For the first time, a study shows that, after the eruption of Vesuvius in AD 79, the Roman city of Herculaneum it was destroyed by a cloud of ash so fiery that it incinerated the bodies and vitrified the brains of the victims it found in its path. The thermal impact was more immediate and more virulent, because it was closer to the volcano than in its neighbor Pompeii, with temperatures that exceeded 550 °C.
The first pyroclastic flow that reached Herculaneum was a short-lived ash cloud, with temperatures of 495–555 °C, capable of causing the instant death of people, leaving only a few centimeters of ash on the ground. The analysis of the optical properties, technically defined as “reflectance”, of the remains of charred wood allowed the recording of multiple and ephemeral extreme thermal events, thus revealing for the first time the actual thermal impact of the eruption of Vesuvius over Herculaneum.
The study, published in the journal ‘Scientific Report’, has been carried out by a group of geologists led by Guido Giordano, from the Roma Tre University, and by the anthropologist Pier Paolo Petrone, from the Federico II University of Naples. He says the research that the temperature of pyroclastic currents (flows composed of gases and solid material such as ash and rocks of various sizes) is a crucial parameter for assess its lethal power.
Scientists have shown that this first red hot ash cloud (495–555 °C), after entering the city, it then reached the beach, still over 500 degrees, causing the instantaneous death of all the people who took refuge in the city buildings and those nearby Sea.
The interaction between the scorching ash cloud and the seawater caused the cloud will increase in volume. Shortly after, the cooled ashes were deposited, enveloping the bodies of people who had already died from the extreme heat. Later pyroclastic flows, at relatively lower temperatures, progressively buried the city of Herculaneum under a blanket of ash and other volcanic deposits up to 20 metros of thickness. The reason why no victims were found in Herculaneum in the characteristic positions of Pompeii is due precisely to the incandescent pyroclastic flow, which destroyed the soft tissues of the body.
Precisely, the chronology of these events would be at the base of the transformation into vitreous material of the brain tissue of an inhabitant of Herculaneum, which was discovered in the service areas of the “Collegium Augustalium” (religious building) and described in 2020 in the ‘ New England Journal of Medicine’. That is, the extreme brevity of the first pyroclastic flow would have prevented the brain tissue from being completely vaporized, while the lapse of time before the next allowed the rapid cooling necessary for the transformation into glass.
Improvements for future eruptions
In addition to the new reconstruction of the thermal events of the Vesuvius eruption, the study results also offer important suggestions for improving the safety of people currently living on the red zone next to a volcano. The lethal impact of pyroclastic flows, with ash clouds as demonstrated in Herculaneum, although they are short-lived, can cause serious damage to buildings and a large number of casualties.
From a perspective of risk mitigation in the Vesuvius area and elsewhere, the study authors suggest that buildings in the red zone should be reinforced to protect people from thermal shock of the ash cloud, in case a complete evacuation is not possible. In a future eruption, peripheral areas less exposed to pyroclastic flows could be affected by short-lived detached ash clouds. In this case, survival depends on the ability of shelters to prevent infiltration of scorching gas and red hot ashes. This type of intervention -concludes the study- would allow people who did not have the opportunity to be evacuated in time to survive and wait for help, or to leave before other pyroclastic flows reach the area.