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An old 'a'a flow on top of an old pahoehoe flow. You can easily compare their differences in this photo. The pahoehoe consists of innumerable flow units each less than a meter thick, and the surface is smooth and glassy. The 'a'a, on the other hand, consists of a single thick flow with a very rough surface. This particular 'a'a flow is about 3 meters thick.
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