Economy at risk if Central Plateau volcanoes blow

If one of the Central Plateau volcanoes went off with a real bang Waikato would be faced with not only widespread ash falls, but significant economic fallout.

The region's civil defence emergency management group plan, updated last year, puts volcanic activity at the top end of Waikato's potential natural disaster risks.

The region is vulnerable to several volcanic hazards from various sources both in and outside the region, but the most frequent and likely volcanic hazard is ash fall. Near-source volcanic hazards would include ballistic projectiles, ash fall, lahars, lava flows, pyroclastic flows and surges and debris avalanches.

Civil defence planners estimate 10mm-50mm of ash fall across the region in a worst-case event, with possible death, injury, damaged buildings, electricity and water failure and economic losses to agriculture and tourism.

Waikato University earth sciences lecturer and vulcanologist Adrian Pittari says the latest awakening of Mt Tongariro was at the "safer" end of erupting volcanoes.

Dr Pittari said the composite cone volcanoes of the Central Plateau - Tongariro, Ngauruhoe and Ruapehu - were relatively active, "every few years to few decades" while calderas such as Lake Taupo erupted every few to tens of thousand years.

The two types differed dramatically in their eruptive impacts, he said. "[Composite cone] activity is significant but relatively low explosivity producing anywhere from lava flows to lava fountains to ash explosions and volcanic plumes, which is what we saw [on Monday] night. The most widespread hazard is volcanic ash falling from the sky, and lahars, which we saw at Ruapehu in 2007."

By comparison, were a caldera such as Taupo or Okataina to erupt, large pyroclastic flows - superheated avalanches of ash, gas and pumice - covering the landscape, ash covering much of the North Island and long climatic impacts could be expected.

"Still, you wouldn't have wanted to be on the Tongariro Crossing at midnight when that eruption occurred, because there could have been flying blocks, and any volcanic blasts which could have occurred within a few kilometres of the vent," Dr Pittari said.

Volcanic ash was not usually life-threatening but it could be a nuisance, he said.

"What we saw [on Monday] night . . . was an explosive blast, but depending on how the magma decides to act it could change to lava fountaining, lava flows, anything is possible. But the assessment that we don't know what will happen next is correct."