Scientist warns Mt Taranaki could erupt with 'very brief warning'

Scientists have warned that although the risk of eruption at Mt Taranaki is low it could erupt with little warning and cause a loss of billions of dollars of economic activity. 

Mt Taranaki last erupted about 150 years ago, according to GeoNet, with the effects being a minor explosive ash eruption.

GeoNet notes that the largest recent eruption occurred in about 1655 with widespread tephra falling across the central North Island.

Scientists have found the probability of an eruption is low with analysis of nearly 230 eruptions over the last 30,000 years finding the chances of a new Taranaki explosion to be between 1-1.3 percent each year. 

But if an eruption did occur the damage could be significant. 

There are over 85,000 people who live within 30km of the mountain and 40,000 of whom are in high-priority evacuation areas.

One estimate puts the net losses in economic activity from a brief Taranaki eruption at between $1.7 billion and $4b or between $13b and $26b over a decade of volcanism.

Now a new study published earlier this month in the Bulletin of Volcanology by scientists from the universities of Auckland, Otago and Paris Cité has concluded that an eruption could occur with fewer warning signs. 

Reconstructions of events over the past 1000 years have shown as little as a day to a week of seismic activity before hot magma reaches the surface to erupt.

"That's a very brief period of warning," associate professor Phil Shane of the University of Auckland School of Environment said. 

Mt Taranaki is going through an unusually quiet period, which will likely come to an end as has happened "at other volcanoes around the world, sometimes surprising local populations". 

The reconstructions also found that some eruptions showed no sign of new magma, which suggests no rumbling and no warning, according to Professor Shane. 

He speculates that a build-up of volcanic gases deep in the earth's crust may have been enough to cause an explosion.

"Ash from past eruptions has been spread across New Zealand, suggesting that future eruptions could be nationwide hazards," he said. 

"For that reason, we need to have a sense of how much warning we will get and unfortunately it seems like it may not be much."

The research relied on the clues left by microscopic crystals in erupted rock. It found microscopic patterns like tree-rings form in the crystals in response to changes in temperature, pressure, and magma composition.

Professor Shane said depending on whether magma rises rapidly or temporarily stalls in the earth's crust on the way up, the crystals grow differently.

He said modelling allowed calculations of the time from a crystal's formation to an eruption, indicating how long magma took to surface.