Breakdown

Pieces of lava tube ceiling or wall that have broken off and fallen to the floor. Much of this breakage occurs during the cooling phase, as shrinkage of the tube’s lining occurs and contraction cracks form. Some ceilings in lava tubes contain very unstable rock that has not yet fallen to the floor, and caution must be taken during exploration not to dislodge it.

Breakdown of the ceiling may also make an opening to the surface, or skylight.

In the lefthand photo we see breakdown that came from the collapse of a red pahoehoe floor of an upper level. Clearly, the breakdown occurred after the last streams of molten lava had coursed through the lower level, or they would have been covered and possibly become welded breakdown.

In the righthand photo is the huge passage of Skull Ice Cave in Lava Beds National Monument, California. Virtually every surface in the cave has shed some rock, so that none of the original passage walls or ceilings are intact. Note the small person for scale in the green pants.