Apr 21, 2005 - How is the physical form of a landscape linked to the wide array of processes that shape it? Such processes range from the pat- ter of tiny ...
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Striking a chord frequencies when perturbed, like the strings of a guitar that generate chords when struck at different fret ow is the physical form of a positions. landscape linked to the wide So instead of being seen as continarray of processes that shape uous or catastrophic, I propose that it? Such processes range from the patlandscapes be considered in terms of ter of tiny raindrops, and the shuffling their response times to various perof beetles and rodents on hill slopes, turbations, such as the lateral growth to major slope failures that generate of an active fault, or the change in the colossal debris flows, and cratering rate at which a major fault slips due caused by the impacts of meteorites. to the accumulated effect of earthTo some, the issue is encapsulated in quakes. Landscapes can be viewed the conflicting imagery of ‘cataas ‘buffered’ when their response strophism’ and ‘continuity’ (or ‘unito rapidly repeating perturbations formitarianism’) — two opposing is slow, but ‘reactive’ when their views in a debate that is almost as old response is rapid compared to the as the discipline of geology itself. Making waves: a digital reconstruction of a landscape from frequency of the disturbance. LandTo take an analogy from the world Kentucky, United States, reveals regular transverse catchments. scapes can also be termed ‘steady’ of art,the uniformitarian or continuity school of thought sees the geological and geo- feature of these two interacting systems is when their response time is smaller than morphological canvas as being made up of that they each operate at a range of temporal the interval between a repeated perturbathe gradual accumulation of minute brush and spatial scales.We can think of landscapes tion, or less than the time passed since a strokes — much like that produced when a as being perturbed by variations in both change in the prevailing tectonic or climpointillist, such as Georges Seurat, adds the internal and external mechanisms. The atic conditions, but ‘transient’ when the unremarkable dots to his picture.Landscapes results of these perturbations might be reverse is true. One of the goals of the burgeoning viewed in this way are orderly, predictable, seen as changes in the morphometric properties of the critical interface or as changes in field of research working on Earth surface reassuring,perhaps even benevolent. But another kind of canvas, such as a van the mass fluxes of rock, particulate sediment processes must be to improve measurements of the rates of tectonic, sedimentologic and Gogh, is characterized by wild brush strokes or solutes through and over it. In the past, tectonic movements were geomorphologic processes using the rapidly of vivid colour, and is restless, unpredictable and disconcerting. This catastrophic view somewhat statically viewed as effectively improving tools that are available. Such tools sees landscapes, and indeed Earth history, instantaneous events causing uplift of the include the digital description of landscapes; as being shaped predominantly by excep- land surface, and upon which geomorphic the dating of geomorphic surfaces and tional events. If this is the case, present-day agents worked over eons of time. It is now events; the elucidation of the spatial patterns processes cannot fully explain the physical clear that in regions of active tectonics, the of denudation at different timescales; and nature of the world around us: most large- rates at which tectonic and geomorphic the use of numerical methods in coupled scale events have not been experienced by processes operate are similar. Consequently, geomorphic tectonic–climate modelling of humans, and can only be surmised from the possibilities for complex coupling of the integrated systems routing sediment from two processes are enormous. erosional mountain slopes to depositional geological studies of ‘deep time’. An exciting avenue of research at present basins. With these new data, we will be better The irony of the catastrophism versus continuity debate is that no real dividing line is evaluating the response of landscapes to able to establish what meaningful informaexists between the ‘ordinary’ and the ‘excep- perturbations, such as changes in tectonic tion about the forces shaping landscapes can tional’. For example, plots of data from thou- and climatic conditions. For example, the be interpreted from its geometrical form, sands of landslides that occurred during the landscapes of the Basin and Range province mass fluxes and sedimentary deposits. Put past 60 years in the Southern Alps of New in western United States are continually simply, we may learn how to read the epic Zealand describe a beautiful power law, adjusting to the tectonic deformation of the poem of Earth history that is written in supporting the view that landscape change underlying crust. This tectonic deformation sediments and landscapes. ■ follows a continuum in magnitude and fre- is essentially caused by the growth of faults Philip Allen is in the Department of Earth Sciences, quency. This implies that it is impossible to to accommodate extension of the crust, and ETH-Zentrum, CH-8092 Zurich, Switzerland. separate the exceptional from the ordinary, is responsible for uplifted ranges with steep and invites us to realign our thinking and transverse rivers that feed cone-shaped ‘fans’ FURTHER READING develop a new vocabulary when investigat- of sediment in adjoining basins. Although Summerfield, M. A. (ed.) Geomorphology and Global such a system appears, at first, to be rela- Tectonics (Wiley, 2000). ing the processes that shape landscapes. We can consider landscapes as the chang- tively simple, its operation is determined Allen, P. A. & Allen, J. R. Basin Analysis: Principles and ing geometrical form of a critical interface by the timescales over which several inter- Applications 2nd edn Ch. 7, 221–265 (Blackwell, 2005). between two interacting systems: an internal acting geomorphic subsystems respond to This essay is based on the William Smith Lecture given at the system driven essentially by tectonic fluxes perturbations. Rather than resonating like Geological Society of London’s conference ‘Earth’s Dynamic of rock, and an external system dominated a single hand-held bell, landscapes are Surface: Catastrophe and Continuity in Landscape Evolution’, by the effects of climate. The challenging polychromatic, resonating with different 4–5 October 2004.
Philip Allen
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NATURE | VOL 434 | 21 APRIL 2005 | www.nature.com/nature
961 ©2005 Nature Publishing Group
P. ALLEN
Landscapes: when perturbed by climatic and tectonic changes, landscapes resonate with a range of frequencies.
