The eruption of Vesuvius occurred in successive phases, spewing varying types of matter and consequently leaving varying types of archaeological evidence.  The ash that fell from Vesuvius was deposited in layers of different thickness around the region.  By analyzing how thick the layers are, what type of ash they contain, and how the deposits were distributed, geologists have been able to reconstruct the different phases of the eruption.

The initial eruptive column that formed around mid-day on August 24 was a sight to behold for the Herculaneans, since the prevailing winds blew the ash and pumice in a southeasterly direction, away from their town.  But when the column—which rose over eighteen miles into the sky—collapsed, the first pyroclastic surge of the eruption came barreling down the slopes of the volcano.

A pyroclastic surge is a mixture of searing gas and rock fragments that takes the form of a great cloud.  This cloud moves easily up and over ridges and obstacles, and its effects are far-reaching in their devastation.  In Herculaneum, on the northern edge of the area affected by the volcano, the surge burned away the victims’ skin and muscle tissue, leaving fairly well-preserved skeletons.  The presence of groundwater in the sea level area of Herculaneum created mud that enclosed and further preserved these skeletons.

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