Explosions don't cause a pillar of fire, they cause explosions.
The question is why did Mashima have Gildratz,Bixlow and Fried leave this early? Gildratz up to something, why would he not wait to see who passes this trial that is only done once a year?
If you missed what others said already, here are what the 2 choices that are more likely to happen with Gildratz group. I might had miss some though.
1. Gildratz group comes back to the island just near end when all the major fights have already finished or about to finish. In other words before they are killed they make it just in time.
2. They never show up FT gets utterly defeated and Zeref is the new Blackbeard in other words Natsu's enemy/rival that probably won't be resolved till the end of the series.
Could be a mixture of 1 and 2 but fuck it type that later.
They won't be coming back any time soon, definitely not before Gajeel.
Do you purposely say what won't happen?