Tafsir of Al-Muzzammil 73:14

(The earth and the mountains shall tremble). It is said that [this phrase] is connected to "leave Me" (dhar-ni); it is also said that it is an adjective describing "a punishment"; and it is said that it is connected to "painful" (alīman). A group has chosen the view that it is connected to the "settling" (istiqrār) to which "with Us" (ladaynā) is connected—meaning: that punishment shall settle with Us and manifest on the day the earth and mountains are convulsed and shaken. Zayd read "shall tremble" (turjafu) in the passive voice.

And the mountains, despite their hardness and height, shall be a heap of sand (kathīb)—that is, gathered sand, from the phrase "I heaped the thing" (katabtu al-shay') meaning I gathered it. It is as if it were originally a fa‘īl form in the meaning of maf‘ūl (passive participle), then it became dominant until it acquired the status of rigid nouns. The statement is a case of hyperbolic metaphor.

It is said that there is no prevention to it being literal sand. "Pouring" (mahīl) is said to mean soft and yielding, such that when a foot treads upon it, it slips from beneath it. It is also said that it means "scattered," derived from "poured out" (hāla) when something is scattered and made to flow. Its being a "heap" refers to the state it was in before being scattered, so there is no contradiction between it being gathered and being scattered. It is not intended to mean that it is merely in the potentiality or state of becoming so, as has been suggested.