aa

The word “aa” means a type of volcanic lava that cools into a rough, jagged, broken-up surface, as opposed to smooth pahoehoe lava. In plain terms, a geology term for chunky, jagged lava rock, most famously seen in Hawaii.

“Aa” is pronounced AH-ah (/ˈɑ.ɑ/ in IPA).

Here is “aa” used in a sentence: “The hikers wore thick boots to cross the sharp aa field near the crater.”

“Aa” is a noun.

“Aa” is a fairly common word in modern English.

Aa is lava that cools into a rough, jagged, broken-up surface, while pahoehoe cools smooth and often ropy-looking. The two can come from the same volcano and even the same eruption; the contrast lies in how the lava behaves as it flows and cools, not in its chemical makeup.

Yes. Aa is accepted in the major Scrabble word lists used in both North American and international play, which makes it one of the most useful two-letter words in the game. It scores only two points on its own but is prized for fitting into tight corners of the board.

Aa was borrowed from Hawaiian, where the word describes exactly this rough, clinkery type of lava; its companion term pahoehoe is Hawaiian as well. Geologists adopted both words as standard terms because Hawaii's volcanoes display the two lava types so clearly.

Lava tends to form aa when it is cooler, stiffer, or losing gas quickly, causing the surface to tear into jagged, broken blocks. Hotter, more fluid lava spreads gently and skins over into smooth pahoehoe. A single flow can start as pahoehoe and change into aa as it cools and thickens downslope.