AA-LAVA

Synonyms

aʻa; ʻaʻa; a-aa

Definition

Aa (derived from the Hawaiian word a’a’ for rough lava) lavas is the term for cooling textures of a highly viscous lava flow and was introduced as a technical term by Clarance Dutton in the year 1883. Aa is characterized by a rough rubbly surface composed of broken lava blocks called clinkers. When the molten rock cools, the lava flow increases its viscosity due to degassing and crystallization of minerals. At some point, the lava flows so slowly that it allows a thick crust to form. As flow continues to move, the cooler and brittle skin surface breaks into rough jagged blocks, or clinkers. The clinkers are carried along at the surface. At the leading edge of an aa flow, however, these cooled fragments often tumble down the steep front and are subsequently buried by the advancing flow. This process 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 (Figure 1).

AA-...