Raining black: How much ash would a Taupō eruption spread?

Lake Taupō fills the hole left by the Oruanui eruption, more than 25,400 years ago. Photo / Marian Robertson

Layers of ash centimetres thick could fall on Auckland - and even on the top of the South Island - if New Zealand's long-slumbering supervolcano had an angry awakening.

The Taupō Volcano, hidden by the water-filled caldera that is Lake Taupō, has produced some of the largest eruptions on the planet.

One of its most recent, the devastating Hatepe event 1800 years ago, spewed more than 120 cubic km of material into the atmosphere and obliterated the landscape surrounding it.

Now, a just-published paper has modelled how much ash would be dispersed in different types of future eruptions.

Ashfall can affect huge areas, with even small amounts of volcanic ash causing widespread damage, as was seen in the Mt Ruapehu eruptions in the mid-1990s.

"From the geological record, we know that future eruptions from Taupō could be many times larger than any historic events at other New Zealand volcanoes," said Dr Simon Barker, a postdoctoral research fellow at Victoria University.

"If repeated, such eruptions would generate voluminous ashfall which could have major consequences for New Zealand."

As part of an EQC-funded study, Barker and his team modelled ashfall distribution from five hypothetical eruption scenarios that factored in scale and weather.

In a big explosive event, ash would begin accumulating on the ground at major cities within a matter of hours.

For some of the smallest eruptions, the model projected that ashfall around 1cm thick would be largely confined to the Central North Island.

Only major towns in the eastern North Island between Tauranga and Hastings could expect more than 1mm of ash.

These maps show the probability contours of ash deposits across the North Island for an eruption scenario that assumed an eruptive volume of 0.1 cubic kilometres of material - the smallest of the five scenarios explored. The maps show where ashfall would be greater than 1cm thick (a) and 1mm thick (b). Image / Supplied

But in bigger eruptions, the probabilities of ash reaching damaging levels of up to 10cm thick of course rose - especially for nearby regions like Gisborne, Hawke's Bay, Bay of Plenty, Waikato and Manawatu.

"With increasing eruption size, ash dispersal becomes less dependent on weather, as the formation of a major umbrella cloud may force ash upwind or cross-wind," Barker said.

Scenarios two and three, mapped here, respectively assume eruptive volumes of 1 and 5 cubic kilometres of material. Image / Supplied

"For the largest eruptions possible, ash thicknesses associated with major damage - 100mm - or severe structural damage – more than 300mm - can be expected at high probabilities in most major towns or cities in the North Island, even as far away as Auckland or Wellington."

Ash thicker than 1cm could even fall in the upper South Island for such large eruptions.

In the largest of the five scenarios, these consider the eruptive volumes of 50 and 500 cubic kilometres of material. Image / Supplied

"However, strong stratospheric winds may still play a major role in controlling medium- to long-range dispersal and which particular areas of New Zealand receive significant levels of ashfall," he said.

"The power of the eruption and strength of the umbrella cloud may also greatly affect the amount of time it takes for ash to start falling at major towns or cities."

While Taupō was capable of massive destruction, over its history it had more commonly produced smaller eruptions which had less of an impact.

Barker's study, just published in the international journal Geochemistry, Geophysics, Geosystems, built off decades of research on Taupō's eruptive history, much of it led by renowned volcanologist Professor Colin Wilson, who served as one of the co-authors.

These past studies had revealed how, over the last 12,000 years, there had been at least 25 eruptions from Taupō spanning three to four orders of magnitude in size.

Most of the smaller eruptions were of a similar size or smaller than the 1980 Mt St Helen's eruption.

The Hatepe event, however, was among the most violent observed on Earth in the past 5000 years.

Its massive eruption plume was estimated to have reached altitudes of more than 30km.

To build their model, Barker and his colleagues fed data from past events into dispersal computer models, which also captured range of eruption sizes and a full range of regional weather patterns.

The team then used a supercomputer operated by the Geological Survey to run 1000 eruption simulations for each eruption size against random weather, based upon actual readings recorded since 2016.

Beyond ashfall, Barker pointed out that a Taupō eruption could unleash many other damaging effects – notably fast-moving, deadly pyroclastic flows, which left everything within 80km of the Hatepe eruption site covered in ignimbrite.

"Even without an actual eruption occurring, if there is a noticeable unrest, there could be many negative side effects on the New Zealand economy," Barker said.

"There is still a lot of uncertainty around determining if any particular future unrest at Taupō will lead to an eruption."

In turn, estimating the likelihood and timing and impacts of future blows - and isolating the probability of another supereruption – remained a huge challenge.

But scientists, emergency managers and communities across the Central North Island were trying to do just that under the five-year, $8.2m ECLIPSE Programme.