Sometimes the amount of lava flowing into an active tube suddenly surges, or is blocked by a sudden breakdown, and backs up through windows into upper levels and side passages, where it may cool and fill the passage completely. Often it will drain back down, leaving a new layer of material. It may then appear that the lava in a down-sloping passage was actually moving in the wrong direction, which it was for a time!

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	Created: August 4, 2000 Author: Dave Bunnell