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{{Short description|Natural processes removing soil and rock}}
{{about||morphological image processing operations|Erosion (morphology)|use of in dematopathology|Erosion (dermatopathology)}}
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{{Use British English|date=February 2021}}
] on an ] field in ] ]. This phenomenon is aggravated by poor agricultural practices because when ]ing, the furrows were traced in the direction of the slope rather than that of the terrain ]s.]]
'''Erosion''' is the action of surface processes (such as ] or ]) that removes ], ], or dissolved material from one location on the ] and then ] it to another location where it is ]. Erosion is distinct from ] which involves no movement.<ref name="Brittanica">{{cite encyclopedia | url=https://www.britannica.com/science/erosion-geology | title=Erosion | encyclopedia=Encyclopædia Britannica | date=2015-12-03 | access-date=2015-12-06 | archive-url=https://web.archive.org/web/20151221064037/https://www.britannica.com/science/erosion-geology | archive-date=2015-12-21 | url-status=live }}</ref><ref>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |isbn=9780199653065 |edition=Fourth |chapter=Erosion}}</ref> Removal of rock or soil as ] ] is referred to as ''physical'' or ''mechanical'' erosion; this contrasts with ''chemical'' erosion, where soil or rock material is removed from an area by ].<ref>{{cite journal |last1=Louvat |first1=P. |last2=Gislason |first2=S. R. |last3=Allegre |first3=C. J. |title=Chemical and mechanical erosion rates in Iceland as deduced from river dissolved and solid material |journal=American Journal of Science |date=1 May 2008 |volume=308 |issue=5 |pages=679–726 |doi=10.2475/05.2008.02|bibcode=2008AmJS..308..679L |s2cid=130966449 }}</ref> Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.


Agents of erosion include ]fall;<ref name="Hysteretic sediment fluxes in ra">{{cite journal | last1 = Cheraghi | first1 = M. | last2 = Jomaa | first2 = S. | last3 = Sander | first3 = G.C. | last4 = Barry | first4 = D.A. | year = 2016 | title = Hysteretic sediment fluxes in rainfall-driven soil erosion: Particle size effects | url = https://infoscience.epfl.ch/record/222767/files/Cheraghi%20ET%20AL.%202016WR019314_MANS.pdf | journal = Water Resour. Res. | volume = 52 | issue = 11 | page = 8613 | doi = 10.1002/2016WR019314 | bibcode = 2016WRR....52.8613C | s2cid = 13077807 }}{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> ] wear in ]s; ] by the sea and ]; ] ], ], and scour; areal flooding; ] abrasion; ] processes; and ] processes in steep ]s like ]s and ]s. The rates at which such processes act control how fast a surface is eroded. Typically, physical erosion proceeds the fastest on steeply sloping surfaces, and rates may also be sensitive to some climatically controlled properties including amounts of water supplied (e.g., by rain), storminess, wind speed, wave ], or ] (especially for some ice-related processes). ]s are also possible between rates of erosion and the amount of eroded material that is already carried by, for example, a river or glacier.<ref name="Hallet1981">{{cite journal | first1=Bernard | last1=Hallet | title=Glacial Abrasion and Sliding: Their Dependence on the Debris Concentration In Basal Ice | journal=Annals of Glaciology | year=1981 | volume=2 | issue=1 | pages=23–28 | issn=0260-3055 | doi=10.3189/172756481794352487| bibcode=1981AnGla...2...23H | doi-access=free }}</ref><ref name="SklarDietrich2004">{{cite journal | first1=Leonard S. | last1=Sklar | url=http://eps.berkeley.edu/~bill/papers/sklaranddietrich20042003WR002496_121.pdf | title=A mechanistic model for river incision into bedrock by saltating bed load | last2=Dietrich | first2=William E. | journal=Water Resources Research | year=2004 | volume=40 | issue=6 | pages=W06301 | issn=0043-1397 | doi=10.1029/2003WR002496 | bibcode=2004WRR....40.6301S | s2cid=130040766 | access-date=2016-06-18 | archive-url=https://web.archive.org/web/20161011172333/http://eps.berkeley.edu/~bill/papers/sklaranddietrich20042003WR002496_121.pdf | archive-date=2016-10-11 | url-status=live }}</ref> The transport of eroded materials from their original location is followed by deposition, which is arrival and emplacement of material at a new location.<ref name="Brittanica"/>
]
'''Erosion''' is the process of ] and transport of solids (], ], ] and other particles) in the natural environment or their source and deposits them elsewhere. It usually occurs due to ] by wind, water, or ice; by down-slope ] of soil and other material under the force of ]; or by living organisms, such as burrowing animals, in the case of ].


While erosion is a natural process, human activities have increased by 10–40 times the rate at which ] is occurring globally.<ref>{{Cite journal|last=Dotterweich|first=Markus|date=2013-11-01|title=The history of human-induced soil erosion: Geomorphic legacies, early descriptions and research, and the development of soil conservation – A global synopsis|journal=Geomorphology|volume=201|pages=1–34|doi=10.1016/j.geomorph.2013.07.021|bibcode=2013Geomo.201....1D|s2cid=129797403 }}</ref> At agriculture sites in the ], intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region.<ref>{{Cite journal|last1=Reusser|first1=L.|last2=Bierman|first2=P.|last3=Rood|first3=D.|title=Quantifying human impacts on rates of erosion and sediment transport at a landscape scale|journal=Geology|volume=43|issue=2|pages=171–174|doi=10.1130/g36272.1|bibcode=2015Geo....43..171R|year=2015}}</ref> Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in ] and (on ]s) ], both because of loss of the nutrient-rich upper ]. In some cases, this leads to ]. Off-site effects include ] and ] of ], as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of ]; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant ] worldwide.<ref name=BlancoConservation>{{cite book|last1=Blanco-Canqui|first1=Humberto|last2=Rattan|first2=Lal|chapter=Soil and water conservation|title=Principles of soil conservation and management|pages=1–20|date=2008|publisher=Springer|location=Dordrecht|isbn=978-1-4020-8709-7}}</ref>{{rp|2}}<ref name="toy-2002-p1">{{cite book|last1=Toy|first1=Terrence J.|last2=Foster|first2=George R.|last3=Renard|first3=Kenneth G.|title=Soil erosion : processes, prediction, measurement, and control|date=2002|publisher=Wiley|location=New York|isbn=978-0-471-38369-7}}</ref>{{rp|1}}<ref name="Apollo 10 (4)">{{cite journal | last1=Apollo |first1=M. |last2=Andreychouk |first2=V. |last3=Bhattarai |first3=S.S. | title=Short-Term Impacts of Livestock Grazing on Vegetation and Track Formation in a High Mountain Environment: A Case Study from the Himalayan Miyar Valley (India) | journal=Sustainability | volume=10 | issue=4 | date=2018-03-24 | issn=2071-1050 | doi=10.3390/su10040951 | pages=951| doi-access=free }}</ref>
Erosion is a natural process, but it has been increased dramatically by human ], especially ], ], and ].<ref name="dirt-book">{{cite book|last=Montgomery|first=David|title=Dirt: The Erosion of Civilizations|publisher=University of California Press|date=October 2, 2008|edition=1st|isbn=978-0-520-25806-8}}</ref><ref>{{cite book|last=Kötke|first=William|title=The Final Empire|publisher=AuthorHouse|date=November 30, 2007|isbn=978-1-4343-3130-4}}</ref> Land that is used for industrial agriculture generally experiences a significantly greater rate of erosion than that of land under natural vegetation, or land used for ] practices. This is particularly true if tillage is used, which reduces vegetation cover on the surface of the soil and disturbs both soil structure and plant roots that would otherwise hold the soil in place. However, improved land use practices can limit erosion, using techniques such as ]-building, conservation tillage practices, and tree planting.


