A’a (pronounced “ah ah”) is the Hawai’ian word for one type of volcanic lava, specifically the kind that comes out as small rocks with jagged edges. It contrasts with pahoehoe, which is lava that comes out more as a flow, kind of like molasses; when it hardens, it looks like a cow patty. I have a friend who grew up in Hawai’i on the Big Island, and that’s what he told me, anyway. He said when he was a kid he picked coffee, and could walk on the a’a, despite it having sharp edges (his word was “crinkly”).
from The Sound of One Hand Typing