"aa" is more properly written with an okina to show there is a glottal stop. - a'a.
And in case you're having a problem with pahoehoe : it is pronounced Pah-hoy-hoy like the hoy in Ahoy.
Pahoehoe is smooth and, like a'a, black and retains the heat of that tropic sun! Some people turn back from treks because the heat coming up through even the thick soles of walking/running sneakers is too much for them.
A'a can cut the soles of even very sturdy shoes all to heck so if you're planning a walk ... *don't *go *barefoot!