Page 36 - Fundamentals of Geomorphology
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WHAT IS GEOMORPHOLOGY?         19


                                                        when all slopes, both hillslopes and river slopes, are
              Geomorphic system dynamics: equilibrium
              and steady state                          adjusted to each other (p. 10). In practice, this early
                                                        notion of dynamic equilibrium was open to ques-
              As defined by John T. Hack, a steady-state land-  tion (e.g. Ollier 1968) and difficult to apply to
              scape is one in which land-surface form stays the  landscapes. In consequence, other forms of equilib-
              same despite tectonic uplift adding material and a  rium were advanced (Howard 1988) (Figure 1.10). Of
              constant set of geomorphic processes removing it.  these, dynamic metastable equilibrium has proved
              An erosional landscape in dynamic equilibrium arises  to be salutary. It suggests that, once perturbed by






































              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)
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