What do the volcanic conditions at Mt Ruapehu suggest?

Monday, 21 March 2022

The crater lake Te Wai ā-Moe on Mount Ruapehu.

The volcanic alert level at Mt Ruapehu has been raised, with a possibility conditions at the volcano could lead to something like the comparatively small 2007 eruption, although it’s also possible there won’t be any eruptive activity at all.

Lifting Ruapehu’s volcanic alert level to level 2 on Monday, GeoNet said activity at the volcano was consistent with elevated volcanic unrest.

There was an increased likelihood of eruptive activity, and while level 2 was mostly associated with such hazards as steam discharge and hydrothermal activity, eruptions could still happen with little or no warning, GeoNet said.

Indications suggest the amount of hot gas flowing through the volcano’s system is increasing, but the temperature of Te Wai ā-moe – Crater Lake – is not rising as fast as would normally be expected.

**READ MORE:

Niwa's climate stations recorded a brief spike in air pressure as the atmospheric shock wave from Tonga's volcanic eruption pulsed across New Zealand.

* Unusual number of cluster earthquakes beneath Ruapehu's summit, but no change to alert level, scientists say

* Weekend tremors a sign Mt Ruapehu's crater lake is heating up

* Ruapehu's crater lake heating up, tremors continue

That has led volcanologists to suspect there’s a partial blockage in the vent beneath the lake, which could allow pressure to build within the volcano.

It was hard to tell what any eruptive activity might be like, and it was equally or more likely there wouldn’t be any activity. “There’s a pretty good chance it won’t lead to any eruption,” GNS Science duty volcanologist Craig Miller said.

Possibly the best comparable eruption was the relatively small 2007 event, when there was also a blockage in the vent. “Just one bang and that’s it, back to normal.”

GNS described the 2007 event as an explosive eruption that lasted about seven minutes. Explosions spread ash, rocks and water across the summit area, producing lahars in two valleys. Unlike eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area.

Clues pointing to an increase in hot gas flow through the Ruapehu system are the heating of the lake – even if it is less than expected – and strong levels of volcanic tremor.

If a volcano's about to erupt in Auckland, scientists will know about it between five and 15 days beforehand, research shows.

Miller said volcanic tremor was a continuous ground shaking that was not strong enough to be felt by humans. The vibration was caused by most likely hot gas, or possibly liquid, being pumped under pressure through cracks inside the volcano.

The tremor now was strong compared to the usual range recorded at Ruapehu. While not the strongest seen at the volcano, it was in the top 1-2 per cent of levels recorded probably in the past 15 years, since the introduction of modern recording systems.

The crater lake went through heating and cooling cycles. The cycles were caused by the flux of hot fluids coming into the bottom of the lake, which was a big pool and took a while to heat up.

Longer cycles could go from peak to trough to peak in 12 to 15 months, with lake temperatures ranging from maybe 15-45 degrees Celsius, Miller said.

Shorter cycles could last maybe three months, or so, with temperatures ranging from about the mid-20s to mid-30s. Each cycle was a bit different.

If the lake temperature dropped below about 15C it probably indicated the vent was fully blocked. That was the case before the 2007 eruption, when the temperature of the lake dropped to about 13C.

At the other end, if the lake temperature climbed above 45C that could be indicating there was some hot magma quite shallow in the system below the crater lake. Fresh magma increased the likelihood of an eruption.

The lake temperature had been around 32C at the beginning of 2022, then had cooled, but only to about 27C last week before climbing to 31C most recently. “Sometimes it kind of gets stuck at this temperature,” Miller said.

“The issue is we know there is a lot of gas input. That’s what the tremor is telling us.

“The problem is the lake hasn’t really responded in the way we would expect. Normally the lake heats up at a much faster rate than we’re seeing now,” Miller said.

“We know the tap’s on at one end, but there’s only a dribble coming out at the other end.”

The vent was the main plumbing system of the volcano, going from the bottom of the lake down maybe a few hundred metres, maybe as much as half a kilometre. Hot fluids were circulating around in that area. Below that was probably a magmatic source providing the heat that was driving it all.