We were going to have lots of seismic activity, and ultimately, there was going to be this big fight between the forces of good and the forces of evil, which ended up in the series manifesting as Jack and The Man in Black, in the midst of magma.
The volcano had been dormant for the duration of the series, but based on moving into this endgame, the island had become unstable and the volcano was going to erupt.
The question was always, how do you basically visualize and dramatize the idea that the island itself is all that separates the world from hellfire and damnation?