In the Roman City of Pompeii, the morning of October 24, 79 A.D., probably seemed like the start of any other autumn day to the population as they went about their daily activities. Very few if any could imagine that a great cataclysm would befall the entire region within a few hours. Certain natural disasters strike without warning giving their victims little chance to escape. Others leave a window that must be discovered and utilized within a certain time period. The trauma began around 1 PM when nearby Mt. Vesuvius erupted sending a giant plume of volcanic ash and pumice nearly twenty miles into the sky. There was some opportunity for evacuation at that point, but the window would be abruptly shut by the next morning.
The Plinian Eruption: The 1 PM eruption was later categorized as a “Plinian” eruption owing largely to the only surviving written record of an eyewitness, Pliny the Elder, as recorded by his 17-year-old nephew, Pliny the Younger. Pliny the Elder was the commander of a fleet of Roman galleys stationed at Misenum, a town about eighteen miles from Pompeii to the west along the Bay of Naples. His observation and recognition of the seriousness of the event motivated him to mobilize his ships into a rescue effort. Pliny the Younger’s description of the eruption is worth repeating here:
“A cloud, from which mountain was uncertain, at this distance (but it was found afterwards to come from Mount Vesuvius), as ascending, the appearance of which I cannot give you a more exact description of than by likening it to that of a pine tree, for it shot up to a great height in the form of a very tall trunk, which spread itself out at the top in a sort of branches; occasioned, I imagine, either by a sudden gust of air that impelled it, the force of which decreased as it advanced upwards, or the cloud itself being pressed back against its own weight, expanded in the manner I have mentioned; it appeared sometimes bright and sometimes dark and spotted, according as it was either more or less impregnated with earth and cinders.”
–Translation by William Melmoth, 1746.
A Plinian eruption includes volcanic debris and hot gases along with pumice that is formed when lava with a certain amount of water escapes during the eruption and cools turning into a very light weight rock infused with tiny gas bubbles. The eruption itself can be as short as a number of hours or as long as a number of months. As the lava is ejected, the magma chamber beneath it can become vacated enabling the volcano to collapse into a caldera. Plinian eruptions can include discharges of static electricity in the form of lightning, and/or loud sounds. This was the situation that the people in Pompeii faced on that afternoon with the pumice and ash buildup taking place. It was about 7 AM the next day that the Plinian eruption settled down and the pumice and ash ceased to fall.
Overnight Earthquakes: The pumice shower may have died down during the early morning the following day, but the residents had a difficult time on their hands the night before. Pliny the Younger wrote that some powerful earthquakes struck overnight with great intensity reaching eighteen miles away where he lived. The last quake in the series took place early in the morning before the ash and pumice ceased to fall. Buildings in Pompeii were toppled by the tremors with some people surely killed inside. Although the earthquakes were reported by Pliny, they were not confirmed until modern times when a team of researchers lead by Dominico Sparice, located some buildings in the Pompeii ruins that suffered seismic damage, and that two of the bodies found in one of them died from the collapse of that building and not from any volcanic action. They suffered the same blunt force trauma that victims of modern-day earthquakes are subject to receiving. The wall that killed the earthquake victims was covered with pumice meaning that it probably collapsed with the pre-dawn earthquake that struck sometime before the pumice shower stopped. This would tie with Pliny’s observation that a very violent earthquake struck just before dawn that “the chariots we had ordered to be brought out, though on a level ground, were shaken back and forth and did not remain steady in their places even wedged with stones.” This could be another causative factor as to why large numbers of residents did not escape—that many buildings were destroyed by seismic activity blocking streets with rubble and doubling down on the rescue effort.
The Pelean Eruption: The pumice shower stopped roughly at dawn, and there was perhaps half an hour of respite from volcanic eruption. At this point we could expect that those still inside the city were up surveying the damage thinking that perhaps the trauma was over. They would be able to see that a massive cleanup would be necessary along with retrieving the dead and caring for the injured. What they would not see was that the very worst of the danger was just ahead. About half an hour after the first eruption ended, Vesuvius erupted again, but this time the eruption was drastically different. A huge cloud of ash, debris, and gases came shooting out of the side of the mountain in what is known as a pyroclastic flow aimed directly at Pompeii and Herculaneum. The flow came in via two major surges, and had temperatures ranging from about 430 to 680 degrees Fahrenheit. It is not known today exactly at what speed the flow roared through, but a pyroclastic flow averages about 60 miles per hour and can reach up to as much as 430 miles per hour. Each surge did massive damage and caused many deaths, but the second surge likely was the one that buried both Pompeii and Herculaneum completely. The second day’s eruption is referred to as a “Pelean” eruption named after the 1902 Mt. Pelee eruption and pyroclastic flow that destroyed the town of St. Pierre in Martinique, killed as many as 30,000 people, sank two ships in the nearby harbor, and knocked a 2 ½ ton statue a distance of about 164 feet.
The Escape Route: From the first eruption to the first pyroclastic flow surge that battered Pompeii and Herculaneum, about eighteen hours elapsed. In the early going, the pumice shower was somewhat sticky as the pumice was not entirely cooled. It settled on roof tops causing some of them to collapse, and perhaps started some fires. It was still daylight, and those who chose to leave probably were able to pack a few belongs and make their way to the west toward Herculaneum along the shoreline. The wind blew in from the west and kept the pumice shower from affecting nearby Herculaneum at all. If any of the early refugees stopped and stayed in Herculaneum, they would have been out of harm’s way until the pyroclastic flow the next morning. To be assuredly safe, refugees on foot would need to travel a distance of at least eighteen miles around the bay until they reached Misenum, where Pliny the Elder and his ships were stationed. Pliny sailed a flotilla of rescue ships to the affected region, leading the way in his smaller command ship. The strong offshore flow from the west kept the ash and debris moving in the other direction, but it also kept the ships from easily turning around and sailing back with refugees. Pliny the Elder died of apparent natural causes during the attempted rescue, but Pliny the Younger kept observing and writing down what took place from his standpoint in Misenum. He indicated that some of the gases and ash from the pyroclastic flow made it all the way to Misenum causing little damage but forcing a number of residents there to also evacuate.
Conclusions: The eighteen-mile walk managed at an average speed of 2 MPH would get the refugee from Pompeii to Misenum and out of the worst danger in about nine or ten hours. However, upon reaching Herculaneum, that wasn’t impacted by the first eruption, how many of these refugees would have stopped there believing that they would be safe? Since the event was so dramatic, and fortunately so rare, it would be difficult for the refugees to understand just how far they had to go to get out of harm’s way. Further, the rain of pumice was extraordinary—reaching up to rooftops, and may have caused some of the people to hesitate inside their homes rather than evacuate in time. The nighttime full of earthquakes may have also impeded evacuation with destruction that blocked routes out of town. The ships certainly managed to rescue some refugees, but some ships may also have been damaged or destroyed in the process. Most of the casualties in the entire event occurred during the pyroclastic flow. Had everyone there understood that the particularly acute danger of the pyroclastic surges was so destructive it could not be survivable under virtually any circumstances, perhaps more of the population would have attempted to evacuate earlier when the circumstances permitted it.
Sources:
History.com, A Volcanic Eruption Wasn’t the Only Disaster That Destroyed Pompei, By Dave Roos, July 18, 2024.
Wikipedia, Eruption of Mount Vesuvius in 79 AD.
Wikipedia, Plinian Eruption.
Wikipedia, Pelean Eruption.
Painting by Karl Bryullov, 1830-1833.
Thank you for such a detailed and well-written article.