Geology Reference
In-Depth Information
when all slopes, both hillslopes and river slopes, are
adjusted to each other (p. 10). In practice, this early
notion of dynamic equilibrium was open to ques-
tion (e.g. Ollier 1968) and difficult to apply to
landscapes. In consequence, other forms of equilib-
rium were advanced (Howard 1988) (Figure 1.10). Of
these,
dynamic metastable equilibrium
has proved
to be salutary. It suggests that, once perturbed by
Geomorphic system dynamics: equilibrium
and steady state
As defined by John T. Hack, a steady-state land-
scape is one in which land-surface form stays the
same despite tectonic uplift adding material and a
constant set of geomorphic processes removing it.
An erosional landscape in dynamic equilibrium arises
Figure 1.10
Types of equilibrium in geomorphology. (a) Static equilibrium occurs when a system is in balance over a time
period and no change in state occurs. (b) Stable equilibrium records a tendency to revert to a previous state after a small
disturbance. (c) Unstable equilibrium occurs when a small disturbance forces a system towards a new equilibrium state
where stabilization occurs. (d) Metastable equilibrium arises when a system crosses an internal or external system
threshold (p. 20), so driving it to a new state. (e) Steady state equilibrium obtains when a system constantly fluctuates
about a mean equilibrium state. (f ) Thermodynamic equilibrium is the tendency of some systems towards a state of
maximum entropy, as in the gradual dissipation of heat by the Universe and its possible eventual 'heat death' and in the
reduction of a mountain mass to a peneplain during a prolonged period of no uplift. (g) Dynamic equilibrium may
be thought of as balanced fluctuations about a mean state that changes in a definite direction (a trending mean).
(h) Dynamic metastable equilibrium combines dynamic and metastable tendencies, with balanced fluctuations about a
trending mean flipping to new trending mean values when thresholds are crossed.
Source:
After Chorley and Kennedy (1971, 202)
