Mt Ruapehu has strongest tremor in 20 years amid continued volcanic gas emissions and heightened unrest

A period of heightened unrest continues for Mt Ruapehu. Photo / Bevan Conley

By RNZ

A period of heightened unrest continues for Mt Ruapehu, with continued high levels of volcanic gas emissions and strong tremors, GNS Science says.

Now in its sixth week of raised volcanic activity, over the past three days the temperature of the crater lake -Te Wai ā-moe - has risen to 38C, after four weeks at 36-37C.

The volcanic alert level remains at 2.

GNS Science duty volcanologist Geoff Kilgour says Mt Ruapehu has "exhibited the strongest volcanic tremor in two decades".

"Over the last week, the level of volcanic tremor has varied, with bursts of strong tremor interspersed by short, periods of weaker tremor. This represents a change in character in the tremor, and the driving processes remain unclear."

GNS Science staff have increased the frequency of aerial gas measurements and Te Wai ā-moe sampling and a gas measurement flight on April 28 recorded the sixth-highest sulphur dioxide (SO2) flux of 390 tonnes per day since 2003, Kilgour said.

"Sulphur dioxide is a strong indicator gas and is derived from a relatively shallow magma body, which is perceived to currently exist a few kilometres beneath Crater Lake.

"Sampling of Crater Lake was also conducted last week and during that visit, our scientists observed upwelling of Central Vent and reduced upwelling at the Northern vents area. During recent visits, active upwelling has only been observed at the Northern vents. It is important to note that Central Vent is the primary vent, whereas the Northern vents are a subsidiary vent system. We had surmised previously that Central Vent was sealed, blocking the main flow of fluids and gases into Crater Lake, however this vent now appears to be at least partially open."

The period of heightened unrest is driven by molten rock (magma), Kilgour said.

He says that within the next four weeks it's most likely there will be no eruption of Mt Ruapehu, or a minor eruption confined to the lake basin - though as an active volcano, it has the potential to erupt with little or no warning.

"Small eruptions are still able to generate lahars, especially in the Whangaehu River. The next most likely scenario is an eruption that impacts the summit plateau and generates lahars in multiple catchments, similar to what was seen after the September 2007 eruption or older events like those in 1975 and 1969.

"The chance of a prolonged eruptive episode or a larger eruption, such as occurred in 1995-96 with wider ashfall impacts, is higher than it was two months ago, but remains very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions."

The aviation colour code remains at yellow.