The likelihood of Mt Ruapehu erupting has increased - with temperature changes indicating eruptions were "more likely over the next weeks to months".
GNS duty volcanologist Steven Sherburn said yesterday temperatures a few hundred metres beneath the volcano's crater lake were about 800C, but the lake itself was only about 20C. "This suggests the vent is partly blocked which may be leading to a pressure build-up beneath the crater lake," Dr Sherburn said. "A sudden release of the pressure may lead to an eruption."
The Aviation Colour Code has increased from Green to Yellow. Code Yellow indicates a volcano is experiencing signs of elevated unrest above known background levels.
Drifting ash from the Mt Tongariro eruption on August 7 this year forced Hawke's Bay airport to close for a day, cancelling 14 inbound and 14 outbound flights. About 500 passengers were affected after Air New Zealand deemed the risk too great to ignore.
The ash also grounded Skyline Aviation services - including the air ambulance and Lowe Corporation Rescue Helicopter.
When Ruapehu erupted in September 1995, the airport closed for nearly two days after westerly winds blew ash across the region. The eruption impacted greatly on farms across the central North Island.
Impaired lactation affected lamb and calf survival rates, and yields on dairy farms suffered. Losses were greatest in lactating ewes grazing short pasture. A 5mm ash fall on the Rangitaiki Plain killed about 2000 ewes and lambs (2.5 per cent of the population), which ate affected pasture. Necropsies suggested fluorine poisoning or pregnancy toxaemia.
Timing was a crucial factor in Hawke's Bay crops vulnerable to a moderate volcanic ash fall.
Tomatoes during seed emergence and flowering stages were deemed vulnerable, as was stonefruit during early fruit development when fruit skins could be damaged and deformed.
Grapes have three main stages where damage can occur: flowering, when ash could burn plant tissues, reduce pollination and reduce bunch fill; fruit development, where ash would block sunlight and reduce quality; harvest, where ash would be a contaminant with the extra acidity of the ash possibly having an impact on wine quality.
Dr Sherburn said GNS was watching the situation but Ruapehu often didn't give any immediate warning it was going to erupt. Small earthquakes have been occurring about 5km beneath the summit area of Ruapehu since late October, but these may not be directly related to the high temperatures beneath crater lake.