A new study has produced the first detailed estimate of the volcanic gases vented across Auckland’s history, finding the city’s 53 cone and crater field has released around 26,000 kilotonnes of carbon dioxide, 9,000 kilotonnes of sulphur dioxide, 470 kilotonnes of hydrogen chloride and 2,220 kilotonnes of hydrogen fluoride over its 200,000 year lifetime.
The research, published in the Journal of Applied Volcanology, fills a notable gap in the country’s volcanic hazard picture. Until now, the planning scenarios used by emergency managers for a future Auckland eruption have flagged volcanic gases as a serious concern, but no one had put usable numbers on how much gas a future eruption might actually pump out of the ground in the country’s largest city.
The Auckland Volcanic Field sits directly beneath roughly 1.7 million residents and is still considered active. Its youngest cone, Rangitoto, last erupted around 600 years ago, almost certainly while early Māori settlers were already living on the isthmus. GeoNet and the Earth Sciences New Zealand network of monitoring sensors continue to watch the field for any signs of fresh magma movement.
The new figures come from the DEVORA research programme, short for Determining Volcanic Risk in Auckland, a long running collaboration between the University of Auckland, GNS Science, Earth Sciences New Zealand, Auckland Council and the Natural Hazards Commission. Further detail on the wider programme is hosted on the DEVORA website. The team behind the gas paper used a technique called the petrologic method, which works by analysing tiny pockets of molten rock trapped inside crystals when each cone first erupted. Those frozen droplets, known as melt inclusions, preserve the original concentrations of dissolved gases in the magma before it reached the surface. By comparing those starting concentrations with the gas levels left in the cooled lava and ash, geologists can work out how much escaped as gas.
The researchers sampled five eruptions chosen to cover the full chemical range of the field, including some of the larger basaltic eruptions and some of the smaller scoria cones. Those measurements were then scaled up across all 53 known eruptive centres to produce a field wide total.
One of the more striking conclusions in the paper is that the small monogenetic eruptions which built the Auckland field can release gas volumes rivalling those of much bigger volcanoes overseas. The authors note that individual Auckland eruptions, despite lasting only weeks to a few years, can put out gas amounts comparable to single large events at Mount Etna in Sicily, one of the most active and best monitored volcanoes in the world.
Sulphur dioxide is the gas of most immediate concern on the ground. It reacts with water vapour in the atmosphere to form sulphate aerosol, which can briefly cool the climate after a large eruption, and at lower altitudes it irritates lungs and creates acid rain that damages pasture, vehicles and water tanks. Hydrogen fluoride is even more dangerous to livestock when it settles on grass, since cattle and sheep grazing on contaminated paddocks can absorb toxic doses with no obvious warning signs. Hydrogen chloride is corrosive to electronics and infrastructure. Carbon dioxide is not directly hazardous at outdoor concentrations but can pool in cellars and low lying spaces near vents, which is one reason researchers monitor soil gas around the Auckland cones.
The new gas budget feeds directly into the DEVORA scenarios. Those scenarios are the playbooks Auckland Emergency Management uses to test what would happen in a real eruption, from a small vent in the Hauraki Gulf to a larger event in the central isthmus. Until now, gas effects in those scenarios had to be sketched in qualitatively. The numbers from the new study allow planners to model where a sulphur dioxide plume would drift, how much pasture might be hit by hydrogen fluoride, and how quickly an exclusion zone would need to be drawn.
Volcanologists have measured carbon dioxide quietly leaking out of Rangitoto and other Auckland centres for years as a separate research strand. Past surveys, including soil gas measurements across the field, have shown that some of Auckland’s volcanoes are still gently exhaling CO2 from deep mantle sources. A sudden change in those background levels is one of the indicators GeoNet watches as a possible warning sign of magma rising beneath the city, alongside earthquake swarms and ground deformation.
For most Aucklanders, the new totals will sound like a lot. Spread across 200,000 years, however, the carbon dioxide release works out to a tiny fraction of what New Zealand human activity now emits in a single year. The point of the work is not climate accounting but emergency planning. Knowing the upper bound of a future Auckland eruption’s gas budget lets responders plan health advisories, evacuation routes, downwind air quality alerts and animal welfare measures with sharper detail.
The Auckland Volcanic Field will erupt again at some point. Geologists cannot say when or where, only that the next vent is unlikely to open in the same place as any of the existing 53 cones. The new paper means that whenever that vent does appear, planners will have a far better grip on what its plume of gas will do, where it will go and who will need to move out of the way.
Are you confident Auckland is ready for the next eruption? Have you ever thought about which cone is closest to your home? Tell us in the comments below.
See also NZ Volcano Watch — Newswire’s live status board for every active and dormant New Zealand volcano.
See also Taupo Past Eruptions — Newswire’s interactive comparison of the Oruanui, Hatepe and modelled future Taupo eruptions.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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