Does that "lava" (pollution) cause terrain damage like regular pollution if you don't clean it up? Since old lava flows are actually quite fertile if there is no further activity, maybe after a century or so a new forest or grassland should spring up if you leave it.
A note to Firaxis: how about some different kinds of pollution/devasted tiles in Civ 4? Radiation for nuclear fallout (causes unit damage) useless mud flats (if floods/hurricanes are brought back), chemical spills, that kind of thing.
