How to Escape From a Volcano Eruption

By | September 8, 2020

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.

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