وَإِذَا ٱلْأَرْضُ مُدَّتْ
Wa izal ardu muddat
Kiedy ziemia będzie wyrównana
and when the earth is stretched out,
Madda is the verb at 13:3, 15:19 and 50:7 for the earth being spread out at its making, with firm mountains set down onto it afterwards. The same verb returns here for its undoing, so what was spread and weighted is pulled flat and left holding nothing, and the line following has it emptying out whatever it contained. Sky and earth are each given one verb of undoing and then the identical clause of consent.