noun[mass noun] Geology
Basaltic lava forming very rough, jagged masses with a light frothy texture. Often contrasted with pahoehoe.
- Roads of black ash traverse it, winding through the otherwise untraversable aa.
- A serpentine flow meanders through formations of smooth pahoehoe lava and rough aa lava, the same forms common in basalt lavas.
- Very different from jagged aa, pahoehoe is the other general texture of newly solidified lava.
Mid 19th century: from Hawaiian 'a-'a.
Definition of aa in: