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Geomorphology

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Surface of the Earth

Geomorphology (from Greek: γῆ, ge, "earth"; μορφή, morfé, "form"; and λόγος, logos, "study") is the scientific study of landforms and the processes that shape them, and more broadly, the evolution of processes controlling the topography of any planet. Geomorphologists seek to understand why landscapes look the way they do, to understand landform history and dynamics, and to predict future changes through a combination of field observation, physical experiment, and numerical modeling. Geomorphology is practiced within geography, geology, geodesy, engineering geology, archaeology, and geotechnical engineering, and this broad base of interest contributes to a wide variety of research styles and interests within the field.

The form of the Earth's surface evolves in response to a combination of natural and anthropogenic processes, and responds to the balance between processes that add material and those that remove it. Such processes may act across very many lengthscales and timescales. On the broadest scales, the landscape is built up through tectonic uplift and volcanism. Denudation occurs by erosion and mass wasting, which produces sediment that is transported and deposited elsewhere within the landscape or off the coast.[1] On progressively smaller scales, similar ideas apply, where individual landforms evolve in response to the balance of additive (tectonic or sedimentary) and subtractive (erosive) processes. Modern geomorphology can be thought of as the study of the divergence of flux of material on a planetary surface, and as such is closely allied with sedimentology, which can equally be seen as the convergence of that flux.

Geomorphic processes are influenced by tectonics, climate, ecology, and human activity, and equally, many of these drivers can be affected by the ongoing evolution of the Earth's surface, for example, via isostasy or orographic precipitation. Many geomorphologists are particularly interested in the potential for feedbacks between climate and tectonics mediated by geomorphic processes.[2]

Practical applications of geomorphology include hazard assessment including landslide prediction and mitigation, river control and restoration, and coastal protection.

History

With some notable exceptions (see below), geomorphology is a relatively young science, growing along with interest in other aspects of the Earth Sciences in the mid 19th century. This section provides a very brief outline of some of the major figures and events in its development.

Ancient geomorphology

Perhaps the earliest one to devise a theory of geomorphology was the polymath Chinese scientist and statesman Shen Kuo (1031-1095 AD). This was based on his observation of marine fossil shells in a geological stratum of a mountain hundreds of miles from the Pacific Ocean. Noticing bivalve shells running in a horizontal span along the cut section of a cliffside, he theorized that the cliff was once the pre-historic location of a seashore that had shifted hundreds of miles over the centuries. He inferred that the land was reshaped and formed by soil erosion of the mountains and by deposition of silt, after observing strange natural erosions of the Taihang Mountains and the Yandang Mountain near Wenzhou. Furthermore, he promoted the theory of gradual climate change over centuries of time once ancient petrified bamboos were found to be preserved underground in the dry, northern climate zone of Yanzhou, which is now modern day Yan'an, Shaanxi province.

Early modern geomorphology

The first use of the word geomorphology was likely to be in the German language when it appeared in Laumann's 1858 work. Keith Tinkler has suggested that the word came into general use in English, German and French after John Wesley Powell and W. J. McGee used it in the International Geological Conference of 1891.[3]

An early popular geomorphic model was the geographical cycle or the cycle of erosion, developed by William Morris Davis between 1884 and 1899. The cycle was inspired by theories of uniformitarianism first formulated by James Hutton (1726–1797). Concerning valley forms, uniformitarianism depicted the cycle as a sequence in which a river cuts a valley more and more deeply, but then erosion of side valleys eventually flatten the terrain again, to a lower elevation. uplift could start the cycle over. Many studies in geomorphology in the decades following Davis' development of his theories sought to fit their ideas into this framework for broad scale landscape evolution, and are often today termed "Davisian". Davis' ideas have largely been superseded today, mainly due to their lack of predictive power and qualitative nature, but he remains an extremely important figure in the history of the subject.

In the 1920s, Walther Penck developed an alternative model to Davis', believing that landform evolution was better described as a balance between ongoing processes of uplift and denudation, rather than Davis' single uplift followed by decay. However, due to his relatively young death, disputes with Davis and a lack of English translation of his work his ideas were not widely recognised for many years.

