Lava flow composed of rough, jagged chunks, sometimes welded together but often loose, where the fragments are sometimes called clinker. Often a lava flow begins as pahoehoe lava and grades into aa as pieces of it start to cool and are carried along in the flow. 
It is rare for lava tubes to form in a`a, but sometimes aa flows over older flows and enters pre-existing tubes through skylights. Usually this ends up plugging up the passage inside, thereby segmenting a once-continuous tube system.
In the photo below, cavers are preparing to enter a tube in the pahohoe lava. The darker material in the upper left is an a`a flow that came later, covering up portions of the pahoehoe flow. Fortunately for the cavers, it stopped just short of covering up the cave's entrance.


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Created: August 4, 2000
Author: Dave Bunnell