Our Changing World: What makes the volcano Ruapehu tick – and boom?

Want to make your own Ruapehu? Vinegar and baking soda won't cut it here. Instead, you'll need andesite rock (crushed), some crater lake fluid (very acidic) and some gases (volcanic).

Heat your ingredients to 200 degrees Celsius in a shiny, titanium pressure vessel and take frequent pressure, temperature and fluid samples. After 40 to 50 days - if you're lucky - you might arrive at your lab one morning to discover an eruption has taken place overnight, ejecting a small amount of fluid and bits of crushed rock and ash into a waiting syringe.

This will be very exciting.

"I know when something good has happened," volcano seismologist Dr Oliver Lamb says, "Because I'm sitting in my office over there and I can see Lucjan and Geoff walking past, really excited, like kids in a sweet shop."

All three - Oliver, hydrothermal geochemist Dr Lucjan Sajkowski and senior volcanologist Dr Geoff Kilgour - are researchers based at Earth Sciences New Zealand's Wairakei Research Centre.

The cluster of buildings lies just north of Taupō, right next to the Wairakei thermal plant, and less than 100 kilometres away from the real object of the researchers' interest: Mount Ruapehu - the North Island's highest point, and New Zealand's largest active volcano.

It's been just over 30 years since the first of the large 1995-96 Ruapehu eruptions, which grounded planes, caused power outages, and covered large areas of the region with ash.

There has been one smaller eruption since, in 2007, which flung rock into the air that crushed a mountain hut, injuring a climber sheltering there.

Monitoring and modelling of volcanoes has advanced greatly since 1995, but we still can't look into Ruapehu - or any volcano - to see exactly what's going on.

So instead, Lucjan, Oliver and Geoff have recreated Ruapehu in the lab, hoping that the changes they observe in the lead-up to a syringe eruption might help them someday predict when the real thing is going to blow.

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Ruapehu sits within the Taupō Volcanic Zone, which forms part of the Taupō-Hikurangi subduction system. Beneath, the Pacific plate dives beneath the Australian plate, stretching and thinning the Earth's crust and causing the many volcanoes and earthquakes that have shaped this part of the North Island.

Ruapehu is an andesite stratovolcano: a rough cone shape made up of alternating layers of ash and lava flow from successive large eruptions, with a crater lake - Te Wai ā-moe - at the summit. The andesite is the key to Ruapehu's explosiveness. It has a relatively high silica content, making the rock 'stickier' and therefore harder for gases to escape than some other types of rock.

Within the volcano, there's hot magma deep below, which sometimes moves upwards. This shallower magma then heats the rock and fluid under the crater lake. In quiet periods, the volcano's hydrothermal system lets gas and hot liquid seep from vents into the lake. But one theory as to why Ruapehu erupts is that, as the temperature rises, a mineral seal may form in these vents, behind which pressure builds and builds.

When Ruapehu erupts, there's often no, or very little, magma involved, Oliver says. "It's just the eruption of supercritical, super-hot gas and water coming out of the system because of the seals breaking."

They believe the seal formation happens because some minerals form solids as the temperature rises, rather than dissolve - like limescale forming inside a kettle.

Recreating this cycle of heating, mineral precipitation, seal formation, pressurisation and finally bursting, hinges on a large, shiny piece of equipment set up in Earth Sciences New Zealand's experimental hydrothermal geochemistry lab. A series of titanium containers, pumps, ovens and tubes allows the team to flow heated, pressurised fluid through rock over a long time to see what happens.

The heart of the action is the pressure vessel, a chunky bit of metal with crushed Ruapehu andesite inside it.

"We crush rock to increase the surface area," Lucjan says. "If we had a big chunk of rock, which we could have, everything would take just much longer. But because we are crushing it, there is so much more opportunity for interactions of the fluid with rock. Then we don't have to wait for 10 years - we can wait just for a month."

The fluid they use has the same chemical make-up as the crater lake fluid at the top of Ruapehu, and every day they take a 24-millilitre sample of fluid from their lab system for testing.

Once the experiment is over, they can also retrieve and examine the rock to see if any minerals have appeared from reactions with the fluid.

Along with the daily fluid samples, they take temperature measurements every five minutes and collect pressure data every second.

They've so far had three syringe eruptions.

The idea is that the data collected each time will build a picture of the geochemical and other changes that indicate a brewing eruption, ahead of the event.

"We hope that there are going to be specific signatures of this in the fluid that we can see, like, 'Oh, the seal is starting to be created', which will help us to say, 'It's coming'," Lucjan says.

Extrapolating what they learn in the lab to the real Ruapehu is the ultimate goal, and still a long way off, Oliver Lamb says.

But the theory links with evidence from real eruptions. In 2007, rocks blasted from the crater lake were found with yellow sulfur-rich minerals jamming up their pores.

"We can't be 100 percent sure where exactly it came from," Oliver says. "It could have probably come from the top of the lake, where there's no seal, or it could have come from deeper ... but we see this evidence of the sulfur filling all these pore spaces that fluid could otherwise move through."

The team has just started a new experiment, this time with an additional sensor, called a piezoelectric transducer, that's of particular interest to Oliver.

While Lucjan focuses on the geochemical processes and changes that precede an eruption, Oliver's interest lies in the geophysics, especially volcanic earthquakes. There are seismometers on the mountain that continuously monitor for any movement, but the signals can be hard to interpret.

"One of the big problems we have in volcano seismology is that we have these unrest episodes at volcanoes like Ruapehu and Taupō that don't lead to eruptions, but sometimes these signals can actually look identical to what's been seen in the past before an eruption," Oliver says.

An example of that is Ruapehu in 2022. For months, seismometers picked up a series of 'drumbeats' and 'tremors' that made researchers believe that the volcano was going to erupt. They lifted the alert level, but nothing happened.

The piezoelectric transducer is equivalent to a tiny microphone, attached to the titanium cylinder containing the andesite rock and volcanic fluid.

"What that's doing is listening for tiny, tiny micro-cracking and we're measuring these tiny, tiny waves," Oliver says. "And essentially, that's the link between this experiment and what we're measuring at the volcano with the seismic activity and the earthquakes."

The volcano is continuously monitored, and the team hope that what they learn from these experiments can help them better interpret data gathered before past eruptions.

Ultimately, the idea is to combine all the signs and signals together - geochemistry clues, seismic movements, crater lake level and temperature changes - to further develop their model of Ruapehu's inner workings.

One day, they hope to accurately predict - with enough warning - real volcanic activity.

In the meantime, they're waiting patiently for the next lab-grown eruption.

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