Quantitative Geomorphology

While Penck and Davis and their followers were writing and studying primarily in Western Europe, another, largely separate, school of geomorphology was developed in the United States in the middle years of the 20th century. Following the early trailblazing work of Grove Karl Gilbert around the turn of the 20th Century, a group of natural scientists, geologists and hydraulic engineers including Ralph Alger Bagnold, John Hack, Luna Leopold, Thomas Maddock and Arthur Strahler began to research the form of landscape elements such as rivers and hillslopes by taking systematic, direct, quantitative measurements of aspects of them and investigating the scaling of these measurements. These methods began to allow prediction of the past and future behavior of landscapes from present observations, and were later to develop into what today is known as the subdiscipline of Quantitative Geomorphology, or Geomorphometry.

Contemporary geomorphology

Today, the field of geomorphology encompasses a very wide range of different approaches and interests. Modern researchers aim to draw out quantitative "laws" that govern Earth surface processes, but equally, recognize the uniqueness of each landscape and environment in which these processes operate. Particularly important realizations in contemporary geomorphology include:

1) that not all landscapes can be considered as either "stable" or "perturbed", where this perturbed state is a temporary displacement away from some ideal target form. Instead, dynamic changes of the landscape are now seen as an essential part of their nature.[4][5]

2) that many geomorphic systems are best understood in terms of the stochasticity of the processes occurring in them, that is, the probability distributions of event magnitudes and return times.[6] This in turn has indicated the importance of chaotic determinism to landscapes, and that landscape properties are best considered statistically.[7] The same processes in the same landscapes does not always lead to the same end results.

Processes

Grand Canyon, Arizona

Modern geomorphology focuses on the quantitative analysis of interconnected processes. Modern advances in geochemistry, in particular cosmochemistry, isotope geochemistry and fission track dating, have enabled us for the first time to measure the rates at which geomorphic processes occur at geologically relevant timescales.[8][9] At the same time, the use of more precise physical measurement techniques, including differential GPS, remotely sensed digital terrain models and laser scanning techniques, have allowed quantification and study of these processes as they happen.[10] Computer simulation and modeling may then be used to test our understanding of how these processes work together and through time.

Most geomorphically relevant processes can be thought of as either erosive, transportive, or some combination thereof. Depositional processes are mostly thought of as within the field of sedimentology, but also frequently considered as part of geomorphology. Weathering is the chemical and physical disruption of earth materials in place on exposure to atmospheric or near surface agents. The products of these near surface changes can subsequently be transported away by various agents of erosion.

The nature of the processes investigated by geomorphologists is strongly dependent on the landscape or landform under investigation and the time and length scales of interest. However, the following non-exhaustive list provides a flavor of the landscape elements associated with some of these.

Primary surface processes responsible for most topographic features include wind, waves, chemical dissolution, mass wasting, groundwater movement, surface water flow, glacial action, tectonism, and volcanism. Other more exotic geomorphic processes might include periglacial (freeze-thaw) processes, salt-mediated action, or extraterrestrial impact.

Fluvial processes

Main article: Fluvial

Rivers and streams are not only conduits of water, but also of sediment. The water, as it flows over the channel bed, is able to mobilize sediment and transport it downstream, either as bed load, suspended load or dissolved load. The rate of sediment transport depends on the availability of sediment itself and on the river's discharge.[11]

Rivers are also capable of eroding into rock and creating new sediment, both from their own beds and also by coupling to the surrounding hillslopes. In this way, rivers are thought of as setting the base level for large scale landscape evolution in nonglacial environments.[12][13] Rivers are key links in the connectivity of different landscape elements.

As rivers flow across the landscape, they generally increase in size, merging with other rivers. The network of rivers thus formed is a drainage system and is often dendritic, but may adopt other patterns depending on the regional topography and underlying geology.

See also: Hack's law and Sediment transport

Aeolian processes

Wind-eroded alcove near Moab, Utah

Aeolian processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments. Although water and mass flow tend to mobilize more material than wind in most environments, aeolian processes are important in arid environments such as deserts.[14]

Mesquite Flat Dunes in Death Valley looking toward the Cottonwood Mountains from the north west arm of Star Dune (2003)

Hillslope processes

Example of mass wasting at Palo Duro Canyon, Texas

Soil, regolith, and rock move downslope under the force of gravity via creep, slides, flows, topples, and falls. Such mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, Titan and Iapetus.

