Scientists do not believe that volcanic activity at Tongariro and Ruapehu this year are related, but are not excluding the idea.
GNS Science announced Friday that pressure was building up inside Ruapehu's Crater Lake and there was an increased likelihood of an eruption.
Tongariro, which is located near Ruapehu, erupted in August, the first time in more than 100 years. White Island, off the coast of the Bay of Plenty, erupted the same month.
While the White Island eruption could not have been related to the volcanic activity at Ruapehu and Tongariro, scientists were not ruling out a link between the latter, GNS volcanologist Nico Fournier said.
"We don't have evidence at the moment that the activity of the two volcanoes are related," Fournier said. "But we can't exclude it entirely."
The fact the two volcanoes had experienced increase activity within a few months of each other was a point of interest among his colleagues, Fournier said.
But GNS vulcanologists were focusing their research more on the connection between slow earthquakes and volcanic activity.
Earthquakes rattled the ground below Tongariro in the weeks leading up to the August eruption.
Earthquakes have also been occurring below Ruapehu.
Ruapehu hasn't shown any increase in activity over the weekend, but scientists are hoping samples they plan to gather this week will provide more clues as to what is happening inside the volcano, Fournier said.
The temperature deep below the Crater Lake was about 800 degrees Celsius, a few hundred degrees higher than what it was a few months before, he said.
The Crater Lake, however, was cool, which indicated something was blocking it, and meant pressure was building up beneath the surface.
Scientists hoped to gather more samples by air from the lake on Wednesday or Thursday and have the results by next week, Fournier said.
It was likely that sulphur had caused a clay-like layer which was blocking the heat from rising.
"What worries us is the combination of the temperature and that heat is getting trapped and those gases are becoming trapped under the lake."
Pressure build-up was thought to have caused the 2007 eruption and a smaller eruption in 2006.
- Fairfax Media
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