Reading the comments to Spica’s latest topic, it’s obvious that most people seem to think that in order to be truly dangerous, a volcano must have a potential for a very large, caldera-forming eruption with an exceptionally high VEI, preferably at least a VEI 7. I think it’s high time we had a little chat and sorted out what is the most likely type of volcano and eruption to be our “killer volcano”.
To use an analogy, you would be just as dead if a terrorist’s home-made pipe bomb went off next to you as you would be from a megaton thermonuclear device dropped on your home town. Even if we can’t exclude the possibility of the latter ever happening, it hasn’t happened yet and seems rather unlikely to do so in the foreseeable future. To date, however, thousands if not tens of thousands of people have died in terrorist attacks where, relatively speaking; crude explosive devices have been utilised. Thus you are far more likely to be killed by a device made using primitive black powder than by way of fission of Uranium atoms or fusion of Deuterium nuclei. In spite of this, it’s a safe bet that you’re far, far more scared of nuclear weapons than you are of fireworks or the bags of ammonium nitrate fertiliser easily obtainable from your local ironmonger’s. It’s human nature.
The same is true of volcanic eruptions. You are far, far more likely to be in the wrong place when a very small eruption takes place than from a caldera-forming event. Roughly speaking, “mega-colossal eruptions” (VEI 8) seem to occur about once or twice every 100,000 years. “Super-colossal eruptions” (VEI 7) happen once every one to five thousand years. Last century, there were three “colossal” VEI 6 eruptions, albeit at the low end of the scale, but looking back at the records it seems to be unusual. About one VEI 6 every 100 years seems to be the norm.
At the other end of the volcanic spectrum there are hundreds if not thousands of small explosive eruptions (VEI 0 – 1) every year. These can terminate you just as efficiently if you’re unfortunate enough to be too close to one. Or rather, far more likely as the probability of coming too close, both temporally and spatially, to such a volcano is vastly greater. There are several VEI 2 eruptions every year and there seems to be at least one VEI 3 or “severe eruption” as well. VEI 4, “cataclysmic eruptions”, are slightly rarer but still there are some three to five every decade while the “paroxysmal” VEI 5 eruptions seem to happen once every 10 to 20 years.
Just work out the numbers! For every caldera-forming eruption (VEI 6++), there will have been more than 5 million smaller eruptions. With human proliferation being what it is, quite a few of those will have occurred in locations where multiple factors combine to put a great many people at risk. The two greatest volcanic disasters of the 20th century – Saint Pierre, Martinique, 1902 (Mt Pelée, ~30,000 fatalities) and Armero, Colombia, 1985 (Nevado del Ruiz, ~23,000 fatalities) – were not very large eruptions at VEI 4 and VEI 3 respectively.
From this, it ought to be clear that we need not worry about the very largest eruptions as they’re not likely to happen during our lifetime, nor during those of our very distant descendants, even if we were to live relatively close to a restless caldera. What we should seriously worry about though are the smaller explosive eruptions, especially if you live in a city built inside an active volcanic complex such as Auckland, Rabaul or Naples. Let me introduce you to my nightmare scenario!
Five hundred years ago, in 1538, the ground north of Naples around Pozzuoli inflated an astonishing six metres in only seven days. On the seventh day, there was a seven-day long eruption that resulted in the formation of the 123 metre high Monte Nuovo. Even if the onset was dramatic, there had been signs of unrest long before. As early as 1502, residents of Pozzuoli noted the emergence of new land from places formerly occupied by the sea. From the early 1530s, this uplift was accompanied by unusual seismicity that reached a first climax in the spring of 1534. During the next four years, seismic activity continued at a lower level until there was a dramatic increase during September 1538 which culminated with the Monte Nuovo eruption beginning on the 29th. The rapid uplift recorded, six metres in seven days, is not unique to Campi Flegrei. The same remarkably large rapid inflation, six metres, was noted during the night, yes night, before the 1994 eruption of several volcanic vents including the Tarvurvur and Vulcan stratovolcanoes of the Rabaul Caldera, Papua New Guinea.
This illustrates that even if the build-up to the eruption lasted more than three decades, it was only during the last seven days that the signs, primarily the rapid and extreme inflation, became so alarming that modern-day volcanologists would have had a chance to persuade local authorities that an eruption was imminent and that an evacuation was necessary. Even with the Aquila disaster in mind, volcanologists would probably not dare alert authorities on “Day One” of rapid uplift. At what point would you ring the bell? After the total inflation reached 80 cm on Day One? 160 cm on Day Two? 210 cm on Day Three? Somehow, I doubt that the order to evacuate would be given even two days before the onset of an eruption and if it were as rapid as at Rabaul, it is doubtful whether even a public warning would reach the unfortunate inhabitants, much less a full-scale, orderly evacuation be initiated.
“The crater of Astroni is part of the volcanic complex of Agnano and of the bigger caldera of the Phlegrean Fields. A treasure that is just a few steps from the urban centers of Agnano and Pozzuoli. Ferdinando D’Aragona in the ‘400’s made the area a royal hunting preserve. During World War II it was a deposit for weapons and finally, at the end of the 90’s, a WWF oasis for protection and shelter of the resident and migratory fauna. The crater’s bottom is situated 10 meters under sea level and contains three bodies of water: the big Lake, the small Cofaniello and the big Cofaniello. Chestnuts, elms, oaks and other plants, typical of elevated areas, have conquered the crater’s bed and then change into Mediterranean scrubs when going higher up the crater’s sides. It feels like when one is “going down to the mountains”. The Astroni include, infact, a very particular inverse vegetation that benefits from a microclimate and the volcanic nature of the area. The fauna is rich and complex as well. Many birds populate the woods among which the red-headed woodpecker, the oasis’s symbol, blackcaps, robin redbreasts and chaffinches. There is also an enormous variety of butterflies as well as the moretta tabaccata, one of the most rare species of duck in Europe.”
Very idyllic, but as recently as 4.8 to 3.8 thousand years before present, there was a series of seven extremely nasty eruptions a scant ten kilometres (six miles) from modern Naples city centre. The total output is perhaps not very impressive, a scant 0.45 cubic kilometres DRE (dense rock equivalent) or approximately two to two and a half cubic kilometres of ash/pumice. What is truly frightening is the manner of these eruptions.
The activity of the volcano was dominated by explosive, mostly phreatomagmatic eruptions, with only subordinate lava effusion, meaning that most of the material ejected was in the form of ash and pumice with a very high water content. To make it even nastier, the eruptions seem to have been intermittent meaning there was not a sustained blast lifting the water-rich, super-heated mixture high in the air, but the eruption column would regularly collapse sending out vast pyroclastic base surges in all directions, many kilometres from the vent. Were this to happen today and insufficient warning given for an effective evacuation to have taken place, hundreds of thousands would be in the direct path of 600 – 1,000 centigrade hot pyroclastic flows as at Herculaneum in 79 AD. The prospect of survival would be very bleak indeed.
With a restless caldera such as Campi Flegrei that has suffered two episodes of great uplift and also sustained periods of earthquake activity during the last 40 years, this is perhaps the greatest volcanic threat to the largest number of people anywhere on Earth at present. It may never happen, certainly, and even if it did, the chances are that it would be in a different and not so dangerous location of the caldera and that the eruption a repeat of the “gentler” Monte Nuovo type. But given the history, recent and past, of Campi Flegrei, it could happen much sooner than never. Let us fervently hope it does not!