Hawaii is a series of islands composed, primarily, of lava. Lava isn’t all the same. Two main types are A’a (ah-ah) and pahoehoe (paw-hoey-hoey). There is also a third type, but you’re not likely to encounter it as it forms during submarine eruptions, this is called ‘pillow’ lava.
The dynamics of a lava flow generally dictate which type of lava forms. A’a lavas are associated with high discharge rates and steep slopes, while pahoehoe flows are associated with lower discharge rates and gentle slopes. Geology aside, pahoehoe is usually darker and a’a tends to be lighter and brownish to reddish. The reddish comes from oxidation of the iron to iron oxide.
Pahoehoe tends to be smooth. You can generally walk on it without shoes. A’a on the other hand is chunky and sharp – think of the sound you’d make when trying to walk on it bare foot!