], ], ]s, ] and ] are amongst the most significant human activities in regard to their effect on stimulating erosion.<ref>{{Cite book|author=Julien, Pierre Y.|title=Erosion and Sedimentation|publisher=Cambridge University Press|year=2010|isbn=978-0-521-53737-7|page=1|url=https://books.google.com/books?id=Gv72uiVmWEYC&pg=PA1}}</ref> However, there are many ] practices that can curtail or limit erosion of vulnerable soils.
A certain amount of erosion is natural and, in fact, healthy for the ]. For example, ]s continuously move downstream in watercourses. Excessive erosion, however, causes serious problems, such as receiving water ]ation, ecosystem damage and outright loss of soil.


] produced by the wind erosion of differentially weathered rock in Jebel Kharaz, ]]]
Erosion is distinguished from ], which is the process of chemical or physical breakdown of the minerals in the rocks, although the two processes may occur concurrently.
], in Jinshitan Coastal National Geopark, ], ], China]]


==Physical processes==
==Causes==
===Rainfall and surface runoff===
]
] and water being ] by the impact of a single ]]]
] produced by erosion of differentially weathered rock in Jebel Kharaz (])]]
]fall, and the ] which may result from rainfall, produces four main types of ]: ''splash erosion'', '']'', '']'', and ''gully erosion''. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four).<ref name="toy-2002-p1"/>{{rp|60–61}}<ref>{{cite book|author= Zachar, Dušan|chapter=Classification of soil erosion|title=Soil Erosion|volume=10|publisher=Elsevier|year=1982|isbn=978-0-444-99725-8|page=48|chapter-url=https://books.google.com/books?id=o8ny2dUkpM8C&pg=PA48}}</ref>
The rate of erosion depends on many factors. Climatic factors include the amount and intensity of ], the average temperature, as well as the typical temperature range, and seasonality, the wind speed, storm frequency. The geologic factors include the sediment or rock type, its porosity and permeability, the slope (gradient) of the land, and whether the rocks are tilted, faulted, folded, or weathered. The biological factors include ground cover from ] or lack thereof, the type of organisms inhabiting the area, and the land use.


In ''splash erosion'', the ] creates a small crater in the ],<ref name="Fig. 4">See Figure 1 in {{cite journal |title=Confined Shocks inside Isolated Liquid Volumes – A New Path of Erosion?|journal=Physics of Fluids|volume=23|issue=10|pages=101702|year=2011|arxiv=1109.3175|bibcode=2011PhFl...23j1702O|last1=Obreschkow|first1=D.|last2=Dorsaz|first2=N.|last3=Kobel|first3=P.|last4=De Bosset|first4=A.|last5=Tinguely|first5=M.|last6=Field|first6=J.|last7=Farhat|first7=M.|doi=10.1063/1.3647583|s2cid=59437729}}</ref> ejecting soil particles.<ref name="Hysteretic sediment fluxes in ra"/> The distance these soil particles travel can be as much as {{Convert|0.6|m|ft|abbr=on}} vertically and {{Convert|1.5|m|ft|abbr=on}} horizontally on level ground.
In general, given similar vegetation and ecosystems, areas with high-intensity precipitation, more frequent rainfall, more wind, or more storms are expected to have more erosion. Sediment with high ] or ] contents and areas with steep slopes erode more easily, as do areas with highly fractured or weathered rock. Porosity and permeability of the sediment or rock affect the speed with which the water can percolate into the ground. If the water moves underground, less runoff is generated, reducing the amount of surface erosion. Sediments containing more ] tend to erode less than those with sand or silt. Here, however, the impact of atmospheric sodium on erodibility of clay should be considered.<ref>Schmittner Karl-Erich and Pierre, 1999. The impact of atmospheric sodium on erodibility of clay in a coastal Mediterranean region. Environmental Geology 37/3: 195-206.</ref>


If ], or if the rainfall rate is ] into the soil, surface runoff occurs. If the runoff has sufficient ], it will ] loosened soil particles (]) down the slope.<ref name="FAO-1965-pp23-25">{{cite book|author=Food and Agriculture Organization|chapter=Types of erosion damage|title=Soil Erosion by Water: Some Measures for Its Control on Cultivated Lands|publisher=United Nations|year=1965|isbn=978-92-5-100474-6|pages=23–25|chapter-url=https://books.google.com/books?id=6KeL3ix6ZqQC&pg=PA23}}</ref> '']'' is the transport of loosened soil particles by overland flow.<ref name="FAO-1965-pp23-25" />
The factor that is most subject to change is the amount and type of ground cover. In an undisturbed forest, the mineral soil is protected by a litter layer and an organic layer. These two layers protect the soil by absorbing the impact of rain drops. These layers and the underlying soil in a forest are porous and highly permeable to rainfall. Typically, only the most severe rainfall and large hailstorm events will lead to overland flow in a forest. If the trees are removed by fire or logging, infiltration rates become high and erosion low to the degree the forest floor remains intact. Severe fires can lead to significantly increased erosion if followed by heavy rainfall. In the case of construction or road building, when the litter layer is removed or compacted, the susceptibility of the soil to erosion is greatly increased.
] covered in rills and gullies due to erosion processes caused by rainfall: ], ]]]
''] erosion'' refers to the development of small, ] concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit ] physics very different from water flowing through the deeper, wider channels of streams and rivers.<ref>{{cite journal | last1 = Nearing | first1 = M.A. | last2 = Norton | first2 = L.D. | last3 = Bulgakov | first3 = D.A. | last4 = Larionov | first4 = G.A. | last5 = West | first5 = L.T. | last6 = Dontsova | first6 = K.M. | year = 1997 | title = Hydraulics and erosion in eroding rills | journal = Water Resources Research | volume = 33 | issue = 4| pages = 865–876 | doi=10.1029/97wr00013|bibcode = 1997WRR....33..865N | doi-access = free }}</ref>


{{Anchor|gully erosion|ephemeral gully erosion}}
Roads are especially likely to cause increased rates of erosion because, in addition to removing ground cover, they can significantly change drainage patterns, especially if an embankment has been made to support the road. A road that has a lot of rock and one that is "hydrologically invisible" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion.
'']'' occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.<ref name=Boardman>{{cite book|editor-last1=Boardman|editor-first1=John|editor-last2=Poesen|editor-first2=Jean|title=Soil Erosion in Europe|date=2007|publisher=John Wiley & Sons|location=Chichester|isbn=978-0-470-85911-7}}</ref><ref>{{cite book |author=J. Poesen |author2=L. Vandekerckhove |author3=J. Nachtergaele |author4=D. Oostwoud Wijdenes |author5=G. Verstraeten |author6=B. Can Wesemael |chapter=Gully erosion in dryland environments|pages=229–262|editor=Bull, Louise J. |editor2=Kirby, M.J.|title=Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels|publisher=John Wiley & Sons|year=2002|isbn=978-0-471-49123-1|chapter-url=https://books.google.com/books?id=qjHoYZXQee0C&pg=PA229}}</ref><ref>{{cite book|author=Borah, Deva K.|chapter=Watershed sediment yield|editor=Garcia, Marcelo H.|title=Sedimentation Engineering: Processes, Measurements, Modeling, and Practice|publisher=ASCE Publishing|year=2008|isbn=978-0-7844-0814-8|page=828|chapter-url=https://books.google.com/books?id=1AsypwBUa_wC&pg=PA828|display-authors=etal}}</ref> A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations.<ref>{{Cite journal|last1=Vanmaercke|first1=Matthias|last2=Panagos|first2=Panos|last3=Vanwalleghem|first3=Tom|last4=Hayas|first4=Antonio|last5=Foerster|first5=Saskia|last6=Borrelli|first6=Pasquale|last7=Rossi|first7=Mauro|last8=Torri|first8=Dino|last9=Casali|first9=Javier|last10=Borselli|first10=Lorenzo|last11=Vigiak|first11=Olga|date=July 2021|title=Measuring, modelling and managing gully erosion at large scales: A state of the art|url=https://linkinghub.elsevier.com/retrieve/pii/S0012825221001379|journal=Earth-Science Reviews|language=en|volume=218|pages=103637|doi=10.1016/j.earscirev.2021.103637|bibcode=2021ESRv..21803637V|hdl=10198/24417|s2cid=234800558|hdl-access=free}}</ref>


