cataclysm

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A violent upheaval that causes great destruction or brings about a fundamental change.
• noun A violent and sudden change in the earth's crust.
• noun A devastating flood.

from The Century Dictionary.

• noun A deluge or an overflowing of water; a flood; specifically, the Noachian flood.
• noun In geology, an inundation or deluge, or other violent and sudden physical action of great extent, supposed to have been the efficient cause of various phenomena (as of the deposition of different formations of diluvium or drift) for which the gradual action of moderate currents, or that of ice, is considered to have been inadequate.
• noun Figuratively, a sudden or violent action of overwhelming force and extended sweep.

from the GNU version of the Collaborative International Dictionary of English.

• noun An extensive overflow or sweeping flood of water; a deluge.
• noun (Geol.) Any violent catastrophe, involving sudden and extensive changes of the earth's surface.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sudden, violent event.
• noun geology A sudden and violent change in the earth's crust.
• noun A great flood.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a sudden violent change in the earth's surface
• noun an event resulting in great loss and misfortune