Mount Vesuvius: The Monster Behind Pompeii
When Mount Vesuvius cataclysmically erupted in 79 A.D., the ancient Romans didn’t truly understand volcanoes. When their world plunged into suffocating darkness it was an act of the gods, not a side effect of subduction and plate tectonics. We like to think we’ve come a long way since that time, despite insurance companies still invoking the “act of God” logic, but have we? Our scientific knowledge of the Earth’s processes has come forward in leaps and bounds, yet we still do not have the ability to predict a volcanic eruption with certainty and volcanologists can get things dramatically wrong, with deadly consequences. In this post I don’t want to ramble on about the ruins of Pompeii (there are a million blogs out there already, and a gallery of my own attached). Instead I want to talk about the sleeping giant that ruined this city in the first place, because up to three million people are living within its danger zone today. It’s not something every traveller thinks about, but when you travel around Pompeii you are in the shadow of one of the most dangerous volcanoes on Earth.
The Plinian* eruption of 79 A.D. was a display of raw, devastating, hell-like, savage, deadly, earth shaking, fear provoking power. I hope I’ve used enough adjectives to make myself clear, because to be honest I’m running out of worthy synonyms. This eruption of Mount Vesuvius was powerful enough that a thick column of gas, ash and pulverised rock extended into the stratosphere, generating thick blankets of ash that plunged Pompeii into darkness, and then engulfed it whole. Don’t just read that; think about it. This column was greater than twenty-five KILOMETRES high!
*Interesting side note: Massive eruptions forming ash columns that extend into the stratosphere are named Plinian eruptions after a fellow named Pliny the Younger observed Vesuvius erupt, destroying Pompeii, and described it in a letter. Sadly his uncle, Pliny the Elder, was killed in the eruption.
If a bit of ash isn’t enough to scare you, consider the real evil menace of a volcanic eruption: pyroclastic flows. Personally I don’t think that the term pyroclastic “flow” truly captures their nature. “Flow” conjures up images of something peaceful, like hippies or flowers or serene flowing water. These “flows” are HOT and FAST, reaching up to 800°C and travelling up to 720 km/h.
You do not out run this. You die. It’s terrifying.
When people say, “go with the flow”, they certainly don’t mean a pyroclastic one… unless they’re advising you to go on a frenzied rampage where you brutally murder all your friends, your family, the family next door, and then yourself… In which case you should probably not take their advice.
If what I’ve said so far just isn’t enough for you to truly grasp the force of the eruption, just use your imagination in the picture of Vesuvius at the top of the page. Extrapolate the two outer slopes of the mountain upwards so they meet in the air. That is the height of Mount Vesuvius before it’s caldera collapsed. That amount of power is petrifying, but is now largely ignored as we wander around taking happy snaps of the Pompeii ruins, because Vesuvius has not erupted since 1944.
Contrary to the assurances of a drunken backpacker I met, Vesuvius is not a dormant volcano. Since its last eruption in 1944 it’s been in a phase of quiescence, but quiet does not equate to dormant. Let me put it in a way a drunken backpacker might understand. When you go to the doctor and he asks “are you sexually active?” the answer is yes, even if you haven’t “blown” for a month. It’s the same concept for a volcano, just over more time. The concept may be simple, but the consequences are more difficult.
How long will this abstinence last? What sort of event will int-erupt the silence?
That’s the job of the Vesuvius Observatory – to constantly monitor the volcano for signs of a re-awakening. They hope to be able to predict an eruption based on seismic activity, gas emissions, ground uplift, geophysical anomalies and other impressive sounding sciencey things. In the event of a re-awakening the Italians have also developed an evacuation plan that you can read about here. The plan divides the area around Vesuvius into different risk zones, with area immediately around the volcano termed the red zone and considered the most at risk of deadly pyroclastic flows and therefore the first to be evacuated if an eruption appears imminent. This zone alone consists of over half a million people; the property values there must be higher than a Plinian ash column.
There are a few tiny issues with the risk map and evacuation plan. Firstly, prediction of volcanic behaviour is hard and history shows us that predictions and maps can be wrong. A good example is the story of Mount St. Helens. In April of 1980 a hazard map was produced, but when the volcano erupted in May of that same year the surge was much larger than anticipated and killed 58 people (Baxter et al. 2008). Secondly, the Italian plan is modelled on the occurrence of an eruption similar to a smaller sub-plinian eruption that occurred in 1631. What if Vesuvius erupts like it did in 79 A.D.? There is only a 4% chance of that happening (Baxter et al. 2008), but deaths would probably be in the 10,000’s. Is it okay to have no plan in place for this dramatic and deadly scenario? (Hint: no).
Admittedly these scenarios are not easy. Is it sensible to order a mass evacuation based on the “worst case scenario”? This would involve the evacuation of 3 million people. Is such a large-scale evacuation response logical for such a small probability of occurrence? Alternatively, consider the costs and economic down time of such a mass evacuation if there is ultimately no eruption, never mind how much of a pain in the ash it is. That leads to angry residents and politicians and probably a few red-faced volcanologists (who I very much feel sympathy for). I acknowledge that it’s complicated, but while false evacuations are undesirable, the alternative is worse: many people die.
I hope by reading this that if you ever visit Pompeii you not only consider the 16,000 people who died to create this snapshot of history for you (such ash-ame), but all the people who are living day to day with the fear of an event of that magma-tude happening again. The three million people living near Mount Vesuvius have so far escaped its wrath, but that is no reason to become complacent. If an eruption happens, nobody will be lahar-ghing.
My Pompeii Gallery:
Baxter, P.J et al. 2008, Emergency planning and mitigation at Vesuvius: A new evidence-based approach, Journal of Volcanology and Geothermal Research, vol. 178, pp 454-473