Objection 1. It would seem that this cleansing will not be effected by fire. For since fire is a part of the world, it needs to be cleansed like the other parts. Now, the same thing should not be both cleanser and cleansed. Therefore it would seem that the cleansing will not be by fire.
Objection 2. Further, just as fire has a cleansing virtue so has water. Since then all things are not capable of being cleansed by fire, and some need to be cleansed by water — which distinction is moreover observed by the Old Law — it would seem that fire will not at any rate cleanse all things.
Objection 3. Further, this cleansing would seem to consist in purifying the parts of the world by separating them from one another. Now the separation of the parts of the world from one another at the world's beginning was effected by God's power alone, for the work of distinction was carried out by that power: wherefore Anaxagoras asserted that the separation was effected by the act of the intellect which moves all things (cf. Aristotle, Phys. viii, 9). Therefore it would seem that at the end of the world the cleansing will be done immediately by God and not by fire.
On the contrary, It is written (Psalm 49:3): "A fire shall burn before Him, and a mighty tempest shall be around Him"; and afterwards in reference to the judgment (Psalm 49:4): "He shall call heaven from above, and the earth to judge His people." Therefore it would seem that the final cleansing of the world will be by means of fire.
Further, it is written (2 Peter 3:12): "The heavens being on fire will be dissolved, and the elements shall melt with the burning heat." Therefore this cleansing will be effected by fire.
I answer that, As stated above (Article 1) this cleansing of the world will remove from it the stain contracted from sin, and the impurity resulting from mixture, and will be a disposition to the perfection of glory; and consequently in this threefold respect it will be most fitting for it to be effected by fire. First, because since fire is the most noble of the elements, its natural properties are more like the properties of glory, and this is especially clear in regard to light. Secondly, because fire, on account of the efficacy of its active virtue, is not as susceptible as the other elements to the admixture of a foreign matter. Thirdly, because the sphere of fire is far removed from our abode; nor are we so familiar with the use of fire as with that of earth, water, and air, so that it is not so liable to depreciation. Moreover, it is most efficacious in cleansing and in separating by a process of rarefaction.
Reply to Objection 1. Fire is not employed by us in its proper matter (since thus it is far removed from us), but only in a foreign matter: and in this respect it will be possible for the world to be cleansed by fire as existing in its pure state. But in so far as it has an admixture of some foreign matter it will be possible for it to be cleansed; and thus it will be cleanser and cleansed under different aspects. and this is not unreasonable.
Reply to Objection 2. The first cleansing of the world by the deluge regarded only the stain of sin. Now the sin which was most prevalent then was the sin of concupiscence, and consequently it was fitting that the cleansing should be by means of its contrary, namely water. But the second cleansing regards both the stain of sin and the impurity of mixture, and in respect of both it is more fitting for it to be effected by fire than by water. For the power of water tends to unite rather than to separate; wherefore the natural impurity of the elements could not be removed by water as by fire. Moreover, at the end of the world the prevalent sin will be that of tepidity, as though the world were already growing old, because then, according to Matthew 24:12, "the charity of many shall grow cold," and consequently the cleansing will then be fittingly effected by fire. Nor is there any thing that cannot in some way be cleansed by fire: some things, however, cannot be cleansed by fire without being destroyed themselves, such as cloths and wooden vessels, and these the Law ordered to be cleansed with water; yet all these things will be finally destroyed by fire.
Reply to Objection 3. By the work of distinction things received different forms whereby they are distinct from one another: and consequently this could only be done by Him Who is the author of nature. But by the final cleansing things will be restored to the purity wherein they were created, wherefore created nature will be able to minister to its Creator to this effect; and for this reason is a creature employed as a minister, that it is ennobled thereby.
Source: Summa Theologica (New Advent)