Mt Ruapehu has again felt the rumble of an avalanche - this time at its crater lake.

It comes as scientists are keeping a heightened eye on the mountain-top lake amid concerns around cooling temperatures in the water and the small possibility of a minor eruption.

GeoNet last night confirmed that a moderately large avalanche - approximately 30,000 tonnes of ice and snow - had flowed into the lake.

This GeoNet graph, measuring the temperature of Mt Ruapehu's crater lake, shows the sudden drop that came with the avalanche of ice and snow into the water.
This GeoNet graph, measuring the temperature of Mt Ruapehu's crater lake, shows the sudden drop that came with the avalanche of ice and snow into the water.

GeoNet seismographs near the summit of the mountain recorded the ground vibration caused by the avalanche, which appeared as an unusual earthquake that in some ways looked like a volcanic earthquake.

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"Our crater lake temperature sensor recorded a rapid drop in the temperature of the lake, from 15C to 8C," GNS Science volcanologist Brad Scott said.

"Over the next few hours the lake reheated to 15C and appeared unaffected by the avalanche."

In the past, drops in the lake temperature had also occurred following small eruptions when large volumes of ice and snow were washed into the lake, entrained by water ejected outside the lake and draining back in.

"Although the lack of signal on our air pressure sensors strongly suggested that no eruption had occurred on Tuesday night, it was important to confirm the cause, GeoNet duty volcanologist Dr Tony Hurst said.

GeoNet and the Department of Conservation staff flew over the crater on Wednesday to ascertain the effects and confirm the cause of the rapid drop in the lake temperature.

Substantial avalanche scars and deposits confirmed that the observed seismic signal and change in the lake temperature overnight were related to the avalanche depositing thousands of tonnes of ice and snow into the lake.

"Although the lake has been relatively cold over the past weeks, at 15C, heat is always being added to the lake from the vents on the lake floor," Mr Scott said.

"Because of this sustained heating, the lake quickly recovered from the sudden inflow of cold snow and bounced back up to its pre-avalanche temperature," he said. "If the heat flow wasn't present the lake would have cooled down and stayed cooler."

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Mt Ruapehu's volcanic alert level remains at level 1.