Pothole (landform)

The Earth scientists, who study rivers and streams, define a pothole as a smooth, bowl-shaped or cylindrical hollow, generally deeper than wide, found carved into the rocky bed of a watercourse. Other names used for riverine potholes are pot, (stream) kettle, giant's kettle, evorsion, hollow, rock mill, churn hole, eddy mill, and kolk.[1] Although somewhat related to a pothole in origin, a plunge pool (or plunge basin or waterfall lake) is the deep depression in a stream bed at the base of a waterfall; it is created by the erosional forces of turbulence generated by water falling on rocks at a waterfall's base where the water impacts.[2] Potholes are also sometimes referred to as swirlholes. This word was created to avoid confusion with an English term for a vertical or steeply inclined karstic shaft in limestone. However, given widespread usage of this term for a type of fluvial sculpted bedrock landform, pothole is preferred in usage to swirlhole.[3]

The term pothole is also used to refer to other types of depressions and basins that differ in origin.[1] For example, some authors[4][5] refer to panholes found in the Colorado Plateau also as potholes. Other terms used for panholes are gnamma (Australia), opferkessel (German, roughly “sacrificial basin”), armchair hollows, weathering pans (or pits) and solution pans or solution pits.[6] In another case, the term pothole is used to refer to a shallow depression, generally less than 10-acre (4.0 ha) in area that occurs between dunes or on subdued morainic relief on a prairie, as in Minnesota and the Dakotas, and often contains an intermittent pond or marsh that serves as a nesting place for waterfowl.[1]

River Orchy erosion potholes
River Orchy, Scotland, showing erosion potholes in bedrock

Origin

The consensus of geomorphologists and sedimentologists is that fluvial potholes are created by the grinding action of either a stone or stones or coarse sediment (sand, gravel, pebbles, boulders), whirled around and kept in motion by eddies within and force of the stream current in a given spot. Being a spectacular feature of bedrock river channels, they have been and still are studied extensively and considered as a key factor in bedrock channel development and morphology and important factor in the incision of bedrock channels.[3][7][8]

References

  1. ^ a b c Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2011. Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 800 pp. ISBN 0-922152-89-6
  2. ^ Marshak, Stephen, 2009. Essentials of Geology, W. W. Norton & Company, 3rd ed. ISBN 978-0393196566
  3. ^ a b Richardson, K. and Carling, P., 2005. A typology of sculpted forms in open bedrock channels. Special Paper 392. Boulder, Colorado: Geological Society of America. 108 pp. ISBN 0-813723-92-2
  4. ^ Chan, M.A., Moser, K., Davis, J.M., Southam, G., Hughes, K. and Graham, T., 2005. Desert potholes: ephemeral aquatic microsystems. Aquatic Geochemistry, 11(3), pp. 279-302.
  5. ^ Hughes, Kebbi A., 2012. Bacterial Communities and their Influence on the Formation and Development of Potholes in Sandstone Surfaces of the Semi-Arid Colorado Plateau University of Western Ontario - Electronic Thesis and Dissertation Repository. Paper 543.
  6. ^ Twidale, C.R., and Bourne, J.A., 2018. Rock basins (gnammas) revisited. Géomorphologie: relief, processus, environnement, Articles sous presse, Varia, mis en ligne le 08 janvier 2018, consulté le 01 juillet 2018.
  7. ^ Das, B.C., 2018. Development of Streambed Potholes and the Role of Grinding Stones. Journal of Environmental Geography. 11 (1–2), 9–16.
  8. ^ Whipple, K.X., Hancock, G.S., and Anderson, R.S., 2000. River Incision Into Bedrock: Mechanics And Relative Efficacy Of Plucking, Abrasion And Cavitation. Geological Society Of America Bulletin. 112, 490–503.
Kettle (landform)

A kettle (kettle hole, pothole) is a depression/hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating glaciers, which become surrounded by sediment deposited by meltwater streams as there is increased friction. The ice becomes buried in the sediment and when the ice melts, a depression is left called a kettle hole, creating a dimpled appearance on the outwash plain. Lakes often fill these kettles, these are called kettle hole lakes. Another source is the sudden drainage of an ice-dammed lake. When the block melts, the hole it leaves behind is a kettle. As the ice melts, ramparts can form around the edge of the kettle hole. The lakes that fill these holes are seldom more than 10 m (33 ft) deep and eventually become filled with sediment. In acid conditions, a kettle bog may form but in alkaline conditions, it will be kettle peatland.

Pothole (disambiguation)

A pothole is a surface disruption in a roadway, caused by fatigue and erosion.

Pothole may also refer to:

A small deep hole dug to look for buried public utilities, see vacuum excavation

Pothole (geology), a phenomenon encountered in the platinum mining industry in South Africa

Pothole (landform), evorsion, swirlhole, or giant's kettle, a smooth, bowl-shaped or cylindrical hollow created by water erosion of bedrock

Pothole, or panhole, a shallow solution basin, or closed depression, found on flat or gently sloping rock

Pothole, a shallow depression, often containing an intermittent pond or marsh and serving as a nesting place for waterfowl in the Prairie Pothole Region of North America

Pothole, or pit cave, a predominantly vertical cave system

The Pothole (Seinfeld)

Wetland methane emissions

Contributing approximately 167 Tg of methane to the atmosphere per year; wetlands are the largest natural source of atmospheric methane in the world, and therefore remain a major area of concern with respect to climate change. Wetlands are characterized by water-logged soils and distinctive communities of plant and animal species that have evolved and adapted to the constant presence of water. This high level of water saturation creates conditions conducive to methane production.

Most methanogenesis, or methane production, occurs in oxygen-poor environments. Because the microbes that live in warm, moist environments consume oxygen more rapidly than it can diffuse in from the atmosphere, wetlands are the ideal anaerobic environments for fermentation as well as methanogen activity. However, levels of methanogenesis can fluctuate as it is dependent on the availability of oxygen, temperature of the soil, and the composition of the soil; a warmer, more anaerobic environment with soil rich in organic matter would allow for more efficient methanogenesis.Fermentation is a process used by certain kinds of microorganisms to break down essential nutrients. In a process called acetoclastic methanogenesis, microorganisms from the classification domain archaea produce methane by fermenting acetate and H2-CO2 into methane and carbon dioxide.

H3C-COOH → CH4 + CO2Depending on the wetland and type of archaea, hydrogenotrophic methanogenesis, another process that yields methane, can also occur. This process occurs as a result of archaea oxidizing hydrogen with carbon dioxide to yield methane and water.

4H2 + CO2 → CH4 + 2H2O

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