with that terrain pcx set & the reduced size image it's hard for me to see what is mountains and what is volcano. Could you post a full size image of a smaller area - maybe the part around the cities & with volcanos in as well? A brief description of which specific terrains are "space", "satellites", and so on would help me understand what you are trying to do as well.
Just occurred to me: People have figured out how to reduce the frequency of volcanic eruptions to the minimum but it seems to be hard-coded that we cannot completely eliminate them. It may be that a line of supernovas exploding and leaving behind some sort of debris is something you want to indicate a dangerous region of space. If not, selecting another terrain to represent the impassable space may be necessary.
As to other alternatives: Using a terrain other than volcanoes avoids the eruption problem. The key thing is making it impassable. Since you will be changing the look anyway as far as settings go it doesn't really matter which terrain you choose. The Impassable flag is the same for both standard and landmark versions of the terrains. The graphics can look completely different, but that setting will be the same for both. From the minimap it looks like you are using both Tundra and Desert in the bottom part of the map - so they are probably not good choices for impassable terrain. If you are using Sea and Ocean for space, and they both look the same then you could use Ocean for the impassable terrain. That might be the simplest solution since the "space terrain" sets are already using the water terrains you would just need to change the settings without manipulating any graphics. If you don't want to go in that direction with your design then one of the terrains that covers grassland is the choice: Volcanoes, Mountains, Hills, Forests, Marshes, Jungles ... There are versions of forest that overlay plains and tundra but some of the forest settings apply to all types. Size and shape of Marsh and Jungle are less restricted than Mountains and Hills, so I'd recommend one of them. Look at the pcx - it's easier to see and understand quickly than to thoroughly explain in words. Basically - the graphic extends beyond the tile designated by the game engine. The graphic can be made large enough that it always covers the underlying tile (Grassland). By making a graphic that blends with the surrounding "space" terrain it can be effectively invisible. alternatively you could make some kind of visible barrier, such as cloudy nebulae. A long straight line of cloudy nebulae might look odd, however. Which brings us back to considering possibilities of visible variations between the landmark and standard versions of the Ocean terrain ...
I'm sure you get the main point: you can use any terrain even if the landmark version looks quite different from the standard one. As long as you know both versions will be impassable. And
the terrain you choose can be made to look like just about anything.