Sixteen years after that telltale earthquake, in August 79 A.D., Mount Vesuvius erupted again. The blast sent a plume of ashes, pumice and other rocks, and scorching-hot volcanic gases so high into the sky that people could see it for hundreds of miles around.
(The writer Pliny the Younger, who watched the eruption from across the bay, compared this “cloud of unusual size and appearance” to a pine tree that “rose to a great height on a sort of trunk and then split off into branches”; today, geologists refer to this type of volcano as a “Plinean eruption.”)
As it cooled, this tower of debris drifted to earth: first the fine-grained ash, then the lightweight chunks of pumice and other rocks. It was terrifying–“I believed I was perishing with the world,” Pliny wrote, “and the world with me”–but not yet lethal: Most Pompeiians had plenty of time to flee.