Volcanic 'hot zone' on a crawl to capital

New Zealand's volcanic "hot zone" is moving south and could spawn volcanoes in Wellington in about five million years.

The southernmost existing volcano, Ruapehu, may also be evolving into a huge "caldera" volcano such as the one that created Lake Taupo.

Auckland University geologist Ian Smith told the New Zealand Geological Society's annual conference in Taupo this week that Ruapehu had only erupted material from the top 12km of the Earth's crust in the past 2500 years.

But 10,000 years ago it saw an eruption of molten rock from much greater depths - and that was likely to recur.

"Eight thousand years ago it was producing huge lava flows that formed the Whakapapa skifield," Dr Smith said. "It might evolve into a Taupo-style caldera. We've found indications in some of the crystals that they are seeing melt compositions the same as the Taupo rhyolite.

"It's an indication that, with time, it may develop into that kind of a volcano."

He said volcanoes were active in Northland 15 million years ago, became established in the central North Island two million years ago, and Ruapehu was only 250,000 years old.

The whole volcanic zone was moving south in line with the Pacific plate, a vast area of seafloor covering much of the Pacific, which was diving underneath the Australian plate just east of the North Island.

As the Pacific seafloor spread westwards from a mid-Pacific spreading ridge, it was pulled southwestwards where it hit the Australian plate on a line from Tonga down the North Island's east coast and southwest to Nelson.

Dr Smith said: "Volcanism will follow it."