Vesuvius Erupted, but When Exactly?

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.