Volcanoes: the most underestimated natural hazard in New Zealand

They grace our post cards. We ski down their slopes and they feature in numerous New Zealand movies. Ngati Tuwharetoa hold the volcanoes of the central North Island in high regard in their geneaology. But how well do we understand these magnificent fire mountains and (not so magnificent)hazards they pose?

Volcanism in New Zealand largely exists because the Pacific tectonic plate is subducting under the Australian plate, which the North Island sits on, thereby causing earthquakes, but also large amounts of rock to melt. As the melt accumulates in magma chambers it changes its chemistry, which determines whether  an eruption will be quiet or explosive; whether the volcano erupting will be a graceful stratovolcano like Ngauruhoe or a vast hole in the ground like Taupo.

A dozen known volcanoes are visible in New Zealand to the naked eye. Another 50 or so are hidden under the sea, and include Rumble I-V, which stretch from northeast of White Island out to the Kermadec Islands. They occupy nearly the entire spectrum of volcanic structures, magma types and most know eruption styles.

Auckland is perhaps the most problematic volcanic hazard. Whilst sticking to fairly reliably to its 800-1000 year repose period, when it does erupt the style varies significantly – it could be a single phreatomagmatic event where the entire magma supply goes into creating a large crater such as Pukaki or Orakei and then becomes extinct, or it could construct a cinder cone such as One Tree Hill or Mount Eden in an eruption that lasts 10-15 years and displaces a large number of Aucklanders.

Perhaps the other extremity is a full blown Yellowstone like apocalypse that centres on one of the caldera volcanoes. In this case it would devastate New Zealand beyond recognition, induce a sort of volcanic winter and cause geomorphological changes to the hydrology of pretty much the entire central North Island. Fortunately such an eruption seems to only happen once every 25-40,000 years, and is not likely in the next 10,000.

Well before then we will be entertained by the stratovolcanoes of the Tongariro National Park and Mount Taranaki. Who knows we might even get another one pop up somewhere. Whilst the eruptions are likely to be moderate, they have the potential to cause damage in the hundreds of millions of dollars. Most will come from volcanic ash drifting down wind and disrupting air traffic, corroding electronics and getting into infrastructure such as Rangipo underground power station, which shut down in 1995 for a time because of ash from Mount Ruapehu. Other disruption will be from lahars, especially on Mt Ruapehu which has one of the most active lahar paths in the world.

Because a volcano can be dormant for hundreds or even thousands of years they keep a lower profile as natural hazards go in New Zealand. However that does not mean they pose less risk. On the contrary, volcanism is probably our most under estimated natural hazard.

Predictions for the future:

  • The caldera volcanoes will stay quiet for another few hundred years, and when they awake it will not necessarily be a full blown apocalypse, but possibly dome or cinder cone building and thick rhyolitic flows
  • Auckland volcanic field will remain dormant for another couple hundred years, and the new feature will probably be a cinder cone
  • The strato volcanoes will feature prominently – Ngauruhoe will continue rebuilding and erupt again in the near future; Taranaki will stick to its time table and erupt sometime in the next hundred years; Ruapehu will be the most regular performer with moderate eruptions involving lahars and tephra; Tongariro will have further small ash eruptions and may produce small scale lava flows
  • Curtiss and Raoul Islands may have larger eruptions, with Raoul erupting out of its known vents
  • White Island will probably have small ash eruptions at some point in the near future – a larger one seems unlikely whilst the volcano can safely degas

Generally there will be be some warning before a significant eruption occurs. However small eruptions that are not fully magmatic in origin can occur with little warning. Geonet monitor the volcanoes around the clock, with cameras trained on the stratovolcanoes plus White Island because these are the most prone to erupting. All have seismometers to record earthquakes. Occasional testing of the water, gas discharge and ground deformation is done to determine whether magma movement is happening.

So, have fun and enjoy the volcanoes. By all means get on a helicopter flight or boat trip to White Island or go on guided trip up Mount Ruapehu. Climb to the top of the cinder cones in Auckland or swim in Lake Taupo. Just be aware that when the forces that put these wonderful natural features go bad, they can do so in a really big way.

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