Settled Crust
When lava is ponded, as in a lava lake, the top portion cools first and forms a crust. The molten material below may then drain out, or simply shrink during cooling, and the unsupported crust may collapse under its own weight. In the first photo, a relatively round lava lake’s crust has collapsed into large, pie-shaped wedges. In the other two photos, a lake crust in a passage segment has collapsed along the axis of the passage.
AUTHOR: Dave Bunnell