On the Earth, gravity- and radiation-driven geomorphic systems interact independently, so that their two types of activity can mingle under conditions of periodic random dominance. Thus, peak energy expenditures engendered by each type of system may or may not coincide geographically. Maximum rates of landform change occur where active orogenesis mingles with changing climates. Minimal change occurs where epeirogenic regions are occupied by morphogenic areas that are in states of dynamic equilibrium. In this arrangement of interacting geomorphic systems, there is clearly a place for both catastrophe and gradualism. There also is a place for cycles of erosion of several kinds and for dynamic equilibrium, either as an end phase of enduring climatic morphogenesis and/or as an end phase of relief and elevation reduction by denudation following orogenesis.

The concept of periodic random dominance as an aspect of landform evolution carries with it the implication of polygenetic landforms and landscapes where geomorphic system dominance fails to develop. Indeed, dominance becomes the special case because it is dependent on a particular juxtaposition of tectonic and/or climatic elements over a protracted interval in a given area. One estimate places polygenetic landforms over approximately 80 percent of the Earth’s land surface. Perhaps 20 percent is experiencing some type of geomorphic system dominance—less than 10 percent if Antarctica is omitted from the calculations.

Process geomorphology and systems equilibria

Details of landform evolution within a given geomorphic system are matters of process behaviour and terrain response. In the context of geomorphic system dominance versus systemic alternation, two general situations exist: (1) those agencies operating in contact with relicts that they are modifying, often quite rapidly, and (2) those in contact with equilibrium features that they have created and have little or no ability to modify further. The principal surficial geomorphic agencies on Earth—wind, running water, glacial ice, and gravity—in any given geomorphic system induce processes that tend to evolve toward a situation of least work. Polygenetic terrain is usually some combination of hillslopes and “flats,” and either topographic type may dominate in the latter part of a geomorphic cycle, depending on whether the system tends to generate relief or reduce it.

Natural geomorphic systems operating along the Earth’s surface are classified as open, since they are powered by external energy sources. Because the rates of both endogenetic and exogenetic energy input vary, the coordinate agencies experience changes analogous to power surges in an electrical system. Thus, rivers receiving excess runoff periodically flood. The atmosphere locally builds up excess heat, and the transfer of this heat is expressed in storms. Glaciers, normally the epitome of slowness, can acquire a mass-energy excess and consequently surge. In all instances, energy available for erosion, transportation, and deposition of sediment varies greatly over time. In addition, the interaction between solids, fluids, and gases results in turbulence, eddy formation, shearing and vortex activity, and periodic local stagnation.

In response to the foregoing situations, process associations within individual geomorphic systems exhibit typical systems phenomena, including “feedback,” “threshold reactions,” and evolution toward dynamic equilibrium (least-work) modes. Where a system is periodically perturbed, processes can pass back and forth between disequilibrium and steady-state conditions rather frequently.

The behaviour and apparent process direction of an individual agency may not reflect the evolution of the overall geomorphic system. For example, a 10,000-year-long episode leading to the formation of an alluvial fan may be seen to include numerous incidents of fan-head trenching that are separately destructive but subordinate to depositional events dominating the trend. Similarly, a river such as the Mississippi that is reworking a relict alluvial deposit in a valley may be seen to be depositing gravel on point bars on the insides of bends. The long-term consequence of the river’s activity, however, will be to remove the entire alluvial deposit in its path, including the point bars, unless subject to systemic interruption. (Humankind has of course “short-circuited” the natural evolution of the Mississippi and that of many other rivers with engineering modifications.)

From the foregoing, it seems evident that the direction of landform evolution can only be grasped from the study of geomorphic process if the character and role of relict landforms and deposits are clearly understood. This is an obvious complication in the application of Hutton’s doctrine of uniformitarianism.

The concept of periodic geomorphic system dominance provides the rational potential end point of landform evolution under a particular set of conditions. Ideally, it may yield either modified or unmodified tectonic landscapes. These in turn may be either orogenic or epeirogenic. Where modified, they may express marine effects and/or glacial, arid, or humid morphogenesis. Antithetically, where more common polygenetic morphogenesis occurs, some mixture of tectonic, marine, or climatic effects is superimposed on the setting, and a hybrid suite of landforms results.