Let’s say you were visiting the Roman town of Pompeii on the morning of August 24, 79 AD. And let’s say you arrived sometime between the hours of 9 and 10 am. That should give you enough time to explore the port town and maybe even grab a loaf of bread at the local bakery (see map below for directions). But it would also put you in Pompeii in time to experience a 5.9 magnitude earthquake, the first of many, and watch the black cloud rise from Mount Vesuvius as the mountain began to erupt 1.5 million tons of molten rock per second and release 100,000 times the thermal energy of the bomb dropped on Hiroshima. All while you were standing a mere 6 miles away.
Your situation would seem challenging–but, surprisingly, not hopeless! When I emailed Pier Paolo Petrone, a forensic anthropologist at the University of Naples Federico II, asking if any Pompeiians survived the eruption, he wrote back to say that many did. “But likely only those who took immediate action.”
Unfortunately, instead of immediately evacuating, some Pompeiians took shelter from the falling ash. This may seem prudent, but it is a mistake. Buy that bread. And get it to go.
You have some time, because the initial stages of Vesuvius’ eruption were not the most dangerous. The pressurized magma beneath Vesuvius contains dissolved gasses, and cracking Vesuvius’ vent has the same effect as cracking a gargantuan can of soda. The hot gases rush out of solution and through the narrow vent. The effect is like a jet engine. The forceful eruption blows lava pieces and gas a few miles high, sucking in and heating surrounding air to create a light, hot cloud lifting high into the atmosphere.
This is good. The cloud is hot enough to melt lead, and the high atmosphere is the safest place for it. The molten rock chunks blasted up and out will eventually cool and fall, and because the wind at both lower and higher altitudes over Vesuvius on August 24 blew south-southwest, they will drop on Pompeii. Though the initial pieces are small and will fall like rain, eventually these pieces of pumice come down with enough size and ferocity to collapse houses–but not yet. You still have time.
But do not linger. Within Mount Vesuvius, a dangerous process is beginning to take place. Because the gassiest magma exits first, as the eruption enters its later phases, less gas is forced through Vesuvius’ vent and its jet loses power. This may sound like a positive development. It is not. Instead of rising miles into the atmosphere, the dense mix of searing hot ash and gas will rise only a few hundred yards and then fall, picking up velocity so that when it reaches the ground, it hugs and flows like a superheated sandstorm moving at autobahn speeds. These “pyroclastic flows” can be 1,800 degrees F, dense enough to suffocate you, and they flow for miles. In the early morning hours of the 25th, a surge will kill everyone remaining in Pompeii. You need to leave long before then.
As to where to go, you have two choices. Mountains block your path to the east, and the Mediterranean Sea blocks your escape to the west. You could try to wait for a boat at the beach, but (a) archaeologists have found a large group of bodies in a boathouse in nearby Herculaneum who appear to have attempted that, (b) the prevailing winds are against you, and (c) tsunamis.
That leaves north, toward the volcano and eventually Naples, or south, toward the town of Stabia. These are your only two viable options Petrone tells me, though he says even then there are issues with both.
Fortunately, none of those issues involve melting in a river of lava.
This fear is natural in any eruption, but generally misplaced. Depending on its composition, lava ranges from 10,000 to 100 million times as viscous as water. This means even the runniest molten rock has the viscosity of room temperature honey. Unless you’re on a very steep slope, you can generally outrun it. Stationary objects like houses can be flattened by these fiery rivers, but “usually people can move out of the way,” says Stephen Self, a volcanologist at UC Berkeley.
Instead, it’s the magma beneath the mountain, and its precise composition, that should deeply concern you. The more viscous the magma, the more gases it contains and the more explosively it will erupt. Unfortunately for you, the magma inside Vesuvius was unusually viscous, which partly explains why its eruption registered as a formidable 5 out of 8 on the logarithmically scaled volcanic explosivity index.
A volcano’s explosive power largely depends on how its magma formed, which, contrary to what one might assume, is not from the earth’s molten core. Instead, it’s created when unusual heat or circumstances contrive to melt the mantle or lower crust. Because pressure increases the melting point of the mantle by strengthening its chemical bonds, the mantle is almost entirely solid even in the extreme heat at its lower depths. Magma creation requires something unusual. It requires either unusual heat, an unusual drop in pressure, or an unusual pollutant (generally water) to enter the mantle that lowers its melting temperature.1
>
Your worrisome situation in Vesuvius can be blamed on the latter, which is the process responsible for many of the world’s most powerful volcanoes. This intrusion of water is the result of an oceanic plate sliding beneath a continental one. In this case, the sliver of the African plate covering the Adriatic sea slid (and indeed continues to slide) beneath the Eurasian plate along Italy’s east coast. In slabs beneath oceans, a small amount of water can infiltrate into their structure, and because water lowers a plate’s melting temperature, this seemingly innocuous seepage is the first step of an incredibly volatile reaction that has led to some of the most catastrophic eruptions in history (see Krakatoa, 1883).
Once the water and heat combine to partially melt the rock of the mantle, the lighter magma bubbles to the surface, melts the surrounding crust, and picks up new components. This doesn’t always increase a volcano’s destructive power, but as luck would have it Mount Vesuvius is located on a thick bed of limestone. Limestone (CaCO3) + heat results in the volcanically unfortunate combination of calcium oxide and CO2. In other words, standing in Pompeii places you in the hazard zone of carbonated magma.
