Scientists keeping tabs on Ruapehu, White Island
A rare "lava dome" has formed around the crater of New Zealand's most active volcano, as scientists raise the volcanic watch level on White Island.
A lava dome is when magma, which has lost much of its gas, bubbles out of the crater of a volcano.
It cools quickly and builds up from the bottom. GNS scientists likened it to "congealed toothpaste" found round the edge of the tube.
The Bay of Plenty volcano is New Zealand's most active and since a rating system has been in place, it has never rested at zero.
GNS today raised its alert level to two, indicating there had been minor eruptive activity.
Constant plumes of steam on the island generally made it difficult to see anything close to the crater, but GNS scientists said conditions on Monday allowed them to get a closer look.
Vulcanologist Brad Scott said he had "never seen anything like it" in his 30 years visiting the island.
"We were really lucky the steam was not too thick and got great views. The dome is probably 20-30 metres across and has spines sticking up."
It is thought the lava dome was a result of an ash eruption recorded on August 5.
Scientists also today confirmed Ruapehu remains at a heightened level of unrest and that an eruption is "more likely than normal".
GNS Science head vulcanologist Gill Jolly said analysis was still showing higher than normal temperatures beneath the crater lake.
"We think this reflects a partially sealed zone a few hundred metres beneath the lake which might be causing a pressure build up behind it. That pressure would make an eruption more likely than normal."
It doesn't mean that an eruption is inevitable, Jolly said.
"If the sealed zone fails suddenly an eruption could occur, probably with little or no warning. If it fails more gradually then the pressure would probably be released more slowly and the likelihood of an eruption would revert to normal."
Eruptions in 1988, 2006 and 2007 are believed to have occurred as a result of sudden failure of a seal beneath the crater lake.
"We never have the whole story so there is always uncertainty in our assessment of what might happen at volcanoes.
"It's like detective work without all the clues. But when we see something that might increase the chance of an eruption we have to be more cautious," Jolly said.
Small earthquakes 3 to 5 kilometres beneath the crater lake in late October and early November had now stopped.
GeoNet said it was not clear if those earthquakes were related to the high temperatures estimated a few hundred metres beneath the lake.
The crater lake is quiet and its temperature has remained relatively low, 20 - 25 degrees Celsius, since March.