Our biggest blows: How did prehistoric 'super-eruptions' transform NZ?

Lake Taupo fills the hole left by the Oruanui eruption more than 25,400 years ago. Photo / Mark Mitchell

It blasted masses of volcanic material into the environment, carpeted the country in ash, and carved out an enormous crater today partly filled by Lake Taupō.

The Oruanui eruption, which transformed prehistoric Aotearoa's environment some 25,000 years ago, was so large that scientists consider it a "supereruption".

Such is the scale of these behemoth blows that even the 1991 eruption of Mt Pinatubo in the Philippines – large enough to have a cooling effect on the global climate for a year afterward – would have stood about 100 times smaller in size.

Now, for the first time, a team of New Zealand scientists aim to quantify the precise environmental impacts of these ancient cataclysms.

"New Zealand is the ideal natural lab to study the impacts of big eruptions as we've had lots of them, in recent geological times," said Dr Simon Barker, a Victoria University senior research fellow leading the new programme.

Along with Oruanui, they've included the Kidnappers event near Mangakino around a million years ago, which proved one of the biggest ignimbrite eruptions ever to shake the North Island's Taupō Volcanic Zone (TVZ).

They're also studying the Whakamaru eruption, 350,000 years ago, which was the largest-volume blow known to have ever originated from the hyper-productive TVZ – and the Southern Hemisphere's biggest bang in recent geological history.

"Some of the events ejected more than 1000 cubic kilometres of pumice and ash across New Zealand," Barker said.

"That is a mind-blowing amount of material and the equivalent of covering the whole North Island in about 10 metres of ash."

Over recent years, volcanologists like Victoria University's renowned Professor Colin Wilson have been shedding more light on the dramatic supereruptions of our past.

It's now believed the entire area of volcanic activity between Kawerau and Tokaanu as one single but complex supervolcano system, which last saw a bout of unrest at Taupō in 2019.

While volcanoes in this area experience more episodes of unrest, perhaps every decade, eruptions are far less frequent, occurring every 500 to 1000 years.

Supereruptions are rarer still: in the past two million years, there are only around 14 of them recorded globally, four of them in our Central North Island.

Yet their out-sized hazard means scientists are all the more eager to understand them.
 
"We are building off decades of knowledge gained by volcanologists through studying the pumice and ash deposits from these eruptions," said Barker, whose team has just received a $913,000 grant through the Marsden Fund.

"However, we are using new techniques, new technology and new environmental records where we have found ash deposits to study these events from a new angle."

This map shows the known extent of ash spread by three prehistoric "super-eruptions" to be explored in a new study. Image / Supplied

For instance, while it was known that past super-eruptions wiped out a large area of the New Zealand landscape, with thick ash deposits and pyroclastic flows that destroyed a large portion of the North Island, it remained unclear how long it took for the environment to recover.

"We also know that big volcanic eruptions can cool global climate for several years," he said.

"Yet nobody has ever managed to estimate the amount of cooling that occurred from past supereruptions. We want to figure this out for the first time."

That involved three approaches.

First, the team will spend time in the field measuring supereruption deposits, to help build numerical simulations to reconstruct the amount of ash injected into the atmosphere.

This work, which will be carried out with top scientists at the United States Geological Survey, will tell them where ash went over the ocean and how long it circulated the globe for.

Next, they'll analyse samples retrieved from lake and sediment cores from different places around New Zealand, such as Onepoto in Auckland and the Chatham Islands, where the Oruanui event left an ash layer 18cm thick.

"We have found ash layers from these events that represent the exact point in time when the eruptions occurred," Barker said.

"We'll then analyse the mud in the cores at millimetre precision and study the change in pollen contents, which reflects the plant species that were growing in the area at the time.

"We will use this information to see how the forests changed in the aftermath of the eruptions and how long it took to recover."

Lastly, they plan to draw on ice cores from Antarctica, where scientists recently discovered tiny glass shards and large sulphur anomalies linked to Oruanui.

"We will look at the change in ice water chemistry that can be used as a proxy from temperature and also for conditions in the Southern Ocean in the aftermath of the eruption," he said.

"We'll then be able to use this information to look at the months, years and decades of climate adjustments following the supereruption."

None of this had ever been done before, directly using such strong evidence from the geological record.

"We're also really pushing the limits in terms of environmental reconstruction and temporal resolution." he added.

Researchers observe pyroclastic flow deposits on the side of State Highway 1, near Taupo, which mark prehistoric volcanic surges that knocked down trees and burnt them to charcoal. Photo / Colin Wilson

"For example, we have a special machine that samples sediment at 1mm resolution so that we can look at environmental changes over years to decades after the eruption."

In a global sense, the team was keen to see whether these impacts were scalable - such that the biggest events have the largest and longest-lasting consequences.

"It is also useful for understanding the global climate impacts of big eruptions, whether from New Zealand or anywhere else in the world," he said.

"If we do experience another big volcanic eruption - not just a supereruption - how long could we expect to have cooling for? This is still a branch of science where there are many questions to answer."

For our country, they sought to better understand how supereruptions might have shaped our own geological and biological heritage.

It's thought Oruanui might even have been powerful enough to have caused the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea.

"These events are so big that they likely influenced the distribution of plant and bird species around New Zealand that we see today."