noun: lava having a rough surface
From Hawaiian aa (to burn).
Earliest documented use: 1859
Usage (from Wordsmith)
“The type of lava most prevalent on Bartolomé Island is aa lava; it has a lower temperature and less gas content, which makes for a slower flow, which causes broken flows and sharp edges.”
Ilene Cox; One More Day in the Galapagos Islands; Redlands Daily Facts (California); Sep 25, 2005.
Notes (from Wordsmith)
Aa is one of the two kinds of lava typically found in Hawaiian volcanoes. The other kind is pahoehoe, one with a smooth, ropy surface.