يَومَ تَرجُفُ الأَرضُ وَالجِبالُ وَكانَتِ الجِبالُ كَثيبًا مَهيلًا

The day when the earth and the mountains will quake, and the mountains will be like dunes of shifting sand.

EXEGESIS

The imperfect tense verb tarjufu means to tremble, quake, and shake.^{‎[1]Arabic-English Dictionary of Qur’anic Usage, p. 350.} Rāghib defines the verbal noun of this verb, which is rajf, to mean: a violent and intense turbulence, turmoil, and disturbance.^{‎[2]Mizan, 20/68.}

The word kathīb means a heap of sand, a sand dune,^{‎[3]Arabic-English Dictionary of Qur’anic Usage, p. 798.} while the word mahīl means to be made to pour down, to be made to trickle down, to heap up, and made to avalanche.^{‎[4]Arabic-English Dictionary of Qur’anic Usage, p. 999.} Derived from h-y-l, it means to pour down, to slide down, and avalanching sand dunes, or, to fill something soft and fine such as sand onto something, and so the meaning intended here is soft fine sand that is not stable or settled or steady.^{‎‎[5]Amthal, 19/140; Arabic-English Dictionary of Qur’anic Usage, p. 999.}

EXPOSITION

Verses 12 and 13 make a promise of heavy chains and a burning fire, choking food, and a painful chastisement, but when? The reply is in this verse: The day when the earth and the mountains will quake and the mountains will be like dunes of shifting sand.

Thus the meaning is that mountains will vanish or disappear and in its place will be soft, fine sand only, such that if one were to tread or step on it, the feet would sink in and disappear.

The shaking of the earth is mentioned in 99:1, 56:4, and 89:21, and the destruction of the mountains is mentioned in other verses, such as 20:105, 56:5, 77:10, 18:47, 101:5, 70:9, 78:20, 69:14, and 81:3.

[1] Arabic-English Dictionary of Qur’anic Usage, p. 350.
[2] Mizan, 20/68.
[3] Arabic-English Dictionary of Qur’anic Usage, p. 798.
[4] Arabic-English Dictionary of Qur’anic Usage, p. 999.
[5] Amthal, 19/140; Arabic-English Dictionary of Qur’anic Usage, p. 999.