500,000 people in lava's line of fire (+ graphic)

Mangere Mountain in Mangere Bridge. Photo / Martin Sykes

New modelling has for the first time revealed which areas of volcano-rich Auckland would be obliterated by lava flows in an eruption - and a wide spread of major suburbs are in the firing line.

Eruption scenarios among Auckland's hotbed of 55 volcanoes include rocks being hurled 1.5km, surges of fast-moving lava 3km long, toxic gases settling in low-lying areas, ashfall, and 500,000 people needing evacuation.

Areas at risk of lava flows are pinpointed in a study by Massey University PhD student Gabor Kereszturi, allowing authorities to identify high-risk areas and safe zones.

Eruption events in hilly northern and central areas could result in lava streaming down Mt Eden or One Tree Hill and into surrounding valleys and basins, laying waste to everything in its path.

Lava flows would be channelled through these areas by the rough landscape, meaning the majority of ridges could be relatively safe from lava. In South Auckland, lava would probably flow a shorter distance but move more freely across the area's flatter terrain.

By mapping topography of landscapes and then applying existing research to create a cross-section, Mr Kereszturi was able to measure the maximum flow lengths from 15 major volcanoes.

Flows from Mt Wellington, Three Kings and One Tree Hill were capable of reaching distances of up to 6.5km, 5.7km and 4.4km from the vents respectively.

The average length of a typical Auckland lava flow was 2.5km and maximum lengths past the 1km mark were also possible at McLennan Hills (Otahuhu), Browns Island, Mt Albert, Puketutu, Mt Roskill, Mt Eden, Mt Mangere, and Rangitoto.

Lava could also roll over parts of St Johns, Remuera, Meadowbank, Royal Oak, Takapuna, Mangere, Favona and Papatoetoe, and could flow at more than 80m thick at One Tree Hill, 76m thick at Mt Eden and more than 180m thick at Rangitoto, the scene of Auckland's most recent eruption 600 years ago.

Future eruptions would be likely to strike within the city limits or on its outskirts, but even with seismology, scientists could not forecast where.

"This means that localisation of a future eruption site is only likely to occur within a few hours or days of an eruption," Mr Kereszturi wrote.

Making matters worse, the city's low-lying topography also allowed few opportunities to prevent lava flows.

"It would just set fire to things and push everything over like a big bulldozer," said co-author Professor Shane Cronin, director of Massey University's Volcanic Risk Solutions.

"But basically because we don't know where things would happen, it's difficult to say which area is at the greatest risk. I would say you've got several hotspots in terms of main depressions, where we could expect lava flows to occur."

The research had been presented to the Determining Volcanic Risk in Auckland (DEVORA) group and would be factored into planning, Mr Cronin said.

The Auckland Volcanic Field has been active for the past 250,000 years, with at least 50 monogenetic volcanoes.

Monogenetic volcanoes have one short eruption, as opposed to other volcanoes which have ongoing 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.

In the event of a major eruption - estimated at a 4 per cent chance within the next 50 years - planners have predicted significant damage to infrastructure, airport closure and insured losses of up to $2 billion.

- additional reporting: APNZ