global geomorphology Global geomorphology refers to the study of regional topography and processes that shape regional topography over geological time. Geomorphology at these scales reflects the sum total of local processes acting upon the landscape. During the past century of research, geomorphologists have altered their approach, from emphasizing time-dependent evolution of regional landforms, based on W. M. Davis's Geographical Cycle, to focusing on local time-independent process–response studies. Davis's model, with its elegant progression of landscapes from ‘youth’ to ‘maturity’ to ‘old age’, is often blamed for leading generations of geomorphologists on fruitless searches for low-relief peneplains. In retrospect, the greater disservice of the Geographical Cycle may have been that the backlash against it caused additional generations of researchers to eschew regional synthesis and turn blind eyes to the role of time in geomorphological systems. More recently, however, researchers have begun to utilize new techniques to re-examine regional topography and long-term landscape evolution (see landscape evolution).

Examining landscape at a global scale (Fig. 1), we see that the Earth's topography reflects the operation and interaction of three major subsystems: tectonics, surface processes, and climate. The Earth's area–altitude distribution is distinctly bimodal, with one mode near sea level and the second at depths of roughly 4–6 km. This bimodal distribution reflects the tectonic differentiation of oceanic and continental crust. Tectonics is responsible for the most dramatic contrasts in the long-wave length topography visible at this scale, including the deepest spots on the surface—the oceanic trenches—and the highest peaks. The most dramatic manifestations of tectonic activity, at least to our human, land-dwelling eyes, are the Earth's mountain ranges. Most mountain ranges are intimately associated with the boundaries of the mobile lithospheric plates. Mountain building can be viewed as the tectonic addition of mass to the crust, through either compression or magmatism.

At one time or another during the eons of geological time, tectonic activity has acted upon all of the Earth's continental crust (to some, this action defines continental crust and explains its origin). If mountain building has been so pervasive in the past, why then do the high-altitude areas in Fig. 1 represent such a minuscule portion of the continents? The answer, of course, is that erosion acts over time to remove mass, transport it, and eventually deposit that material close to sea level. The low- to mid-altitude areas of the Earth generally represent regions where geomorphological processes have predominated for long periods of time; hundreds of millions of years or longer in some areas. In addition, geomorphological processes are responsible for generating relief. Whereas tectonic activity may supply the thickened crust and the high surface altitudes, most of the Earth's dramatic scenery, from sea cliffs to continental escarpments to deep mountain valleys, is cut by erosional processes.

Climate is also central to the creation and evolution of regional-scale topography. Solar insolation and atmospheric and oceanic circulation determine the patterns of temperature and precipitation across the Earth's surface, which in turn determine the distribution and rates of virtually all geomorphological processes. Under otherwise equal conditions, temperature will determine whether glacial or fluvial processes will predominate in a region, and precipitation rates will determine whether a limestone outcrop is the most resistant unit in a landscape or the least. Climatic conditions also control the rates at which most geomorphological processes operate. For example, precipitation rates tend to increase and average temperatures decrease with increasing altitude. Measurements suggest that, as a result, rates of erosion tend to be greater in high-altitude drainage basins. This effect is also partially the result of the fact that high-altitude, mountainous regions have increased topographic relief, and taller and steeper slopes result in faster rates of gravity-driven processes. Integrated over time, the increased rates of erosion at high altitudes mean that growing mountain ranges tend to be self-destructive—the higher they grow, the more potent becomes the force of climate-driven erosion in wearing those mountains away.

The ‘cybernetic’ model of regional landscape

The new model of regional topography and long-term landscape evolution has been called ‘cybernetic’, and not because the model implies that electronic or mechanical forces are at work. The term comes from the Greek ‘kybernetes’, which refers to piloting or steering, as in a ship. ‘Cybernetic’ refers to the command-and-control elements in an integrated and self-regulating system. The model of mountain building is called ‘cybernetic’ because it emphasizes the strong interactions and feedback between tectonics, surface processes, and climate in the geomorphic system. Changes in any one of these three sub-systems seem to be capable of causing major changes elsewhere in the system.

