Plural of aa; a type of rough, jagged lava with a crumbly surface, as opposed to smooth ropey lava.
From Hawaiian 'a'ā,' referring to the rough lava type. Hawaiian volcanologists distinguished between 'aa' (sharp, rough) and 'pahoehoe' (smooth, ropy). The term entered English geological vocabulary in the 19th century through contact with Hawaiian islands.
Geologists borrowed Hawaiian words directly because the Hawaiians had centuries of detailed vocabulary for lava types—'aa' literally means 'hurt' in Hawaiian because walking barefoot on it causes pain, which is a perfect descriptive name!
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