Mt Taranaki's explosive history has just been re-written. This is why it matters

Taranaki began erupting about 130,000 years ago, with large eruptions occurring on average every 500 years and smaller eruptions about 90 years apart. Photo / Alan Gibson

Scientists have re-written the explosive history of Mt Taranaki – and now have a clearer picture of what the next big blow might look like.

A just-published study has found the last eruption at the picture-postcard volcano likely took place more recently than long thought.

Taranaki began erupting about 130,000 years ago, with large eruptions occurring on average every 500 years and smaller eruptions about 90 years apart.

While minor volcanic events were recorded to have taken place in the mid-1800s, the last explosive-medium sized ash eruption was estimated to have occurred around 1755AD.

But the new study, which drew on new paleomagnetic methods, has shifted that date to somewhere between 1784 and 1795.

The study also revealed the volcano was in its second-longest break between events in more than 1200 years of records – an important finding, given larger slumbers could make for bigger eruptions.

Further, the work showed how the last event created a lava dome much larger than scientists had thought, covering much of the upper northwestern part of the volcano.

University of Auckland volcanologist Professor Shane Cronin said scientists had been long frustrated in trying to learn more about that eruption.

That was due to the dangers of high elevation and extreme weather, snow and the site being too young to date with standard radiometric methods.

Interestingly, the key to getting sharper picture proved to be the knowledge that the Earth's magnetic pole migrated and varied in strength over time.

After they developed a map of the pole changing over time, the researchers turned to those iron-bearing, magnetic minerals found within old erupted volcanic lavas.

"As soon as the lava cools, the magnetic minerals align themselves to the magnetic field active at the time," Cronin said.

"By collecting about 50 small, oriented samples from the summit lava dome on Mt Taranaki, we could heat them up in the laboratory and progressively strip away the magnetic field stored in the samples - all the while measuring the magnetic field of the sample in very sensitive equipment."

The large number of samples allowed the scientists to gain a very accurate estimate of the strength and orientation of the magnetic field of New Zealand at the time of lava cooling.

"We could then compare this value with that of the master New Zealand pole-wandering map," he said.

"As a check on the field conditions and age, we also had the benefit of direct measurements of the magnetic pole made by Captain James Cook as he passed the west coast of New Zealand in 1769 to 1770.

"This resulted in our best ever age estimate for the last eruption between 1784 and 1795."

That revised estimate also gave the greatest certainty about the possibilities of future eruptions.

"Previously we had used two main scenarios for the last eruption, spanning our only constraints of 1755 and 1840," Cronin said.

"The new date also tells us that the volcano has been in one of its longest ever dormant periods."

Past activity had shown that, after long quiet periods at the volcano, the next eruptions were generally large or long lived.

In the last eruption, a lava cap created a large mantle of lava that spanned the current summit dome and spread all the way down the volcano's northwestern flank, connecting with the Turtle feature that's still there today.

Much of the lava mantle down the northwestern flanks collapsed away during and soon after the eruption, launching devastating pyroclastic flows toward present-day Okato.

What might happen next time?

"A new eruption if it appeared today would firstly have to clear a new path to the surface," Cronin said.

"This means that the current summit would be reshaped by magma intruding through it, perhaps gently, but more likely violently, with explosions created as the magma traps water and blasts a vent opening."

That intrusion of magma would also generate earthquakes and landslides from the upper volcano, culminating in an eruption.

And once an eruption began, what happened next depended on the volume and state of magma as it rose to the surface.

"In the best case, small amounts of lava would build up, forming lava flows and small pyroclastic flow deposits, with small and thin ash falls across the region," Cronin said.

"In the worst case months, if not years and decades of eruptions could begin - leading to large explosive eruptions and many tens of centimetres of volcanic ash dispersed across farmland in the region and across all of the North Island.

"We know of at least two eruptions over the last 1000 years that lasted for more than 50 years, with one possibly lasting 200 years with on-off activity throughout that time.

"This situation would force us to think of ways people and businesses in Taranaki could continue to thrive with ongoing volcanism - and live alongside an active rather than passive mountain."

The study has been published in the Bulletin of Volcanology.