Cutting an imposing feature on Auckland’s eastern horizon, 260m-high Rangitoto remains something of an outlier in size within the local volcanic field and its 200,000-year eruptive history. Photo / Brett Phibbs
Cutting an imposing feature on Auckland’s eastern horizon, 260m-high Rangitoto remains something of an outlier in size within the local volcanic field and its 200,000-year eruptive history. Photo / Brett Phibbs
A new study has backed the theory that Auckland’s largest, youngest volcano once rumbled on for centuries at a time – carrying important implications for future eruptions in the region.
Nearly a decade ago, University of Auckland scientists drilled into Rangitoto to retrieve evidence that challenged long-held assumptions about the island volcano’s fiery past.
Rangitoto was widely believed to have blown only twice throughout a 700-year existence, but their 2014 study findings instead suggested it had erupted in prolonged episodes, over a period of between 500 and 1500 years ago.
“At the time, it caused a bit of a stir in the scientific community because the radiocarbon ages obtained suggested that the volcano may have been active for centuries or longer, rather than the commonly held belief of just days to months,” its lead author, University of Auckland volcanologist Associate Professor Phil Shane, said.
Now, fresh data has added more weight to that intriguing hypothesis.
Standing 260m high, Rangitoto is something of an outlier among the more than 50 centres of Auckland’s volcanic field, most having been short-lived and erupted only once.
When scientists drilled into Rangitoto’s flank, however, they were surprised to find evidence of nearly 50 different lava flows – leaving them to investigate how just when, and for how long, this period of activity had lasted.
Rangitoto is the youngest and largest of more than 53 centres in Auckland's volcanic field. Photo / Martin Sykes
To reconstruct the past, the study team used an intriguing approach – comparing evidence from some 200 drill core samples taken from within the volcano, to a record of known changes in the Earth’s magnetic field when Rangitoto was thought to be last active.
“Earth’s magnetic field changes all the time, but it is a slow process, on timescales of more than a year,” explained study co-author Dr Andreas Nilsson of Sweden’s Lund University.
The theory was that if there had been shifts in the magnetic field between each lava flow they revealed, it could be assumed at least a few years had passed between each eruption.
After applying a specially-developed statistical model to their data, they concluded Rangitoto had been built up over a phase that likely lasted more than 100 – and perhaps even several hundred – years.
That this process took so long raised the possibility that the wider volcanic field had transitioned into a “new phase”, Nilsson said, which only underscored the need to understand where, when and how Auckland’s next eruption might unfold.
“Instead, it is possible that volcanoes that form in the future may be active [for] centuries rather than less than one year or so.”
In the meantime, he said it was still unclear just when Rangitoto first erupted.
University of Auckland scientists drilled into the flank of Rangitoto volcano in 2014, to retrieve ancient evidence of eruptions hundreds to thousands of years ago. Photo / University of Otago
“We need better radiocarbon age dates and more materials to date to see if we can refine the timing.”
Another geologist, Bruce Hayward, has suggested that several thousand years before Rangitoto was born, a small volcano had already been there in the form of a small, cratered island.
The new study comes after University of Auckland and GNS Science researchers recently published evidence to reveal how a major fault may have acted as a channel for Rangitoto’s magma to flow upward.
Similarly, those findings had ramifications for the wider field, by raising the need to investigate how local fault systems interacted with its many volcanoes.
Study sheds new light on Dunedin’s long-lost volcano
Meanwhile, another new study has reached back even further into Aotearoa’s prehistoric past, to when Dunedin and its surrounding harbour and hills were covered by a massive volcano.
When it was active, between 11 and 16 million years ago, the Dunedin Volcano would’ve been an imposing feature in our southern landscapes.
Shaped like a shield, it stood about four times the height of modern Rangitoto, before its lower slopes, once covered by ancient forests, were gradually drowned by the sea and other large chunks eroded away.
The remnants of its volcanic core are today marked by Dunedin landmarks like Mt Cargill, Signal Hill and Harbour Cone, while one of its last explosive vents lay across an area between Port Chalmers and Portobello.
Dunedin and its surrounding harbour and hills were once covered by a massive volcano. Photo / Pawel Opaska
Over three main phases, the volcano also unleashed the full range of eruptive effects, from flowing lava and clouds of ash, to fast-moving, devastating pyroclastic surges.
With studies stretching back to the early 1900s, the volcano and its complex, unusual history have long been of interest to scientists, University of Otago researcher Rachael Baxter said.
Not only was it extraordinarily long-lived, the volcano produced an “amazingly wide range” of unique lava compositions.
Further, it wasn’t born from the usual processes that create volcanoes – namely two tectonic plates smashing into each other at a boundary like a subduction zone.
“It’s a kind of intraplate volcanism, away from a plate boundary, that is not yet well understood.”
For geologists, having only fragments of it left to work with had its pros and cons, she said.
“On one hand, the old part of the volcano is now exposed along the cliffs at Allans Beach on the Otago Peninsula,” she said.
“But on the other, a lot of the volcanic material and information those rocks contained, has been eroded away.”
To reconstruct what would have been its magma “plumbing system” in this area – today a rugged, windswept spot home to seals and yellow-eyed penguins – Baxter and her colleagues turned to techniques that revealed the rocks’ “magnetic fabric”.
“Depending on the way magnetic grains within these rocks are distributed, a magnetic pattern can be measured,” she explained.
Allan's Beach, where the study was carried out, is today home to sea lions and yellow-eyed penguins. Photo / Dunedin NZ
“If the pattern is chaotic or well organised, this can tell us about how these rocks were created, and if they are near or far from their source.
“In volcanic rocks, the shape of the magnetic signal from the fabric can tell you the direction they travelled in, especially if you have samples from multiple sites which let you triangulate the origin.
“Essentially, you can find out what was happening in the eruption while [the rock] was being formed.”
But the eruption her team studied wasn’t a typical one – and happened to have stemmed from a vent in the volcano that’d been underwater at the time.
Eventually, their detective work led them to the location of that vent, sitting less than 500m off the current coastline.
“It also confirmed earlier interpretations that this eruption was creating the underwater equivalent of the pyroclastic density currents – gravity-driving mixtures of hot volcanic gas and particles which can be very dangerous,” she said.
“And we demonstrated that early in the eruption, a lot of material was being dumped out close to the vent in a very chaotic manner.”
For volcanologists around the world, the first-of-its-kind study also proved that the technique involved – called anisotropy of magnetic susceptibility, or AMS – could be used to analyse remains of submarine eruptions.
As for the Dunedin Volcano itself, there were countless more questions for scientists to answer about this giant of our past.
“There’s other really cool ongoing work being done to investigate the volcano further, with the question in mind of what this could mean for our volcanoes that are still active today.”
