Something volcanic is stirring close to the Italian city of Naples and it has nothing to do with the looming presence of Mount Vesuvius. Another volcano lurks close by, one that’s much harder to spot, but potentially far more dangerous than its neighbouring cousin. And it’s getting restive.
Rather than forming a peak, the Campi Flegrei volcano is marked by a 13km-wide (8 miles) giant crater, or caldera, formed following colossal eruptions during prehistoric times. It’s the closest thing we have to a supervolcano in Europe and it sits directly beneath the port town of Pozzuoli, just to the west of Naples.
Campi Flegrei's biggest eruption, which happened 36,000 years ago, wasn’t quite large enough to qualify as ‘super’, but it was still Europe’s greatest volcanic blast in at least 200,000 years. It dumped ash across the Mediterranean region and spawned a bitter volcanic winter across eastern Europe, with temperatures reduced by up to 9°C (16.2°F).
There have been plenty of smaller eruptions since the last big one 15,000 years ago (the most recent was in 1538), and the area teems with small craters, vents, hot springs and bubbling pools.
Since the 1950s, the caldera has also been restless, with worrying episodes of uplift raising the town by an extraordinary 4m (13ft) and periods of increased earthquake activity. Understandably, the resident population of 360,000 are on edge and getting more so by the minute.
Since the early years of this century, the caldera floor, and the town, have been rising by 3–4cm (1–1.5in) a year, accompanied by periodic swarms of small quakes. In May this year, the area was struck by a magnitude 4.4 earthquake – the biggest in 40 years and, at the time, the latest of more than 1,200 seismic events over the preceding weeks. Residents fled their homes and many camped outside in fear of further shaking.
While there have been earthquake swarms before, alongside ground swelling, there’s concern that this time, things are different.
“If the ground continues rising for long enough, the crust must eventually give way. The big unknown is how much more stretching the crust can take,” says Prof Christopher Kilburn, an expert on the volcano based at University College London. He also points out that the uplift was caused either by molten rock or the gas escaping from it, but it’s impossible to tell which.
In 2017, he warned that “further unrest will increase the possibility of an eruption,” and urged the authorities to be prepared. They seem to have taken his advice and evacuation exercises are slated to take place this summer.
Restless giants
Campi Flegrei might erupt in the months or years ahead, or it may return to its slumber. But it’s just one of a number of restless volcanic giants across the planet, some of which dwarf the Neapolitan volcano both in terms of size and magnitude of past eruptions.
Of these, the Yellowstone supervolcano is by far the best known and is rarely out of the news. The ground above it rises and falls, and small earthquakes periodically shake the region. The focus of numerous television documentaries, and at least one drama, it hogs centre stage whenever there’s a debate about where the next super-eruption will tear the crust apart.
Yellowstone has erupted three times in the last 2.1 million years, most recently 640,000 years ago, and each time has left a caldera dwarfing that beneath Pozzuoli, the biggest being more than 80km (50 miles) across.
The Long Valley caldera in eastern California formed in a colossal eruption 750,000 years ago, has also been restless since 1980, and is recognised by the United States Geological Survey as posing a very high threat.
Yellowstone and Long Valley are two of 20 or so volcanoes that are known to have sourced super-eruptions in the past. That is, explosive blasts that eject at least 1,000km3 (240 miles3) of magma in the form of ash, rock and debris. Others include Taupo in New Zealand and Toba in Sumatra.
There’s a lot we know about supervolcanoes, most notably that their biggest eruptions are capable of obliterating everything within a radius of at least 100km (62 miles), while the prodigious quantities of sulphur gases lofted into the stratosphere can bring about episodes of serious global cooling, known as volcanic winters.
We know they’re fed by sticky, silica-rich magma, which means that they can only form in certain parts of the world – within the Pacific Ring of Fire and a few other places, such as Yellowstone in the US interior. There’s also, however, much more that we’re only just starting to understand.
Activity at the longest-lived and best-studied supervolcanoes, such as Yellowstone and Toba, follows a distinctive pattern. The accumulation of a huge volume of magma results in a super-eruption that causes the crust above to collapse and form a caldera as the magma is evacuated. After a period of quiet, the arrival of fresh magma beneath causes uplift of the caldera floor.
This process, known as resurgence, has been termed ‘the after-party after the big dance’ by foremost supervolcano expert, Prof Shan de Silva of Oregon State University. Campi Flegrei, Yellowstone, Long Valley and Toba are all in the resurgence stage, during which ground deformation, earthquake swarms and small-scale eruptions are common.
Ultimately, once a sufficient volume of magma has accumulated, the whole cycle starts over. But plenty of questions still need answering.
Questions like, how are such huge magma bodies kept hot for the many thousands of years needed for them to accumulate? And what’s stopping the magma coming out in dribs and drabs during those years? But probably the most important questions from a disaster preparedness point of view, are what actually triggers the eruption, and how can we spot the warning signs?
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Bloated with magma
It does seem as if supervolcanoes are beginning to give up more of their secrets, many of which have been revealed by de Silva and his team, using an approach he terms supervolcano forensics.