Extreme gully erosion can progress to formation of ]. These form under conditions of high relief on ] in climates favorable to erosion. Conditions or disturbances that limit the growth of protective vegetation (]) are a key element of badland formation.<ref>{{cite journal |last1=Moreno-de las Heras |first1=Mariano |last2=Gallart |first2=Francesc |title=The Origin of Badlands |journal=Badlands Dynamics in a Context of Global Change |date=2018 |pages=27–59 |doi=10.1016/B978-0-12-813054-4.00002-2|isbn=9780128130544 }}</ref>
Many human activities remove vegetation from an area, making the soil susceptible to erosion. ] can cause increased erosion rates due to ], exposure of ] ], for example roads and landings. However it is the removal of or compromise to the forest floor not the removal of the canopy that can lead to erosion. This is because rain drops striking tree leaves coalesce with other rain drops creating larger drops. When these larger drops fall (called ]) they again may reach ] and strike the ground with more energy then had they fallen in the open. Terminal velocity of rain drops is reached in about 8&nbsp;meters. Because forest canopies are usually higher than this, leaf drop can regain terminal velocity. However, the intact forest floor, with its layers of leaf litter and organic matter, absorbs the impact of the rainfall.<ref></ref>


===Rivers and streams===
], China, farmer is gradually losing his land as the edge of the ] is eroded away]]
Heavy ] can reduce vegetation enough to increase erosion. Changes in the kind of vegetation in an area can also affect erosion rates. Different kinds of vegetation lead to different infiltration rates of rain into the soil. Forested areas have higher infiltration rates, so precipitation will result in less surface runoff, which erodes. Instead much of the water will go in subsurface flows, which are generally less erosive. Leaf litter and low shrubs are an important part of the high infiltration rates of forested systems, the removal of which can increase erosion rates. Leaf litter also shelters the soil from the impact of falling raindrops, which is a significant agent of erosion. Vegetation can also change the speed of surface runoff flows, so grasses and shrubs can also be instrumental in this aspect.


{{further|topic=water's erosive ability|Hydraulic action}}
One of the main causes of erosive soil loss in the year 2006 is the result of ] treatment of ] ]. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive. For example, on the ] high central ], comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of ], with gully erosive furrows typically in excess of 50&nbsp;meters deep and one kilometer wide. ] is a farming system which sometimes incorporates the ] method in some regions of the world. This degrades the soil and causes the soil to become less and less fertile.
], Scotland, showing two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the stream's banks are ] that was left behind as ice age glaciers flowed over the terrain.]]
] exposed by a river eroding through them]]
]
''Valley'' or ''stream erosion'' occurs with continued water flow along a ] The erosion is both ], deepening the ], and ], extending the valley into the hillside, creating ] and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V-shaped cross-section and the stream gradient is relatively steep. When some ] is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream ]s across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, ]s, and ]s can also act erosively as they traverse a surface, in a process known as ''traction''.<ref>Ritter, Michael E. (2006) {{webarchive|url=https://web.archive.org/web/20120506040721/http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/geologic_work_of_streams.html |date=2012-05-06 }} ''The Physical Environment: an Introduction to Physical Geography'' University of Wisconsin, {{OCLC|79006225}}</ref>


'']'' is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as ''scour''. Erosion and ] may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.<ref>{{Cite book |chapter-url=https://books.google.com/books?id=_PJHw-hSKGgC&pg=PA113 |title=Stream hydrology: an introduction for ecologists |author=Nancy D. Gordon |chapter=Erosion and Scour |isbn=978-0-470-84357-4 |date=2004 |publisher=John Wiley and Sons }}</ref>
==Effects==
Approximately 40% of the world's agricultural land is seriously degraded.<ref></ref> According to the ], an area of fertile soil the size of Ukraine is lost every year because of ], ] and ].<ref>I AM FROM SCOTLAND</ref> In ], if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025, according to ]'s Ghana-based Institute for Natural Resources in Africa.<ref></ref>


''Thermal erosion'' is the result of melting and weakening ] due to moving water.<ref name="nsidc_thermal">{{cite web|url=http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion |title=Thermal Erosion |work=NSIDC Glossary |publisher=] |access-date=21 December 2009 |archive-url=https://web.archive.org/web/20101218124656/http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion |archive-date=2010-12-18 |url-status=live }}</ref> It can occur both along rivers and at the coast. Rapid ] observed in the ] of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials.<ref name="lena">{{cite journal|doi=10.1002/esp.592|title=Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)|year=2003|last1=Costard|first1=F.|last2=Dupeyrat|first2=L.|last3=Gautier|first3=E.|last4=Carey-Gailhardis|first4=E.|journal=]|volume=28|pages=1349–1359|bibcode = 2003ESPL...28.1349C|issue=12 |s2cid=131318239 }}</ref> Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the ], where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a {{convert|100|km|mi|abbr=off|adj=on}} segment of the ] shoreline averaged {{convert|5.6|m|ft|abbr=off}} per year from 1955 to 2002.<ref name="jones_arctic">{{cite journal|last=Jones|first=B.M.|author2=Hinkel, K.M.|author3=Arp, C.D.|author4=Eisner, W.R.|year=2008|title=Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska|journal=Arctic|volume=61|issue=4|pages=361–372|url=http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|doi=10.14430/arctic44|url-status=dead|archive-url=https://web.archive.org/web/20130517101602/http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|archive-date=2013-05-17|hdl=10535/5534|hdl-access=free}}</ref>
]s, Yauhanna, South Carolina]]
When land is overused by animal activities (including humans), there can be mechanical erosion and also removal of vegetation leading to erosion. In the case of the animal kingdom, this effect would become material primarily with very large animal ]s stampeding such as the ] on the ] plain. Even in this case there are broader material benefits to the ecosystem, such as continuing the survival of grasslands, that are indigenous to this region. This effect may be viewed as anomalous or a problem only when there is a significant imbalance or ] of one species.


Most river erosion happens nearer to the mouth of a river. On a river bend, the longest least sharp side has slower moving water. Here deposits build up. On the narrowest sharpest side of the bend, there is faster moving water so this side tends to erode away mostly.
In the case of human use, the effects are also generally linked to ]. When large number of hikers use trails or extensive off road vehicle use occurs, erosive effects often follow, arising from vegetation removal and furrowing of foot traffic and off road vehicle tires. These effects can also accumulate from a variety of outdoor human activities, again simply arising from too many people using a finite land resource.


Rapid erosion by a large river can remove enough sediments to produce a ],<ref name="mont">{{cite journal|last=Montgomery|first=David R.|author2=Stolar, Drew B. |title=Reconsidering Himalayan river anticlines|journal=Geomorphology|date=1 December 2006|volume=82|issue=1–2|pages=4–15|doi=10.1016/j.geomorph.2005.08.021|bibcode = 2006Geomo..82....4M }}</ref> as ] raises rock beds unburdened by erosion of overlying beds.
One of the most serious and long-running water erosion problems worldwide is in the ], on the middle reaches of the ] and the upper reaches of the ]. From the ], over 1.6 billion tons of sediment flows into the ocean each year. The ] originates primarily from water erosion in the ] region of the northwest.


===Coastal erosion===
==Processes==
{{main|Coastal erosion}}
===Gravity===
{{See also|Beach evolution}}
]hi im from scotland
] caused by erosion of cliffs by the sea, at ] in South Wales]]
'']'' is the down-slope movement of rock and sediments, mainly due to the force of ]. Mass movement is an important part of the erosional process, as it moves material from higher elevations to lower elevations where other eroding agents such as streams and ]s can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a ]. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a ] slope.
] (of ] age) along cliffs of ] Bay, Yorkshire, England]]
Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and ] but sea level (tidal) change can also play a role.
] beach, ]]]


'']'' takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. '']'' is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. '']'' or '']'' is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with ''corrosion''). '']'' is the dissolving of rock by ] in sea water.<ref>Geddes, Ian. "Lithosphere". Higher geography for cfe: physical and human environments, Hodder Education, 2015.</ref> ] cliffs are particularly vulnerable to this kind of erosion. ''Attrition'' is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as ] and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping and grinding of organisms, a process termed '']''.<ref>Glynn, Peter W. "Bioerosion and coral-reef growth: a dynamic balance". Life and death of coral reefs (1997): 68–95.</ref>
'']'' happens on steep hillsides, occurring along distinct fracture zones, often within materials like ] that, once released, may move quite rapidly downhill. They will often show a spoon-shaped ], in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along ]s where it is a regular occurrence.


