Once again tsuwm has a wwftd worth a bit of further discussion.

AA is a kind of lava that is fragmented into small sharp prisms that hurt your bare feet so that you say "A! A!" as you gingerly pussyfoot across it.

Pahoehoe is the more common lava that has a relatively smooth surface.

For a discussion of them, here is a URL with some excellent pictures and description:

http://volcano.und.nodak.edu/vwdocs/vwlessons/lava.html

Dear Jackie: I cannot remove offensive word from heading in your post or in your text.