Symbolism of Fire and Burning

Many scriptures are based on symbolism. Part of the Lord's purpose in this, I believe, is to help us understand more clearly his purposes by relating principles or events to objects. Since His wisdom, knowledge and understanding is infinitely greater than our own, He has to simplify His teachings so our mortal brains can comprehend. Christ taught in parables so people would remember basic principles of the gospel, such as charity, through the story of the good Samaritan. The book of Revelation from the New Testament is one almost universally proclaimed as difficult to understand without proper study. Isaiah also wrote in symbolism. One source states that the book of Malachi was written by Isaiah. Judging by the writing style in "Malachi," I would not be surprised if Isaiah was the author. This brings us to the topic of discussion. Chapter 4 of Malachi says: "For, behold, the day cometh, that shall burn as an oven, and all the proud, yea, and all that do wickedly, shall be as stubble: and the day that cometh shall burn them up, saith the Lord of hosts, that it shall leave them neither root nor branch." Going back to the allegory of the vineyard and the olive trees, the people of the earth are the individual trees. Those without strong roots resisted the nourishing and pruning of the Lord, and produced no fruit. The Lord of the vineyard subsequently cast off the wild and unproductive branches of the trees into the fire. My question is, what exactly is burning? The lack of roots and good branches are a result of pride and wickedness, a personal choice. The parallel is obvious there. But what is the fire? Will those who have done evil, or have a lack of good work be literally tossed into a bonfire? Reason makes me think not. Nuclear war that only the righteous who have followed counsel and hidden in proper places will be saved from? Or is this burning a state of mind? When Christ comes, they will not be able to stand His presence, feel a burning of conscience and ask to leave the earth? What say you?