Cataclysm, n.
Etymology: [L. cataclysmos, Gr. cataclysme.]
1. An extensive overflow or sweeping flood of water; a deluge.
2. (Geol.)
Any violent catastrophe, involving sudden and extensive changes of the earth's surface.
Shades of meaning
Fernald has no entry of its own for cataclysm; he lists it under catastrophe.
Where he writes about it:
A cataclysm or catastrophe is some great convulsion or momentous event that may or may not be a cause of misery to man. — from catastrophe