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...
