Flight planned to keep eye on Ruapehu's heating crater lake

Thursday, 24 December 2020

Scientists plan to keep a close eye on Mt Ruapehu’s warming crater lake after the volcanic alert level was raised to two on Monday.

When weather allows, they plan to make a plume gas measuring flight and collect a lake water sample via helicopter but both measures were dependent on an updated hazard assessment, duty volcanologist Michael Rosenberg told Stuff on Thursday.

GeoNet monitors Mt Ruapehu via a network of seismic and acoustic sensors, satellite position receivers, and sensors in the lake, as well as physical visits to collect water samples, and flights to measure volcanic gases in the plume.

An airborne view of the Crater Lake on top of New Zealand's largest active volcano, Mt Ruapehu.

According to recent GNS updates, the current heating cycle in the lake – an announcement on Monday put it at 43C – seems to have started in mid October from a low point in September.

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A GNS Science technician takes measurements at Mt Ruapehu's Crater Lake as part of regular monitoring of the volcano.

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In the previous cycle, the lake temperature reached a high of 41C in April before cooling steadily to 12C by late September.

The lake has a long history of heating and cooling, with each cycle usually lasting several months.

While the heating phases indicated that the volcano was active, said Rosenberg, eruptions of Ruapehu had occurred both when the lake was in heating phases (above 40C) and, more rarely, when it was in cooler phases (10-15C).

Times between peak and lowest temperatures varied, he said.

Lake temperatures have been measured for more than 50 years, with the most recent past major eruptions occurring in 2007, 1995, 1975, 1969 and 1945.

While the VAL Volcanic Alert Level had been raised to level 2 and the Aviation Colour Code to yellow (the second level of four on the scale), GNS stressed on Monday that the increases were “not an indication that an eruption was on its way”.

“As a monitoring team we are alert but not overly concerned,” Rosenberg said.

Ruapehu was last at Volcanic Alert Level 2 in 2016. The level 2 phase lasted about two months – May 11 to July 5 that year.

VAL level 2 indicates the primary hazards are those expected during volcanic unrest; steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While it is mostly associated with environmental hazards, eruptions can still occur with little or no warning.

In line with its risk management plans the Department of Conservation has closed access to the area in a two-kilometre radius from Te wai-ā-moe in the Tongariro National Park.