New Rangitoto volcano research prompts re-think

By Cassandra Mason

An aerial view of Auckland City with Rangitoto in the background. Photo / Brett Phibbs
An aerial view of Auckland City with Rangitoto in the background. Photo / Brett Phibbs

A new discovery that shows that Rangitoto erupted "semi-continuously" for about 1000 years is prompting scientists to re-think what the volcano could do in the future.

The most recent volcano to erupt in Auckland, Rangitoto was thought to be close to 550-years-old and to have erupted once or twice in its lifetime.

However, new University of Auckland research shows that the volcano actually erupted "intermittently" or "semi-continuously" from about 1500 years to 500 years ago, smashing traditionally-held beliefs about volcanoes here and around the globe.

The findings are also prompting scientists to re-think how Auckland's volcanoes will behave in the future.

"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," said lead researcher Associate Professor Phil Shane.

"This needs to be revisited in light of the new Rangitoto history of activity."

While the scientists acknowledge that Rangitoto's surroundings are unique, the findings mean that long-lived activity cannot be ruled out, and Auckland needs to prepare for that.

"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," said Shane.

He said future planning would have to consider living with active volcanism for a long period of time.

"It's not as bizarre as it sounds, if you think about Iceland and Hawaii - societies have gotten used to living with volcanic activity for generations.

"At the moment, people think that volcanoes in Auckland would probably occur at a location that hasn't had a volcano in the past and that it would be short lived .. and then the activity would be over."

The discovery was made by examining volcanic ash in the build-up of sediment in nearby Lake Pupuke. The sediment, which can be very accurately dated, revealed a "chemical fingerprint" of tiny volcanic glass shards. Studying this material showed researchers when the eruptions happened.

Up until now, scientists have used radiocarbon dating to determine the age of samples taken directly from the volcano. But sediment examination has proved to show much more detailed evidence of volcanic activity that has been buried by further eruptions.

The study is part of an ongoing research project into the history of volcanic eruptions and lakes in the Auckland region. The research has been published in the latest issue of the Journal of Volcanology and Geothermal research.

- nzherald.co.nz

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