Should We Fear A Supervolcano Outburst?

Naples’ slumbering supervolcano has started rumbling again. Authorities are alarmed. The real and imagined dangers of a supervolcano eruption.

The volcano in the centre of the picture is Mt. Vesuvius. The Phlegraean Fields are near the left edge. Naples is in the thick of it. The red areas are unpopulated. NASA/METI/AIST/Japan Space Systems,
and U.S./Japan ASTER Science Team

Naples is preparing itself for a fiery apocalypse. In October last year, the civil defence presented a new volcano-evacuation-plan: Using 500 coaches and 220 trains, authorities want to bring 700,000 people out of the city — in only 72 hours. Almost half of them, 360,000, live not only near the infamous Vesuvius but in fact sit atop Europe’s largest supervolcano, the Campi Flegrei caldera.

Almost 3 kilometres below the surface lies the volcano’s magma chamber. While that doesn’t mean the residents get free underfloor heating, they surely get rising sea levels: When the magma pocket swells, it puts the surrounding rocks under pressure, which makes them brittle. This is the moment the ground rises. After some time, the magma then subsides.

Until now, many geologists thought that when the molten rock in the chamber eventually slumps down, also the surrounding rocks’ stress would relax.

But in a new study published on Monday in Nature Communications, volcanologists Christopher Kilburn, Giuseppe De Natale and Stefano Carlino beg to differ: From drilling 501 metres into the volcano, they know the stress slowly builds up in Earth’s crust:

When tension becomes too high, the brittle rocks crack. The magma then flows into the crevices, where it solidifies and again builds up pressure. Every uplift/subsidence-cycle weakens the crust, until it is so thin-walled that one more swelling could tear it open — you can guess what follows.

The Campi Flegrei have only erupted once before. In 1538, the volcano spat out enough magma to sand up the 123-metre-high hill Monte Nuovo. That eight-day-long eruption did not come unheralded: The uplifts antedating the eruption lifted the ground by about 17 metres.

After that, the Campi Flegrei slumbered for 400 years.

But in the 1950s, the Campi Flegrei woke up and started rumbling again. Since then, the ground underneath the supervolcano has risen by three metres. But uplifts are not the only indicator that helps geologists like Kilburn, De Natale and Carlino understand better how volcanoes tick.

A myriad of small-scale earthquakes often announced and accompanied the awakening of volcanoes. Recently, the volcanologists recorded 26,000 micro-earthquakes with a magnitude no greater than 2.5 on the Richter scale. Nevertheless, these quakes further rupture the rock that surrounds the magma.

Atop all of these quakes and rumblings sits the town Pozzuoli, 20 km west of Naples, 83.000 residents. If the crust breaks and spews lava and ash, the blast would obliterate Pozzuoli immediately and kill 500.000 more people that live in its vicinity. But the entire population of the Metropolitan City of Naples (that’s three million people) would be at risk of annihilation.

That’s a large-scale catastrophe, and it can’t get worse than that, right?

It can.


In fact, a supervolcano outbreak could theoretically wipe us out for good — but not with a super blast that incinerates us all. The reason why the Campi Flegrei volcano qualifies as a supervolcano is its size: It can spew more than 100 km³ of volcanic material, along with a smoke column that reaches higher than 20 km into the atmosphere.

There is a somewhat objective scale to classify volcanoes based on how much material they could eject — the Volcanic Explosivity Index VEI. Volcanoes with a VEI of 7 (Campi Flegrei) or 8 (Yellowstone) are supervolcanoes. For comparison, Mount Vesuvius is only a 5, and the ever-active Stromboli is a 2 on the VEI.

Volcanic Explosivity Volume Graph. The scale is logarithmic: If the VEI number increases by one, the amount of volcanic material increases tenfold. Credit: US Geological Survey

The problem with high-reaching ash pillars huge is that they could veil the stratosphere with layers of soot particles that darken the sky and block the sunlight. In such a volcanic winter, crops like maize, wheat and rye would stop growing — and we’d run into trouble feeding the world.

