When will the next big eruption happen?
After all, though there have been 25 "little" volcanic eruptions in the central North Island in geologically recent times, it is 1800 years since the last big one.
Geological and Nuclear Sciences (GNS) volcanologist Dr Gill Jolly says Ruapehu's 1995 eruption, although impressive to the layman, was just one of the little ones.
"We're talking 1000 to a million times less than the amount involved in Taupo," she says.
The Taupo she refers to is the Taupo eruption of 1800 years ago, which is still considered the most violent known in the world in the last 5000 years. But even that pales when compared with the Oruanui eruption of 26,500 years ago, which first helped form Lake Taupo.
That eruption buried much of the central North Island in up to 200 metres of ignimbrite – what most people would consider to be lava flows, made up of gas, pumice and ash – and changed the shape of the country forever. The ignimbrite reached almost to Cambridge, covering everything in its 133-kilometre path from where today the waters of Lake Taupo fill what is left of the collapsed volcano, or caldera.
A caldera forms when large eruptions drain the magma reservoir beneath the volcano, causing the surrounding ground to collapse into the void. Add water and you have Lake Taupo.
According to GNS, the Taupo volcano first began erupting about 300,000 years ago and it is a given that it will do so again one day. A repeat eruption of the size of either the Oruanui or Taupo eruptions would "devastate most of the North Island", according to GNS.
Taupo is part of a belt of active volcanoes known as the Taupo Volcanic Zone that runs from Ruapehu northeast to White Island and on towards the Kermadec Islands. Together they form part of a chain that includes most of the world's 600 active volcanoes in a ring circling the Pacific Ocean. It is known as the Pacific Ring of Fire. The ring follows the shape of the Pacific tectonic plate, which meets the Australian tectonic deep under New Zealand, and includes volcanoes such as Mt Pinatubo in the Philippines, Mt Saint Helens in Washington, Mt Fujiyama in Japan, Krakatoa in Indonesia and Kilauea in Hawaii.
Jolly says the denser Pacific plate is slowly moving under the Australian plate below New Zealand, and twisting relative to it. They are moving about 36mm a year.
Volcanoes are basically holes in the Earth's crust caused when two tectonic plates collide. The meeting of those plates – fault lines – is responsible for cataclysmic events such as earthquakes, tsunamis and volcanic eruptions and involves hard-to-comprehend forces. For example, a 7.8 magnitude quake in Fiordland in March effectively nudged the bottom of the South Island 30 centimetres closer to Australia.
Recent global events, mostly centred on the Pacific Ring of Fire, have raised questions about whether they are linked – the tsunami which hit Samoa, American Samoa and Tonga on September 30, the 7.6 magnitude quake that struck the Indonesian island of Sumatra on the same day, another 7.1 magnitude quake that hit Java – also in Indonesia – on September 2.
To the untrained, it could easily be assumed they were all somehow connected.
And while there seems to be a link, Jolly says they are not directly related. It is simplistic to think there is a common cause, she says. Like all good scientists, she deals only in what can be proved, and a connection between those events cannot.
"Statistics are very difficult," she says. "It's all part of the same system but the problem is connecting them.
"We've got a lot of earthquakes and not very many volcanic eruptions."
As well, when considering such a small time frame – geologically speaking – statistics would indicate there will be a period of time when a lot of events happen close to each other.
"So many different factors contribute to how a volcano works," Jolly says.
"We need to be careful when we have a lot of quakes and volcanic eruptions.
"It's a matter of looking at the observables."
Those observables include monitoring seismic activity, ground movement and gas.
Chemistry measurements are made on Mt Ruapehu to ascertain gas levels and mixtures, but there are none around Taupo, though the lake does have seven seismographs and six continuous GPS (CGPS) stations dotted around it.
Seismographs are effective but can be influenced by other sources of ground vibration, so their sites need to be carefully chosen. Jolly says early seismographs "would have filled a room". Those used today are much smaller and are basically metal weights suspended in a coil of wire. When the ground moves, the weight does too, changing the electromagnetic field, which can then be measured.
In the last two months, those seismographs have recorded dozens of earthquakes in and around Lake Taupo, although Jolly says that is nothing unusual.
