Aa

Aa (pronounced "ah-ah") describes a volcanic terrain with a particularly rough and jagged surface, resulting from a lava that cools and solidifies while still in motion. As the forward edge of the flow cools, the interior is yet viscous — this unsolidified lava breaks the surface crust, so that the front of the flow becomes a mass of broken and jagged lumps, with pieces of lava crust rolling down slope, adding to the rough and jumbled character.

Aa is also formed when a lava flow comes in contact with ground water, which is quickly turned to steam and erupts violently, breaking up the lava crust.

The incredibly spiny surface of solidified aa makes walking very difficult and slow, disallowing any stride greater than 1 for humanoids and most creatures, except those, like jellies, which are unaffected by aa's nature.

See also,
Jungle (range)
Pacific Islands