Mt Ruapehu crater lake warming up, rise in volcanic tremor

Wednesday, 6 June 2018

The temperature at Crater Lake, Mt Ruapehu, is rising.

The temperature of Crater Lake at Mt Ruapehu is rising, and there is an increase in volcanic tremor in the area. GeoNet said the activity was 'consistent with minor volcanic unrest behaviour'.

The aviation alert level remains at 1, corresponding to minor unrest, and the aviation colour code stays at green.

The lake, Te Wai ā-moe, had been cooling as part of a normal cycle, until Tuesday last week, when it began heating by about 1 degrees Celsius each day, duty volcanologist Geoff Kilgour said.

'Coincident with the increasing lake temperature, we have also noticed that the level of volcanic tremor has increased. This is a characteristic feature of a heating cycle and represents the increased flow of hydrothermal fluids into the lake. Previous heating cycles have shown this increased tremor to last for days to weeks,' he said.

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'The Crater Lake has undergone many heating and cooling cycles in the past and we don't see any unusual signs of increased unrest.'

Observations were consistent with minor unrest behaviour, but it was useful to remember eruptions could happen with little or no warning, Kilgour said.

Temperatures ranged from about 12-40C over about a year in the cycle seen at the lake since 2003. During a recently relatively long period of elevated temperature the lake had reached about 38C.

It had cooled as expected during the past two months to reach about 20C, but then started heating again.

As part of normal routine monitoring, it is planned to visit the lake this week - weather permitting - to take water and gas samples.

'The key thing is we have seen these changes. It's part of the normal cycle, completely expected,' Kilgour said.

With the ski season about to start, GeoNet wanted to make it clear the activity at the lake was part of the normal process.

The temperature of the lake was recorded continuously, but that did not tell scientists much about what, if anything, was changing at the volcano. That's why it was necessary to go up to the lake to get samples of water and gases emitted from the lake.

The chemistry of the water and gases provided a better indication of any changes under way. 'The chemistry can tell us if there's new magma rising under the volcano, or if it's just part of the normal hydrothermal process.'

So far there was no indication of any new magma, Kilgour said.

In the past nine or 10 years the temperature had dropped below 20C about 10 times, but it tended to only go a few degrees lower. It was rare for the temperature to drop below 15C.