Tafsir of An-Naba' 78:20

"And the mountains are set in motion," meaning in the atmosphere while retaining their form after being pulverized and uprooted from their foundations, as indicated by the words of the Almighty: And you see the mountains, thinking them solid, while they pass like the passing of clouds. Included within this is a similitude of the mountains to the threads of clouds in terms of the rarefaction and fluffiness of their particles, as expressed by the words of the Almighty: And the mountains will be like wool, teased.

"So they become a mirage," meaning that after being set in motion, they become like a mirage. You see them after they have been crumbled and elevated into the air as if they were mountains, yet they are not mountains; rather, they are thick, accumulated dust seen from a distance as if they were a mountain, just as a mirage is seen as if it were a sea, for example, while it is not. The speech is thus a case of tashbih baligh (eloquent simile). The common factor is that both the mountains and the mirage are seen in the shape of something, while they are not that thing.

It is permissible that the point of resemblance is rarefaction, as they become, after being set in motion, scattered dust, just as the Almighty said: And the mountains are pulverized with a pulverizing, and become dust dispersed. Among the wonderful observations in the natural sciences by Muhammad al-Harawi is that a mirage is air whose lower layer—that which is adjacent to the earth—is heated by the earth warming from the heat of the sun. Thus, it becomes rarefied, and a portion of it rises to the layers above, which are denser than what is below them. Due to this heating, it departs from its natural position on the earth, and the reflection and refraction of light rays occur within it in a specific manner explained in the aforementioned book. With the reflection of the color of the sky, it is thought to be water, and one sees in it the image of things inverted. Sometimes one sees in it swimming images like palaces, pillars, beautiful and strange dwellings, and moving specters whose forms change at every moment and shift from their places, then vanish. They are nothing but images resulting from the reflection of visible objects that are very distant, or those superimposed in layers of air of varying density. Therefore, considering only rarefaction as the point of resemblance is not without scrutiny.

However that may be, this is after the second trumpet blast upon the gathering of creation. Allah, the Almighty and Majestic, sets the mountains in motion and makes them scattered dust, and levels the earth on that day, as expressed by His words: And they ask you about the mountains, so say, "My Lord will blow them away with a blast. And He will leave the earth a level plain; you will not see therein any crookedness or complexity." On that day they will follow the Caller... and the words of the Almighty: On the day the earth will be replaced by another earth as will the heavens, and they will come out before Allah, the One, the Prevailing. The following of the Caller—who is Israfil, peace be upon him—and the emergence of creation before Allah the Almighty, does not occur except after the second blast. As for the crumbling and splitting of the mountains, that is at the first blast. It has been said that their setting in motion and becoming a mirage is also at the first blast, but the literal appearance of the verse rejects this. Yes, if the sentence were considered a circumstantial clause—meaning: "You will come in multitudes while the mountains have been set in motion and become a mirage"—that would be possible.

The apparent meaning is that they become a mirage for the purpose of leveling the earth, and it is not unlikely that there are other wisdoms therein. The claim of some that they flow like water, moving as water moves like a mirage—thereby increasing the agitation of those thirsty at the place of gathering and intensifying their longing for water—is contrary to the apparent meaning.