In 79 CE, a thick layer of ash and other volcanic debris rained down on the residents of Pompeii and Herculaneum when Mount Vesuvius cleared its throat. But that may not have killed the towns’ occupants. A new study published in Scientific Reports points to an earlier and short-lived surge of hot ash and volcanic gas that baked the ancient Romans at temperatures higher than 500°C.
“It’s the prototype of any explosive eruption.”
The Vesuvius eruption is iconic, said Guido Giordano, a volcanologist at the University of Roma Tre in Italy. “It’s the prototype of any explosive eruption.” As such, historians and volcanologists have long studied how the event played out.
The new study reported that skeletons of some victims showed signs of being heated to temperatures higher than the 350°C-400°C (662°F-752°F) suggested by previous studies. And in the remains at Herculaneum, the brain of one of the victims was turned into glass—a transition that requires temperatures as high as 570°C (1058°F).
Pliny the Younger, who witnessed the eruption from across the Bay of Naples, described clouds flowing over the ground. These clouds were pyroclastic density currents (sometimes called pyroclastic flows)—ground-hugging cascades of volcanic gas and particles, mostly ash but also ejected matter a centimeter or larger.
In a Plinian eruption (so named after Pliny’s eyewitness account) pyroclastic density currents are produced by the sudden collapse of the volcano’s eruption column. Driven by their density, which can be a thousand times greater than the density of air, these currents can move at speeds of hundreds of kilometers per hour, Giordano said.
Smothering pressure isn’t the only hazard posed by pyroclastic flows. They can also be incredibly hot. But they’re hard to study because they often leave only centimeters of ash, which tend to be washed away within weeks unless the deposit is immediately buried. “They are very rarely preserved in the geological record,” Giordano said.
The ash deposit corresponding to the first pyroclastic flow from Vesuvius ranges from 20 to 80 centimeters (8 to 30 inches) deep, and luckily for researchers, it was quickly covered by tens of meters of more volcanic material. In the early deposit, Giordano and his colleagues looked for hints about the eruption’s events and their temperature profiles.
Thermal Sketches in Charcoal
The researchers sampled charcoal fragments from buildings, homes, and furniture at several sites in Herculaneum, as well as trees downed by the volcanic flow. With sufficient heat, wood turns to charcoal, and charcoal’s ability to reflect light—its reflectance—increases with the temperature at which this transformation takes place. Researchers can therefore use charcoal’s reflectance as a thermometer.
Past studies of charcoal from the Vesuvius eruption had averaged reflectance measurements, blurring signals that could reveal multiple flows of hot ash.
“These people would have been killed even if the eruption would have stopped then and no other pyroclastic flows would have buried the town.”
In the new study’s hundreds of reflectance measurements, the researchers noticed that the temperatures clustered around three distinct values, suggesting three separate events heated the area, the first being the hottest, the team reported.
The first event scorched residents’ homes at temperatures of up to 495°C–555°C (923°F-1031°F). This blast was brief, leaving wood partially transformed to charcoal. “These people would have been killed even if the eruption would have stopped then and no other pyroclastic flows would have buried the town,” Giordano said.
At least two later flows followed, raining ash and debris onto residents. These pulses had lower temperatures, reaching 390°C–465°C (734°F–869°F) and 315°C–350°C (599°F–662°F).
Planning to Survive
“This is a great piece of work,” said Eric Breard, a volcanologist at the University of Edinburgh who was not involved in the study. Realizing the thermal history ingrained in the charcoal samples allowed the researchers to extract new clues about an eruption that has been fascinating volcanologists for some 2,000 years, he said.
It’s challenging to study the temperatures of these events, Breard explained. Researchers don’t have sensors to survey the internal heat, and because pyroclastic flows mix efficiently with air, they vanish quickly as their temperature and density drop.
Once Breard read the study, he whipped up a model to estimate how quickly people would go into shock from a 550°C (1022°F) blast. “It’s very fast—we’re talking under 30 seconds.” Even a few seconds of exposure could cause permanent damage, but tens of seconds would cause a person to fall (and, in Vesuvius’s shadow, be buried in the scorching materials of the pyroclastic flow).
Better temperature estimates of past events will help inform the mathematical models used to study these flows, Breard said. And models estimating temperatures of pyroclastic flows could help to assess the likelihood of survival, providing important information for hazard planning.
Understanding the conditions in which people could survive might make it possible to create heat shelters in places vulnerable to an eruption. If the people at Herculaneum had such a heat shelter, Giordano suggested, more could have survived—at least until they were buried by ash.
—Carolyn Wilke (@CarolynMWilke), Science Writer