Glowing `a`a flow front advancing over pahoehoe on the coastal plain of Kilauea Volcano, Hawai`i.

`A`a flow `A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified `a`a flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an `a`a flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an `a`a flow.

Related photoglossary terms:

• Accretionary lava balls as large as 3 m are common on `a`a flows
• Pahoehoe, a Hawaiian word for the other type of basaltic lava flow
• Transition from pahoehoe to `a`a