] is transported along the coast in the direction of the prevailing current (]). When the upcurrent ] is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of ]. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a buildup of eroded material occurs forming a long narrow bank (a ]). ] beaches and submerged offshore ] may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.<ref>Bell, Frederic Gladstone. "Marine action and control". Geological hazards: their assessment, avoidance, and mitigation, Taylor & Francis, 1999, pp. 302–306.</ref>
''Surface creep'' is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0&nbsp;mm in diameter by wind along the soil surface.


Erosion of a coastal surface, followed by a fall in sea level, can produce a distinctive landform called a ].<ref name="Pinter2010">{{cite web| last1=Pinter |first1=N |date=2010 |title=Exercise 6 - Coastal Terraces, Sealevel, and Active Tectonics |url=http://www.geology.siu.edu/people/pinter/pdf/CoastalExercise.pdf |access-date=2011-04-21 |url-status=dead |archive-url=https://web.archive.org/web/20101010230028/http://www.geology.siu.edu/people/pinter/pdf/CoastalExercise.pdf |archive-date=2010-10-10 }}</ref>
===Water===
], ], ].]]
'''Splash erosion''' is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soil.


===Chemical erosion===
'''Sheet erosion''' is the detachment of soil particles by raindrop impact and their removal downslope by water flowing overland as a sheet instead of in definite channels or rills. The impact of the raindrop breaks apart the soil aggregate. Particles of clay, silt and sand fill the soil pores and reduce infiltration. After the surface pores are filled with sand, silt or clay, overland surface flow of water begins due to the lowering of infiltration rates. Once the rate of falling rain is faster than infiltration, runoff takes place. There are two stages of sheet erosion. The first is rain splash, in which soil particles are knocked into the air by raindrop impact. In the second stage, the loose particles are moved downslope by broad sheets of rapidly flowing water filled with sediment known as sheetfloods. This stage of sheet erosion is generally produced by cloudbursts, sheetfloods commonly travel short distances and last only for a short time.
{{see also|Karst topography}}
<!--why is chemical erosion listed here as a physical (mechanical) process?-->
Chemical erosion is the loss of matter in a landscape in the form of ]. Chemical erosion is usually calculated from the solutes found in streams. ] pioneered the study of chemical erosion in his work about ] published in 1960.<ref>{{cite journal |last1=Dixon |first1=John C. |last2=Thorn |first2=Colin E. |date=2005 |title=Chemical weathering and landscape development in mid-latitude alpine environments |journal=] |volume=67 |issue=1–2 |pages=127–145 |doi= 10.1016/j.geomorph.2004.07.009|bibcode = 2005Geomo..67..127D }}</ref>


Formation of ]s and other features of karst topography is an example of extreme chemical erosion.<ref>{{cite journal|author = Lard, L. |author2=Paull, C. |author3=Hobson, B. |year = 1995|title = Genesis of a submarine sinkhole without subaerial exposure|journal = Geology|volume = 23|issue = 10|pages = 949–951|doi = 10.1130/0091-7613(1995)023<0949:GOASSW>2.3.CO;2|bibcode = 1995Geo....23..949L }}</ref>
'''] erosion''' refers to the development of small, ] concentrated flow paths, which function as both sediment source and ] delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically on the order of a few centimeters or less and slopes may be quite steep. These conditions constitute a very different hydraulic environment than typically found in channels of streams and rivers. Eroding rills evolve morphologically in time and space. The rill bed surface changes as soil erodes, which in turn alters the hydraulics of the flow. The hydraulics is the driving mechanism for the erosion process, and therefore dynamically changing hydraulic patterns cause continually changing erosional patterns in the rill. Thus, the process of rill evolution involves a feedback loop between flow detachment, hydraulics, and bed form. Flow velocity, depth, width, hydraulic roughness, local bed slope, friction slope, and detachment rate are time and space variable functions of the rill evolutionary process. Superimposed on these interactive processes, the sediment load, or amount of sediment in the flow, has a large influence on soil detachment rates in rills. As sediment load increases, the ability of the flowing water to detach more sediment decreases.


===Glaciers===
Where precipitation rates exceed soil infiltration rates, ''runoff'' occurs. Surface runoff turbulence can often cause more erosion than the initial raindrop impact.
] (''Pirunpesä''), the deepest ground erosion in ],<ref>{{cite web| url = https://www.pizzatravel.com.ua/eng/finland/117/devils_nest| title = The Devil's Nest, the deepest ground erosion in Europe}}</ref> located in ], ], ]]]
] above ], in ]]]
]s erode predominantly by three different processes: abrasion/scouring, ], and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes, such as erosion also control cross-valley variations. In a homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created. Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in ]s. Scientists also provide a numerical estimate of the time required for the ultimate formation of a steady-shaped ]—approximately 100,000 years. In a weak bedrock (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, the amount of over deepening is limited because ice velocities and erosion rates are reduced.<ref>{{Cite journal|last1=Harbor|first1=Jonathan M.|last2=Hallet|first2=Bernard|last3=Raymond|first3=Charles F.|date=1988-05-26|title=A numerical model of landform development by glacial erosion|journal=Nature|language=en|volume=333|issue=6171|pages=347–349|doi=10.1038/333347a0|bibcode=1988Natur.333..347H|s2cid=4273817}}</ref>


Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the ]. Differences in the height of mountain ranges are not only being the result tectonic forces, such as rock uplift, but also local climate variations. Scientists use global analysis of topography to show that glacial erosion controls the maximum height of mountains, as the relief between mountain peaks and the snow line are generally confined to altitudes less than 1500&nbsp;m.<ref>{{Cite journal|last1=Egholm|first1=D. L.|last2=Nielsen|first2=S. B.|last3=Pedersen|first3=V.K.|last4=Lesemann|first4=J.-E.|title=Glacial effects limiting mountain height|journal=Nature|volume=460|issue=7257|pages=884–887|doi=10.1038/nature08263|pmid=19675651|bibcode=2009Natur.460..884E|year=2009|s2cid=205217746}}</ref> The erosion caused by glaciers worldwide erodes mountains so effectively that the term '']'' has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges.<ref name="reference">{{cite journal | last1 = Thomson | first1 = Stuart N. | last2 = Brandon | first2 = Mark T. | last3 = Tomkin | first3 = Jonathan H. | last4 = Reiners | first4 = Peter W. | last5 = Vásquez | first5 = Cristián | last6 = Wilson | first6 = Nathaniel J. | year = 2010 | title = Glaciation as a destructive and constructive control on mountain building | journal = Nature | volume = 467 | issue = 7313| pages = 313–317 | doi = 10.1038/nature09365 | pmid = 20844534 |bibcode = 2010Natur.467..313T | s2cid = 205222252 | hdl = 10533/144849 | hdl-access = free }}</ref> As mountains grow higher, they generally allow for more glacial activity (especially in the ] above the glacial equilibrium line altitude),<ref>{{cite journal | last1 = Tomkin | first1 = J.H. | last2 = Roe | first2 = G.H. | year = 2007 | title = Climate and tectonic controls on glaciated critical-taper orogens | url = http://earthweb.ess.washington.edu/roe/Publications/TomkinRoe_Glaciers_EPSL07.pdf | journal = Earth Planet. Sci. Lett. | volume = 262 | issue = 3–4 | pages = 385–397 | doi = 10.1016/j.epsl.2007.07.040 | bibcode = 2007E&PSL.262..385T | citeseerx = 10.1.1.477.3927 | access-date = 2017-10-24 | archive-url = https://web.archive.org/web/20170809121522/http://earthweb.ess.washington.edu/roe/Publications/TomkinRoe_Glaciers_EPSL07.pdf | archive-date = 2017-08-09 | url-status=live }}</ref> which causes increased rates of erosion of the mountain, decreasing mass faster than ] can add to the mountain.<ref>Mitchell, S.G. & Montgomery, D.R. "Influence of a glacial buzzsaw on the height and morphology of the Cascade Range in central Washington State". ''Quat. Res''. 65, 96–107 (2006)</ref> This provides a good example of a ]. Ongoing research is showing that while glaciers tend to decrease mountain size, in some areas, glaciers can actually reduce the rate of erosion, acting as a ''glacial armor''.<ref name="reference"/> Ice can not only erode mountains but also protect them from erosion. Depending on glacier regime, even steep alpine lands can be preserved through time with the help of ice. Scientists have proved this theory by sampling eight summits of northwestern Svalbard using Be10 and Al26, showing that northwestern Svalbard transformed from a glacier-erosion state under relatively mild glacial maxima temperature, to a glacier-armor state occupied by cold-based, protective ice during much colder glacial maxima temperatures as the Quaternary ice age progressed.<ref>{{Cite journal|last1=Gjermundsen|first1=Endre F.|last2=Briner|first2=Jason P.|last3=Akçar|first3=Naki|last4=Foros|first4=Jørn|last5=Kubik|first5=Peter W.|last6=Salvigsen|first6=Otto|last7=Hormes|first7=Anne|title=Minimal erosion of Arctic alpine topography during late Quaternary glaciation|journal=Nature Geoscience|volume=8|issue=10|pages=789|doi=10.1038/ngeo2524|bibcode=2015NatGe...8..789G|year=2015}}</ref>
{{Anchor|gully erosion|ephemeral gully erosion}}
'''Gully erosion''', also called ''ephemeral gully erosion'', occurs when water flows in narrow channels during or immediately after heavy rains or melting snow. This is particularly noticeable in the formation of ], where, prior to being tarmacked, an old rural road has over many years become significantly lower than the surrounding fields.


These processes, combined with erosion and transport by the water network beneath the glacier, leave behind ]s such as ]s, ]s, ground moraine (till), ], kames, kame deltas, moulins, and ]s in their wake, typically at the terminus or during ].<ref>Harvey, A.M. "Local-Scale geomorphology – process systems and landforms". ''Introducing Geomorphology: A Guide to Landforms and Processes''. Dunedin Academic Press, 2012, pp. 87–88. EBSCO''host''.</ref>
A ] is sufficiently deep that it would not be routinely destroyed by tillage operations, whereas rill erosion is smoothed by ordinary farm tillage. The narrow channels, or gullies, may be of considerable depth, ranging from 1 to {{convert|2|ft|m}} to as much as 75 to {{convert|100|ft|m}}. Gully erosion is not accounted for in the ].


The best-developed glacial valley morphology appears to be restricted to landscapes with low rock uplift rates (less than or equal to 2mm per year) and high relief, leading to long-turnover times. Where rock uplift rates exceed 2mm per year, glacial valley morphology has generally been significantly modified in postglacial time. Interplay of glacial erosion and tectonic forcing governs the morphologic impact of glaciations on active orogens, by both influencing their height, and by altering the patterns of erosion during subsequent glacial periods via a link between rock uplift and valley cross-sectional shape.<ref>{{Cite journal|last1=Prasicek|first1=Günther|last2=Larsen|first2=Isaac J.|last3=Montgomery|first3=David R.|date=2015-08-14|title=Tectonic control on the persistence of glacially sculpted topography|journal=Nature Communications|language=en|volume=6|doi=10.1038/ncomms9028|issn=2041-1723| pmc=4557346 |pmid=26271245|page=8028|bibcode=2015NatCo...6.8028P |bibcode-access=free |doi-access=free }}</ref>
''Valley'' or ''stream erosion'' occurs with continued water flow along a linear feature. The erosion is both ], deepening the valley, and ], extending the valley into the hillside. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical '''V''' cross-section and the stream gradient is relatively steep. When some ] is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream ]s across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, ]s and ]s can also act erosively as they traverse a surface.


===Floods===
At extremely high flows, ]s, or ] are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called ]s. Examples can be seen in the flood regions result from glacial ], which created the ] in the ] region of eastern ].<ref>{{cite book
] in ] after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode; leaving behind a tall sand bank in its place]]
|first=David
At extremely high flows, ], or ] are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called ]s. Examples can be seen in the flood regions result from glacial ], which created the ] in the ] region of eastern ].<ref>See, for example: {{cite book|author=Alt, David|title=Glacial Lake Missoula & its Humongous Floods
|last=Alt
|publisher=Mountain Press|year=2001|isbn=978-0-87842-415-3|url=https://books.google.com/books?id=s4y3c8fxeEwC}}</ref>
|isbn=978-0-87842-415-3
|location=
|title=Glacial Lake Missoula & its Humongous Floods
|publisher=Mountain Press Publishing Company
|year=2001
}}</ref>


===Wind erosion===
'''Bank erosion''' is the wearing away of the banks of a ] or ]. This is distinguished from changes on the bed of the watercourse, which is referred to as ''scour''. Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.<ref>{{Cite book |url=http://books.google.com/?id=_PJHw-hSKGgC&pg=PA113 |title=Stream hydrology: an introduction for ecologists |author=Nancy D. Gordon |chapter=Erosion and Scour |isbn=978-0-470-84357-4 |date=2004-06-01 |postscript=}}</ref>
], a rock formation in the ], ] sculpted by wind erosion]]
{{main|Aeolian processes}}
Wind erosion is a major ] force, especially in ] and ] regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as ], ], and ].<ref>{{Cite book|author=Zheng, Xiaojing |author2=Huang, Ning|title=Mechanics of Wind-Blown Sand Movements|publisher=Springer|year=2009|isbn=978-3-540-88253-4|pages=7–8|url=https://books.google.com/books?id=R6kYrbA3XSAC&pg=PA7|bibcode=2009mwbs.book.....Z}}</ref><ref>{{cite book|author=Cornelis, Wim S.|chapter=Hydroclimatology of wind erosion in arid and semi-arid environments|editor=D'Odorico, Paolo |editor2=Porporato, Amilcare|title=Dryland Ecohydrology|publisher=Springer|year=2006|isbn=978-1-4020-4261-4|page=141|chapter-url=https://books.google.com/books?id=rUsUPZbFHK8C&pg=PA141}}</ref>


Wind erosion is of two primary varieties: '']'', where the wind picks up and carries away loose particles; and '']'', where ] are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) '']'', where larger, heavier particles slide or roll along the ground; (2) '']'', where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) '']'', where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%).<ref name=BlancoWind>{{cite book|last1=Blanco-Canqui|first1=Humberto|last2=Rattan|first2=Lal|chapter=Wind erosion|title=Principles of soil conservation and management|date=2008|publisher=Springer|location=Dordrecht|isbn=978-1-4020-8709-7|pages=54–80}}</ref>{{rp|57}}<ref>{{Cite book|author=Balba, A. Monem|chapter=Desertification: Wind erosion|title=Management of Problem Soils in Arid Ecosystems|publisher=CRC Press|year=1995|isbn=978-0-87371-811-0|page=214|chapter-url=https://books.google.com/books?id=uS62XNzDZDsC&pg=PA214}}</ref>
====Shoreline====
{{main|Coastal erosion}}
{{See also|Beach evolution}}
] at ], ].]]
] caused by erosion of cliffs by the sea, at ] in South ]]]
Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.


