Mt Ruapehu: What volcanic unrest and the increased alert level means
Tuesday, 22 December 2020
The increased alert level and heightened activity at Mt Ruapehu’s Crater Lake isn’t an indication that an eruption will happen – but it is possible.
Crater Lake, Te Wai ā-moe, heated to 43 degrees Celsius on Monday, resulting in GeoNet upgrading its Volcanic Alert Level (VAL).
Alert level 2, which indicates moderate to heightened volcanic unrest, remains in place.
That means volcanic unrest means hazards may occur on or near the active volcano. It has the potential to erupt with little or no warning.
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An eruption may include explosions, flying rocks, fast-moving hot ash clouds, lava flows and domes, landslides, volcanic gases, lightning and or earthquakes.
“The Volcanic Alert Level reflects the current level of volcanic unrest or activity and is not a forecast of future activity,” a statement from Michael Rosenberg, duty volcanologist at GeoNet, read.
How often does the lake heat up?
The Crater Lake has heated up like this many times before without resulting in an eruption, and it often fluctuates between 15C and 45C throughout the year.
Increases over 45C may result in an eruption, but it’s not guaranteed.
Te Wai ā-moe warmed from 24C to around 40C in February this year, and in April, 2019, it reached 42C. The highest temperature ever recorded in the lake was 60C in 1968.
The alert level in the area hasn’t been raised to level 2 since 2016, when the lake heated to 46C. The alert remained in place for four months at that time.
When did it last erupt?
The last eruption was on September 25, 2007. The explosive eruption lasted around 7 minutes and caused ash, rocks and water to be spread across the summit area.
It produced lahars – mudflows caused by lake water ejecting onto the ice and snow – in two valleys, including the Whakapapa ski field.
There was no high ash plume, unlike the eruption in 1996.
What do scientists look for to prompt an alert level change?
The rising temperature of the lake, an increase of short-lived pulsing volcanic tremors, and gas output prompted the alert level change on Monday.
The Aviation Colour code has also changed from green to yellow, indicating signs of elevated unrest.
An exclusion zone, stretching 2km around the mountain’s peak, has made the area off limits for guided walks. The Tongariro Alpine Crossing is not affected and remains open, the Department of Conservation (DOC) confirmed.
“Tongariro National Park is an active volcanic area, so eruptions can occur with little or no warning. The closer people are to the Park’s active volcanic vents, the higher the risk,” said DOC’s Tongariro operations manager Connie Norgate.
A recent gas flight showed the lake was “well-mixed” with some flow over the outlet. The gas output had increased due to the heating, with the largest amounts of carbon dioxide and sulphur gases recorded in the last 20 years, according to GeoNet.
An increase of energy input into the lake in the last month – around 200 megawatts to 400 MW – has also been recorded.
What happens next?
GNS Science and the National Geohazards Monitoring Centre are monitoring the volcano, looking for further signs of activity.
It is monitored by a network of seismic and acoustic sensors, GPS receivers, and sensors in the lake as well as visits to the area and flights.
If unrest continues to escalate and the probability of eruption increases, the summit hazard zone – a 3km radius from the lake – may be closed.