A global famine seems like a natural implication. Prices would skyrocket. Wars over the remaining food stockpiles could take down the world’s economy. Existential risk researchers consider this scenario an X-risk, that is one of the low-probability but high-impact events that could annihilate humanity.

X-risk researchers study scenarios like supervolcanic winters at institutions like the Future of Humanity Institute (FHI)in Oxford or the Centre for the Study of Existential Risk (CSER) in Cambridge. Against anthropogenic X-risks, we can come up with policies and control how we design and build them (AI, for example). But we’d be helpless when a supervolcano erupts. The global temperatures would cool off for years.

Sounds a bit far-fetched?

Not quite. Two centuries ago, a much smaller-scale eruption has given us a foretaste of how a supervolcanic apocalypse might unfold. A volcano with the same explosive potential as the Campi Flegrei (VEI-7) engulfed the world in a volcanic winter for months:

The Indonesian Mt. Tambora’s outburst in 1815 was the most violent in the last 10,000 years. It spew 50 km³ of tephra (=whatever ejects through a volcanic pipe, like gas, ashes and lava). The death estimates vary a bit, but about 100,000 people asphyxiated, burnt to ashes or drowned in the tsunamis that followed the eruptions.

But people also died 13,000 km away, although with some delay: The Tambora’s ash column had injected ash particles (sulphur dioxide aerosols) into the stratosphere where they dimmed the sunlight for months, acting as a giant solar filter. Temperatures plummeted by up to 0.7 degrees Celsius. The summer of 1816 was canceled: Snow, heavy rainfalls and frosty cold killed off most of the crops in Europe and North America.

This was the “Year Without A Summer”: Crops froze, harvests failed and food prices went through the roof (oats cost 760 per cent more in the US). The people who didn’t starve to death rioted. Because the monsoon was delayed, China and India suffered from floods, and Cholera spread easily.

Yet, it could have been much worse. Twice as worse, to be exact: The Tambora only ejected half of the tephra he could have spat out.

Now, back to Naples and the Campi Flegrei. Because so many publications claim that a volcanic apocalypse must be imminent and we better prepare for doomsday, I decide to call up volcanologist Giovanni Chiodini and clarify this once and for all.

A simulation of what a Campi Flegrei eruption could look like, created by INGV geologists in 2015. Note how little time it takes the lava to spread out. Credit: INGV via Youtube

Chiodini is the research director of the Italian National Institute of Geophysics and Volcanology (INGV) and has published a study in December that shows how gases (steam and carbon dioxide) emanating from magma render the surrounding rocks brittle, hence increasing the odds of an eruption.

What’s more, some scientists think a huge eruption at the Campi Flegrei 39,000 years ago killed off an entire human (sub)species, the Neanderthals. But that theory is highly debated.

As for a new volcanic winter, Chiodini tells me that the question is not only if the Campi Flegrei erupt in the future, but what the scale of such an eruption is.

A volcanic winter caused by the Campi Flegrei? In the near future, that’s only a remote possibility. The whole scientific community expects that IF the Campi Flegrei become active again, it most probably won’t be a super-eruption.

Alright, so he isn’t worried about a volcanic winter, at least not from the Campi Flegrei. But how would he know there won’t be a super-eruption? Chiodini says there is no definitive answer, but:

We do look at indicators like rock deformations and earthquakes. But essentially we base our assumption on the eruptions of the last 10,000 years: Most of them didn’t produce more than one km³ of volcanic material.

Before he hangs up, Chiodini laughs and tells me to make sure my piece won’t be alarmist. I will.

So, a global apocalypse caused by a supervolcanic winter seems somewhat improbable. Noted.

Yet, it makes a lot of sense that Naples’ civil defence tests its 72-hour-evacuation plan for a volcanic outburst, even if the plan is not tailored to a super-eruption. After all, not one but two volcanoes imperil the city. One of them are the Campi Flegrei.

The other one, Mt. Vesuvius, although no supervolcano, has shown that it has the potential to be a quick killer almost 2,000 years ago. In 79, it took the Vesuvius’ pyroclastic surges (fast-moving, hot ash clouds) only 18 hours to reach the Roman town Pompeii, 18 kilometres distant.

If that happens again, a 72-hour evacuation comes too late anyway.