It gets worse. It so happened that after Vesuvius’ last eruption (believed to have occurred in 217 BC), lava may have cooled somewhere in its piping system to create a plug, dramatically increasing the amount of pressure on the magma before it dislodged the rock, shook the earth, and exited the vent on the top of Mount Vesuvius.
As the lava depressurized, carbon dioxide and sulfuric gases rapidly came out of solution to create the magma-fueled jet engine, and the cloud rose into the sky in a formation called a Plinian column (named after Pliny the Younger, who documented Vesuvius’ eruption from across the bay of Naples).
The height of a volcano’s Plinian column is determined partly by its explosive power, so it can be used to help determine its volcanic explosivity index. Vesuvius’ VEI of 5 was the same as Mount Saint Helens’ eruption in 1980—which is significant but well short of the top of the scale. (Fortunately, VEI 8 eruptions are quite rare. The most recent one occurred on New Zealand’s Mount Taupo 26,500 years ago, and it destroyed an area the size of El Salvador.)
The trouble with Mount Vesuvius’ AD 79 eruption wasn’t just its size, but the fact that so many people lived so close. When I asked James Moore, a volcanologist and scientist emeritus at the US Geological Survey, how best to escape an erupting volcano, he said it’s quite easy: “Don’t live near one!”
But that is far more difficult than it might seem. It wasn’t just bad luck that Romans built their city at the base of a volcano. Instead, volcanoes tend to attract human societies because their long-ago eruptions can produce fantastic soils. If the Pompeiians had done any excavation, they might have found evidence of the massive Vesuvius eruption in 1995 BC and the Bronze Age population it destroyed. But they didn’t.2
>
So instead, you and the rest of the Pompeiians find yourselves 6 miles from the vent of Vesuvius with only two options: Run north, or run south.
If you run south and away from Vesuvius, you have two concerns. First, it’s a little unclear how far you need to go. We know that you need to at least run past the town of Stabia–around 4.5 miles away–because Pliny the Elder (the Younger’s uncle) died here on the morning of the 25th. If you run south, you’ll also be running in the direction of the prevailing winds, which means the Plinian cloud will continuously rain ash and pumice on you. As the eruption continues, this problem will only worsen. Eventually, the cloud will become so thick that day will appear as night. Pliny the Younger described the darkness in his notes as “not so much a moonless or cloudy night, but as if the lamp had gone out in a locked room.”
When I asked Petrone where the survivors of Pompeii went, he wrote that there’s evidence of successful escapes to both the north and south. However, he suggests you run north toward Naples–and toward the eruption. He says the road between Pompeii and Naples was well maintained, and the written records of those who survived suggest that most of the successful escapees went north–while most of the bodies of the attempted escapees (who admittedly left far too late) have been found to the south.
But if you do run north, you’ll need to move quickly, because you’ll pass through the small Roman resort town of Herculaneum on your way to Naples–and Herculaneum is hit by the first pyroclastic flow.
Herculaneum sits barely 4 miles east of the volcanic vent, but for the first few hours of the eruption the prevailing winds largely spare it from most of the ash and pumice. Unfortunately, when Vesuvius first taps into the deeper magma and develops its first pyroclastic flow, the heated gas and ash will move directly into Herculaneum and kill everyone almost instantly.
>
Archeologists have found scorch marks in the city that suggest the cloud may have been as hot as 930 degrees Fahrenheit, and because its victims were encased in negative spaces of ash, archeologists can see their final, frozen poses. These poses show almost no signs of the boxer-like defensive stance typically taken in extreme heat, which suggests to Petrone that the victims in Herculaneum may have been killed so quickly that they did not even consciously register discomfort. Petrone even found a glassy piece of brain-matter in the skull of one Herculaneum victim, suggesting the cloud heated this person’s brain so quickly it vitrified. Nevertheless, you can avoid vitrification if you follow these instructions carefully.
Purchase your bread no later than 10 am and exit Pompeii as soon as you see the black cloud. Luckily, the bakery conveniently puts you on the road to Herculaneum. All you have to do is head north.
Herculaneum is a little more than 9 miles from Pompeii, and the pyroclastic flow doesn’t hit until the late afternoon. We’ll say you have a deadline of 2 pm, to be safe. That gives you four hours, which means you can maintain the average walking pace of 3 miles per hour and arrive in Herculaneum just after 1:00.
>
Herculaneum is a beach resort town for the Roman elite. You’ll find large, beautiful houses clad in marble. Some of these may seem enticing places to wait out the troubles. But obviously, that would be a mistake.
Instead, you need to pass through. If you want to be sure of your survival, you need to at least reach the outskirts of Naples–another 4 miles away. And to be extra safe, you should probably make it there within the hour, which means averaging a fast walk or jog.
The speed might seem manageable, but the total distance from Pompeii to Naples is 13 miles. Avoid overexertion and take any opportunity to drink fresh water. Crowds and obstacles may slow you down, but it won’t be that hard to grab that loaf and get out of the city itself. Many residents initially take shelter, so you’ll have an open window for your departure, which is critical for your survival. “Probably only those who managed to understand from the beginning the gravity of the situation escaped in time,” Petrone wrote to me in an email. And, perhaps to motivate any rubbernecking time travelers, he attached a link to the vitrified brain.