The huge magma body beneath a supervolcano is able to stay substantially molten because fresh magma is constantly being fed into it from below. The reason it doesn’t erupt magma in small batches is because the enormous amount of heat generated by the magma body keeps the rock around it plastic.
This means that the crust above can inflate to accommodate new magma, but it doesn’t fracture (as it would if it were cooler and brittle), so paths aren’t easily opened to the surface.
Eventually, however, once the magma body has become so bloated that its volume is between 10,000–100,000km3 (2,400–24,000 miles3) in volume, the crust above becomes unstable. Circular fractures develop at the surface and propagate downwards until they intersect the magma body, when all hell breaks loose.
The central block of crust delineated by the circular faults pushes down under gravity, acting – according to de Silva – like a plunger, forcing the magma upwards through the fractures and driving it out at supersonic speeds, and with colossal violence, from multiple locations along the fractures. Ultimately, the central block of crust stops subsiding, ending the eruption and leaving behind a caldera.
So far so good, but some key questions remain, and they’re big ones. For example: what warning signs can we expect to see when a supervolcano is building towards an eruption? And how can we tell if an eruption is going to be ‘super’ or something smaller?
It’s possible to work out how big a magma reservoir is, using various geophysical techniques, but only a fraction of the magma comes out, even during the biggest blasts. It’s also possible to estimate how much of a magma body is molten. For example, about 16–20 per cent of Yellowstone’s magma reservoir is molten.
Whether or not all of this would come out in one blast, however, depends on any number of factors – the most important one being that all of the molten material needs to be together in one place, rather than scattered between a number of disconnected masses.
Even regular volcanoes never erupt without warning. Accumulating or rising magma needs to make space for itself, and this can be detected and monitored using geodetic techniques such as GPS and satellite radar interferometry.
Rising magma also needs to break rock to get to the surface, leading to swarms of characteristic earthquakes, detected using seismometers.
Is it time to start worrying?
Since everything about supervolcanoes is at a super-scale, you might think that the build-up to an exceptionally large eruption might also take far longer.
Research undertaken at Long Valley suggests, however, that the build-up to its biggest eruption, around 750,000 years ago, may have been less than a year. From a disaster-preparedness point of view, this is very bad news, not least because a super-eruption can have an enormous impact on global society and economy.
The eruption of at least 2,800km3 (671 miles3) of ash, debris and gas from Sumatra’s Toba volcano around 74,000 years ago, led to several years of severe cooling that, if it happened today, would likely devastate harvests around the world.
Having less than 12 months to prepare for such an event would make it extremely difficult to put in place the measures to stockpile and ration food, which would be required to minimise the impact.
But realistically, do we need to worry about a super-eruption in the near future? They are, after all, extremely rare. The last one devastated New Zealand’s North Island 26,500 years ago and the average return period of a super-eruption is estimated at about 100,000 years. So, can we breathe easy for another 75,000 years or so?
Well, not really. Earth just doesn’t work like that.
This is a long-term average, so there could easily be clusters of super-eruptions in time, separated by much longer periods with none. In fact, the aforementioned Toba eruption means that there have already been two super-eruptions within the last 100 millennia, but this is no reason to think that another won’t happen in the near- to medium-term.
As to where we should look for the next super-eruption, there are, of course, the usual suspects (Yellowstone, Toba and Long Valley); but there are other volcanoes worth keeping an eye on too.
In Chile, Laguna del Maule, which has hosted huge caldera-forming eruptions in the past, has been swelling at an extraordinary rate – up to 30cm (almost 1ft) a year, over the last two decades.
Concerns have been voiced that this might mark the build-up to a major eruption. Monitoring suggests, however, that the volume of new magma being emplaced is nowhere near enough to feed a super-eruption, at least not yet.
In Bolivia, Uturuncu volcano is also restless. This is located in the Altiplano-Puna Volcanic Complex, a big cluster of volcanic centres, underpinned by a massive magma body that has fed several super-eruptions in the past.
Since the 1960s, an area of more than 1,000km2 (385 miles2) centred on the volcano has been uplifting, attracting considerable interest. Uturuncu has never hosted a super-eruption before, and its last eruption was 250,000 years ago. In addition, assuming the uplift is caused by rising magma – rather than gas or hot fluids – the volume, as at Laguna del Maule, is small.
Nonetheless, there has been speculation that we may be seeing the early stages of a build-up to a future super-eruption. If this is the case, the chances are we’ll have quite a wait.
The reality is that the chances of a super-eruption happening anywhere on the planet, within a human lifetime, are around 1 in 1,400, so I wouldn’t lose any sleep over it. Then again, someone wins the lottery jackpot every week at odds of millions-to-one.
The only thing that we can be sure of is that another super-eruption is certain to happen, sometime. And we had better be ready.
About our expert
Prof Christopher Kilburn is an expert on volcanology based at University College London. His work on the study of volcanoes has been published by Earth and Planetary Science Letters, Communications Earth & Environment, and Bulletin of Volcanology.
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