The Oruanui eruption blasted masses of volcanic material into the environment, carpeted the country in ash, and carved out an enormous crater today partly filled by Lake Taupō. Photo / Peter Wilton
The world’s most recent “super-eruption” was gigantic enough to form Lake Taupō and spread ash as far as Antarctica - yet didn’t appear to plunge the planet into a long-term cooling episode.
That’s according to new research drawing on prehistoric traces unearthed from an ancient Auckland basin - and challenging long-held assumptions about the climatic fallout of Earth’s very biggest blows.
Such was the scale of the Ōruanui event at Taupō 25,500 years ago, that it was 100-fold larger than even the 1991 eruption of Mt Pinatubo in the Philippines - noted for cooling the global climate for a year afterward.
With successive studies, scientists have been revealing more about the cataclysm, one of just a few dozen ever to have topped the eruption-measuring Volcanic Explosivity Index.
Victoria University researcher Stephen Piva said super-eruptions were known to eject as much as 1000 cubic kilometres of material with a single eruption – and Ōruanui spewed enough out to leave much of the country carpeted in thick deposits of ash.
Yet, there was much more to learn about how it transformed our ancient environment, and influenced the diversity of plant and bird species that we see today.
“We know that past super-eruptions have changed the New Zealand landscape, with pyroclastic flows and thick ash deposits being responsible for widespread destruction across the North Island,” Piva said.
“But it’s not known how severe these impacts were - or how long they lasted for - in different parts of the country.”
Fortunately, modern techniques meant scientists could reconstruct entire eruptions – as well as the environment they occurred in – with unprecedented resolution, using geological information stored in intact ash deposits.
In a newly-published study, supported by the Marsden Fund, Piva and colleagues turned to a centimetres-thick layer of Ōruanui ash, preserved in a lake sediment core taken from Auckland’s Onepoto maar.
It also recorded around 150,000 years of environmental and climatic change – enabling the team to analyse any ongoing effects it may have triggered.
“An interesting aspect of this event was that it occurred during the Last Glacial Maximum, where for example, the climate of the Auckland region was drier, windier, and at least 4C cooler than present,” Piva said.
“While volcanic eruptions can be responsible for planetary cooling, the effect of this super-eruption on the climate was unknown.”
Within the deposits themselves, the most important clues happened to be found in chemical changes in sediments and fossilised pollen.
“By noting the number and type of fossilised pollen and spores present, we were able to see how the vegetation responded in the aftermath of the eruption, and the time taken to recover.”
Another key puzzle piece was the traces of ash and sulfur that’d been quite literally frozen in time within Antarctica – and recovered in deep ice cores.
“By looking at changes in ice water chemistry from Antarctic records and the changes in pollen in Auckland, we have been able to examine the extent and duration of climatic response.”
The results were eye-opening.
“Previously, it has been assumed that super-eruptions may have caused long-term cooling on Earth because of the huge volumes of ash and sulfur that are ejected into the atmosphere,” Piva said.
“However, our results define surprisingly subtle and short-lived volcanic impacts that occur over timescales of only years to decades.”
Rather than having forced any permanent climate shift, Ōruanui’s volcanic cooling effects were limited to less than 10 years – comparable to smaller historic volcanic eruptions.
As for its impact on our landscapes, the researchers found the eruption would have partly stripped beech forest canopy trees then blanketing the Auckland site – allowing more small trees, shrubs, grasses and ferns to grow more.
It also led to more wetland and aquatic plants at the site for around 60 years after the eruption, after which, the region’s vegetation had recovered and returned to normal.
Piva said the same techniques in the study could also be used to shed fresh light on two other North Island super-eruptions, known to have occurred at the Whakamaru and Mangakino calderas 340,000 and one million years ago respectively.
For Ōruanui, meanwhile, the study proved the first time that scientists had been able to quantify a super-eruption’s short-term impacts.
“And it shows that these events do not necessarily cause long-term changes in regions far away from the volcano,” he said.
“However, it wouldn’t have been a good time to be around as it took many hundreds of years for the central North Island to recover - and the huge volumes of ash would have been blowing around during the Last Glacial Maximum, in very cold conditions.”
