Aa, noun

A brittle, scoriaceous substance consisting of sand, earth, stones, and melted lava, cooled and broken up. (Hawaiian).

Aa, any of several small rivers of Europe, especially, two in Latvia, both emptying into the Gulf of Riga, and two in NW Germany.
Funk and Wagnalls, Standard Dictionary, International Edition ©1976

Aa (pronounced "ah-ah" - a Hawaiian term), is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.
Source: Volcanic Materials

Aa, Volcanic
Aa: Lava that has a rough, jagged surface.

AA, a geological and volcanological term which designates a peculiarly rough and jagged type of lava-flow surface. The word has a Polynesian (Hawaiian) origin, where it first came into usage, in contrast to smooth lava flow surface referred to as pahoehoe. Lava occurs when the flow cools, solidifying while in motion. The upper surface and forward edge of the flow cools leaving the interior viscious; the crust of the surface and front of the flow is broken by the movement of the unsolidified lava, resulting in a mass of jagged and broken clumps and pieces of the lava crust roll downward a slope, which contributes to its rough appearance. Aa is also formed when lava comes in contact with water, which is quickly converted to steam and violently erupts, breaking up the crust into the characteristically jagged form of aa.
Colliers Encyclopedia, ©1960, paraphrased

  • aa, noun.lava hardened into a rough, slaggy, brittle mass.
  • AA (no periods),an abbreviation or symbol for the following: 1. achievement age. 2. air-to-air. 3. anti aircraft. 4. author's alteration. 5. (in financial rating) the second highest rank. 6. the second in a series of narrow shoe widths (followed in degree of narrowness by AAA).
  • A.A.,an abbreviation for the following: 1. Alcoholics Anonymous. 2. antiaircraft. 3. Associate in Arts. 4. Automobile Association (of Great Britain).