For hundreds of thousands of years, they’ve sat mere kilometres apart, looming high over the fiery heart of the Central North Island.
While their geological histories are intertwined, Ruapehu and Tongariro each pack their own hidden labyrinth of magmatic systems – and their own explosive records.
Now, virtually everything scientists know about the neighbouring wonders has been condensed into a just-published special review, rounding up nearly 200 publications and 30 years of work by hundreds of scientists.
It’s described a surprisingly wide range of eruptions from the stratovolcanoes, producing everything from lava flows, lahars and fast-moving pyroclastic flows, to ash clouds large enough to cover the most of the North Island.
“This is a comprehensive review of the state of scientific knowledge of Ruapehu and Tongariro volcanoes – a first of its kind for the iconic volcanoes of Tongariro National Park,” said lead author Dr Graham Leonard, of GNS Science.
While the wider Taupo Volcanic Zone has been active for around two million years, the two volcanoes are much younger: the oldest rock at Tongariro and Ruapehu have been respectively dated at 350,000 and 200,000 years old.
Only over recent times have scientists been able to completely understand how the volcanoes had been gradually shaped into rough, undulating cones that are more common features in Iceland.
Leonard described their geological evolution as “a fight between fire and ice”.
As much of the planet had been in various degrees of cooling over the past 200,000 years, the volcanoes had been capped with ice, with vast, valley-filling glaciers fanning out in all directions.
Magma repeatedly erupted through and built up alongside this thick glacial ice, before it eventually melted away to leave the dramatic but messy landforms we see today.
“They were never nice, simple cone volcanoes in the first place,” he said.
“These forms contrast with Ngāuruhoe, which is an almost perfect cone, because it is young and formed after the ice retreated about 10,000 years ago.”
Large valleys such as the Whangaehu, which drains Te Wai ā-Moe/Crater Lake – and the Mangatepōpō that first greets trampers walking the Tongariro Alpine Crossing track – weren’t carved out by glaciers, but formed between lava flows built up next to the ice.
“The thin lava flows from the younger Ngāuruhoe that trampers walk across on the crossing are the first lavas that were able to get into the Mangatepōpō valley bottom,” he said.
“Rocks on the top of Tongariro, next to Red Crater and all the way across to Tongariro’s high peak look like those you would find in lakes or streams, not what you would expect at the top of a steep mountain.
“They formed when lava erupted into ice and meltwater lakes that were trapped in the ice,” said review co-author Rosie Cole of Otago University, whose PhD study focused on this process.
As for the volcanoes’ fiery innards, years of geophysical studies and chemical analysis have helped scientists form a clearer picture of the dense systems within.
“There is probably a Swiss cheese of small magma chambers down there supplying heat that creates the present geothermal features, and slowly cooling until they are rejuvenated periodically from deep in the crust.”
Each of the volcanoes had separate plumbing “narrowly focused” under each mountain – much unlike the sprawling magma systems that drive huge caldera volcano systems found from Taupō to Tarawera.
Both volcanoes have generated eruptions that could have been calamitous had they happened today.
But the most frequent tended to be smaller blows, striking on scales of years to decades.
Many of them – even Ruapehu’s famous 1995-96 episodes, and its surprise 2007 eruption that threw ash, rocks and water across the summit area – weren’t even large enough to feature in the geological record.
Unlike quick-fire, steam-driven blasts like 2007, the larger, magmatic eruptions were often preceded by warning signals.
They were also likely to drag out over long periods of unrest and eruption – although trying to forecast such scenarios in the future was made difficult by the fact there so few historic examples to draw on.
Instead, scientists have turned to geological remnants to reconstruct a wide range of large, ancient blows, and even estimate how much tephra had been spewed out.
“The detailed work in recent mapping and geology has led to a detailed understanding of the life histories of both volcanoes,” he said.
“This means we can better estimate the long-term chances of eruption at different scales.
“We also have a better picture of the eruption processes and therefore the types of hazard during different types and scales of eruption.”
In some cases, deposits left by violent pyroclastic currents had offered enough clues to infer major eruptions.
But these high-speed surges of hot gas, ash and rock could be triggered by even small explosive events – and Ruapehu’s skifields were well within hazard zones.
During Ruapehu’s 1945 tantrum, a pyroclastic flow was recorded among a gamut of other effects – notably the forming of a tephra dam that collapsed eight years later, causing the tragic Tangiwai rail disaster.
More than half a century on, the review noted the national park’s popularity with tourists meant that not all risk could be avoided, and that there’d “always be some probability of adverse impacts including severe injury and death”.
“Historic activity at both Ruapehu and Tongariro remind us of the ever-present chance of sudden onset steam-driven eruptions,” Leonard said.
“These are at the smallest end of the eruption spectrum, but due to their sudden nature they present an ongoing and challenging hazard.
Through the GeoNet programme, GNS Science experts keep a constant watch on the volcanoes using earthquake and ground deformation data, chemistry analysis, satellite remote sensing and web cam monitoring.
Weeks before Tongariro’s midnight eruption on August 6, 2012, scientists had observed an increase in quake activity and tell-tale chemical changes that prompted its Volcanic Alert Level to be raised.
It proved Tongariro’s first event other than from Ngāuruhoe in more than a century, and played out at the same crater that was formed during a large eruption in 1869.
Another eruption 23 years later belched an immense quantity of steam, mud and boulders, and ejected material rose 600m to 900m, before rushing down the mountainside.
Leonard pointed out that GNS Science didn’t make calls on how to respond to such threats, but provided the scientific analysis needed.
“The Department of Conservation (DoC) is the primary risk manager in Tongariro National Park, and they have plans in place to respond to changes in volcanic unrest or eruptions,” he said.
“DoC regularly conducts lahar warning system exercises, to test the system. With support from GNS scientists, DoC observes how well people hear the warning, and how quickly they move to safety.”
Even after a mass of work, and new insights gleaned through sophisticated new technology, Leonard said there was much scientists still had to learn about the volcanoes.
“We still don’t fully understand how the wide plain of lahar and river deposits around the volcano relates to times of eruption,” he said.
“It looks like some of the thickest lahar deposits have come down from ash and lava deposited on to glaciers during periods of widespread ice – but this still needs more work.”
He and colleagues were also eager to discover why Tongariro was traversed by more active faults than Ruapehu – and how this quake-making network interacted with the magma system.
“It looks like the faults may help spread out the places that magma erupts, thus the wider range of vent locations across Tongariro,” he said.
“New geophysics in the future will hopefully help to narrow down more precisely where individual magma chambers are, to then improve our understanding of how they then erupt.
“The range of datasets we can collect to understand what’s going on inside these volcanoes is growing steadily, as is the back catalogue of data we’ve collected.”
In the coming decade, scientists may even be able to build models of the volcano plumbing system as a whole – something that could help understand its state, and perhaps even some of the situations that it’s more likely to erupt in.
“Above the ground, New Zealand scientists are working to better model the probabilities of eruption, the different eruption styles, the hazards from those styles and what the impacts might be at different distances,” Leonard said.
“In future, there is an opportunity to bring all of these aspects together to further support DoC and emergency managers to understand, communicate and manage the risk from eruptions.”
The new review, published in the New Zealand Journal of Geology and Geophysics, comes ahead of the next International Association of Volcanology and Chemistry of the Earth’s Interior global volcanology congress, planned for Rotorua in 2023.
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