Smaller conduits carrying lava into a larger passage often seal off completely as the lava cools, if they haven't drained completely. In some cases a seal may form when breakdown blocks a passage and the flow is insufficient to overflow or carry off the breakdown.
In the photo below, it appears that at least four different flows were carried through an infeeding conduit before it sealed completely.

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Created: August 4, 2000
Author: Dave Bunnell