Wind erosion is much more severe in arid areas and during times of drought. For example, in the ], it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.<ref>{{Cite book|author=Wiggs, Giles F.S.|chapter=Geomorphological hazards in drylands|editor=Thomas, David S.G.|title=Arid Zone Geomorphology: Process, Form and Change in Drylands|publisher=John Wiley & Sons|year=2011|isbn=978-0-470-71076-0|page=588|chapter-url=https://books.google.com/books?id=swz4rh4KaLYC&pg=PA588}}</ref>
'']'' takes place when air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. '']'' is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. '']'' or ''corrasion'' is caused by waves launching seaload at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with ''corrosion''). '']'' is the dissolving of rock by ] in sea water. ] cliffs are particularly vulnerable to this kind of erosion. ''Attrition'' is where particles/seaload carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as ] and sand. Another significant source of erosion, particularly on carbonate coastlines, is the boring, scraping and grinding of organisms, a process termed '']''.


===Mass wasting===
] is transported along the coast in the direction of the prevailing current (]). When the upcurrent amount of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form. These banks may slowly migrate along the coast in the direction of the ], alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a build up of eroded material occurs forming a long narrow bank (a ]). ] beaches and submerged offshore ] may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.
] in ], Israel, showing gravity collapse erosion on its banks]]
{{Main|Mass wasting}}
''Mass wasting'' or ''mass movement'' is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of ].<ref>{{cite book|author=Van Beek, Rens|chapter=Hillside processes: mass wasting, slope stability, and erosion|editor=Norris, Joanne E. |display-editors=etal |title=Slope Stability and Erosion Control: Ecotechnological Solutions|publisher=Springer|year=2008|isbn=978-1-4020-6675-7|chapter-url=https://books.google.com/books?id=YWPcffxM_A0C&pg=PA17|bibcode=2008ssec.conf.....N}}</ref><ref>{{cite book|author=Gray, Donald H. |author2=Sotir, Robbin B.|chapter=Surficial erosion and mass movement|title=Biotechnical and Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control|publisher=John Wiley & Sons|year=1996|isbn=978-0-471-04978-4|page=20|chapter-url=https://books.google.com/books?id=kCbp6IvFHrAC&pg=20}}</ref>


Mass wasting is an important part of the erosional process and is often the first stage in the breakdown and transport of weathered materials in mountainous areas.<ref name=Nichols>{{cite book|author=Nichols, Gary|title=Sedimentology and Stratigraphy|publisher=John Wiley & Sons|year=2009|isbn=978-1-4051-9379-5}}</ref>{{rp|93}} It moves material from higher elevations to lower elevations where other eroding agents such as streams and ]s can then pick up the material and move it to even lower elevations. Mass-wasting processes are always occurring continuously on all slopes; some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a ]. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a ] slope.{{Citation needed|date=April 2012}}
===Ice===
'''Ice erosion''' can take one of two forms. It can be caused by the movement of ice, typically as glaciers, in a process called ''glacial erosion''. It can also be due to freeze-thaw processes in which water inside pores and fractures in rock may expand causing further cracking.


'']'' happens on steep hillsides, occurring along distinct fracture zones, often within materials like ] that, once released, may move quite rapidly downhill. They will often show a spoon-shaped ], in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along ]s where it is a regular occurrence.<ref name="P.2007">{{cite book|author=Sivashanmugam, P.|title=Basics of Environmental Science and Engineering|url=https://books.google.com/books?id=aTJ31ycKQeEC&pg=PA43|year=2007|publisher=New India Publishing|isbn=978-81-89422-28-8|pages=43–}}</ref>
]s erode predominantly by three different processes: abrasion/scouring, ], and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. These processes, combined with erosion and transport by the water network beneath the glacier, leave ]s, ]s, ground moraine (till), kames, kame deltas, moulins, and ]s in their wake, typically at the terminus or during ].


''Surface creep'' is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles {{convert|0.5|to|1.0|mm|abbr=on|2}} in diameter by wind along the soil surface.<ref>{{Cite web|url=http://library.eb.com/levels/referencecenter/article/27828|title=Britannica Library|website=library.eb.com|language=en|access-date=2017-01-31}}</ref>
Cold weather causes water trapped in tiny rock cracks to freeze and expand, breaking the rock into several pieces. This can lead to gravity erosion on steep slopes. The ] which forms at the bottom of a steep mountainside is mostly formed from pieces of rock (soil) broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads, because morning thaws can drop hazardous rock pieces onto the road.


===Submarine sediment gravity flows===
In some places, water seeps into rocks during the daytime, then freezes at night. Ice expands, thus, creating a wedge in the rock. Over time, the repetition in the forming and melting of the ice causes fissures, which eventually breaks the rock down.
] of submarine canyons in the ] off the coast of New York and New Jersey]]
On the ], erosion of the ocean floor to create channels and ]s can result from the rapid downslope flow of ]s, bodies of sediment-laden water that move rapidly downslope as ]s. Where erosion by turbidity currents creates oversteepened slopes it can also trigger underwater landslides and ]s. Turbidity currents can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock.<ref>{{cite journal |last1=Halsey |first1=Thomas C. |title=Erosion of unconsolidated beds by turbidity currents |journal=Physical Review Fluids |date=15 October 2018 |volume=3 |issue=10 |pages=104303 |doi=10.1103/PhysRevFluids.3.104303|bibcode=2018PhRvF...3j4303H |s2cid=134740576 }}</ref><ref>{{cite journal |last1=Mitchell |first1=Neil C. |title=Bedrock erosion by sedimentary flows in submarine canyons |journal=Geosphere |date=October 2014 |volume=10 |issue=5 |pages=892–904 |doi=10.1130/GES01008.1|bibcode=2014Geosp..10..892M |doi-access=free }}</ref><ref>{{cite journal |last1=Smith |first1=M. Elliot |last2=Werner |first2=Samuel H. |last3=Buscombe |first3=Daniel |last4=Finnegan |first4=Noah J. |last5=Sumner |first5=Esther J. |last6=Mueller |first6=Erich R. |title=Seeking the Shore: Evidence for Active Submarine Canyon Head Incision Due to Coarse Sediment Supply and Focusing of Wave Energy |journal=Geophysical Research Letters |date=28 November 2018 |volume=45 |issue=22 |pages=12,403–12,413 |doi=10.1029/2018GL080396|bibcode=2018GeoRL..4512403S |s2cid=134823668 |doi-access=free }}</ref> Almost all continental slopes and deep ocean basins display such channels and canyons resulting from sediment gravity flows and submarine canyons act as conduits for the transfer of sediment from the continents and shallow marine environments to the deep sea.<ref>{{cite journal |last1=Harris |first1=Peter T. |title=Seafloor geomorphology—coast, shelf, and abyss |journal=Seafloor Geomorphology as Benthic Habitat |date=2020 |pages=115–160 |doi=10.1016/B978-0-12-814960-7.00006-3|isbn=9780128149607 }}</ref><ref>{{cite journal |last1=Bührig |first1=Laura H. |last2=Colombera |first2=Luca |last3=Patacci |first3=Marco |last4=Mountney |first4=Nigel P. |last5=McCaffrey |first5=William D. |title=A global analysis of controls on submarine-canyon geomorphology |journal=Earth-Science Reviews |date=October 2022 |volume=233 |pages=104150 |doi=10.1016/j.earscirev.2022.104150|bibcode=2022ESRv..23304150B |s2cid=251576822 |doi-access=free }}</ref><ref>{{cite book |title=Seafloor Geomorphology as Benthic Habitat |date=2012 |doi=10.1016/C2010-0-67010-6|isbn=9780123851406 |s2cid=213281574 }}</ref> ]s, which are the sedimentary deposits resulting from turbidity currents, comprise some of the thickest and largest sedimentary sequences on Earth, indicating that the associated erosional processes must also have played a prominent role in Earth's history.


