When Mount Vesuvius erupted in A.D. 79, fiery avalanches of ash and pumice assaulted Pompeii, displacing some 15,000 inhabitants and killing at least 1,500 more. Volcanic debris “poured across the land,” wrote the Roman lawyer Pliny the Younger, and blanketed the town in a darkness “like the black of closed and unlighted rooms.” Within two days Pompeii had vanished, leaving little more than a legend until 1748, when the chance discovery of a water line prompted the first deliberate excavation.

In his late-18th-century travelogue “Italian Journey,” Goethe observed that no calamity in history had given greater entertainment to posterity than the eruption that had buried Pompeii. For scholars and armchair archaeologists, that entertainment has involved wrangling over pretty much every facet of the disaster. They still can’t agree on the day Vesuvius blew its top, the height of the umbrella-shaped cloud or the length and the aggression of the blasts. Two new research projects add kindling to those embers.

A report published by the Archaeological Park of Pompeii resurrected the once widely accepted belief that the cataclysm began to unfold on Aug. 24, the date put forward by Pliny, who was 17 when he witnessed the event from a villa across the Bay of Naples. His letters to the historian Tacitus, written more than 25 years after the fact, are the only surviving firsthand account and the only documents that offer a precise date.

We no longer have the original letters, only translations and transcriptions of copies, the first of which was made in the fifth century A.D. “Many manuscripts of Pliny’s letters came down to us with differing dates,” said the classicist Daisy Dunn. Her 2019 biography of Pliny, “The Shadow of Vesuvius,” is the definitive guide to him and his uncle, the naturalist Pliny the Elder, who died during the eruption. “Aug. 24 was chosen as the most secure on textual grounds,” Dr. Dunn said.

In sticking by Pliny, the park walked back some of the recent enthusiasm for Oct. 24 as a possible start date for the eruption, a theory that had been fueled by the 2018 discovery of a scrap of graffiti on a wall of the site’s freshly excavated House of the Garden. The charcoal scrawl records a date that translates to Oct. 17 in the modern calendar, suggesting that the eruption might have occurred after this time. The find, which did not specify a year, seemed to corroborate other unearthed clues that pointed to cooler weather than is typical in August: remnants of unripe autumnal fruits such as chestnuts and pomegranates; heavy wool clothing found on bodies; wine in sealed jars, indicating that the grape harvest was over; and wood-burning braziers in homes.

Massimo Osanna, general director of the park at the time of the discovery, was convinced that the graffiti was idly doodled a week before the explosion. “This spectacular find finally allows us to date, with confidence, the disaster,” he said. Dr. Dunn found it improbable that Pliny would have forgotten such a momentous date; still, she said, “in my view, the traditional date of Aug. 24 is just too early in the year to be accurate.”

The dating game

The park’s recent about-face from October to August relied in part on a forensic analysis of Pliny’s letters by Pedar Foss, a classicist at DePauw University in Indiana. For his 2022 book, “Pliny and the Eruption of Vesuvius,” Dr. Foss examined 79 early hand-copied manuscripts of the letters and mapped out how textural errors had been compounded. He concluded that a simple scribal mistake, made in the 1420s, of switching a “u” for an “n” had resulted in an incorrect eruption date of Nov. 1. The error appeared in the second print edition of Pliny’s letters, in 1474, and gave rise to further misreadings, misunderstandings and misuses.

By the 20th century, seven different possibilities were in circulation — eight, counting Nov. 9, which Mark Twain casually proposed in “Innocents Abroad,” his 1869 travel narrative. “Those many options gave the appearance of doubt concerning what Pliny actually wrote but, upon examination, I was able to explain away each of the mistaken alternatives,” Dr. Foss said.

He also explained away each of the archaeological alternatives to Aug. 24, some of which he believes fail based on the evidence; some, on the basis of faulty reasoning. He argued that the pomegranate rinds were used for dyeing, not eating; that the Romans commonly used braziers for cooking, not just heating; that wool clothing was standard gear for Roman firefighters; and that Roman agricultural and storage practices allowed for the preservation of fruits beyond their natural harvest seasons.

