Well you have to define the area.
int aid = rmCreateArea(string area name);
rmSetAreaSize(aid, int min size, int max size);
rmSetAreaTerrainType(aid, string terrain type);
rmSetAreaLocation(aid, x,z);
rmBuildArea(aid);
Now the area is built. The area IDs do work in rms; however, I believe Matei said they do not work in triggers. You could find the id for the area first, outside of the trigger and then use the variable with id first. If that doesn't work you could try placing units on the lava terrain you made and then running a loop over a bunch of integers until you find the area that has units in it. You could even do this at the beginning of the game place some strange Gaia unit on the lava and do a loop over terrain ids and query that unit. Once you've found it you can save the area ID.