==Factors affecting erosion rates==
===Wind===
{{Geology sidebar}}
], ] sculpted by wind erosion.]]
] near ].]]
{{main|Aeolian processes}}
In arid climates, the main source of erosion is wind.<ref name="Erosion">{{cite web|author=Vern Hofman and Dave Franzen|year=1997|url=http://www.ag.ndsu.edu/disaster/drought/emergencytillagetocontrolerosion.html|title=Emergency Tillage to Control Wind Erosion|publisher=] Extension Service|accessdate=2009-03-21|archiveurl = http://www.webcitation.org/5uLai90jn |archivedate = 2010-11-18|deadurl=no}}</ref> The general wind circulation moves small particulates such as dust across wide oceans thousands of kilometers downwind of their point of origin,<ref name="Gobi">{{cite web|author=James K. B. Bishop, Russ E. Davis, and Jeffrey T. Sherman|year=2002|url=http://www-ocean.lbl.gov/people/bishop/bishoppubs/paparobots.html|title=Robotic Observations of Dust Storm Enhancement of Carbon Biomass in the North Pacific|work=Science 298|pages=817–821|accessdate=2009-06-20|archiveurl = http://www.webcitation.org/5uLajGMw8 |archivedate = 2010-11-18|deadurl=no}}</ref> which is known as deflation. Erosion can be the result of material movement by the wind. There are two main effects. First, wind causes small particles to be lifted and therefore moved to another region. This is called deflation. Second, these suspended particles may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation. An example is the formation of sand ], on a beach or in a desert.<ref>{{cite web|author=]|year=2004|url=http://geomaps.wr.usgs.gov/parks/coast/dunes/index.html|title=Dunes – Getting Started|accessdate=2009-03-21|archiveurl = http://www.webcitation.org/5uLak8smG |archivedate = 2010-11-18|deadurl=no}}</ref> Loess is a homogeneous, typically nonstratified, porous, ], slightly coherent, often calcareous, fine-grained, ]y, pale yellow or buff, windblown (aeolian) ].<ref>{{cite book|author=F. von Richthofen|year=1882|title=On the mode of origin of the loess|work=The Geological Magazine, Decade II, 9(7)|pages=293–305}}</ref> It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.<ref>{{cite book|coauthors=K.E.K. Neuendorf, J.P. Mehl, Jr., and J.A. Jackson|year=2005|title=Glossary of Geology|publisher=Springer-Verlag, New York|page=779|isbn=978-3-540-27951-8}}</ref> Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.<ref>{{cite book|name=Arthur Getis|coauthors=Judith Getis and Jerome D. Fellmann|title=Introduction to Geography, Seventh Edition|year=2000|publisher=McGraw Hill|isbn=978-0-697-38506-2|page=99}}</ref> Loess deposits are geologically unstable by nature, and will erode very readily. Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.<ref name="Erosion"/>


===Thermal=== ===Climate===
{{see also|Climatic geomorphology}}
The amount and intensity of ] is the main ] governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by ]. This might be during periods when ] leave the soil bare, or in ] regions where vegetation is naturally sparse. Wind erosion requires strong winds, particularly during times of drought when vegetation is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties. In general, given similar vegetation and ecosystems, areas with more precipitation (especially high-intensity rainfall), more wind, or more storms are expected to have more erosion.


In some areas of the world (e.g. the ]), rainfall intensity is the primary determinant of erosivity (for a definition of ''erosivity'' check,<ref>{{Cite book|title=Encyclopedia of Natural Hazards|last1=Zorn|first1=Matija|last2=Komac|first2=Blaž|chapter=Erosivity |date=2013|publisher=Springer Netherlands|isbn=978-90-481-8699-0|editor-last=Bobrowsky|editor-first=Peter T.|series=Encyclopedia of Earth Sciences Series|pages=289–290|language=en|doi=10.1007/978-1-4020-4399-4_121}}</ref>) with higher intensity rainfall generally resulting in more soil erosion by water. The size and velocity of ]s is also an important factor. Larger and higher-velocity rain drops have greater ], and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops.<ref name=BlancoWater>{{cite book|last1=Blanco-Canqui|first1=Humberto|last2=Rattan|first2=Lal|chapter=Water erosion|title=Principles of soil conservation and management|date=2008|publisher=Springer|location=Dordrecht|isbn=978-1-4020-8709-7|pages=21–53 }}</ref>
Thermal erosion is the result of melting and weakening ] due to moving water.<ref name="nsidc_thermal">{{cite web|url=http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion|title=Thermal Erosion|work=NSIDC Glossary|publisher=]|accessdate=21 December 2009|archiveurl = http://www.webcitation.org/5uLakeCcx |archivedate = 2010-11-18|deadurl=no}}</ref> It can occur both along rivers and at the coast. Rapid ] observed in the ] of ] is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials.<ref name="lena">{{cite journal|doi=10.1002/esp.592|title=Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)|year=2003|last1=Costard|first1=F.|last2=Dupeyrat|first2=L.|last3=Gautier|first3=E.|last4=Carey-Gailhardis|first4=E.|journal=Earth Surface Processes and Landforms|volume=28|page=1349}}</ref> Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the ] coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a 100-kilometer segment of the Beaufort Sea shoreline averaged 5.6&nbsp;meters per year from 1955 to 2002.<ref name="jones_arctic">{{cite journal|last=Jones|first=B.M.|coauthors=Hinkel, K.M., Arp, C.D. and Eisner, W.R.|year=2008|title=Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska|journal=Arctic|publisher=]|volume=61|issue=4|pages=361–372|url=http://digitization.ucalgary.ca/arctic/index.php/arctic/article/view/44/115}}{{dead link|date=July 2010}}</ref>


In other regions of the world (e.g. ]), runoff and erosion result from relatively low intensities of ] falling onto the previously saturated soil. In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water.<ref name=Boardman/> According to the climate change projections, erosivity will increase significantly in Europe and soil erosion may increase by 13–22.5% by 2050 <ref>{{Cite journal|date=2021-10-01|title=Projections of soil loss by water erosion in Europe by 2050|journal=Environmental Science & Policy|language=en|volume=124|pages=380–392|doi=10.1016/j.envsci.2021.07.012|issn=1462-9011|last1=Panagos|first1=Panos|last2=Ballabio|first2=Cristiano|last3=Himics|first3=Mihaly|last4=Scarpa|first4=Simone|last5=Matthews|first5=Francis|last6=Bogonos|first6=Mariia|last7=Poesen|first7=Jean|last8=Borrelli|first8=Pasquale|doi-access=free|bibcode=2021ESPol.124..380P }}</ref>
==Soil erosion and climate change==
{{Main|Land degradation}}


In ], where typhoon frequency increased significantly in the 21st century, a strong link has been drawn between the increase in storm frequency with an increase in sediment load in rivers and reservoirs, highlighting the impacts ] can have on erosion.<ref name=":0">{{Cite journal|last1=Montgomery|first1=David R.|last2=Huang|first2=Michelle Y.-F.|last3=Huang|first3=Alice Y.-L.|date=2014-01-01|title=Regional soil erosion in response to land use and increased typhoon frequency and intensity, Taiwan|url=https://www.cambridge.org/core/journals/quaternary-research/article/div-classtitleregional-soil-erosion-in-response-to-land-use-and-increased-typhoon-frequency-and-intensity-taiwandiv/377D2EDD04D595F781FC81F2D064D37A|journal=Quaternary Research|volume=81|issue=1|pages=15–20|doi=10.1016/j.yqres.2013.10.005|issn=0033-5894|bibcode=2014QuRes..81...15M|s2cid=53649150|access-date=2017-02-23|archive-url=https://web.archive.org/web/20170224053537/https://www.cambridge.org/core/journals/quaternary-research/article/div-classtitleregional-soil-erosion-in-response-to-land-use-and-increased-typhoon-frequency-and-intensity-taiwandiv/377D2EDD04D595F781FC81F2D064D37A|archive-date=2017-02-24|url-status=live}}</ref>
The warmer atmospheric temperatures observed over the past decades are expected to lead to a more vigorous hydrological cycle, including more extreme rainfall events.<ref>IPCC. 1995. Second Assessment Synthesis of Scientific-Technical Information relevant to interpreting Article 2 of the UN Framework Convention on Climate Change. Intergovernmental Panel on Climate Change. Geneva, Switzerland. 64 pp.</ref> In 1998 Karl and Knight reported that from 1910 to 1996 total precipitation over the contiguous U.S. increased, and that 53% of the increase came from the upper 10% of precipitation events (the most intense precipitation).<ref>Karl, T.R. and R. W. Knight. 1998. Secular trend of precipitation amount, frequency, and intensity in the United States. Bulletin of the American Meteorological Society 79:231-242.</ref> The percent of precipitation coming from days of precipitation in excess of 50&nbsp;mm has also increased significantly.


