Auckland's next big eruption likely to come from a volcano that doesn't yet exist, scientists say

Friday, 3 March 2017

Auckland is built on one of the densest volcanic fields in the world, with more than 50 vents dotted around the city.

Boiling lava is spewing from a mountain in south Auckland, rocks are being thrown through the air and a thick cloud of ash covers the sky - would you know what to do?

This was the question behind new research from GNS Science, which analysed what would happen if one of Auckland's 52 volcanoes erupted.

Rangitoto is Auckland's youngest volcano, having erupted 600 years ago.

The research paper, released in the Journal of Volcanology and Geothermal Research this week, is based on a hypothetical two-month-long period of unrest and eruption near Mangere Bridge, in south Auckland.

Geoscientists from GNS Science, the University of Canterbury and Massey University looked at the immediate hazards of an eruption and the ongoing impact an eruption could have on Auckland's critical infrastructure.

The Mangere Bridge area has one of Auckland's major power circuits running through it, which would cause rolling outages across Auckland and Northland for up to a year after an eruption, Deligne said.

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Lead author of the paper, Natalia Deligne, said that while scientists had no idea where the next eruption would be, they could be better prepared by looking at how to respond.

Their eruption scenario included a new volcanic cone being created, a series of small volcanic earthquakes, ash fall, lava flows, and rocks being hurled into the air - all of which were considered realistic possibilities for an eruption in Auckland, Deligne said.

Deligne, a volcanic hazard and risk modeller at GNS Science, said the wider study looked at fuel, roads, rail, ports, aviation, water supply and telecommunications to get a dynamic picture of how Auckland would hold up.

The Auckland volcanic field is the most densely populated field of its type in the world.

The field is 250,000 years old and there have been 55 recorded eruptions, the most recent being Rangitoto around 600 years ago.

Most Auckland volcanoes are monogenetic, meaning they only erupt once, so it is very likely that the next volcanic vent in Auckland will erupt in an entirely new location.

No spatial patterns have been detected within Auckland's volcanic field - the oldest and youngest volcanoes, Pupuke and Rangitoto, are next to each other - so the location of a future eruption cannot be predicted through pattern.

While they can't say where the next eruption will be, GNS scientists say they are certain that there will be a future eruption.

By looking at the infrastructure network and the impact an eruption would have, people can better understand how they can live with this kind of risk in their backyard, Deligne said.

'If we can understand what is possible and credible, we can better plan for it.'

The geography of Auckland means there are only three main electricity lines supplying Auckland and Northland with power.

The lines run through Otahuhu to Mangere, Penrose and Southdown, and extend northward. The Mangere line provides power to Auckland Airport, Middlemore Hospital and the Mangere Wastewater Treatment Plant.

This meant that a power disruption to the Mangere line could affect the entire region, the paper said.

'An eruption would be challenging for all sectors,' Deligne said, but some would handle it better than others.

Deligne said that if electricity supplies to the city were comprised during an eruption, residents in Auckland and Northland could experience rolling outages from anywhere between a month to a year or more.

The paper is the first complete eruption scenario developed for Auckland since the 1990s.

In November 2007 and March 2008, Civil Defence carried out 'Exercise Ruaumoko', which tested nationwide preparedness during the 'unrest' period leading up to an eruption, but didn't assess the aftermath.

WHAT GNS SCIENCE'S 'MAKE-BELIEVE' ERUPTION LOOKED LIKE

The volcanic activity in the study spanned 10 weeks: two weeks of nonactivity, four weeks of low to heightened activity and four weeks of eruptive activity.

An 800m-diameter volcanic vent emerged, expanding over course of the eruption to around 1200m.

The size was modelled on the Maungataketake (Ellet's Mount) site, which had a 76m high cone and 100m wide crater before it was quarried.

The first pyroclastic surge was 'worst case scenario', resulting in complete destruction within 2.5km of the vent, severe damage to most structures and destruction of weaker structures as far as 6km away.

Lava flow was assumed to be 10m deep, and 2m-high tsunami waves were predicted for the Manukau Harbour shoreline.