Ongoing hillslope processes can change the topology of the hillslope surface, which in turn can change the rates of those processes. Hillslopes that steepen up to certain critical thresholds are capable of shedding extremely large volumes of material very quickly, making hillslope processes an extremely important element of landscapes in tectonically active areas.[15]

On Earth, biological processes such as burrowing or tree throw may play important roles in setting the rates of some hillslope processes.[16]

Glacial processes

Features of a glacial landscape

Glaciers, while geographically restricted, are effective agents of landscape change. The gradual movement of ice down a valley causes abrasion and plucking of the underlying rock. Abrasion produces fine sediment, termed glacial flour. The debris transported by the glacier, when the glacier recedes, is termed a moraine. Glacial erosion is responsible for U-shaped valleys, as opposed to the V-shaped valleys of fluvial origin.[17]

The way in which glacial processes interact with other landscape elements, particularly hillslope and fluvial processes, is an important aspect of modern landscape evolution and its sedimentary record in many high mountain environments. Environments which have been relatively recently glaciated but are no longer may still show elevated rates of landscape change compared to those which have not. Nonglacial geomorphic processes which nevertheless have been conditioned by past glaciation are termed paraglacial processes. This concept contrasts with periglacial processes, which are directly driven by formation or melting of ice or frost).[18]

Tectonic processes

Tectonic effects on geomorphology can range from scales of millions of years to minutes or less. The effects of tectonics on landscape are heavily dependent on the nature of the underlying bedrock fabric that more less controls what kind of local morphology tectonics can shape. Earthquakes can, in terms of minutes, submerge large extensions creating new wetlands. Isostatic rebound can account for significant changes over thousand or hundreds of years, and allows erosion of a mountain belt to promote further erosion as mass is removed from the chain and the belt uplifts. Long-term plate tectonic dynamics give rise to orogenic belts, large mountain chains with typical lifetimes of many tens of millions of years, which form focal points for high rates of fluvial and hillslope processes and thus long-term sediment production.

Features of deeper mantle dynamics such as plumes and delamination of the lower lithosphere have also been hypothesised to play important roles in the long term (> million year), large scale (thousands of km) evolution of the Earth's topography. Both can promote surface uplift through isostasy as hotter, less dense, mantle rocks displace cooler, denser, mantle rocks at depth in the Earth.[19][20]

Igneous processes

Both volcanic (eruptive) and plutonic (intrusive) igneous processes can have important impacts on geomorphology. The action of volcanoes tends to rejuvenize landscapes, covering the old land surface with lava and tephra, releasing pyroclastic material and forcing rivers through new paths. The cones built by eruptions also build substantial new topography, which can be acted upon by other surface processes.

Subsurface movement of magma also plays a role in geomorphology. Migrating melts beneath the surface can cause the inflation and deflation of the land surface, and a partially molten crustal layer beneath Tibet has even been implicated in controlling the geomorphology of the Tibetan plateau across many thousands of kilometres.[21]

Biological processes

The interaction of living organisms with landforms, or biogeomorphologic processes, can be of many different forms, and is probably of profound importance for the terrestrial geomorphic system as a whole. Biology can influence very many geomorphic processes, ranging from biogeochemical processes controlling chemical weathering, to the influence of mechanical processes like burrowing and tree throw on soil development, to even controlling global erosion rates through modulation of climate through carbon dioxide balance. Terrestrial landscapes in which the role of biology in mediating surface processes can be definitively excluded, are extremely rare, but may hold important information for understanding the geomorphology of other planets, such as Mars.[22]

Scales in geomorphology

Different geomorphological processes dominate at different spatial and temporal scales. Moreover, the scales on which processes occur may determine the reactivity or otherwise of landscapes to changes in driving forces such as climate or tectonics.[23] These ideas are key to the study of geomorphology today.

To help categorize landscape scales some geomorphologists might use the following taxonomy:

