Mm. Knowing the difficulty helps to, because on Plymouth, your base/the lava flow actually change depending on difficulty. On easy your base shouldn't be affected, but on hard you're f***ed.
For Plymouth Starship, Just expand east/west. Going up or down will lead you into the lava flow. Play it a few times and learn the flow and where not to go. On medium, the lava flow will rape your smelter, and I haven't survived long enough to determine where the second volcano flows to (I haven't played this mission on medium in a good 6 years so my old strategies blow hardcore). On hard, well, your guess is as good as mine.
For Plymouth Population, you're fine on Easy. Just don't go too far south and micro-manage your trucks (or use waypoints) until the lava stops flowing. On medium/hard, you have to relocate. I personally recommend moving to the Australia (south-east corner) location, as you have 2 commons and a rare. However, the truck routes are plagued by vortexes so you have to keep your smelters rather far away. I'll post a picture about a good location if you'd like. Other than that, no other location is decent and has rare ore. On medium, the lava will only take out your starting agridome and your mine. On hard, it will smother your entire base.