SUPERVOLCANO

One of the deadliest forces on Earth is waking up. Are we prepared?

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 is 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 13-km wide giant crater, or caldera, formed following colossal eruptions during prehistoric times. It is 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. Its biggest eruption, which happened 36,000 years ago, was not quite large enough to qualify as 'super', but let's not quibble. It was still Europe's greatest volcanic blast in at least 200,000 years, dumping ash across the Mediterranean region and as far as Russia, and spawning a bitter volcanic winter across eastern Europe, with temperatures reduced by up to 9°C.

There have been plenty of smaller eruptions since the last big one 15,000 years ago, the most recent 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 four metres, 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 – 4 cm a year, accompanied by periodic swarms of small quakes. As I write this, the area has just been struck by a magnitude 4.4 earthquake, the biggest in 40 years, and the latest of more than 1200 seismic events in the last several weeks. Residents fled their homes and many are camped outside in fear of further shaking.

While there have been earthquake swarms before, alongside ground swelling, there is concern that this time things are different. Professor Christopher Kilburn, an expert on the volcano based at UCL, told me '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'. Prof. Kilburn also pointed out that the uplift was caused either by molten rock or the gas escaping from it, but it was not possible 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 for the end of this month.

Campi Flegrei might erupt sometime in the months to years ahead, or it may return to its slumbers, at least for a time. But it is 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 the magnitude of past eruptions. Of these, the Yellowstone Supervolcano is by far the best known and is rarely out of the news as the ground rises and falls, and small earthquakes periodically shake the region. Focus of numerous television documentaries, and at least one drama, it hogs centre stage whenever there is a debate about where the next super-eruption will tear the crust apart. Yellowstone has hosted three in all over the last couple of million years, the last 640,000 years ago, and each has left behind a caldera that dwarfs that beneath Pozzuoli, the biggest more than 80km across. Long Valley caldera in eastern California, formed in a colossal eruption three-quarters of a million years ago, has also been restless since 1980, and is recognised by the USGS (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 1,000 km³ of magma in the form of ash, rock and debris. Others include Taupo in New Zealand and Toba in Sumatra. There is a lot we know about supervolcanoes, most notably that their biggest eruptions are capable of obliterating everything within a radius of at least 100km, 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 that they are 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 is also, however, much that we are 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, Professor Shan de Silva of Oregon State University. Campi Flegrei, Yellowstone, Long Valley and Toba are all currently in the resurgence stage during which ground deformation, earthquake swarms and small-scale eruptions are all commonplace. Ultimately, once a sufficient volume of magma has been assembled, the whole thing starts over. But plenty of questions still need answering. How are such huge magma bodies kept hot for the many thousands of years needed for them to accumulate, and what is stopping the magma coming out in dribs and drabs, rather than in one go? Probably most important, from a disaster preparedness point of view, what – after an enormously long build-up – actually triggers the eruption, and how can we spot the warning signs.

At last, it does seem as if supervolcanoes are beginning to give up more of their secrets, many of which have been decoded by Prof. 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 was cooler and brittle, so paths aren't easily opened to the surface. Eventually, however, once the magma body has become so bloated that it's volume is between 10,000 and 100,000 cubic kilometres 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 downwards under gravity, acting – according to Prof. 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 do remain, and they are big ones. What warning signs can we expect to see when a supervolcano is building towards an eruption. And how can we tell if a coming eruption is going to be 'super' or something smaller. After all, not all eruptions at supervolcanoes are colossal. It is 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 is also possible to estimate how much of a magma body is molten. For Yellowstone's magma reservoir, for example, the figure is between 16 and 20 percent. Whether or not all of this would come out in one go, however, depends on any number of factors, key amongst them being that all or most of the molten material needs to be together in one place, rather than forming a number of scattered and disconnected masses.

Even common-or-garden volcanoes never erupt without warning signs. 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 that can be detected using seismometers.

Because 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 three-quarters of a million 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 2,800 km³ 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. The 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 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 neighbouring Bolivia, Uturuncu volcano is also very 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 1,000 km² centred on the volcano has been uplifting, attracting considerable interest. Uturuncu has never hosted a super-eruption before, and its last eruption was a quarter of a million 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 very early stages of a build-up to a future super-eruption. If this is the case, the chances are we will 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 1400, 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.

An illustrated version of this article was published on July 9th 2024 in BBC Science Focus magazine