By GREGG WYCHERLEY
Tourists at a popular geothermal area near Taupo were treated to a surprise eruption yesterday when a crater spewed hot ash and pumice over hundreds of metres.
Part of the Craters of the Moon began erupting about 11am, blanketing the area in a 25cm-thick layer of fine pumice ash.
About 100,000 people visit the area each year, and yesterday busloads of tourists received a coating of ash as a souvenir.
Department of Conservation spokesman Ralph Turner said he closed some walking tracks and boardwalks after tourists reported the eruption.
Scientists from the Institute of Geological and Nuclear Sciences were investigating and parts of the field would remain closed until it was certain the crater was stable.
"It's only a few metres from the boardwalk. The whole thing could collapse at any time under the pressure," said Mr Turner.
"We'd like it to settle down before we let anyone near it again."
He said an eruption occurred in the area about once every 18 months, normally during a spell of fine weather after heavy rain.
"But it's bigger than usual - as big as anything I've seen here.
"It's pretty spectacular. It might blow like this for two or three days and then quieten down."
Mr Turner said he did not recommend people get too close.
"I walked through it and got absolutely covered in pumice and ash. You can't breathe once you're in there in the cloud. You just shut your eyes and your mouth and keep going."
Institute geothermal scientist Chris Bromley said the eruption was one of the largest he had seen at the 40ha site in the past 12 years.
He said the eruption was caused by high-pressure steam breaking through a new fissure, generating huge amounts of heat and shooting pumice 50m into the air.
Although the steam would be fatal if someone got too close, the main danger was probably the risk of slipping on the mud thrown out by the eruption.
"It's a bit slippery at the moment and you get really dirty, and nobody wants dirty tourists."
The Craters of the Moon is at Wairakei, one of the country's largest geothermal fields.
By GREGG WYCHERLEY