I think that volcanic eruptions will not spare any area. There seems to be no pattern as to where and they blow up. As to when, you know that there is an impending eruption when the lava starts to shrink.
And yes, it would be nice to actually have a physical volcano. When it degrades it could become a mineral deposit of some kind.
Commonwealth of Groddle
Groddle | Bell's Pyramid | Glitch
Nope but it's easier if they come in new places letting the old lava cool down