One of the most significant feedback mechanisms at work is isostasy. This refers to the processes and mechanisms by which topography at the surface is supported by the buoyancy of the crust beneath. As a result of isostasy, surface processes appear to be capable of guiding or even accelerating tectonic processes deep within the crust. The removal of mass at the surface by regional erosion causes an upward force of isostatic uplift as the lithosphere acts to re-establish buoyant equilibrium. The rate of isostatic uplift is always less than the rate of denudation so that, in the absence of any additional tectonic uplift, the average altitude of topography can only decline in response to the combined effects of erosion and isostasy.

The significance of isostatic uplift becomes most apparent when one considers those few regions where it is essentially absent; for example, the Tibetan Plateau. Continental collision of India with southern Asia has uplifted the plateau to a mean altitude above 5000 m, and some researchers have suggested that this is as high as the Tibetan Plateau can go. With little precipitation and no large-scale redistribution of mass by erosion—because of internal drainage—the mass of the plateau may act as a ‘gravitational cork’, blocking any additional tectonic uplift. But how can 5000 m be a gravitational limit when the Himalayas just to the south have peaks of nearly 9000 m? The average altitude throughout the Himalayas is about equal to that of the Tibetan Plateau, with the high peaks matched by an equal area of deeply incised gorges. The difference between the Tibetan Plateau and the Himalayas, it is suggested, is that the latter is characterized by through-going river systems that cut the deep valleys and promulgate erosion through the region. Isostatic compensation to erosional mass removal apparently unplugs the gravitation cork and allows tectonic uplift to continue.

Evolution of regional topography over time

Recent recognition of the significance and impacts of the linkages between tectonics, surface processes, and climate has led to greatly improved qualitative and quantitative models of long-term landscape evolution. W. M. Davis's Geographical Cycle was based on the flawed assumption that tectonic construction occurs in brief, powerful spasms and is followed by the much slower effects of erosion. Geologists have since learned that tectonic activity may persist for long periods of geological time, that erosional denudation may be just as rapid as tectonic uplift, and that the two sets of processes almost always act simultaneously.

Japanese researchers were among the first to formulate a comprehensive model of landscape evolution rich in feedback (Fig. 2). This model bears a superficial resemblance to the Geographical Cycle in that it, too, suggests a three-stage progression. During the first, or ‘developing stage’, uplift rates exceed denudation rates so that average altitudes increase over time. In Japan, low-relief surfaces of Tertiary age diminish in extent as altitudes increase, relief increases, and erosion rates rise. During the second, or ‘culminating’ stage, uplift rates and denudation rates are regionally balanced. If tectonic activity is sufficiently long-lived, and if climatic conditions allow erosion rates that are high enough, then the topography may enter a period of dynamic equilibrium with no change in regional average altitude. The third, or ‘declining’ stage could commence with either a reduction in tectonic uplift or a sustained increase in denudation. A landscape in decline would be characterized by declines in average altitude and lithospheric thickness—rapid at first but gradually decelerating—as both the high-standing topography and the associated isostatic root were gradually consumed by erosion over time.

Numerical models of regional topography based on a strongly coupled tectonic–geomorphological–climatic system have yielded strikingly realistic simulations of landscape evolution under various conditions. Perhaps for the first time, researchers are bridging the gap between process-based empiricism and regional synthesis. Future research is likely to add additional degrees of detail to these models, incorporating such complexities as lithological heterogeneity and climatic variability over time.

Nicholas Pinter

Bibliography

Keller, E. A. and and Pinter, N. (1996) Active tectonics: earthquakes and landscape. Prentice Hall, Upper Saddle River, NJ.
Ohmori, H. (1985) A comparison of the Davisian scheme and landform development by concurrent tectonics and denudation. Bulletin of the Department of Geography, University of Tokyo, 17, 19–28.
Pinter, N. and and Brandon, M. T. (1997) How erosion builds mountains. Scientific American, 276, (4), 74–9.

geomorphology, global see global geomorphology