Sunday, January 14, 2007

Auckland volcanoes pose an important threat!

"That amount of ash would cause chaos in Auckland," said GNS Science volcanologist Graham Leonard. "From a hazard point of view, that is pretty significant".

The research – which casts new light on volcanic threats to New Zealand's biggest city and a third of the nation's population – springs from the Auckland Maar Volcano Drilling Programme.

A partnership between Auckland University and GNS Science it was started to collect drill cores providing a record of longterm climatic changes, but the drill cores have also thrown new light on past eruptions.

The barge-mounted drill rig anchored in about 2 metres of water is expected to finish its work today.

Some of the cores in the Orakei Basin have shown the Orakei volcano is probably three times as old as previously thought, and by drilling 81m into the sediment beneath the basin the scientists have identified ash layers from 90 eruptions in about the past 90,000 years.

Until now, scientists believed the Orakei volcano was about 20,000 years old.

One ash layer more than 10cm thick was from the Lake Rotoiti area in the Bay of Plenty, and thinner layers came from eruptions at Mayor Island, Taupo, Mt Tongariro, and Mt Taranaki.

The scientists have identified at least 50 different ash layers from Auckland volcanoes such as Little Rangitoto, Mt Hobson, The Domain, Mt St John, and One Tree Hill.

Each of the 50 volcanic cones scattered around Auckland has been fed by a plume of molten basalt which can force its way to the surface quite quickly, at speeds of about 5kph, according to Auckland University research published on the GNS website.

There appears to be a trend towards an increase in the average size of eruptions, which have been of two main types.

Phreatomagmatic eruptions – violent explosions of steam, gas, lava and rock triggered when water and magma mix. These typically create a maar (CRRCT) volcano with a crater up to 1km, such as the Orakei Basin, and make up 72 per cent of the Auckland volcanoes. The other main type of volcano is the scoria cones such as One Tree Hill.

Experts have predicted that in future explosive eruptions in Auckland volcanic ash falling from the air may slash visibility or even cause complete darkness, damage electrical equipment, immobilise motor vehicles, and short-circuit communication systems and electricity supplies.
Even small amounts can damage vegetation and affect the breathing of people and animals, while attempts to wash it away will clog drainage systems.

Over 70 per cent of the Auckland volcanoes have also had fire-fountains of incandescent magma, and 30 of the volcanoes have had lava flows.

Auckland has a big volcanic eruption every 5000 years on average and the last one was Rangitoto, about 600 years ago.

According to the university's head of geology Ian Smith, monitoring will give a warning of "days to weeks" before the next eruption.

The 360sqkm covered by the Auckland volcanic field stretches from Rangitoto Island in the north to Manurewa in the south.

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