Rangitoto may be much older - and more explosive - than previously believed.
A new study has led scientists to reassess how volcanoes may behave in the future and could be a large step toward unlocking Auckland's mysterious volcanic past.
Contrary to the long-held belief that Rangitoto was formed less than 700 years ago and has erupted only twice, University of Auckland researchers now suspect there may have been intermittent activity from between 1500 years ago to 500 years ago.
Alongside basaltic ash from the island volcano's most recent eruption between 500 and 550 years ago, sediment samples taken from Lake Pupuke have revealed evidence of minor eruptions 922 years ago, 1040 years ago and 1500 years ago.
"That's much longer than we've traditionally believed for basaltic volcanoes of this kind, not only in Auckland but anywhere in the world," said lead researcher, Associate Professor Phil Shane.
The findings, published in the Journal of Volcanology and Geothermal Research, were important for understanding the risk posed by volcanoes in the Auckland region - and perhaps elsewhere, Dr Shane said.
"The old paradigm was that these volcanoes erupt suddenly in a new location each time, and only live for months to a year or two. This needs to be revisited in light of the new Rangitoto history of activity."
Future long-lived activity was a possibility, either at Rangitoto or at other eruption sites across the Auckland Volcanic Field, home to more than 50 volcanoes.
"We cannot rule out long-lived activity in the future, or eruptions at sites that have experienced previous activity," Dr Shane said. "The Auckland volcanic field could be going into a new mode of operation. If so we need to think about hazard planning and risk in a very different way."
The new study is part of an ongoing research project into the history of volcanic eruptions and lakes in the Auckland region involving Dr Shane, Associate Professor Paul Augustinus and PhD student Ola Zawalna-Geer.
The largest and most recently active of Auckland's volcanoes, 260m-high Rangitoto has long proven an enigma for scientists trying to unravel what eruptions have occurred in Auckland. More importantly, it may be the key to understanding where the next blow might take place.
Rangitoto's most recent eruption, between 500 to 550 years ago, happened near the site of the field's oldest eruption, Lake Pupuke, leaving no obvious northward or southward trend to draw a pattern from.
Scientists believe the answer to Rangitoto's riddle is that either its activity was an anomaly and had now ceased, that the field's activity has now moved to Rangitoto, or that future eruptions may occur at Rangitoto and elsewhere in the field for the next 500,000 years.
Unfortunately, the new study did not solve the puzzle, said University of Auckland volcanologist Dr Jan Lindsay.
"Will it continue erupting in 500 years' time, will the activity switch somewhere else, or will it occur in connection to activity somewhere else? We don't know."
When and wherever the next eruption does come, Civil Defence planners believe sufficient warning time would result in no casualties, with even a three-day lead-up providing plenty of time for evacuation.
Research by Market Economics estimated that in a worst-case eruption near the CBD, the Auckland region could suffer a 47 per cent reduction in gross domestic product.
But this could be reduced by 40 per cent if businesses had effective preparedness measures in place.
New Zealand would suffer a 14 per cent decline in GDP. However, relocated business could bring about a 3 per cent rise in GDP in the rest of the country.