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Surface of the EarthGeomorphology is the study of landforms, including their origin and evolution, and the processes that shape them. The underlying question is: Why do landscapes look the way they do? The term is derived from the Greek geo, meaning Earth, and morph, meaning form. Geomorphologists seek to understand landform history and dynamics, and predict future changes through a combination of field observation, physical experiment, and numerical modeling. The discipline is practiced within geology, geography, archaeology and civil and environmental engineering. Early studies in geomorphology are the foundation for pedology, one of two main branches of soil science.
Landforms evolve in response to a combination of natural and anthropogenic processes. 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. Landscapes are also lowered by subsidence, either due to tectonics or physical changes in underlying sedimentary deposits. These processes are each influenced differently by climate, ecology, and human activity.
Particular applications of geomorphology include landslide prediction and mitigation, river control and restoration, coastal protection, and even assessing the presence of water on Mars.
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History
Geomorphology was not originally differentiated from the rest of geography. The first 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 evolution, and was depicted as a sequence by which a river would cut a valley more and more deeply, but then erosion of side valleys would eventually flatten out the terrain again, now at a lower elevation. The cycle could be started over by uplift of the terrain. The model is today considered too much of a simplification to be especially useful in practice.
Walther Penck developed an alternative model in the 1920s, based on ratios of uplift and erosion, but it was also too weak to explain a variety of landforms.
Processes
Modern geomorphology focuses on the quantitative analysis of interconnected processes, such as the contribution of solar energy, the rates of steps of the hydrologic cycle, and plate movement rates from geophysics to compute the age and expected fate of landforms. The use of more precise measurement technique has also enabled processes like erosion to be observed directly, rather than merely surmised from other evidence. Computer simulation is also valuable for testing that a particular model yields results with properties similar to real terrain.
Primary surface processes responsible for most topographic features include wind, waves, weathering, mass wasting, ground water, surface water, glaciers, tectonism, and volcanism.
Fluvial Geomorphology
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 mobilise sediment and transport it downstream, either as bedload or suspended load. The rate of sediment transport depends on the avilability of sediment itself and on the river's discharge.
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 dentritic, but may adopt other patterns depending on the regional topography and underlying geology.
Glacial Geomorphology
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.
Weathering
Main article: Weathering.
This results from chemical dissolution of rock and from the mechanical wearing of rock by plant roots, ice expansion, and the abrasive action of sediment. Weathering provides the source of the sediment transported by fluvial, glacial, aeolian, or biotic processes.
Taxonomy
Different geomorphological processes dominate at different spatial and temporal scales. To help categorize landscape scales some geomorphologists use the following taxonomy:
- 1st - Continent, ocean basin, climatic zone (~10,000,000 km²)
- 2nd - Shield, e.g. Baltic shield, or mountain range (~1,000,000 km²)
- 3rd - Isolated sea, Sahel (~100,000 km²)
- 4th - Massif, e.g. Massif Centralor Group of related landforms, e.g., Weald (~10,000 km²)
- 5th - River valley, Cotswolds (~1,000 km²)
- 6th - Individual mountain or volcano, small valleys (~100 km²)
- 7th - Hillslopes, stream channels, estuary (~10 km²)
- 8th - gully, barchannel (~1 km²)
- 9th - Meter-sized features
Its use, however, is rare and may be misleading - the nature of landscape change may be better viewed as a continuum of coupled processes.
References
- M. J. Selby, Earth's Changing Surface (Oxford University Press, 1985) ISBN 0198232527
- Richard Chorley, Stanley Schumm, and David Sugden, Geomorphology (Methuen, 1984)
See also
- Base level
- Biogeology
- Bioerosion
- Coastal erosion
- Denudation
- Drainage system
- Erosion
- Erosion prediction
- Fluvial landforms of streams
- Geologic modeling
- Landslide
- Lithosphere
- Meander
- Peneplain
- Soil
- Soil conservation
- Soil mechanics
- Soils retrogression and degradation
- Stream capture
- Watershed
- Important publications in geomorphology