Geomorphology
Geomorphology 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
γη, ge, meaning
Earth, and
μορφή, morfé, 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,
geodesy,
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 assessing the presence of water on
Mars.
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 which were first formulated by
James Hutton (1726-1797). Concerning
valley forms, the cycle 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.
G. K. Gilbert was an important early
American geomorphologist.
|
Age of seafloor crust. Red is youngest. |
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,
suspended load or dissolved load. The rate of sediment transport depends on the availability 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 dendritic, 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
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.
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 Central or 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 featuresIts use, however, is rare and may be misleading - the nature of landscape change may be better viewed as a continuum of coupled processes.
*
Base level*
Biogeology*
Bioerosion*
Biorhexistasy*
Coastal erosion*
Drainage system*
Erosion prediction*
Fluvial landforms of streams*
Geologic modeling*
Lithosphere*
Regolith*
Soil*
Soil conservation*
Soil mechanics*
Soils retrogression and degradation*
Stream capture*
watershed*
Important publications in geomorphology*
M. J. Selby,
Earth's Changing Surface. ISBN 0198232527,
Oxford University Press, 1985
*
Richard Chorley,
Stanley Schumm, and
David Sugden,
Geomorphology. Edition
Methuen, 1984
*
Bernhard Edmaier,
Earthsong. A collection of breathtaking arial photographs.
Phaidon, 2004.
*
Adrian E. Scheidegger,
Morphotectonics. ISBN 3-540-20017-7, Springer-Verlag Berlin 2004.
*
The Geographical Cycle, or the Cycle of Erosion (1899)
