In a Plinian eruption, volcanic ash and pumice initially rain on surrounding areas. At Pompeii, scientists can distinguish the various layers of debris.
White and gray pumice layers, found at the
bottom of the excavations, confirm this type of debris fell first. Pompeii was not destroyed because of pumice
debris, however. In fact, human remains found above the ash and pumice layers indicate people came back to their homes after the
ashfall
stopped. Without the knowledge they needed
to survive, people placed themselves and their children into death’s path.
Annihilation occurred hours later, when Vesuvius spewed out pyroclastic surges and pyroclastic flows. A surge can turn pristine lakes and forests - like
Lake Spirit at the foot of Mt. St. Helens - into a kind of
moonscape. A pyroclastic flow can bury surrounding areas - like the Marella River
Valley near Mt. Pinatubo - to depths of 50-200 meters.
A pyroclastic flow first moves into canyons and river beds. At the bottom of the volcano, it can sweep away (or bury) anything in its path, including populated areas. That’s what happened to Pompeii. This link to a U.S. Geological Survey animation (you will need Quick Time) demonstrates a flow’s speed and ferocity.
Nothing standing in its way will survive.