From Looney Pyramid Games Wiki

The Eruption is the essential player move in Volcano and its large family of game variants.

A stack of pyramids erupts when a cap (a small pyramid of a designated color, typically opaque) is moved off of the stack. The eruption takes place in a straight line determined by the direction that the cap is moved. The cap itself moves a single space. The pyramid in the stack directly beneath the cap moves two spaces, jumping over the cap to land in the next square. The next pyramid down in the erupting stack moves three spaces, jumping over the two pieces already moved. And so forth.

Eruption continues until there are no pyramids left in the stack on the square that erupted, or until the path of the eruption encounters an obstacle like a board edge or another cap.

In original Volcano and most variants, erupting pyramids are captured if they land on other pyramids of matching sizes.