geomorphological threshold

A point at which a landform becomes unstable. An intrinsic threshold indicates that changes are possible independently within the system; for example, a surging glacier, while an extrinsic threshold indicates change brought about by change in an external factor; for example, the formation of gullies with the progressive removal of the vegetation cover. However, the changes which initiate discontinuities may be different from those needed to maintain or reverse them; glacial surging again providing an example.

Such sudden discontinuities may be mathematically modelled using catastrophe theory.