In the days leading up to the eruption, Pompeii and Herculaneum were experiencing significant seismic activity. Springs dried up. The earth shook. They tried to rationalize "it's just the mountain clearing it's throat. it's always been there". The physical reality was a pressurized Plinian column waiting to collapse. Vesuvius was a brutal reminder that civilization is just a thin, temporary veneer slapped onto a volatile planetary crust. You can build all of the marble columns you want. The thermal plume doesn't care.