Aa (from Hawaiian ‘a‘a, rough lava, ) is one of three types of flow lava. The other two types of flow lava are pillow lava characterized by smooth rounded pillows formed where lava contacts water, and pahoehoe characterized by ropy surface.
Aa is characterized by a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified aa 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 aa 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 aa flow. Accretionary lava balls as large as 3 m are common on aa flows. Aa is usually higher viscosity than pahoehoe. Pahoehoe can turn into aa if it becomes turbulent due to meeting impediments or steep slopes.
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