()  [Hawaiian 'a-'.] 

    A rough, scoriaceous lava, one of the two chief forms of lava emitted from volcanoes of the Hawaiian type, the other being PAHOEHOE.
 
  1859 R. C. HASKELL in Amer. Jrnl. Sci. & Arts 2 Ser. XXVIII. 70 We..saw ‘pahoihoi’ or solid lava forming, and also ‘aa’ or clinkers. 1880 Encycl. Brit. XI. 531/2 The a-a or spongy lava, which, on account of its extreme roughness and hardness, is carefully avoided by all travellers. 1883 C. E. DUTTON in U.S. Geol. Surv. 4th Ann. Rep. 95 The second form of the lavas is called by the natives a-a, and its contrast with pahoehoe is about the greatest imaginable. It consists mainly of clinkers sometimes detached, sometimes partially agglutinated together with a bristling array of sharp, jagged, angular fragments. 1914, 1920 [see block lava s.v. BLOCK n. 23]. 1944 C. A. COTTON Volcanoes iv. 27 Cooling and solidification frequently takes a different course..in lava flows, producing the clinker-like ‘aa’ lava.