Lessons to be learned from Chile eruption

MICHAEL DALY
Last updated 15:20 13/06/2012

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Researchers studying the after-effects of a volcano which erupted in Chile last year see lessons for this country, including the continuing impact fine ash can have months after it is erupted.

The Puyehue-Cordon Caulle volcanic complex erupted a year ago, and among its impacts it disrupted flight schedules in this country and elsewhere in the southern hemisphere.

Prevailing westerly winds carried most of the ash from the event into Argentina.

A field team, with members from Canterbury University, Britain and Argentina, travelled to the affected area in February and March to look at what could be learned about reducing vulnerability to volcanic eruptions.

The Chilean eruption is considered to have similarities to a potential large eruption in this country.

Among the places visited by the researchers was the town of Jacobacci, which received about 5cm of ash. It is on the windy Patagonian steppe about 240km from the volcanic vent. Because it is downwind of thick ash deposits closer to the volcano, it has continued to be affected by fine wind blown ash since the eruption.

Dr Carol Stewart of Canterbury University said the team visited about eight months after the eruption, on a windy day. Ash in the air reduced visibility to about 200 metres and destroyed a team member's camera.

"It was just miserable," Stewart said.

While she said a similar eruption in New Zealand would leave a blanket of ash, Stewart also pointed out that the steppe was drier and windier than New Zealand.

In this country ash would be likely to wash into the soil more quickly, although there was a risk it could form a cement-like layer.

The visit showed the need to draw up plans for preparing for ash fall before an eruption occurs. In particular cleaning up ash from urban environments can be challenging and requires forward planning, Stewart said.

A fleet of trucks and earth moving machinery needed to be mobilised, and a place needed to be identified where ash could be moved to.

In San Carlos de Bariloche, a city of about 110,000 people 100km from the vent of the Chilean eruption, it had taken two months to clean up after nearly 5cm of ash fell. With tourism an important industry for the city, a considerable effort had been made to get the job done.

Dr Tom Wilson of Canterbury University said a major risk with widespread deposition of fine ash was that it could erode and wash into waterways.

"It will get into rivers and potentially could cause flooding, and in extreme cases this could generate lahars," he said.

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Ash coming down from the high country close to the volcano could block rivers and cause flooding of farm land in Hawke's Bay, Bay of Plenty and Taranaki.

Ash would also cover farm animals' feed and water, and in extreme cases it could be necessary to send stock away, or even cull some animals. But in most cases, with some planning and support, most farms would successfully manage an ash fall.

A major project was under way to find out how Auckland would be affected by volcanic activity.

The city was built on a volcanic field where small volcanoes could come up, causing problems in the immediate vicinity. Core samples taken from lake beds also showed ash from the central North Island had reached the Auckland area in the past.

Cone volcanoes, such as Ruapehu and Ngauruhoe tended to have quite frequent small eruptions., Wilson said.

Taranaki was another cone volcano, although it had not erupted since at least the 19th century and possibly not since the end of the 1700s.

While such volcanoes tended to have small eruptions, there was evidence they had infrequent larger events, during which around one cubic km of material was ejected. That was about 1000 times more material than in the smaller eruptions.

Caldera volcanoes, such as Taupo and Okataina which includes Tarawera, erupted "incredibly" infrequently with hundreds of cubic kilometres of material being ejected. Such an eruption would devastate the central North Island.

There were also intra-Caldera eruptions, such as the 1886 Tarawera , when one to 10 cubic km of material was ejected. Those probably happened every 2000 to 10,000 years on average.

The event studied by the team which went to Argentina had been of the size of an intra-caldera eruption or a very large cone eruption.

- © Fairfax NZ News

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