Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Rangitoto gives clues on next blast

Drilling on island suggests it may have been active 1500 years ago and latest eruption only 500 years ago.

As they drill reached deeper, they expected to find where magma had interacted with water in the harbour after flowing out of the ground. Photo / Sarah Ivey
As they drill reached deeper, they expected to find where magma had interacted with water in the harbour after flowing out of the ground. Photo / Sarah Ivey

Digging deep into Rangitoto Island has begun to reveal the explosive secrets of Auckland's youngest volcano - and the risk the city could face in future eruptions.

Drilling began three days ago and has reached more than 70m. It will eventually produce dozens of core samples, giving a picture of the volcano's eruptive history.

The project follows recent findings that suggests Rangitoto may be much older and more explosive than previously believed.

Contrary to the long-held belief that the volcano was formed less than 700 years ago and has erupted only twice, it is now suspected there may have been intermittent activity from between 1500 years ago and 500 years ago.

The study team's leader, Associate Professor of Geology Phil Shane of Auckland University's School of Environment, said the drilling project had found numerous thin lava flows.

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The number of flows, which showed how the volcano had built up over time, had surprised the researchers, he said.

"I suppose it means it has been built up from smaller eruptions, but we can't yet say how much time is involved - that is one of the main aims of the investigation."

As the drill reached deeper, he expected to find where magma had interacted with water in the harbour after flowing out of the ground.

The EQC-funded study, a collaboration with Massey University, ultimately aims to find whether activity has been continuous or intermittent, how long episodes lasted, and whether the character of eruptions has changed.

"If we can get a detailed record of how these volcanoes grow, then we will know much better what to expect with future activity."

The information would help hazard and risk planners, such as Civil Defence, and improve models of future volcanic activity.

"While we can't predict the future, our research raises questions about if there was future activity in Auckland, how long would it last and could it be for longer periods rather than just months?"

Rangitoto, the largest of the 50 volcanoes that make up Auckland's volcanic field, might have been erupting for nearly 1000 years - a long time for that type of volcano, he said.

"That radically changes the perception of potential future hazards in the region and the implications for Auckland as New Zealand's largest city."

- NZ Herald

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