Mt Ruapehu's crater lake is heating up, but here's why there is no eruption on the horizon

Thursday, 11 April 2019

Monitoring the crater lake helps scientists to calculate the volcanic alert level of Ruapehu.

The Mt Ruapehu volcanic system still has a long way to go to get back to the energy levels of the 1960s and 1970s, despite the recent heating of the crater lake.

GeoNet has reported the temperature of the lake - Te Wai ā-moe - had been rising by around 0.5 degrees Celsius a day during the past two weeks, getting to 43C on Thursday.

Volcanologists say so far there's nothing to indicate an eruption is imminent.

The temperature rise of the past few weeks comes after an unusual six months or so during which the lake temperature stayed around 30-32C.

The energy level in the volcanic system at Mt Ruapehu remains low and has yet to recover after the eruptions of 1995-96

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The lake temperature did occasionally get stuck, but the six months around 30C was one of the longest recorded periods, certainly since the lake reformed in 2003 after the 1995-96 eruptions, GNS Science volcano information specialist Brad Scott said.

Normally Ruapehu was more dynamic.

'The best analogy is thinking about a pot of water on an electric stove. If you don't change the element the pot will stay at a steady temperature,' Scott said.

'Ultimately the volcano is fed from molten material at depth. That's the equivalent of the electric element. If more heat is being put into the lake, the only place you can get more heat down there is from new hot rock.'

The threat from Mt Ruapehu

There was a body of plasticky molten material 1-2km underneath the volcano.

'The volumes are very small. A (cone) volcano like Ruapehu only deals with small volumes,' Scott said.

The largest known cone eruptions in New Zealand involved about 1 cubic kilometre of material. More usually it was 0.01cu km - the size of the 1995-96 eruptions - and even as little as 0.001cu km or smaller.

The historic record went back to 1870, and since then there had been three large eruptive episodes at Ruapehu - in the 1890s, 1945 and 1995-96. There were also many smaller events, particularly from 1964-95, with the most recent being in 2006 and 2007.

The highest temperature recorded in the crater lake was 60C in 1968. 'It could happen again but it's very unlikely with the way the volcano is currently. It's in a pretty low energy status currently. In the '60s it was a higher energy status,' Scott said.

During the 1960s and 1970s there was much more molten material associated with the volcano than was the case now.

The 1995-96 eruption basically excavated the shallow magma system 1-2km under the volcano, and so far scientists hadn't seen signs of a recovery. 'If there was we would have higher heat flows and more regular heat flows into the geothermal system that feeds the crater lake.'

The chemistry of the fluids and gases coming into the lake were another important factor. So far monitoring hadn't picked up any large signature of molten rock at depth.

The lake temperature often cycles between 15C and 40C in a nine to 12 month period. Records going back to the 1950s indicate the probability of an eruption rises If the lake temperature gets above 45C. But in 2016, the lake hit 46C without an eruption.

Since continuous recording of the lake temperature started in 2009, it's only been over 42C 1 per cent of the time. The last time the temperature reached a similar level was May 2016.

The intensity of the volcanic tremor level at Ruapehu rose during the past few weeks, although it started easing again two days ago.

Ruapehu produced volcanic tremor signals every day, Scott said. Usually the signals were weak. For the past few weeks they had been moderate strength, but had eased in the past two days to be on the boundary between weak and moderate.

'Volcanic tremor is a continuous signal. The best analogy is an electric jug boiling on a bench. Steam bubbles are being made inside the jug and are rising and collapsing and making a lot of noise. Volcanic tremor is like that.' Gas and steam bubbles were forming and collapsing and flowing into the crater lake, producing the signal.

The volcanic alert level at Ruapehu remained at 1 on Thursday morning, indicating minor volcanic unrest.