The Collaborative International Dictionary of English v.0.48:

Crumble \Crum"ble\ (kr[u^]m"b'l), v. t. [imp. & p. p. Crumbled
   (kr[u^]m"b'ld); p. pr. & vb. n. Crumbling
   (kr[u^]m"bl[i^]ng).] [Dim. of crumb, v. t., akin to D.
   kruimelen G. kr["u]meln.]
   To break into small pieces; to cause to fall in pieces.
   [1913 Webster]

         He with his bare wand can unthread thy joints,
         And crumble all thy sinews.              --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Crumble \Crum"ble\, v. i.
   To fall into small pieces; to break or part into small
   fragments; hence, to fall to decay or ruin; to become
   disintegrated; to perish.
   [1913 Webster]

         If the stone is brittle, it will crumble and pass into
         the form of gravel.                      --Arbuthnot.
   [1913 Webster]

         The league deprived of its principal supports must soon
         crumble to pieces.                       --Prescott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crumble
    v 1: fall apart; "the building crumbled after the explosion";
         "Negotiations broke down" [syn: crumble, crumple,
         tumble, break down, collapse]
    2: break or fall apart into fragments; "The cookies crumbled";
       "The Sphinx is crumbling" [syn: crumble, fall apart]
    3: fall into decay or ruin; "The unoccupied house started to
       decay" [syn: decay, crumble, dilapidate]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "crumble":
   abate, ablate, abrade, atomize, bate, be eaten away, beat,
   bite the dust, bray, break, break down, break up, brecciate,
   canker, cave in, collapse, come apart, come to dust, come unstuck,
   comminute, conk out, consume, consume away, contriturate, corrode,
   corrupt, crack up, crumb, crumble into dust, crumble to dust,
   crush, decay, decline, decompose, decrease, deliquesce, die away,
   diminish, disintegrate, disjoin, disorganize, dissolve, dive,
   droop, drop, drop off, dwindle, ebb, effloresce, erode, fade, fail,
   faint, fall, fall away, fall into decay, fall off, fall to dust,
   fall to pieces, fester, fission, fizzle out, flag, flour, fragment,
   gangrene, give out, give way, go bad, go downhill, go soft,
   go to pieces, grain, granulate, granulize, grate, grind,
   grind to powder, hit the skids, languish, lessen, let up, levigate,
   mash, melt away, mildew, mill, mold, molder, mortify, mush,
   necrose, peg out, pestle, peter out, pine, plummet, plunge,
   poop out, pound, powder, pulverize, putrefy, putresce, rankle,
   reduce to powder, rot, run low, sag, scrunch, shard, shiver, shred,
   shrink, sink, smash, sphacelate, split, spoil, squash, subside,
   suppurate, tail off, taint, topple, triturate, tumble, turn, wane,
   waste, waste away, weaken, wear, wear away, wear thin, wilt,
   yield