Sand formations can collapse catistropically. Watch collapse propagate on the surface of a large dune.

YOUTUBE WzJCkh-vPyQ Sand avalance recorded by YouTube member Velonarr.

The Abelian Sandpile Model is the first discovered example of a dynamical system displaying self-organized criticality. The model is a cellular automaton where each site on a finite grid has an associated value that corresponds to the slope of the pile. This slope builds up as grains of sand are randomly placed onto the pile, until the slope exceeds a specific threshold value at which time that site collapses transferring sand into the adjacent sites, increasing their slope. wikipedia

See also Singing Sand