Throwing dust on one’s head is in Scripture a typical means of expressing deep sorrow (Joshua 7:6; 1 Samuel 4:12; 1 Samuel 2:12; Nehemiah 9:1; Lamentations 2:10). It is a means of expressing sorrow because it involves a confession of mortality, finiteness, unworthiness: I am dust and to dust I shall return
(see Genesis 3:19). The burning of the “Babylon,” which had given them work and purpose, confronted these sailors with reality: they too were dust.
19 And they threw dust on their heads as they wept and mourned, crying out, “Alas, alas, for the great city where all who had ships at sea grew rich by her wealth! For in a single hour she has been laid waste.