By SIMON COLLINS science reporter
Land in the central North Island is constantly rising and falling, according to a scientific study that confirms New Zealand's image as the "Shaky Isles".
The long-term study by the Institute of Geological and Nuclear Sciences has found that land near the northwestern part of Lake Taupo dropped 7cm in the 10 years to 1996 and land on the southeast side of the lake rose 2cm.
The study's authors say the Lake Taupo volcano, which affected the atmosphere in China when it last erupted in AD 181, could start building up to blow at any time.
Mt Taranaki (Egmont) last erupted in 1755 and is considered even more likely to blow soon, with potentially catastrophic effects locally and causing damage as far afield as Auckland.
"Auckland is directly downwind, and we know from peat swamps that there are 14 Egmont ashes found in Auckland," said the institute's volcano surveillance co-ordinator, Brad Scott, one of the study's three authors.
"An eruption of Egmont would shut down parts of Auckland, and certainly may take out the airport. With that type of volcano we'd expect the eruption to last for two or three weeks to a month."
The study, published in the NZ Journal of Geology and Geophysics, is the first long-term report on data from 22 measuring stations installed around the Taupo lake edge between 1979 and 1985, using the lake level as a standard to check shifting land levels.
"It's a relatively unique data set. We pioneered this technology and it's one of the largest data sets in the world," Mr Scott says.
New Zealand's southwest-northeast ranges, from the Southern Alps through to East Cape, are being formed by compression of the land as one of the six great "plates" that make up the Earth's surface, the vast Pacific plate, dives under the Australian plate, he says.
But the edge between these plates bends like an S in the middle of New Zealand, which has the effect of pulling the Australian plate southwards as it rides over the Pacific plate in the middle of the country.
This means the East Cape area is being pulled slowly southwards away from Auckland and Northland. In the middle, the land on the Volcanic Plateau is literally being pulled apart by around 1cm a year.
"If you get two things and pull them apart, the piece in the middle will fracture and drop," says Mr Scott.
"So what we have seen in the Lake Taupo data set is subsidence of the central part of the volcanic zone between Rotorua and Taupo."
On average, geological evidence shows land in that zone has dropped by 4mm to 5mm a year for at least 200,000 years. But the Taupo data shows that particular areas have heaved both downwards and upwards in the past 23 years.
Between 1979 and 1982, the land sank by 4mm a year at Kinloch and rose by 3mm a year at Scenic Bay on the western side of the lake.
In early 1983, Kinloch suddenly started lifting at the fast clip of more than 60mm a year - only to drop by 56mm when a fault line moved in a swarm of earthquakes in mid-year.
The area then resumed its downward trend at a faster rate of 11mm a year until 1996. Since then it has slowed, but is still sinking gradually.
Mr Scott says the land movements will probably give scientists plenty of notice when the area starts building towards another eruption.
Another study says the Taupo volcano has erupted 28 times in the past 26,500 years and is the world's most active volcano of its kind.
"It's all just a matter of time," he says. But that timing is uncertain.
"We have had periods of 2000 to 3000 years of no eruption and then two in 500 years, so you can't just apply the average."
Another institute scientist, Dr David Johnston, has calculated that the relatively small eruptions of Mt Ruapehu in 1995-96 cost the country $130 million, mainly from the closure of the mountain's three skifields and damage to the Rangipo power plant.
Mr Scott says Mt Taranaki is "right at the top of the list of sleeping volcanoes that are likely to wake up" because it has erupted on average about every 300 years.
The institute is in the second year of a 10-year, $50-million programme to modernise and expand its monitoring network. Photos of Mts Ruapehu and Ngauruhoe, and White Island, are updated hourly on: www.geonet.org.nz
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