Most pipe vesicles occur near the base of basaltic flows where they are interpreted to be traces left by ascending gas bubbles. Some basaltic pillows also contain pipe vesicles, but in these pillows pipe vesicles have a radial distribution, indicating that buoyancy cannot be a factor in their formation. Pipe vesicles do not extend through chilled glassy margins at the base of flows or rims of pillows, but rather, they form only where there has been significant crystallization, indicating that the gas forming these vesicles is exsolved from the lava and is not derived from an external source.
Pipe vesicles in flows and pillows are proposed to form by the exsolution of gas onto bubbles that are attached to the zone of solidification. As this zone advances into the cooling lava, continued exsolution of gas causes the bubbles to grow normal to the solidification front as pipes. Gas tubes formed in a similar manner can be seen in most ice cubes. Lava near the advancing tip of a pipe vesicle cools more rapidly than elsewhere because heat is able to transfer along the pipe by radiation. Pipe vesicles, therefore, grow into the lava as “cold fingers.” They cease growing when separate gas bubbles nucleate ahead of them in the ever broadening zone of solidification. Lava need contain no more than 0.01 wt% H2O for pipe vesicles to form in this way.