Geomorphology is the study of present-day landforms, including their classification, description, nature, origin, development, and relationships to underlying structures, as well as the history of geologic changes as recorded by these surface features.
The term is sometimes restricted to features produced only by erosion and deposition. Although geomorphology tends to focus on terrestrial landforms, the surfaces of the Moon and Mars are now sufficiently well-known for morphological analysis to be applied there as well.
Geomorphology is fundamentally inspired by the shapes of the terrain we see every day; the meandering course of a river, the rounded shapes of some hills and the pointed shapes of others, the seemingly-random headlands and bays of a coastline. While it is generally accepted that, for instance, water erodes rock over a long period of time, that doesn't answer the question of whether any particular landform was created by water erosion, how long ago, whether wind played a role also, and so forth. Geomorpology delves into these questions in depth, seeking both to explain origins, and so to provide predictive power that can be used in activities such as civil engineering.
Some geomorphologists identify a taxonomy of landforms, sorted by magnitude:
- 1st - continent, ocean basin, climatic zone (~10,000,000 kmē)
- 2nd - Baltic shield, mountain range (~1,000,000 kmē)
- 3rd - isolated sea, Sahel (~100,000 kmē)
- 4th - Massif Central, Weald (~10,000 kmē)
- 5th - river valley, Cotswolds (~1,000 kmē)
- 6th - individual mountain or volcano, small valleys (~100 kmē)
- 7th - hillslopes, stream channels, estuary (~10 kmē)
- 8th - gully, barchan (~1 kmē)
- 9th - meter-sized features
Different kinds of processes tend to dominate at different magnitudes.
Geomorphology was not originally differentiated from the rest of geology. The first geomorphic model was 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.
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.
- M. J. Selby , Earth's Changing Surface (Oxford University Press, 1985) ISBN 0198232527
- Richard Chorley , Stanley Schumm , and David Sugden , Geomorphology (Methuen, 1984)