The volcano is surrounded by a densely populated area and Naples, the third largest city in Italy lies less than 20 kilometers (12 miles) away from the mountain.

Mount Vesuvius towering over the town of Torre del Greco. Image credits: Tom Wachtel/CCsearch, CC BY-NC-ND 2.0

Vesuvius is one of the many stratovolcanoes around the world, which are known for their explosive eruptions. The volcano, without doubt, is still very active and has erupted around three dozen times since the famous AD 79 eruption, which destroyed the city of Pompeii and Herculaneum, killing as many as 16,000 people.

Although it became silent in the 13th century for around four hundred years, Vesuvius entered a new volcanic phase in 1631 when a major eruption destroyed all villages and towns around the mountain and killed at least 3000 and maybe up to 6000 people.

The last active volcanic cycle of Vesuvius began in 1913 and ended with a massive eruption in 1944.

Since then, the volcano has been quiescent but sooner or later it will wake up. Some experts even think that it’s overdue for another blowout. So, what would happen if Vesuvius erupted again and why would that be so dangerous?

The eruption of 1944 taken from an American B-25 bomber. Image credits: Wikimedia Commons / CC0 1.0

In case of a major eruption, estimates say that more than 3 million people could be at risk and an incredible value of infrastructure would be destroyed. First, a huge explosion would occur which would be followed by a pyroclastic flow. Pyroclastic flows are ground-hugging clouds consisting of hot volcanic gases, ash, and rock fragments. Their velocities can reach 160 km/h (100 mi/h) on average and they burn everything in their path, so outrunning the eruption is definitely not an option.

Scientists believe that during the eruption of AD 79 Vesuvius produced six separate pyroclastic surges heating the temperature to 300 °C (570°F), killing hundreds within just seconds.

The extreme heat of the pyroclastic surges would likely engulf, flash-fry, and blanket the people living near the slopes of the mountain with ash. When the excavations began in Pompeii, archeologists found not only skeletons but also voids, left behind by the decomposed biological material in the solidified ash surrounding them. By pouring plaster into these spaces, researchers were able to recreate the victims’ final posture. The casts imprinted around the bodies so perfectly that the plaster even reveals some of their facial expressions during their demise.

The bodies of Pompeii frozen in time. Image credits: Simon & Vicki/CCsearch, CC BY-NC 2.0

In addition to the explosion and the pyroclastic surge, solidified magma and other debris would rain from the sky. If Vesuvius were to erupt to its full potential, so much pumice and debris could be shot into the air that it would bury Naples. At the same time, a huge pillar of ash and smoke would rise up to the sky, the height of which could reach 32 kilometers (20 miles).

In only the first 15 minutes of the eruption, an area with a 7 kilometers (4 miles) radius of the volcano could be destroyed and become buried under thick layers of volcanic ash.

The damage to the power, water, and gas infrastructure would leave millions of people without water, power, and transportation. Furthermore, an eruption would disrupt air travel on the entire continent and could deal a major blow to tourism in the entire country.

Eruption of Mount Vesuvius by Pierre-Jacques Antoine Volaire (1770). Image credits: Ron Cogswell/Flickr, CC BY 2.0

Every settlement that affected by the eruption would not be suitable for living – there would be nowhere to return for those who had to flee from the wrath of Vesuvius. The cost of the damage caused by an eruption could be as high as $20 billion and tens of thousands of people could lose their lives during the catastrophe.

That’s why the volcano is being monitored 24/7 by the Vesuvius Observatory and the Italian government has an evacuation plan that could, in theory, move 600,000 people out of the so-called “red-zone”. The practical question of how much time the evacuation could take is controversial – the best-case scenario is that it can be completed within 72 hours but according to Marco Di Lello, director of urban planning in the Campania region it could take up to two weeks. Besides, the rumbling from an eruption could severely damage roads and bridges, making evacuation or transportation much harder.

The crater of Vesuvius formed during an eruption, which blew off the top of the mountain. Image credits: Pastorius/Wikimedia Commons, CC BY-SA 3.0

The Italian authorities are actively trying to reduce the population near the danger zone by demolishing illegal buildings and offering financial support to get residents to relocate into safer areas. They even established a national park on the uppermost part of the volcano in 1995, to prevent further constructions.

Let’s hope that their endeavors will meet succes when the inevitable comes.