Mount Vesuvius and Pompeii: Pyroclastic Flows

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Location: 40o 49' 16" N, 014o 25' 33" W.

Because only about ten per cent of the people who lived in Pompeii were buried by the ash, and very few bodies were found in Herculaneum, historians and archeologists long believed that the eruption was not very violent. That changed dramatically when new excavations in Herculaneum, plus improved understanding of the deposits produced by eruptions, showed that the later stages of the eruption were extremely violent.

A few hours after the eruption began, the gas pressure in the volcano began to weaken. The eruption cloud began to collapse under its own weight and rolled down the flanks of the mountain as pyroclastic flows. Pyroclastic flows are clouds of ash, pumice fragments and superheated steam and other gases. Anyone caught in one will burn to death and suffocate almost instantly. Herculaneum, closer to the volcano, was hit first. People on the waterfront hoping to escape by sea were killed and buried, not to be discovered for 1900 years. Pompeii was further from the volcano and was not hit by pyroclastic flows until several hours later.

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Created 09 July 2009, Last Update 15 January 2020