"What happens when a volcano erupts." vs "What will happen when a volcano erupts."
What happens when a volcano erupts = What is the usual course of events what an eruption occurs
(scientific/logical implication, rather than a reference to a potential situation)
Zero conditional
What will happen when a volcano erupts = What is going to happen if one of these volcanos erupts
(results of an actual event)
First conditional
PS. RE: The "coming out" - grammatically, the same distinction applies. However, it sounds like in that case you are referring to the lava.