"Last year we had an increase in activity with a few hundred earthquakes in a couple of months," she says.
"One was 4.4 (magnitude). There were a couple of threes but most were twos and ones. They were fairly regular for a couple of months."
Advancing technology is far more able to measure tremors than it used to be and Jolly says that though New Zealand had been monitoring its volcanoes since the 1960s, there had been rapid improvements in the last decade.
"For 10 years it's been good, before that it was okay, before the 1960s there was nothing."
Experts are also using technology to get people to report when they feel earthquakes, through the Geonet website, www.geonet.net.nz.
The website lists all recorded earthquakes – there have been seven this month already – and Jolly says members of the public are always keen to report tremors.
"If people feel an earthquake, people report it," she says.
The stronger the quake, the more reports that come in.
She says the earthquakes around Taupo last year were an indication that the Taupo volcano is "slumbering, not dormant".
The CGPS units measure ground deformation, the change in shape of land that can take place before, during or after an eruption. Deformation happens when magma enters or leaves a volcano vent. Increases in ground deformation can signal the start of a new eruptive episode.
Jolly says that as the pressure builds underground, the area surrounding the volcanic vent would be expected to rise and move away from the vent itself. The CGPS units are capable of accuracy to within millimetres and Jolly says they have measured the ground "moving up very slightly around the lake. It's small but noticeable and it tells us it's reacting."
During the 1994 eruption of the Rabaul caldera in Papua New Guinea, "there was seven metres of ground movement," Jolly says.
At Rabaul, an island sank into the sea, the bay was covered with floating pumice and volcanic material filled part of the harbour. Ash clouds produced severe lightning and heavy rain, which soaked into the ash fallout on the ground.
This made the ash heavier and caused buildings to collapse. The eruption also triggered flooding and mudflows.
According to a bulletin issued by the US Geological Survey on September 23, 1994, "the rains soaked the thick ash that has fallen on buildings, the combined weight has collapsed an estimated 80 per cent of the buildings in Rabaul. The ash is estimated to be at least 2.5 feet [76cm] thick throughout the city, with other areas having as much as 5-7 feet of accumulated ash."
The airport was closed permanently, five people died and 53,000 people were displaced.
Jolly says that while that could all happen in a matter of weeks from the first signs of activity, it was more likely to occur over months or even years.
"If we saw tens of centimetres of movement (around Taupo) I'd be worried, if it was metres, I'd be very worried," she says.
In 1922 there was a swarm of earthquakes around Taupo and the lake level subsided "by a couple of metres" but nothing more happened.
"If Taupo were to erupt, we would expect to see major ground deformation and thousands of earthquakes, not hundreds," Jolly says.
White Island's 2000 eruption followed a long period of activity and is one of "a couple" of eruptions Jolly has seen.
Even that eruption, though, was "effectively a burp", she says.
Despite all the monitoring taking place, scientists still have no way of accurately knowing when a volcano will erupt and Jolly doubts they will ever get to that stage.
Sometimes, volcanoes just erupt.
"Ruapehu in 2007 came out of the blue. Nothing we were monitoring gave any indication it would happen," she says.
"We won't get to a stage where we can predict," she says, although some volcanoes are more dependable than others. "Montserrat, you could set your time by it, although that is quite rare," and "Etna had a set pattern".
Even with the best monitoring in the world scientific theories about when an eruption may occur remain just that, theories.
And though the chances of a Taupo eruption remain slim, when it does one day happen, it will have "big consequences," Jolly says.
"Seventy kilometres around would be total obliteration. But I can't imagine it in my lifetime."
A bigger risk, to the Waikato in particular, although just about as unlikely, is an eruption from one of Auckland's 50 volcanoes, with Rangitoto blowing its top just 600 years ago.
Direct fallout would be unlikely to affect the region but the human impact would be huge.
"The social consequences would be felt in Hamilton" as people fled Auckland.
A recent study on the impact of an eruption at Mt Eden showed Auckland's GDP would plummet in the aftermath, and Hamilton's would increase due to the increased population.
But, Jolly says, that is unlikely to happen any day soon.
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