Schematic diagram showing the formation of a pit crater from the bottom up. This cross-section is cut perpendicular to the line of a rift zone, and "C" represents the main conduit at 2-3 km depth. Note that the crater is not a piston that has dropped and that the top lavas are the last to fall in (from Walker 1988). This process of a void working its way upward through solid rock (like a bubble) is called "stoping", and it commonly occurs in mines. The void and the eventual crater have a volume greater than the conduit, however, the conduit can continually carry material downrift.

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