The Auckland Volcanic Field is unique among New Zealand's volcanic hazards: it is monogenetic, meaning each of its 53 known volcanoes has erupted exactly once and will not erupt again. The hazard comes from the field as a whole — a new vent could open anywhere across a 360-square-kilometre area that includes Auckland's entire central isthmus. The most recent eruption formed Rangitoto Island, around 600 years ago.
53 volcanoes, one field, one future vent somewhere
The field's volcanoes include Mt Eden, One Tree Hill, Rangitoto, North Head, Mt Victoria, Mt Wellington, Mt Albert, Mt Roskill and about 45 others — most of which have been partly or fully quarried away for scoria over the past 150 years. Each is a small cone or tuff ring that erupted once and then became extinct. The magma source beneath the field is thought to be a deep asthenospheric plume; new magma batches rise to the surface roughly every 1,000–2,000 years on average, but with enormous variation — some periods have seen multiple eruptions within a century, others have seen 10,000-year quiet periods.
Rangitoto is young, and Rangitoto changed everything
Rangitoto Island erupted around 1400 CE and was witnessed by the first generation of Māori settlers in the Auckland area — there is a footprint of an adult and a child preserved in the ash on Motutapu Island, along with destroyed villages. Its eruption was larger than every other Auckland Volcanic Field eruption in the preceding 120,000 years combined: roughly 1.7 cubic kilometres of material, vs typical values around 0.05 for each other vent. Whether Rangitoto was a one-off or marks a shift to larger eruptions is an active research question.
What a new eruption would look like
Unlike an Alpine Fault rupture, where we know roughly what will happen and where, an Auckland Volcanic Field eruption could occur almost anywhere on the isthmus, including directly under downtown Auckland, under the motorway, or under the harbour. The most likely pattern would be a brief period (days to weeks) of escalating earthquakes as magma rises, followed by the opening of a new vent and a VEI 2–3 eruption lasting weeks to months. If the vent opens on land, expect a scoria cone and lava flows within a few kilometres. If it opens in water (very likely if on the harbour), expect violent steam-driven explosions and a tuff ring up to 1 km across. In either case, downwind ashfall would blanket hundreds of square kilometres.
Evacuation is the plan
Auckland Civil Defence has detailed evacuation plans that depend entirely on where the new vent opens — evacuation zones would be defined within hours of the first precursor earthquakes. Auckland Transport has modelled scenarios where 400,000 people need to move within 48 hours, including contraflow on motorways and ferry-based evacuations of North Shore and island communities. The DEVORA programme at the University of Auckland publishes most of the underlying modelling.
How likely is it, really
In any given 50-year window, the probability of a new Auckland Volcanic Field eruption is roughly 0.5–1%. That is lower than the Alpine Fault, lower than Taranaki, lower than a Hikurangi subduction earthquake. But the consequences of a vent opening in central Auckland are enormous, which is why the hazard is taken seriously. The practical steps for Auckland residents are essentially the same as for any urban emergency: know your civil-defence group, have a grab-bag, have a household plan, and follow GeoNet if swarm activity is reported under the city.
Other New Zealand geohazards
This page is part of the NZ Geohazards dashboard — a live tracker of the six major geological hazards New Zealand faces, with probabilities and elapsed time updated continuously.