International Encyclopedia of Systems and Cybernetics

A very quick succession of changes triggered by a minimal perturbation, in a network or a composite system in a critical state.

An avalanche is a percolation phenomenon and is submitted to a power law distribution.

According to P. BAK and K. CHEN: "An avalanche is a type of chain reaction or branching process" (1991, p.28). Good examples are the propagation of an earthquake, of a forest fire or of a mud or snow slide.

In composite systems, the possibilities that such avalanches could propagate, depend on the characteristics of the network and, particularly on the existence of more or less extended stable "islands". (Blocks of frozen elements as described by St. KAUFFMAN, 1993, p.67)

Avalanches imply the existence of conditional interactions between elements in a more or less loose network.

P. BAK writes: "Large avalanches, not gradual change, make the link between quantitative and qualitative behavior, and form the basis for emergent phenomena" (1996, p. 32)

This is significant not only for physical phenomena, but also for economic and sociological ones: "If this picture is correct for the real world, then we must accept instabilitiy and catastrophes as inevitable in biology, history and economics" (Ibid).

History here covers political and social up-heavels.

And: "Because the outcome is contingent upon specific minor events in the past, we must also abandon any idea of detailed long term determinism or predictability" (Ibid).

This does not however seems to be absolute: "detailed" defines the basic meaning of this sentence. We still have a kind of overall long-term stochastic determinism, related to self-organized criticality. Earthquakes for instance are frequent along some known geological fracture lines, and rare elsewhere. In time, their frequency is relatively defined by a specific power law (Gutenberg- Richter Law) (Ibid, p.13)