Ruapehu's crater lake heating up, tremors continue

Monday, 21 December 2020

A two kilometre exclusion zone is in place around the Mount Ruapehu crater lake - Te Wai a-moe - after GeoNet raised its volcanic alert level.

Mount Ruapehu’s crater lake – Te Wai ā-moe – is heating up and is now at 43C.

The Volcanic Alert Level has also been raised to Level 2 but duty volcanologist Michael Rosenberg told Stuff the signs, and increase to alert levels, were certainly “not an indication that an eruption was on its way.”

In a statement on Monday GNS Science said during a gas monitoring flight last week, the lake was a uniform grey colour indicating it was well-mixed and gas wasn't being trapped.

The Crater Lake on top of New Zealand's largest active volcano, Mt Ruapehu.

But while the gas output through the crater lake had also increased markedly, with carbon dioxide and sulphur gases in the plume being the largest measured in the past two decades, a GNS scientist did not see this as beyond normal behaviour for the mountain.

* Rising SO2 gas output, volcanic tremor at White Island could suggest eruptive activity more likely

* More activity on Ruapehu, but don't expect it to blow

The release of gas he saw as a good thing “because it means that the top of the volcano is not being sealed off and you’re not getting that shallow pressurising”.

“So the fact that we can measure a lot of gas coming through means the whole vent system is at least open.”

Gas was also not being trapped by the lake water, as indicated by the mixing, Rosenberg said.

A GNS Science technician takes measurements at Mt Ruapehu's Crater Lake as part of regular monitoring of the volcano (file photo).

GNS was noting bursts of volcanic tremor in combination with the high gas output but “again we have seen this type of thing before so the behaviour is typical for Ruapehu, but on the upper bounds of normal behaviour which is why we have pushed up the alert level.

“As a monitoring team we are alert but not overly concerned.”

Modelling of energy input into the lake has shown an increase during the past month from around 200MW to around 400MW.

Since 2007 the crater lake temperature has exceeded 40C a number of times, without leading to an eruption.

The GNS release said the Volcanic Alert Level reflects the current level of volcanic unrest or activity and is not a forecast of future activity.

Level 2 indicates the primary hazards are those expected during volcanic unrest; steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While it is mostly associated with environmental hazards, eruptions can still occur with little or no warning.

GNS has also raised the Aviation Colour Code to yellow.

The colour code system is based on four ascending activity levels – green, yellow, orange and red – and is only intended to provide the international civil aviation community with information about volcanic activity and the potential presence of volcanic ash in the atmosphere.

Yellow indicates the volcano is experiencing signs of elevated unrest above known background levels, or, if moving down from a higher alert level yellow indicates volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

GNS said it and the National Geohazards Monitoring Centre continue to closely monitor Mt Ruapehu for further signs of activity.

GeoNet monitors the mountain via a network of seismic and acoustic sensors, GPS receivers, sensors in the lake, visits to the lake area and gas flights.