structural:expansion, information:classification
ʻ A ʻ ā

ʻ A ʻ ā (also spelled aa, a ʻ a, ʻ a ʻ a, and a-aa, and pronounced or / ˈ ɑː (ʔ) ɑː /) is one of three basic types of flow lava. ʻ A ʻ ā is basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker. The word is Hawaiian meaning "stony rough lava", but also to "burn" or "blaze"; it was introduced as a technical term in geology by Clarence Dutton.

The loose, broken, and sharp, spiny surface of an ʻ a ʻ ā flow makes hiking 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 ʻ a ʻ ā 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 ʻ a ʻ ā flow.

Accretionary lava balls as large as 3 metres (10 feet) are common on ʻ a ʻ ā flows. ʻ A ʻ ā is usually of higher viscosity than pāhoehoe. Pāhoehoe can turn into ʻ a ʻ ā if it becomes turbulent from meeting impediments or steep slopes.

The sharp, angled texture makes ʻ a ʻ ā a strong radar reflector, and can easily be seen from an orbiting satellite (bright on Magellan pictures).

ʻ A ʻ ā lavas typically erupt at temperatures of 1,050 to 1,150 °C (1,920 to 2,100 °F) or greater.