The Lord descended in fire for further terror to obstinate sinners. Hence this law is called a fiery law, Deuteronomy 33:2. The whole mount quaked greatly, by an earthquake, as appears from Psalms 60:2, Psalms 104:32.
Related Commentaries of Exodus 19:18
Exodus 19:18
18 And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.