As for the House of the Garden doodle, on Oct. 12, 2023, researchers commissioned by Dr. Osanna’s successor, Gabriel Zuchtriegel, left their own charcoal message on the same wall on which the graffito appeared. Ten months later — on Aug. 24, 2024 — the text was still perfectly legible. “The inscription could have been put on the wall during October of any number of previous years,” Dr. Foss said.

So much for dating the disaster with confidence.

Letting off steam

Claudio Scarpati, a volcanologist at the University of Naples Federico II, favors the traditional date. “In my mind, the eruption occurred in August, on a sunny day,” he said. Dr. Scarpati is the lead author of two recent studies of the catastrophe published in The Journal of the Geological Society. One offered an hour-by-hour reconstruction, extending the chronology from the previously estimated 19 hours to 32 hours. The other revealed a dynamic sequence with 17 distinct “pyroclastic density currents,” many of them previously undocumented.

Pyroclastic currents are hot, swift-moving mixtures of volcanic particles — ash, pumice lava fragments and gas — that flow according to their density in relation to the surroundings. Dr. Scarpati said that contrary to popular belief, Pompeians were neither entombed by molten lava nor poisoned by gas. “No lava reached Pompeii, and the gas was predominantly vaporized water and, to a lesser extent, carbon dioxide,” he said. “According to our studies, the victims died primarily from asphyxiation caused by inhaling ash.”

To measure the distribution and the volume of the ash and the pumice layers, the team measured the thickness of the single layers over a 775-square-mile area around Mount Vesuvius. The deposits recorded dramatic, increasingly violent pulses from the volcano.

At noon on Day 1, Vesuvius began to eject a plume of rocky volcanic fragments and gas into the air, known as an eruption column. The mushroom cloud that Pliny observed at 1 p.m. was typical of what is now known as a Plinian eruption, in tribute to his richly detailed testament.

Dr. Scarpati said that the first currents had flowed to the town of Herculaneum, west of Vesuvius, bringing intense heat that essentially roasted inhabitants and, in one documented case, turned human tissue into glass, a process known as vitrification. At Pompeii, south of the volcano, the currents were cooler, and only the final eight penetrated the town.

During the first 17 hours, Dr. Scarpati said, Pompeii was blanketed with pumice lapilli from the column, which fluctuated like a giant fountain through 12 distinct pulses. At 2 p.m., the volcano began to spew pumice mixed with gas. Over the next four hours, roofs began to cave in under the weight of the pumice lapilli, causing some supporting walls to crumble as well. After 17 hours, the debris in Pompeii was up to nine feet thick. Enough was ejected to bury Manhattan roughly 450 feet, or 45 building stories, deep.

The eruption peaked when the column reached its maximum height of 21 miles, at about 1 a.m. on Day 2. “The column rose as long as its density was lower than that of the air, like a balloon,” Dr. Scarpati said. At daybreak, enormous amounts of fine ash and pumice collapsed the eruptive column, forming pyroclastic currents.

During a brief lull, Pompeians presumably tried to flee the town. Then, just after 7 a.m., the 13th and most lethal current struck — a thick concoction of ash was disgorged for nine hours, spreading detritus 16 miles across the plain and into the Lattari Mountains. In Pompeii, many victims of the volcano were found in the streets encased in this layer.

Around 4 p.m., the magma in the volcano’s conduit interacted with groundwater, causing the magma to break up into fine ash. No human remains were found in any of the layers after the 13th, suggesting to Dr. Scarpati that the morning’s devastation left no survivors. The eruption ceased at 8 p.m.

Paul Cole, a volcanologist at the University of Plymouth in England who was not involved with the project, said, “The work places a finer timeline on the events of 2,000 years ago, and also provides fresh evidence for how the hazard from such large, explosive eruptions can change even during the event.”

Rumpus involving Vesuvius may go on endlessly, but unlike Pliny’s letters, the geological history of the eruption seems to have been written in stone.