New date for Mt Taranaki’s last big blast
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 new 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 medium-sized ash eruption was estimated to have occurred around 1755AD.
But the new research, which drew on new palaeomagnetic 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.
University of Auckland volcanologist Professor Shane Cronin said scientists had long been struggling to learn more about the last 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 a 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 ironbearing, 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.
Taking about 50 samples from the summit lava dome on Mt Taranaki, “we could heat them up [and] strip away the magnetic field stored in the samples — all the while measuring the magnetic field of the sample.”
The large number of samples allowed the scientists to accurately estimate 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.
“This resulted in our best ever age estimate for the last eruption between 1784 and 1795.”
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.
That intrusion of magma would also generate earthquakes and landslides from the upper volcano, culminating in an eruption.
“In the worst case months, if not years and decades of eruptions could begin — leading to [volcanic ash] dispersed across farmland in the region and across all of the North Island”.
The study has been published in the Bulletin of Volcanology.