To crumble is to break into small pieces, or to fall to pieces.(verb)
See crumble in Webster's New World College Dictionary
See crumble in American Heritage Dictionary 4
crum·bleverb crumbled crum·bled, crumbling crum·bling, crum·bles verb, transitive
Origin: Alteration (influenced by crumb) of Middle English cremelen, from Old English *crymelen, frequentative of gecrymmian, to break into crumbs, from cruma, crumb.
Learn more about crumble