The description of God's advent to judgment is founded on the idea of a terrible storm and earthquake, perhaps accompanied with volcanic eruption, though evidence of such eruptions in the historical period is not forthcoming.
The description recalls the awful revelation at Sinai (Exodus 19:1-25.). Shall be molten; either by the lightning or the showers of rain that descend from heaven. The mountains, the type of stability and strength, fall away at the presence of the Judge.
Septuagint, σαλευθήσεται, "shall be shaken;" Vulgate, consumentur ( 5:4, 5:5; Psalms 18:7, etc.; Psalms 68:8; Psalms 97:4, Psalms 97:5; Amos 9:5). Be cleft; Septuagint, τακήσονται, "shall melt." The valleys shall be hollowed out into channels by the force of the water, which falls in torrents.
As wax (Psalms 68:2; Psalms 97:5). This belongs to the first clause, "the mountains," etc. As waters. This belongs to the second clause. The cloven plains shall melt away as waters disappear down a precipice.
The idea that underlies this description is that the inanimate creation shares in the effects of the judgment on man, and is used as an instrument in his punishment.