A shit, having the consistency of toothpaste, that contains enough mass so as it stacks upon itself it will create a pile of shit in the toilet bowl that breaches the water level in the bowl. The pile of crap in the toilet will resemble a volcano. (Vesuvius is a famous volcano in Italy.)

When you eat dirt for the first time and leave your family to build a shack in the woods

Large zit looks like it could erupt or explode any second. The near thought of being around one of these are scary.

The Volcano Vaporizer aka "The Vessel"

a great eruption, typically fronm the penis, causing a flow of lava (cum)

When you jack off and your Previn erupts everywhere