Auckland’s volcanoes: Rangitoto discovery adds ‘new dimension’ to city’s hazard-scape

Cutting an imposing feature on Auckland’s eastern horizon, the 260-metre-high Rangitoto remains something of an outlier in size within the local volcanic field and its 200,000-year eruptive history. Photo / NZ Herald

Scientists have gleaned fresh insights into Auckland’s youngest, largest volcano, with a discovery adding a “new dimension” to the city’s hazard-scape.

In a just-published study, University of Auckland and GNS Science researchers have used cutting-edge 3D modelling to reveal how a major fault may act as a channel for Rangitoto’s magma to flow upward for part of its ascent but escape it at the near-surface.

The team say their findings raise a need to better understand how local fault systems interact with the many volcanoes lying beneath the region.

Cutting an imposing feature on Auckland’s eastern horizon, the 260-metre-high Rangitoto remains something of an outlier in size within the local volcanic field and its 200,000-year eruptive history.

Of 53 volcanic centres, it’s been the only one known to be active in recent geological times, last erupting in two phases around 600 years ago.

After successive studies, scientists have come to question whether Rangitoto is the new style of the Auckland Volcanic Field or merely an anomaly.

While the new research didn’t solve these long-standing questions, it did tell us more about the ways magma within its system – and perhaps from other local volcanoes - could travel to the surface in future eruptions.

Rangitoto’s north and south cones happened to be aligned parallel to the Islington Bay Fault: a major feature running between Rangitoto and nearby Motutapu Island.

The presence of this fault zone near the volcano, study co-author and senior lecturer Dr Jennifer Eccles explained, raised the hypothesis that it somehow influenced how lava rose from depth.

“Before our study, we didn’t know much about the subsurface of the volcano, and there were only two such studies, both unpublished, in the 1970s and 1980s.”

In other fields, volcanoes have been seen to erupt directly at fault “traces”, or where faults were visible enough to be mapped at the surface.

In an environment like Auckland - where such features were often covered by buildings, roads, water, sediment or old volcanic deposits – pin-pointing faults and the relationship of volcanoes to them isn’t so easy.

The Islington Bay Fault runs between Rangitoto (foreground) and Motutapu Islands.

But scientists’ understanding had improved dramatically with laser-scanning technology called Lidar, which recently helped reveal hundreds of ancient landslides around Pukekohe and new active faults in Wairarapa.

A decade ago, scientist Jill Kenny used Lidar data and other information gained from boreholes to map faults across the region for Auckland Council.

The fact many volcanoes were found to sit less than 500m of them led researchers to speculate there was indeed an important connection.

Yet Rangitoto, with its vents sitting some 3.5 kilometres from the nearest mapped fault, again didn’t fit the rest of the picture.

That prompted the study team, led by PhD student Alutsyah Luthfian, to take a closer look.

Over the Covid-19 pandemic period, the team carried out ground-based geophysical measurements on Rangitoto and Motutapu with the assistance of Ngāi Tai ki Tāmaki.

This data, combined with pre-existing airborne data, was used to create sophisticated computer 2D and 3D models of the volcano’s subsurface.

The work confirmed that there could indeed be an important link between Rangitoto and the Islington Bay Fault, just as scientists had suspected decades ago.

“It may act as a channel for Rangitoto magma to flow upward, before smaller cracks near the surface divert the magma away from that fault,” Eccles said.

“The diversion of magma away from the fault adds a new dimension to the Auckland volcanic hazard landscape: that volcanoes can erupt several kilometres away from the surface position of a mapped fault.”

This was something that future hazard planning may need to consider, she added, but there was still much more to understand.

“In addition, some of the large basement faults in Auckland are buried between one or two kilometres of younger sediment, making them hard to map.”

As for Rangitoto, the latest study - carried out as part of the collaborative Determining Volcanic Risk in Auckland (Devora) programme - only threw up yet more intriguing questions about its past and future.

Eccles noted geologist Bruce Hayward’s recent suggestion that, several thousand years before Rangitoto was born, a small volcano had already been there in the form of a small, cratered island.

“Could this prior event have used a similar magma pathway as the recent eruptions?”