But are there gasses that we know of on Earth that are volatile enough to react violently with another gas to cause a full-out explosion?
I guess there would be a few, but it most likely won't suddenly happen now without some sort of catalyst. If such things did happen so spontaneously, I would expect the reactants to be used up already. Now I can kinda understand your desire for random disasters all over the map, which of course you do nothing to cause. It just wouldn't be Outpost without them. But I think you need some kind of event that causes these things. Certain disasters just work. Like earthquake and volcanic eruptions. What causes them is beneath the earth and not something the player sees or needs to concern themselves with really. I can't see it going any further than the simple warning system used in Outpost2. But for something like lightning to occur, you must first have the conditions of a storm arise. Maybe if you had something like lightning from a storm causing a violent explosion with surface or atmospheric substances? Then you can reasonably expect such gases or whatever to naturally build up, and stay there until some external force acts on them. It wouldn't be too unexpected for gases to be released by geological activity, but I think they'd either react as soon as they're released, or need some sort of event to cause it.
Hmm, I wonder what would happen if ligtning stuck a vortex of volatile gases?
As for chemical reactions, if you have oxygen on your planet, then any hydrocarbon could explode given an initial spark. The lighter ones would of course be gases, such as methane. Also, methane seems to be rather common, and I believe it's known to come from geological activity. (Someone else want to correct me on that?)
The only other fun reaction I can think of, is cesium and water. But I don't see how that can be worked into the game very well. The best I can think of is maybe having a dry planet with lots of cesium, but no water. Then when you arrive, you supply the water. Clean hydrogen burning Cargo Trucks anybody?
Yeah..., maybe not. Besides, I don't think that's so much of an explosion as just a violent reaction that releases a lot of heat. But I guess that can still be plenty harmful.
Hmm, why not a slower acting disaster like that? It might give you a bit more time to react and try to reduce the damage done even after it's started.