What does aa mean?
Aa is a noun used in geology to describe one of the two principal surface forms of basaltic lava, the other being smooth, ropey pahoehoe. When lava rich in certain conditions cools while still moving slowly, its crust breaks into a jumble of angular, clinkery fragments, producing a landscape of razor-sharp rubble that is notoriously difficult and dangerous to walk across. Borrowed into English from Hawaiian in the nineteenth century, aa is prized by word-game enthusiasts as one of the shortest legitimate entries in standard dictionaries, yet for volcanologists it carries precise descriptive weight: identifying whether a flow is aa or pahoehoe reveals information about how the lava moved and cooled. The word appears chiefly in scientific writing and accounts of volcanic regions such as Hawaii, Iceland, and the western United States.
nounAa is rough, jagged basaltic lava that hardens into a clinkery surface of sharp, fragmented blocks. It contrasts with pahoehoe, the smooth, ropey form of basaltic lava.
- Rough, jagged basaltic lava with a sharp, clinkery surface texture.
"The trail across the aa was so jagged that hikers wrapped their boots in duct tape to protect them."
"The 1950 eruption covered part of the coastline in fresh aa."
"Crossing the field of aa without sturdy boots was nearly impossible."
An uncountable mass noun; it has no plural form.
"The slope was blanketed in fresh aa after the eruption."
"Aa" is one of the only two-letter words allowed in Scrabble — and it isn't even English in origin; it's Hawaiian.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of aa
The word aa entered English in the mid-nineteenth century, borrowed directly from Hawaiian, where it described the same rough lava texture. Geologists studying Hawaiian eruptions adopted the native term alongside pahoehoe, its smooth-surfaced counterpart, giving English a rare pair of technical terms drawn from a Polynesian language. Both words remain standard in volcanological vocabulary worldwide.
How aa is actually used
A technical term used mainly in geology and volcanology; it is always a mass noun and is not capitalized except in all-caps contexts where it could be mistaken for the abbreviation AA. It is frequently mentioned alongside its counterpart pahoehoe when describing Hawaiian lava flows.
Easily confused with aa
"AA" in capitals most often denotes Alcoholics Anonymous or a battery size, whereas "aa" lowercase names a type of rough lava.