Pressure builds under Ruapehu
Pressure is building up under the Crater Lake on Mt Ruapehu, increasing the likelihood of an eruption in the coming weeks, scientists say.
GNS Science said recent measurements taken on the volcano over the last few weeks indicated that eruptions were more likely "over the next weeks to months".
A sudden release of pressure built up under the lake may lead to an eruption.
Pressure build-up was thought to have caused the 2007 eruption and a smaller eruption in 2006.
The Aviation Colour Code has increased from green to yellow as a warning for those flying over the region, but the Volcanic Alert Level remained at level 1.
"We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt," GNS Science Duty Volcanologist Steven Sherburn said.
GNS Science believed the temperature a few hundred metres beneath the crater was about 800 degrees Celsius but the lake itself was only 20degC.
"This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake.''
Small earthquakes have been occurring about five kilometres beneath the summit of Ruapehu since late October, but it was not known whether they were related to the increased temperature below the crater, GNS vulcanologist Brad Scott said.
"We perceive them to be completely unrelated at this stage," he said.
However, the quakes had the potential to build up processes and push magma further to the surface, which would further increase the likelihood of an eruption, he said.
GNS constantly monitored Ruapehu and had discovered over the last few weeks that the chemistry and gases within the Crater Lake were changing, Scott said.
The criteria for the aviation code and alert levels were not the same and the heightened temperature of the lake meant GNS was required to change the aviation code, Scott said.
Alert level 1 indicated that the volcano was in a state of unrest, while an actual eruption was needed to increase it to level 2.
"It's just a heads up," Scott said about GNS' warning of an increased likelihood of an eruption.
"It's really just a reminder that Ruapehu is an active volcano and can erupt, and the chances of that happening is more than what it was last week."
Ruapehu experienced a small scale eruption in 2007 and a series of large scale eruptions in 1995 and 1996.
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