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]