Australasian Science Magazine

By Stephen Luntz

The Rangitoto volcano near Auckland erupted for a period of 1000 years, not 50 years as previously thought, challenging ideas about the behaviour of small volcanoes and causing a rethink of safety plans in New Zealand’s largest city.

Auckland sits among a field of volcanoes, and at some point new ones will probably appear, possibly within the metropolitan area. “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 A/Prof Phil Shane of the University of Auckland. Evidence of two eruptions 550–500 years ago meant Rangitoto was already an outlier, but Shane’s work has extended this dramatically.

Shane has been studying the sediments at the bottom of Lake Pupuke for signs of ancient climate change, and says: “We noticed there was all this volcanic glass there as well”. Sediments from more distant, and reliably dated, volcanic eruptions meant Shane could date the Rangitoto ash with confidence. He found intermittent or continuous deposits for a period of around 1000 years, of which the known eruptions were the most recent.

The sediments could not come from another volcano, Shane says, because “the next youngest volcano in the area is 10,000 years old. Volcanoes also have a chemical fingerprint, and these deposits match Rangitoto.”

Lake bed sediments provided a good record of volcanic activity, Shane argues, as long as the lake is so deep there is little biological activity at the bottom to disturb the ash.

“It’s a good question why no one has done this before,” he says. Previous evidence was based on samples taken directly from the volcano where later eruptions hide the evidence from earlier ones.

Shane is unsure of the implications for future danger. “It was active for 1000 years then stopped for 500. We can’t tell if this is a short break or a sign that it has stopped.”

While a future eruption of Rangitoto would have serious effects on Auckland, Shane believes that a small eruption would be manageable. “In Hawaii people have learned to live by continuously erupting volcanoes,” he points out. A bigger danger would be “the emergence of a new volcano under the central business district”.

Larger volcanoes, such as Vesuvius, or Hawaii-style shield volcanoes can last for centuries, but this work will cause a rethink of the behaviour of small basaltic volcanoes.