Mad About Science

By Brenden Bobby
Reader Columnist

When we picture a volcanic eruption, we envision a huge and awesome fiery explosion and huge columns of smoke spewing into the sky. In the case of Pompeii, many of us imagine the city being leveled by a huge, fiery wave of destruction, as though it were hit by a nuclear weapon. This is not an entirely accurate image; as often is the case, the truth is much more macabre than we could ever imagine.

Pompeii was a Roman city near the base of Mount Vesuvius, the only volcano on the European mainland to have erupted within the past century. The city of Pompeii was founded sometime between the 6th and 7th centuries, BCE — surviving for nearly 800 years before being completely destroyed in 79 CE.

The eruption of Mount Vesuvius was heralded nearly a decade earlier, when a massive earthquake rocked the city of Pompeii. Immense amounts of pressure were created from gasses that were being superheated by magma beneath the Earth’s surface. This pressure exerted from these gasses was enough to move millions of tons of rock, causing the very ground to violently heave and shift beneath the feet of the ancient Romans. Objects expand when they are heated — similarly, they will contract when chilled; these are properties of thermodynamics, to which gas is no exception. Even though we can’t see it, gas is capable of exerting huge amounts of pressure on things when it is heated and expands — that’s why oxygen canisters in hospitals are so dangerous and need to be kept away from sources of heat: oxygen is highly flammable and it violently expands when heated.

In the hours leading up to Mount Vesuvius’s fateful eruption, these tremors would have become very noticeable as the pressure beneath the volcano was building, and the mountain’s structure was beginning to succumb at its weakest points to this pressure. These quakes, and the release of pressurized gas from the mountain would have sounded like large, booming explosions in the distance — a sound that is barely normal today, let alone 2,000 years ago.

At some point, the pressure would have reached a critical point. The eruption of Mount Vesuvius was a violent one, much like Mount Saint Helens in 1980. Vesuvius’s eruption was believed to have blown a huge amount of rock from near its peak, practically turning the mountain into a crater. Most of this rock — but not all — was blasted into silicate dust and plumed into the atmosphere, creating a huge pillar of black smoke from the heart of the mountain. Countless pieces of debris were flung high into the sky, ranging in size anywhere from the size of a pebble to watermelon-sized chunks, and perhaps even larger than that. These chunks of rock didn’t remain airborne for long, and returned to the Earth as a terrifying and destructive hail. Before any wall of superheated gas hit the city, it was likely the denizens of Pompeii were pelted for what potentially may have been hours by this deadly pumice.

Additionally, much of the dust fell around Vesuvius, too, creating massive blankets of snow-like ash. This ash, in large amounts, became extremely heavy and began to collapse the roofs of several buildings throughout Pompeii, burying those that sought shelter indoors.

It is believed that there was an evacuation attempt (though “exodus” is probably a more accurate term) during this stage of the eruption. It has been documented that several of the more prominent members of Pompeian society managed to flee at this point, taking many of their most valuable belongings with them.

As night began to fall on Pompeii, the pyroclastic flows (surges of heated gas) began to strike the city, leveling and incinerating buildings and bodies in their path. This was when the largest number of people are believed to have died. At this point, Vesuvius would have been obscured by a pillar of swirling ash, illuminated from within by the glow of molten lava. The city was barraged by pyroclastic flows for several hours through the night and into the next morning. What few structures still stood were quickly being covered by thick layers of volcanic ash.

Indeed, the ash would not stop falling for almost two days after the initial eruption. Pompeii was so thoroughly covered that many Romans could not identify where the city once stood, and the site would be abandoned and forgotten for almost 1,500 years. It was rediscovered, by accident, in 1599, but not excavated and explored in some detail until the 1700s.

Despite the destruction faced by the city, it is uniquely one of the most well-preserved sites of antiquity to survive into the current era. Many frescoes, wall paintings common in ancient Rome, were well preserved by the layers of volcanic ash — as were the human bodies excavated from the ruins. In fact, one of the most well-preserved pieces of accidental art from Pompeii is actually graffiti scrawled on walls, including a simple piece that states: “Gaius was here” — a statement that becomes extremely poignant when factoring in the violent utter destruction of the city.

Enjoy the snow, and stay curious, 7B.