FYI: lava can ONLY flow on lava-rock, which is the darkgrey tiles. If you switch to the altitude minimap, it's colored black & lightgrey (darkgrey spots are non-passable, so they dont concern you anyway). As for how lava flows, it's controlled by the mission script: i could set it to flow any specific tile, if i want to, or excluded any tile. This alos means the lava will ALWAYS stop at the same point, no matter how many times you play the mission.
This specific colony game has a small gap in the lava-flow just east of the original base, so you can still move between the 2 base sites TH300 mentioned.
Good luck!