Researchers say the volcano which dominates Waitemata Harbour in Auckland is older and has erupted more often than first thought, which may mean other volcanoes in the city become more active.
Rangitoto was usually thought to have been formed about 500-550 years ago, but University of Auckland Associate Professor Phil Shane says it erupted intermittently or semi-continuously from about 1500 years ago to 500 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," Assoc Prof Shane said.
He said the findings were potentially relevant for the rest of Auckland, which has about 50 volcanoes in the city.
"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," says Dr Shane.
"This needs to be revisited in light of the new Rangitoto history of activity."
He said it was possible there was something unique about Rangitoto, but on the findings they cannot rule out long-lived activity in the future or eruptions at previous sites.
"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."
Two eruptions around 550 and 500 years ago were known about but the earlier activity was not discovered until scientists examined Rangitoto ash found in the sediments of Lake Pupuke, about 5km to the west near Takapuna on Auckland's North Shore.
Assoc Prof Shane said they studied tiny volcanic glass shards in the sediment rather than the more common method of radiocarbon dating to detect the earlier eruptions.