Pahoehoe is a smooth, ropy lava, common on the Hawaiian islands. 'A'a is a more chunky and thick flow, that is most commonly associated with composite volcano eruptions. 'A'a lava flows are also called clinkers, because of their rough and jagged appearance.
The eruptions of submarine volcanoes result in pillow lavas, which form as the lava cools in seawater. Plateau basalts occur as a result of fissure eruptions of basaltic lava. Fissure eruptions occur along elongated cracks in Earth's surface rather than from a single vent or pipe. Magma wells up through the crack, and pours out on either side in effusive eruptions. These eruptions form extensive sheets of basaltic lava. Columnar basalts are another feature that can form on basaltic lava flows when they cool uniformly. The lava cracks into vertical columns, typically hexagonal in shape.
‘A’a`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough surface. The top layers of these flows are composed of broken lava blocks. The sharp and rugged surface of a hardened `a`a flow makes for treacherous and slow walking conditions.
The most active area of the flow is the core, which is much denser than the rough surfaces at the top of the flow, and underneath it. The dense, pasty lava in the core of the flow travels downslope, carrying the jagged chunks along the top of it. The cooler chunks can fall down the leading edge of the flow and be covered with advancing lava. This creates the rough layer below the core of an 'a'a flow.
It is possible for a pahoehoe flow to become and 'a'a flow, but 'a'a flows never become pahoehoe flows.