Aucklanders may get only a day or two warning of a volcanic eruption and will only know hours before where it will explode on the surface, a scientific paper says.

The 1.4 million people living on the Auckland Volcanic Field (AVF) make too much noise for existing instruments to hear what is coming up from below.

"This means that localisation of a future eruption site is only likely to occur within a few hours or days of an eruption," the paper in the Journal of Remote Sensing of Environment said.

Each new vent in a place like Auckland will occur at a new location, rather than an existing volcano, creating uncertainty over where it will hit.

The scientists said they have used a variety of devices and models to explore in a new way the lava flows of the 360sqkm AVF, which holds 50 monogenetic volcanoes – that is single vents that erupt in one go. Polygenetic volcanoes – such as Ruapehu – erupt from the same craters regularly.

They said the city, especially south Auckland, will probably suffer more damage from lava flows than the early stages of the eruption.

"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 paper said Auckland had two different areas for lava danger.

"The southern part of the city is predominantly flat, without hindrance to lava flow, whereas the hilly northern and central part has many ridges that can limit or channelise lavas."

The lava flows in central Auckland were likely to be longer in length as the topography forces lava into river-like channels.

"The volcanism in Auckland in New Zealand differs from large, polygenetic volcanoes because future eruptions will likely take place within a densely populated city, there are no rift zones that indicate areas of elevated hazard, and the future vent area is therefore unknown, and due to the generally low-lying topography, there are few opportunities to use mitigation options, such as artificial dams.

"Hence, future vent forming eruptions will very likely occur within the city limits or its outskirts, allowing few mitigation or preparation options.

"‘The majority of previous scoria cones and lava flows are located in the heart of the city, upon a presently slightly elevated ridge-system."

The paper said most studies so far had been about the location, nature and the possible effect of the future eruptions on the city.

"Detailed evaluation of lava flow hazards and delimitation of potentially safe places from lava flow inundation have not yet been attempted, in spite of the relatively high level of their potential risk."

Studies to date have focused on the creation of the vent and the ash fall over the city.

"Knowledge of lava flow susceptibility is an essential addition, because over half of the eruptions in Auckland have produced lava flows or lakes."

The paper was written by scientists from Massey University’s Institute of Natural Resources, Gabor Kereszturi, Jonathan Procter, Shane Cronin, Karoly Nemeth and Mark Babbington with Auckland University School of Environment’s Jan Lindsay.

They say human pressure has changed the shape of previous volcanoes and their lava flows.

From 15 preserved lava flows the maximum length of the AVF lava flows range between 0.7km for Mount Matakarua and 6.5km for Mount Wellington while the average is 2.5km.

"The longest flow of Auckland is, however, from the Mount St. John volcano with the total length of about 10km."

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