The study comes as scientists have been closely monitoring an episode of unrest at Taupō that began a year ago – and prompted GeoNet to lift the hidden caldera’s volcanic alert level for the first time in the agency’s history.
Yet, Piva said, there were no signals pointing toward an eruption any time soon – let alone a super-eruption.
Meanwhile, scientists have gained major new insights into how another vast caldera volcano – Ōkataina, sprawling out east of Rotorua - might be primed for its next blow.
The system’s most recent eruptive period began around 25,000 years ago, while its last event, at Mt Tarawera in 1886, which buried the world-famous Pink and White Terraces and remains New Zealand’s largest and deadliest eruption in recorded history.
But not all of these eruptions have occurred in the same way: while the Tarawera blow involved basaltic magma, most of its eruptions had been rhyolitic – typically associated with explosive activity.
Mt Tarawera itself had been built up over time by viscous rhyolite erupted from Ōkataina, as had another feature in the caldera, the Haroharo Massif, beneath which scientists recently identified a still-active magma system.
In two recently published studies, Dr Hannah Elms and fellow researchers from Victoria University of Wellington, working under the MBIE-funded ECLIPSE project, set out to find what Ōkataina’s past bangs might tell us about events to come.
“Although quite a lot is known about the deposits of eruptions over the past 50,000 years - the time of the last caldera-forming event - and their chemical compositions, little is known about how long it takes for magma to build before eruptions,” said Elms, now based at Royal Holloway, University of London.
“This is a crucial piece of information for monitoring and preparing for future eruptions.”
The biggest question facing her team was just how long it could take for Ōkataina to shift from its currently-dormant state to being “eruption-ready”.
Answering that question meant finding out how long a batch of magma could sit in the Earth’s upper crust before it was triggered to erupt – but also how swiftly that magma could rise to the surface.
Because these processes typically came with clear signs of unrest – such as earthquakes and ground-swelling – it was assumed that agencies like GeoNet would have plenty of warning before an eruption.
“However, like at Taupō volcano, a lot of unrest will not necessarily result in eruptive activity - especially given that Ōkataina is surrounded by active geothermal systems and faults,” Elms said.
“So, understanding the processes that occurred prior to previous eruptions is key.”
In one study, the researchers analysed materials from those past eruptions to find that magma bodies tended to build up at Ōkataina slowly, over decades to centuries.
This confirmed that magma bodies could sit for long periods in the upper crust, before either erupting or cooling.
They also found these bodies were most likely primed when hotter magma - either basaltic or rhyolitic – was injected into the system from below, which would likely be associated with earthquakes, or perhaps gas being released at the surface.
In another study, they were able to show that magma was most likely to be stored at between 4 to 8km below the volcano, and that the start of the magma’s journey to the surface would likely be slow, taking a few days to a week for the initial stages of ascent.
“Once the magma rise gets fully under way, though, it can rise to the surface in only a few hours, moving at average speeds of about half a metre to two metres per second,” Elms said.
“We found that this rapid final magma ascent is the same for eruptions of different sizes and also for eruptions from different vent sites.
“This is a crucial finding as it shows that once the magma is on the move and about to erupt, warning times may be very short, regardless of the size or location of the event.”
While we could expect to see warning signals, it was difficult to pinpoint just what they’d look like.
“This is an active volcano that has produced plenty of previous eruptions and will likely erupt at some point in the future,” she said.
“When magma starts to move before eruption, we may only get a relatively short amount of notice through elevated unrest.
“This is important for hazard mitigation and shows how rapidly the situation could change.”
The next stage, she said, was to determine whether there was presently a magma body present beneath the volcano, and what the state of the current system was.
While geophysical studies by researchers at GNS have revealed “hot spots” under certain areas of the caldera – such as at the southwestern end of Haroharo - less was known about its eastern and northern parts of its most recently active area.
“Given the nature of past events – which can erupt over an entire vent zone – more geophysical and monitoring work is needed to get a better idea of where a future magma body may be forming.”
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