See also

References

  1. ^ Willett & Brandon, 2002, On Steady States in Mountain Belts, Geology, v. 30(2), p. 175-178.
  2. ^ Roe et al., 2008, Feedbacks among climate, erosion and tectonics in a critical wedge orogen, Am. J. Sci., v. 308(7), p. 815-842.
  3. ^ Tinkler, Heith J. A short history of geomorphology. Page 4. 1985
  4. ^ Whipple, 2004, Bedrock Rivers and the Geomorphology of Active Orogens, Anu. Rev. Earth Planet. Sci., v. 32, p. 151-185.
  5. ^ Allen, 2008, Time scales of tectonic landscapes and their sediment routing systems, Geol. Soc. Lon. Sp. Pub., v. 296, p.7-28.
  6. ^ Benda & Dunne, 1997, Stochastic forcing of sediment supply to channel networks from landsliding and debris flow, Water Resources Res., v. 33(12), p. 2849-2863.
  7. ^ Dietrich et al., 2003, Geomorphic Transport Laws for Predicting Landscape Form and Dynamics, AGU Geophysical Monograph 135, p. 1-30.
  8. ^ Summerfield, M.A., 1991, Global Geomorphology, Pearson Education Ltd, 537 p. ISBN 0-582-30156-4.
  9. ^ Dunai, T.J., 2010, Cosmogenic Nucleides, Cambridge University Press, 187 p. ISBN 978-0-521-87380-2.
  10. ^ e.g., DTM intro page, Hunter College Department of Geography, New York NY, http://www.geo.hunter.cuny.edu/terrain/intro.html
  11. ^ Knighton, D., 1998, Fluvial Forms & Processes, Hodder Arnold, 383 p. ISBN 0-340-66313-8.
  12. ^ Strahler, A.N., 1950, Equilibrium theory of erosional slopes approached by frequency distribution analysis, Am. J. Sci., v. 248, p. 673-696.
  13. ^ Burbank, D.W., 2002, Rates of erosion and their implications for exhumation: Mineralogical Magazine, v. 66, p. 25-52.
  14. ^ Leeder, M., 1999, Sedimentology and Sedimentary Basins, From Turbulence to Tectonics, Blackwell Science, 592 p. ISBN 0-632-0497-6.
  15. ^ Roering, J.J., Kirchner, J.W., and Dietrich, W.E., 1999, Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology, Water Resources Res., v. 35, p. 853-870.
  16. ^ Gabet, E.J., Reichman, O.J., Seabloom, E.W., 2003, The Effects of Bioturbation on Soil Processes and Sediment Transport, Ann. Rev. Earth Planet. Sci., v. 31, p. 249-273.
  17. ^ Bennett, M.R. & Glasser, N.F., 1996, Glacial Geology: Ice Sheets and Landforms, John Wiley & Sons Ltd, 364 p. ISBN 0-471-96345-3.
  18. ^ Church, M. and Ryder, J.M., 1972, Conditioned by GlaciationParaglacial Sedimentation: A Consideration of Fluvial Processes Conditioned by Glaciation, Geological Society of America Bulletin, v. 83, p. 3059-3072.
  19. ^ Cserepes, L., Christensen, U.R., & Ribe, N.M., Geoid height versus topography for a plume model of the Hawaiian swell, Earth Planet. Sci. Lett., v. 178(1-2), p. 29-38.
  20. ^ Seber, D., Barazangi, M., Ibenbrahim, A. and Demnati, A., 1996, Geophysical evidence for lithospheric delamination beneath the Alboran Sea and Rif--Betic mountains, Nature, v. 379 (6568), p. 785–790.
  21. ^ Hodges, K.V., A synthesis of the Channel Flow-Extrusion hypothesis as developed for the Himalayan-Tibetan orogenic system. In: Law, R.D., Searle, M.P. & Godin, L. (Eds.), Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones, Geol. Soc. London Spec. Publ., v. 268, p. 71-90.
  22. ^ Dietrich, W.E., and Perron, J.T., 2006, The search for a topographic signature of life, Nature, v. 439, p. 411-418.
  23. ^ Allen, 2008, Time scales of tectonic landscapes and their sediment routing systems, Geol. Soc. Lon. Sp. Pub., v. 296, p.7-28.
  • Selby, Michael John (1985). Earth's changing surface: an introduction to geomorphology. Oxford: Clarendon Press. ISBN 0-19-823252-7.
  • Chorley, Richard J. (1985). Geomorphology. London: Methuen. ISBN 0-416-32590-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Edmaier, Bernhard (2004). Earthsong. London: Phaidon Press. ISBN 0-7148-4451-9.
  • Scheidegger, Adrian E. (2004). Morphotectonics. Berlin: Springer. ISBN 3-540-20017-7.
  • Needham, Joseph (1954). Science and civilisation in China. Cambridge, UK: Cambridge University Press. ISBN 0-521-05801-5.
  • Kondolf, G. Mathias (2003). Tools in fluvial geomorphology. New York: Wiley. ISBN 0-471-49142-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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