By Jean Balchin 18/07/2017


Two recent studies have found that Auckland’s volcanoes had a rather stormy and temperamental past. At one stage, several large eruptions happened within 4,000 years, whereas at other times there were thousands of years of silence.

The two studies were published this month in the Journal of Volcanology and Geothermal Research and in the Bulletin of Volcanology by a team of researchers from the DEVORA (Determining Volcanic Risk in Auckland) research programme. The research team included scientists from GNS Science, Victoria University of Wellington and The University of Auckland. They determined that the Auckland Volcanic Field has a complex and episodic eruption history.

The Auckland volcanic field is an area of monogenetic volcanoes (volcanoes that erupt only once), that is covered by most of the metropolitan sprawl of Auckland. There are 53 volcanoes in this field (or thereabouts), and each volcano has erupted for just one period, except for Rangitoto Island, which erupted repeatedly. The volcanoes have produced a diverse array of maars (explosion craters), tuff rings, lava flows and scoria cones. The field is fuelled by basaltic magma, and is currently dormant, although it has the potential to become active again in the future.

These studies found that the oldest volcanic eruption (Pupuke) dates back to approximately 200,000 years ago and the youngest (Rangitoto) only 500 years ago. Moreover, at least half of all the eruptions occurred in only the last 60,000 years – which is a relatively short time frame and indicates an increase in the rate of eruptions overall.

Map of the field drawn by Hochstetter in 1859 and published in English in 1864. Courtesy of Wikimedia Commons.

According to Dr Graham Leonard, Senior Scientist at GNS Science who led the research team, “What we now know about the Auckland Volcanic Field, based on this new research, is that some eruptions flare-up over what is, geologically speaking, a short period of time. For example, there can be 6-10 volcanoes erupting within a 4000-year timeframe.” Dr Leonard’s team used new data to decode Auckland’s volcanic past and understand what sort of volcanic events have occurred and when they happened.

“What we now know about the Auckland Volcanic Field, based on this new research, is that some eruptions flare-up over what is, geologically speaking, a short period of time. For example, there can be 6-10 volcanoes erupting within a 4000-year timeframe.

“On the other hand, the volcanic field has also gone quiet for up to 10,000 years in the last 60,000 years, which is quite a long gap. This new research is exciting because it has allowed us to further define when eruptions have occurred which has helped us flesh out an eruption timeline.”

Dating methods

Argon-argon dating, a radiometric dating method, was used in collaboration with the US Geological Survey to increase the number of reliably, directly dated volcanic centres from 12 to 35 – out of a total of 53 in the Auckland Volcanic Field. 40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (39K) into the radioactive 39Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the 40K/40Ar* ratio, and thus to calculate the age of the unknown sample.

High-precision chemistry by volcanic geochemist Jenni Hopkins of Victoria University also ensured that a further 13 volcanic centres were put in their likely place on the timeline. Dr Hopkins, says “We now have an idea of the order and timing of almost all of Auckland’s eruptions, which is an unusual success compared to the state of knowledge on other volcanic fields around the world.’’

“Using both of these new techniques we were able to determine that some eruptions in the Auckland Volcanic Field may be interlinked and that the field as a whole, can be either ‘all on’ or very quiet, sometimes for several millennia,” says Dr Leonard. “However, it’s important to note that the Auckland Volcanic Field is temperamental and we can’t use this study to predict a simple likelihood of a future eruption. What our research has revealed is that the past is complex, so we must wait to see what it will do next.”

GNS Science

GNS Science runs the GeoNet instrument network in Auckland which monitors the volcanoes and provides updates on activity to stakeholders. Auckland Council, Civil Defence, and the DEVORA team are investigating new techniques to improve monitoring and warning systems, and on improving the resilience of the city to allow it to continue to operate safely and minimise disruption in the event of an eruption.

Pigeon Mountain, of the Auckland Volcanic Region. Image courtesy of Wikimedia Commons.

Craig Glover, Head of Strategy and Planning at Auckland Civil Defence, says that Auckland Civil Defence works with the DEVORA team and other experts to plan the city’s response to a volcanic event.

“If this research teaches us anything it’s that Auckland’s volcanic field is unpredictable. We all need to be prepared and while it might seem daunting, planning for a volcanic eruption is no different to planning for any other disaster – have a talk with your loved ones and make a plan”.

About DEVORA

Determining Volcanic Risk in Auckland (DEVORA) is a multi-agency, multi-disciplinary collaborative research programme led by volcanologists at The University of Auckland and GNS Science. Active since 2008, the project aims to improve the volcanic hazard outlook and risk assessment for Auckland. The findings have the potential to improve business decision-making and risk management, as well as make Auckland a safer place.

Image courtesy of Wikimedia Commons: Rangitoto Island as seen from the path around North Head, in Auckland, New Zealand.