Recent measurements at Ruapehu indicate that the likelihood of eruptions has increased, GNS Science said today.
The Aviation Colour Code has increased from Green to Yellow, but the Volcanic Alert Level remains at 1.
GNS Science Duty Volcanologist Steven Sherburn said, “We have been measuring changes at Ruapehu over the last few weeks and believe these changes make eruptions more likely over the next weeks to months”.
“We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt,” Dr Sherburn said.
"We think that the temperature a few hundred metres beneath Crater Lake is about 800 °C, but the lake itself is only about 20 °C. This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake. A sudden release of the pressure may lead to an eruption."
Since late-October small earthquakes have been occurring about 5 km beneath the summit area of Ruapehu, but these may not be directly related to the high temperatures beneath Crater Lake as the earthquakes are much deeper.
Aviation Colour Codes are based on four colours and are intended for quick reference only in the international civil aviation community. Code Yellow indicates that a volcano is experiencing signs of elevated unrest above known background levels.
The Volcanic Alert Level ranges from 0 to 5 and defines the current status at a volcano. Level 1 indicates a departure from typical background surface activity.
A build-up of pressure beneath Crater Lake is thought to have caused the 2007 eruption and a smaller eruption in 2006.
Brad Scott, Steven Sherburn
GNS Science Volcanology