Lava would flow freely in southern Auckland should a volcano erupt, while it would be channelled and travel further in northern and central parts of the city, according to a New Zealand study.
Researchers from Massey and Auckland universities have used analysis of previous lava flows in the Auckland Volcanic Field and aerial surveys to model how lava would flow through Auckland's suburbs following a volcanic eruption.
In the northern and central parts of the Auckland Volcanic Field, lava flows would be channelled by the rough landscape, meaning the majority of ridges could be seen as relatively safe places from future lava flows.
"Extremely long lava flows are hence only expected from future Auckland eruptions located in topographically constrained portions of the central/northern region of the field," the study said.
In the much flatter southern parts of the field, however, the lava would flow more freely.
"The southern parts of the City of Auckland are characterised by watersheds that lack natural topographical boundaries and that have small differences in elevation between the watershed boundaries and channels," the researchers wrote.
"This implies that future lava flows can easily flow over the topography, affecting multiple watersheds."
Analyse of previous eruptions found the average length of lava flows from Auckland's volcanoes is 2.5km, with Mt St John creating the longest total lava flow of 10km.
The researchers said any future vent-forming eruptions would likely occur with the Auckland city limits or its outskirts, meaning there are few mitigation options, such as artificial dams.
"The City of Auckland is highly susceptible to lava flows, which are likely to travel further, and be potentially more destructive to infrastructure over longer periods, than the products of explosive opening phases of monogenetic eruptions at Auckland," the researchers said.
The findings have been published in the journal Remote Sensing of Environment.
One of the study's researchers, senior lecturer in volcanology at Auckland University, Jan Lindsay, said it is hard to predict exactly where the next eruption will come from.
"So Rangitoto erupted at least twice and it's the most recent volcano, so we can't rule out Rangitoto erupting again because it has erupted a couple of times in the past," Dr Lindsay told Newstalk ZB.
She said Auckland has a volcano contingency plan which outlines the Civil Defence plan.
"What will happen is there will be warnings that the eruption is going to occur, in the form of earthquakes, and lots of information broadcast through the media ... about what people should do."
The Auckland Volcanic Field has been active for the last 250,000 years, with at least 50 monogenetic volcanoes. Monogenetic volcanoes have one short eruption event, as opposed to other volcanoes which have on-going volcanic events from the same vents.
Not all of the region's volcanoes produced lava flows, with tuff rings, maars and tuff cones in the north and south of the field formed by hydromagmatic eruptions.
Rangitoto, Auckland's youngest built up scoria cone, last erupted more than 500 years ago.
- Herald Online