Mt Ruapehu's Crater Lake cools down

Mt Ruapehu's Crater Lake. Photo / Jared Savage

Mt Ruapehu is slowly cooling down after its Crater Lake peaked at 41C last month, GNS Science says.

The temperature of the lake has now fallen to about 33C and other monitored indicators show less activity.

However, GNS Science said its aviation alert remained on yellow, or elevated unrest, despite decreasing activity and no signs of an imminent eruption.

Steam might be seen above the crater and the lake colour might change if lake sediments were disturbed by gas and heat flows.

Earlier this month, GNS Science upgraded the alert to yellow following a sustained rise in the temperature of the crater lake.

The lake was entering a cooling phase but GNS scientists would continue to monitor the active volcano. Mt Ruapehu last erupted in 2007.

Meanwhile, Mt Ruapehu will soon be one of a handful of volcanoes worldwide watched around the clock by a "radar eye" aimed at saving lives.

A new high-speed Doppler radar system will mean volcanologists from Massey University and the University of Hamburg in Germany will be able to record every explosive burst from the active volcano.

It is the first monitoring system of its kind in the country and one of only a few in the world.

The instrument has a 1.2m diameter radar dish that focuses its narrow beam precisely over the crater lake.

The experimental radar would be an important addition to the existing range of warning instruments installed at Mt Ruapehu, Volcanic Risk Solutions director Shane Cronin said.

"Once this instrument is calibrated, it can be used for the regular monitoring and warning activities carried out by the Department of Conservation, GeoNet and GNS Science."

The radar should directly detect the volcanic particles thrown out of the crater lake, often at several hundred metres per second.

Vulcanologist Gert Lube said the radar penetrated through darkness, clouds and mist and could give reliable and rapid eruption warnings even if the summit could not be seen.

- NZPA