involve a pyroclastic flow of 10km to 15km, lists effects as deaths, injuries in less severely affected areas, ash-laden roofs collapsing and vegetation and buildings igniting.
Add to that the chaos caused by food and medical supplies being cut off, ash polluting household water tanks and clogging vehicle fuel filters and power blackouts.
It was more possible the next 50 years could bring worrying but benign signs of unrest in one of our three caldera volcanoes - Taupo, Mayor Island and Okataina - whose most recent display was the 1886 Mt Tarawera eruption which killed 153 people and buried the Pink and White Terraces.
Of our sleeping volcanoes most likely to wake next, Mt Taranaki was predicted to be the one to roar to life, with a chance of an eruption probability sitting at around 50 per cent in the next 50 years.
More than 85,000 people live within 30km of the mountain today, 40,000 in high-priority evacuation areas.
The full volcanic arsenal was there - lahars gushing down the mountain's watercourses, pyroclastic flows and sector collapses spreading up to 20km from the vent, lava flows reaching up to 10km and a near-certain ashfall threatening our dairy hinterland and source of natural gas.
A recent estimate of the net losses in economic activity from a brief Taranaki eruption was crudely estimated at between $1.7b and $4b – or between $13 billion and $26b over a decade of volcanism.
Scientists are further exploring this risk in a five-year, $13.7m, Government-funded study.
Just as frightening would be the toll of even a small, localised eruption in the Auckland volcanic field, home to almost 50 volcanic centres and around 530,000 people.
Scenario plans assume buildings and infrastructure within 3km of an erupting vent would be destroyed by an initial surge of hot gas, steam and rocks and ash would fall over most of the greater Auckland area - up to 10cm thick near the vent.
Ash and acid rain would pollute water supplies and probably destroy sewerage infrastructure.
Auckland International Airport would be closed for weeks and insured losses could be in the order of $1 billion to $2 billion.
While none of Auckland's existing volcanoes were likely to erupt again, the Auckland field was geologically young and potentially active.
GeoNet began upgrading seismic monitoring technology of the field in 2006, increasing the number of stations and sites with sensors in deep boreholes.
But although the field was monitored to detect magma movement within the earth's crust, the location of the next vent and the area which would have to be evacuated might not be known until an eruption was imminent.
The threat of a steam-driven bang, just as at White Island, was always there.
"The absolute worst case scenario was one when we saw over the last day and a half - and this is going to send a few shockwaves in terms of how we can better monitor these things," Cronin said.
"But they are so difficult to monitor. It's pretty easy to point the finger and say how can we monitor this better - but, as yet, there is no standard recipe for understanding these systems."
People can learn more about what to do in the event of an eruption here.
White Island (Whakaari)
Has been locked in a near-constant state of unrest since 2012. Has had three eruptive cycles since 1976. The uninhabited island is the visible tip of a 1.6km high, 17km wide submerged volcano, New Zealand's most active. Produces lava flows, minor ashfalls and its crater has collapsed several times. Unleashed an eruption yesterday that may have killed up to 13 people, which would make it the biggest disaster there since 11 miners died in the 1930s. Last eruption: Yesterday.
Mt Tongariro
Dormant until 2012, the volcano produced a large eruption in 1869 which formed the upper Te Mari Crater. Another eruption 23 years later belched an immense quantity of steam, mud and boulders, and ejected material rose 600m-900m before rushing down the mountainside. Like White Island's, its most recent eruptions in 2012 were steam-driven ones, affecting a small isolated area. Last eruptions: August 6 and November 21, 2012.
Mt Ruapehu
New Zealand's largest cone volcano has mostly produced lava and ash in its frequent eruptions. The last explosive eruption lasted seven minutes and spread ash, rocks and water across the summit area, producing lahars in two valleys including one in the Whakapapa ski field. In contrast with the previous eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area. The effects of an eruption on local tourism, particularly skiing, is large and ash can spread over large areas, especially toward the east in prevailing westerly winds. Last eruption: September 25, 2007.
Mt Ngauruhoe
Discharged red-hot rocks of lava and periodic activity that lasted for months when it erupted in 1973. In the next two years, there were explosive eruptions of ash, and blocks of lava were thrown as far as 3km. During the last violent eruption, gases streamed from the crater for several hours, producing a churning plume of ash up to 13km above the crater. This column then collapsed, causing ash and scoria avalanches that swept down the sides of Ngauruhoe, leaving trails of rubble. Last eruption: 1973-1975.
Mt Taranaki
More than 85,000 live within 30km of Mt Taranaki — 40,000 in high priority evacuation areas. Risk of lahar flows, pyroclastic flows (up to 20km from eruption vent), lava flows (up to 10km) and ashfall. Could disrupt dairy farming and petrochemical industries, including reticulated supply to North Island. Last eruption: 1854
Taupo Volcano
Has not erupted since human settlement of New Zealand, but has been one of the most active caldera volcanoes on Earth over 300,000 years. Largest known eruption expelled more than 500sq km of lava, ash, rocks and gas. Effect of even a small eruption could be devastating for the central North Island and its effects would be felt throughout the entire country. As well as direct damage, it would inflict severe losses on tourism, agriculture, forestry and North Island hydroelectric generation industries. Last eruption: 1800 years ago.
Auckland Volcanic Field
Home to nearly 50 volcanic centres. Mass evacuation for an unknown length of time would be essential. Planning scenarios include an ashfall over Greater Auckland, significant damage to infrastructure, airport closure and insured losses of up to $2 billion. Existing volcanoes unlikely to erupt again, but field is young and potentially active. Last eruption: Rangitoto, 600 to 700 years ago.
Bay of Islands and Whangarei volcanic fields
Bay of Islands volcanic field contains 30 vents, mostly comprising scoria cones and lava flows and domes. Little is known about the field but is likely to have erupted 10 times in the past 20,000 years. Its area is not heavily populated but Bay of Islands is a popular tourist destination. Whangarei volcanic field's last eruption included small eruptions of ash, scoria and lava. Last eruptions: Bay of Islands Volcanic Field: 1300-1800 years ago. Whangarei Volcanic Field: 250,000 years ago.
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