Even a moderate-sized event in Auckland could cost the local economy up to $10 billion. Image / Supplied
Even a moderate-sized event in Auckland could cost the local economy up to $10 billion. Image / Supplied
Another eruption in Auckland isn't a question of if, but when - but what if we could influence how it unfolded?
That's something scientists are now exploring in a new million-dollar study, which will use hot wax and a sophisticated model to simulate a big blow beneath our biggest city.
Much of Auckland is at risk - more than 50 volcanoes lie beneath a field stretching across 360sq km - and more than one million people live on areas where an eruption could occur.
An eruption from one of the volcanoes could blast out an explosion crater 1km to 2km across, destroying everything in it, and people could get as little as five days' notice – although planners say there would still be enough time to organise evacuations.
The economic toll would be heavy – even a moderate-sized event could cost the local economy up to $10 billion - and a push towards intensification could make the city even more vulnerable to disaster.
Most of Auckland's volcanoes have only blown once - the most recent event was Rangitoto's eruption around 600 years ago - and the likelihood is the next eruption will be at a new, and so far, unknown location.
Precisely just what an Auckland eruption might look like is also something scientists are trying to better understand.
Professor James White, of Otago University's Department of Geology, said volcanoes like those in Auckland were known to erupt in a range of ways.
Those included incandescent fountains of lava, or "curtains of fire" along a fissure, or a series of powerful explosions that blew out large craters in the ground, while producing blast-like currents that rushed outward over the ground.
"In many eruptions there are different things going on in different places – either along the length of an erupting crack, or from different centres along it, and at different times," White said.
"In short, an eruption can be like a whole fireworks display, not just a single sparkler fountain.
Much of Auckland is at risk of eruption - more than 50 volcanoes lie beneath a field stretching across 360sq km - and more than one million people live on areas where one could occur. Image / Supplied
"We want to know what controls the show."
What scientists were certain of was Auckland eruptions were triggered above cracks that carried magma from deep in the Earth up to the surface.
In many eruptions, two or more small volcanoes, whether cones or craters, formed at different sites along the crack.
A new three-year study being led by White would zero in on just what processes determined where the cones or craters formed along the crack.
"New Auckland volcanoes initiate when magma 'fracks' its way to the surface and erupts," White said.
To do this, White and colleagues would design and then create a small-scale, controllable imitation of the shallow plumbing of small volcanic eruptions that began on a fissure, or open crack.
"We can control the walls of the artificial fissure, and we can measure the changing temperatures and properties of our artificial magma, which will be a kind of professional wax."
This gave the scientists a chance to more closely examine a key process in volcanism called thermodynamic feedback.
"When magma cools, it first flows less easily, then solidifies – this is a feature the wax recreates," White explained.
"The amount of cooling as magma flows through a volcanic crack depends on the rate of magma flow, which the crack's width helps control, as does the temperature of the crack walls.
"With constant magma from below, solidification of cooling magma in one part of the crack makes the magma flow faster in the still-open part.
"This is a feedback, because the more flow becomes concentrated in one or a few places, the less it flows through the others and the faster the magma solidifies and blocks them off."
Auckland's most eruption happened at Rangitoto around 600 years ago. Photo / NZ Herald
By measuring temperature and pressure of the magma and crack walls, the team could work out thresholds for blocking parts of the crack, and focusing the erupting melt into one or a few sites that represented volcanic cones.
"Our study will reveal how contrasts of cold, weak swampy surface rocks versus stable, drier rocks could affect the course of an eruption," he said.
"What happens in the rest of the eruption when weak crack walls collapse inward? When some rocks warm up faster than others, so that magma flows faster through them?
"If hazard managers know the answers to these questions, then by monitoring ground conditions they can better predict what an eruption will do next."
The results might also help pre-eruption hazard mapping based on the cracking potential, and style, for different ground types around the city.
"People can't stop magma rising to the surface to erupt, but if we find that early stages of fissure eruption are highly sensitive to feedback effects, it could be possible to design engineering interventions that could change an eruption's course of events."
The study is being supported through the Ministry of Business, Innovation and Employment's Endeavour Fund.
