Pahoehoe flow (left) flowing over older 'a'a, and in front of an advancing 'a'a flow (background). Note the continuous skin of the pahoehoe being crumpled at the flow front, and compare it to the broken clinker of the active 'a'a. Note also that the pahoehoe flow front is ~30 cm thick whereas the 'a'a flow front behind is almost 2 m high.

VolcanoWorld