Interpreting Ruapehu’s signals is ‘tricky’ task
The temperature of the water in Ruapehu’s crater lake has risen again and GNS Science is keeping a close eye on what this might mean for the risk of a substantial eruption.
The agency’s duty volcanologist, Geoff Kilgour, said yesterday it was pretty much steady as she goes, with the volcanic alert status at Ruapehu unchanged at 2.
But the temperature rise was being monitored closely for a range of reasons.
Kilgour’s explanation illustrates the scientific skill needed to assess the risk of an eruption of volcanic material and ash.
He said the temperature of the crater lake, TeWai a-moe, had gone up from 39 degrees Celsius to 40C since Tuesday’s last volcanic alert level bulletin.
This was not so concerning on its own, as it was continuing a trend that started earlier in the week. But whether the rise meant more or less risk of an eruption was not certain, Kilgour said.
The temperature rise did indicate there wasmore volcanic fluid and gas coming into the lake, which acted as a cap that helped prevent eruptions. Kilgour said the lake had two sets of vents – the central and northern vents. The larger central vent opened only recently for the first time in the latest round of more significant activity at Ruapehu. That central vent was the main source of volcanic fluid and gas heating the water. GNS Science knew the central vent was open because of ripples on the surface of the lake. ‘‘Central vent [produces] a really large bubble, tens of metres across,’’ said Kilgour. On whether the slow warming increased the risk of an actual eruption, Kilgour said: ‘‘That’s a tricky one.’’
The fact that central vent was open could help ease pressure below the lake as ‘‘it is releasing some of the pressure there’’. That pressure reduction could cut the risk of an eruption. But, on the other hand, the fact central vent was open could also mean hot volcanic magma might rise towards the waters of the lake, resulting in the production of more steam and ejection of water. Given the lake acted as a cap, any resulting significant lowering of the lake’s water level would be of concern.
‘‘Then we might see larger eruptions start to happen,’’ said Kilgour.
GNS Science said this week that the chances of a prolonged eruptive episode or a larger eruption, such as occurred in 1995-96, with wider ashfall impacts was higher than it was two months ago but remained very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions.