Italian volcanoes and the difference between hazard and risk
Pompeii. Surely you have seen photos of the destroyed, but perfectly preserved ancient roman city. Even imprints of the inhabitants were preserved in their final moments when a volcanic eruption sealed their fate. This happened almost 2,000 years ago – so, we don’t need to worry about that today, do we?… ( you may hear Geologists chuckle at this… for us, 2,000 years ago is like yesterday).
There are many reasons to visit “Bella Italia” - the food, the climate, the beaches, and amazing cultural heritage. While wine and pizza have a crucial place in any research campaign on Italian grounds, the favourite sights of a volcanologist may differ slightly from yours. From Stromboli, the ever-reliable producer of small-scale photogenic explosions, to “Mamma Etna”, simply put the most beautiful volcano on this planet, to Vesuvius, one of the most dangerous volcanoes in the world, and Europe’s only “Super volcano” Campi Flegrei – Italy has it all!
Ever since its beginnings, Italian volcanoes have shaped European volcanology and it’s not a true volcano conference without Italian accents around. My favourite volcanological term “lapilli” - lava fragments between 2 and 64 mm in size – is Italian, as is the origin of the word “Volcano”.
So, let’s have a more detailed look at Italy’s volcanic inventory, working our way from South to North along its most famous volcanoes…
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Etna
Towering almost 3,500 m over Catania is Europe’s highest and most active volcano: Etna. If you want to escape the burning Sicilian summer at sea level, make your way up the mountain. When the vegetation gradually gives way to the high altitude, you find yourself in a fantastical outer-world landscape, shaped for 60,000 years by lava and explosions, beautifully accentuated with the stark contrast between remaining snow and the freshest, blackest volcanic deposits. Four craters embellish Etna’s summit, continuously reminding visitors of who’s in charge with their rumbling and degassing. The ground under your feet scrunches wonderfully when lapilli and ash of recent eruptions slowly eat away the soles of your shoes, while your eyes try to grasp the wonders of solidified lava flows and bombs surrounding you in hundred shades of grey and black. And then... you come to the edge and stare into the Valle del Bove and your breath is simply taken away. This huge depression that goes all the way from the summit towards the submarine portion of the volcano was formed by successive landslides about 10,000 years ago and is still slowly moving towards the sea today.
And then, Etna doesn’t only erupt from its summit craters. Its flanks are covered in eruptive cones and fissures, and every few decades, a new eruptive fissure opens somewhere on the flanks. Due to their sheer volume and lower altitude vents, lava flows produced in these events have frequently threatened infrastructure and villages in the surroundings. While Catania is usually safe from lava flows due to the distance from Etna’s active zones, it was reached by lava in 1669. Finally, though rather unusual for Etna, even Plinian eruptions can occur. These are the biggest category of explosive eruptions, with eruptive columns reaching all the way up into the stratosphere. In 122 BC, a Plinian eruption from Etna destroyed parts of ancient Catania.
It is this diversity in behaviour and the frequency of activity that make Etna and its ever-changing landscape so fascinating for scientists. Due to the risk involved for the population, IAVCEI (the International Association of Volcanology and Chemistry of the Earth’s Interior) has named Etna one of the 16 “Volcanoes of the Decade” in the 1990s, a project aiming to promote research and monitoring at these particularly dangerous volcanoes. As a result, Etna is one of the best-monitored volcanoes in the world, with its every movement and breath recorded by a multitude of instruments maintained by INGV (Istituto Nazionale di Geofisica e Vulcanologia) Catania. But Etna is also the core of countless international scientific collaborations and many innovative new monitoring techniques have and are being tested here.
Stromboli
The Aeolian islands offshore the Northern coast of Sicily are all volcanic in origin, but the most famous of them is surely Stromboli. This volcano is the name giver for a specific type of volcanic eruption that can be observed all around the world and is now known as a “Strombolian eruption”. Stromboli produces them rather reliably up to several times per hour. These small-scale explosions that throw out ash and larger, in parts still fluid lava fragments, have a big advantage over their more intense relatives on the volcanic explosivity scale: you can go a lot closer! The ability to bring our instruments in direct vicinity to the eruptive vents, and the reliable activity have made Stromboli a natural laboratory for volcanologists and the summit is sprinkled with instruments of all kinds.
Of course, volcanologists must share this beauty with tourists. Until recently, thousands of tourists visited the summit of Stromboli during the summer season in guided tours to watch the eruptions in the darkness. In 2019, Stromboli reminded everyone that a volcano is neither never perfectly safe nor predictable – no matter how well we have studied it, or how many instruments we equip the summit with. In addition to its typical eruptions, Stromboli can also go bigger in so-called major explosions and paroxysms. These events are a lot stronger and produce high ash columns, pyroclastic flows (hot volcanic avalanches), and the widespread fallout of lava bombs, lapilli and ash. Sometimes, they can be predicted. But in July of 2019, a paroxysm surprised everyone. While usually, a few decades lie between these events, Stromboli also broke those statistics with another paroxysmal event in August of the same year. That summer caused one fatality – and the only reason that this number does not carry additional zeros, is that the eruptions took place in daylight, so before the tourists reached the summit. This experience put an end to the summit tours and tourists now must watch the spectacle from further down.
Let’s pause here….
The island of Stromboli (which, in fact, consists of the upper 950 m of the volcanic edifice) is home to only two small villages. Removing the tourists from the summit region massively reduced the volcanic risk on the island. Speaking of risk – there are many terms thrown around when it comes to natural hazards: hazard, risk, threat, danger.... But they do not all mean the same! So, let’s take a step back and define the most crucial terms: A hazard is the actual phenomenon, e.g., a lava flow, a hurricane, or a flood. When we speak about hazard assessments or catastrophe modelling - I.e., when we try to put numbers on things – hazard refers to the probability that the hazardous phenomenon occurs. Risk, on the other hand, is the probability that a certain amount of damage is caused by said phenomenon. The general formula to calculate risk is Risk = Hazard x Exposure x Vulnerability.
Say, you’re in an area with daily flooding. This means that the hazard is very high. If no one lives there and no infrastructure has been built there, however, the floods will not destroy anything – so the risk is low. This is covered by the term “exposure”: the number of assets exposed to the hazard. On Stromboli, exposure is a lot lower than on Etna – at least when you remove the tourists. And now imagine that I build two houses in my flooding area. One is swimming, the other is made of wood and fixed to the ground. Which one will suffer damages? This aspect is covered by vulnerability, which indicates how intense my hazard needs to be in order for the exposed assets to be damaged.
Why is this important? Because we cannot change the hazard. But, with good land planning and building codes (e.g., quakeproof buildings), we can significantly reduce the risk in hazard prone areas. The next volcano will make this even clearer.
Vesuvius
Vesuvius may be the most famous of all Italian volcanoes, as it destroyed but perfectly preserved Pompeii and Herculaneum by covering them in volcanic deposits in 79 AD. This eruption has another name giving story to tell: Pliny the Younger (the nephew of Pliny the Elder, who died in the eruption) documented the eruption in such detail that his letter may well be seen as the birth of volcanology, and we now call these kinds of eruptions “Plinian”. They represent the most explosive category (of historically documented eruption types) that develops huge eruptive columns and widespread fallout of volcanic deposits.
In contrast to Stromboli and Etna, where gasses and ash are emitted from the craters frequently, Vesuvius prefers to take a proper rest between its eruptions, and unscathed vegetation can be found almost all the way up its flanks. But it is by no means extinct! Vesuvius has been active all throughout historical time and its last eruption was in 1944. And while that one was a lot smaller than the one that destroyed Pompeii, Vesuvius still poses a big threat to the population close-by.
Few volcanoes demonstrate the problem of exposure as well as Vesuvius. Over 6 million people live within 100 km of the vent, and the vibrant city of Naples may well turn into a modern-day Pompeii. While modern monitoring techniques will hopefully provide some warning and evacuation plans are in place, Naples is not a prime example of foresightful land planning. The huge risk associated with Vesuvius guaranteed Italy a second volcano in IAVCEI’s decade list.
Campi Flegrei
If you ask people around the Bay of Naples for a volcano, they’ll all point to Vesuvius. But there is something even bigger close-by. Not all volcanoes are shaped like a picturesque cone. Big eruptions are often followed by a ground collapse due to the hollowed magma chamber at depth. Sometimes, only a massive hole in the ground hints to the (super)natural forces that have been at play in the past. “Campi Flegrei”, or the Phlegrean Fields, is such a caldera.
It was the site of huge eruptions in the past, whose volumes came at least close to what is known as a “super eruption”. A super eruption can produce more than 1015 kg (one billiard / quadrillion) of volcanic material - a phenomenon never observed in historical times, but the massive deposits have left sufficient evidence for geologists to recreate horrifying scenarios for our prehistoric ancestors. Campi Flegrei is Europe’s only (potential) super volcano. The caldera is still active, the last smaller eruption occurred in 1538.
Furthermore, the area is characterised by active degassing and ground deformation – evidence of magmatic processes at depth. Should another eruption like the “Campanian Ignimbrite” (39,000 yrs ago) or the “Neapolitan Yellow Tuff” (15,000 yrs ago) happen today, the consequences would be dramatic for Italy, Europe and potentially around the globe.
This concludes our quick – and non-exhaustive – trip to Italy. Coming up next in our volcano article series: What are the different kinds of volcanic hazards, what damages do they cause and how can we protect our society against them? We also have several additional geographic spotlights planned – let us know your favourite volcano and we’ll work it in!
Your friendly neighbourhood volcanologist
Dr. Karen Strehlow
Volcanologist and Catastrophe Modeller at Mitiga
Further reading:
https://volcano.si.edu/search_volcano.cfm
https://www.eskp.de/en/natural-hazards/the-ultimate-natural-hazard-supereruptions-935936/
