Mt Ruapehu’s Crater Lake/Te Wai ā-moe has entered a new heating episode, although volcanic activity remains low.

A volcanic alert bulletin released by GNS Science today said that the temperature of the Crater Lake has been slowly increasing since mid-October last year – reaching 29ºC.

The colour of the lake has changed from a blue-green tinge, seen when the lake is cooler, to the common grey colour seen when warm. Minor upwellings are present in the lake and sulphur slicks are present on the lake surface.

GNS Science said this colour change is consistent with more gas flow “disturbing sediment” at the base of the lake, which is now suspended in the lake water.

“The scanDOAS gas scanning equipment and gas flights have measured moderate levels of SO2 (sulphur dioxide) gas output as the lake has heated.

“This indicates the vents at the bottom of the lake are at least partially open, allowing hot gas and steam to drive the increase in lake temperature.”

Seismic monitoring equipment at Mt Ruapehu recorded a sequence of small earthquakes under the volcano since late November, peaking at magnitude 2.4.

“The level of volcanic tremor has remained weak during the start of this new heating episode.”

“These earthquakes may be in response to a volcanic process like the injection of new magma at depth under the volcano,” said GNS Science.

“However, the other monitoring parameters like volcanic tremor, heat flow, gas flow, and lake surface behaviour are all typical of a heating episode in the crater lake, which is being driven by shallower processes.

“Data observed to date is typical of the past processes seen in the active hydrothermal system associated with the crater lake.”

Overall, GNS Science found that this is consistent with a low level of volcanic activity usually seen during a heating episode.

As a result, the Volcanic Alert Level remains at Level 1 and the aviation colour code remains green.

Volcanic Alert Level 1 indicates the primary hazards are those expected during volcanic unrest: steam discharge, volcanic gas, earthquakes, landslides, and hydrothermal activity.

Mt Ruapehu is an active volcanic and has the potential to erupt with little or no warning when in a state of minor volcanic unrest.