Research by a doctoral student at Victoria University has revealed a clearer picture of the risks associated with future volcanic eruptions in Auckland.
The Super City's dormant volcanic field -- made up of more than 50 craters -- is expected to erupt again from a new site within the next few hundred years.
Jenni Hopkins, in collaboration with GNS Science and the University of Auckland, reconstructed the eruptive history of Auckland's volcanic field in order to make estimates about the areas that will be affected by future eruptions.
The 53 volcanoes date back to around 200,000 years and Dr Hopkins -- who graduated with her doctorate last week -- said they are almost entirely monogenetic, meaning they generally only erupt once.
"But what was previously unknown was the order in which they erupted. I wanted to find that out so that we could establish the characteristics of the field and get an idea of what a future eruption might be like," she said.
To do this, Dr Hopkins examined the ash deposits taken from a number of lake sediment cores to see the thickness of the layers and the order in which they were deposited.
She also developed ground-breaking geochemical techniques that for the first time allowed the ash deposits to be accurately linked to their source volcanoes.
She said being able to pinpoint the volcano from which each layer of ash was derived meant researchers could see how far ash was dispersed in each eruption.
"We can use this geological evidence to make estimates about the areas that will be affected by eruptions in the future."
Dr Hopkins' research on the past ash deposits shows that for many of the smaller eruptions, the ash fall did not cover the entire area of the city.
However, for the larger eruptions -- for example Mt Eden or One Tree Hill -- the ash fall would likely impact a much greater area.
She used core samples that were drilled by GNS Science and Auckland Council's research programme Devora (Determining Volcanic Risk in Auckland) and analysed the elemental make-up of the ash deposits with an electron micro-probe and laser ablation techniques.
Each ash layer has a unique geochemical "fingerprint" of trace elements that can be matched to lava from the source volcanoes.
Dr Hopkins said the whole point of her research -- which was funded by the Earthquake Commission, Auckland Council through Devora, and GNS Science -- was to provide an improved understanding of the threat posed by the Super City's volcanoes to people and critical infrastructure.
"It's designed to assist in the development of better management practices for evacuations, and to help local authorities work out how best to mitigate the damage to assets like roads, power lines and buildings."
She said the Devora team as a whole was working together to model and simulate different risk scenarios, based on a theoretical eruption from a future volcano.
"These include, for example, ash fall, lava flow, pyroclastic flow and ballistic fall. These risks and potential impacts to the city and infrastructure have been modelled based on the scale and locations of a potential future eruption, for example, in the harbour, or near the airport."
Dr Hopkins will continue to conduct further research on Auckland's volcanic field. She is currently working for GNS Science.
She hopes to apply the skills developed during her PhD research to the ancient super-eruption of Taupo, revealing some of its historic secrets by examining far-flung ash deposits.