===Vegetative cover===
Studies on soil erosion suggest that increased rainfall amounts and intensities will lead to greater rates of erosion. Thus, if rainfall amounts and intensities increase in many parts of the world as expected, erosion will also increase, unless amelioration measures are taken. Soil erosion rates are expected to change in response to changes in climate for a variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include: a) changes in plant canopy caused by shifts in plant biomass production associated with moisture regime; b) changes in litter cover on the ground caused by changes in both plant residue decomposition rates driven by temperature and moisture dependent soil microbial activity as well as plant biomass production rates; c) changes in soil moisture due to shifting precipitation regimes and evapo-transpiration rates, which changes infiltration and runoff ratios; d) soil erodibility changes due to decrease in soil organic matter concentrations in soils that lead to a soil structure that is more susceptible to erosion and increased runoff due to increased soil surface sealing and crusting; e) a shift of winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures; f) melting of permafrost, which induces an erodible soil state from a previously non-erodible one; and g) shifts in land use made necessary to accommodate new climatic regimes.
{{See also|Vegetation and slope stability}}
Vegetation acts as an interface between the atmosphere and the soil. It increases the ] of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of the plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water<ref>{{Cite journal|last1=Gyssels|first1=G.|last2=Poesen|first2=J.|last3=Bochet|first3=E.|last4=Li|first4=Y.|date=2005-06-01|title=Impact of plant roots on the resistance of soils to erosion by water: a review|journal=Progress in Physical Geography|language=en|volume=29|issue=2|pages=189–217|doi=10.1191/0309133305pp443ra|bibcode=2005PrPG...29..189G |s2cid=55243167|issn=0309-1333}}</ref> and wind erosion. The removal of vegetation increases the rate of surface erosion.<ref>{{cite book|author=Styczen, M.E. |author2=Morgan, R.P.C.|chapter=Engineering properties of vegetation|editor=Morgan, R.P.C. |editor2=Rickson, R. Jane|title=Slope Stabilization and Erosion Control: A Bioengineering Approach|publisher=Taylor & Francis|year=1995|isbn=978-0-419-15630-7|chapter-url=https://books.google.com/books?id=3jXg9pyfikQC&pg=4}}</ref>


===Topography===
Studies by Pruski and Nearing indicated that, other factors such as land use not considered, we can expect approximately a 1.7% change in soil erosion for each 1% change in total precipitation under climate change.<ref>Pruski, F. F. and M.A. Nearing. 2002. Runoff and soil loss responses to changes in precipitation: a computer simulation study. Journal of Soil and Water Conservation 57(1), 7-16.</ref>
The topography of the land determines the velocity at which ] will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain is also more prone to mudslides, landslides, and other forms of gravitational erosion processes.<ref name=BlancoWater/>{{rp|28–30}}<ref>{{cite book|author=Whisenant, Steve G.|chapter=Terrestrial systems|editor=Perrow Michael R. |editor2=Davy, Anthony J.|title=Handbook of Ecological Restoration: Principles of Restoration|publisher=Cambridge University Press|year=2008|isbn=978-0-521-04983-2|page=89|chapter-url=https://books.google.com/books?id=moJHjZ9qW_8C&pg=PA89}}</ref><ref>{{cite book|author=Wainwright, John |author2=Brazier, Richard E.|chapter=Slope systems|editor=Thomas, David S.G.|title=Arid Zone Geomorphology: Process, Form and Change in Drylands|publisher=John Wiley & Sons|year=2011|isbn=978-0-470-71076-0|chapter-url=https://books.google.com/books?id=swz4rh4KaLYC&pg=PA209}}</ref>


===Tectonics===
==Tectonic effects==
{{Main|Erosion and tectonics}}
]]]
Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface (e.g., a mountain range) to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to the local slope (see above), this will change the rates of erosion in the uplifted area. Active tectonics also brings fresh, unweathered rock towards the surface, where it is exposed to the action of erosion.
The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the ] and ]. This can cause ] or ] in the region. Research undertaken since the early 1990s suggests that the spatial distribution of erosion at the surface of an ] can exert a key influence on its growth and its final internal structure (see ]).<ref>Willett, Sean D., et al., ''Tectonics, Climate and Landscape Evolution'', Geological Society of America Special Paper 398, 2006 ISBN 978-0-8137-2398-3 </ref>


However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the ] and ]. Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause ] or ] in the region.<ref name=Nichols/>{{rp|99}}<ref>{{cite book|author1=Burbank, Douglas W. |author2=Anderson, Robert S.|chapter=Tectonic and surface uplift rates|title=Tectonic Geomorphology|publisher=John Wiley & Sons|year=2011|isbn=978-1-4443-4504-9|pages=270–271|chapter-url=https://books.google.com/books?id=83FuAvtSwE4C&pg=PT270}}</ref> In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates, for example, beneath the extremely steep terrain of ] in the western ]s. Such a place has been called a "]".<ref>Zeitler, P.K. et al. (2001), Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism, GSA Today, 11, 4–9.</ref>
==Materials science==
In ], erosion is the recession of surfaces by repeated localized mechanical trauma as, for example, by suspended abrasive particles within a moving fluid. Erosion can also occur from non-abrasive fluid mixtures. ] is one example.


=== Development ===
In hard particle erosion, the ] of the impacted material is a large factor in the ] of the erosion. A soft material will typically erode fastest from glancing impacts.<ref>Finnie, I., Erosion of surfaces by solid particles. Wear, 1960. 3(2): p. 87.</ref> Harder material will typically erode fastest from perpendicular impacts. Hardness is a correlative factor for erosion resistance, but a higher hardness does not guarantee better resistance. Factors that affect the erosion rate also include impacting particle speed, size, density, hardness, and rotation. ] can be applied to retard erosion, but normally can only slow the removal of material. Erosion rate for solid particle impact is typically measured as mass of material removed divided by the mass of impacting material.<ref>ASTM G76-07</ref>
Human land development, in forms including agricultural and urban development, is considered a significant factor in erosion and ], which aggravate ].<ref>{{Cite journal|last=Chen|first=Jie|date=2007-01-16|title=Rapid urbanization in China: A real challenge to soil protection and food security|journal=CATENA|series=Influences of rapid urbanization and industrialization on soil resource and its quality in China|volume=69|issue=1|pages=1–15|doi=10.1016/j.catena.2006.04.019|bibcode=2007Caten..69....1C }}</ref> In Taiwan, increases in sediment load in the northern, central, and southern regions of the island can be tracked with the timeline of development for each region throughout the 20th century.<ref name=":0" /> The intentional removal of soil and rock by humans is a form of erosion that has been named '']''.<ref>{{cite book |last=Selby |first=Michael John |author-link=Michael Selby |title=Earth's changing surface: an introduction to geomorphology |publisher=Clarendon Press |location=Oxford |year=1985 |isbn=0-19-823252-7 |url-access=registration |url=https://archive.org/details/earthschangingsu00selb }}</ref>


==Erosion at various scales==
==Figurative use==
The concept of erosion is commonly employed by ] to various forms of perceived or real homogenization (i.e. erosion of boundaries), "leveling out", collusion or even the decline of anything from ] to ]. It is a common ] of the English language to describe as ''erosion'' the gradual, organic mutation of something thought of as distinct, more complex, harder to pronounce or more refined into something indistinct, less complex, ] or (disparagingly) less refined.


==Origin of term== ===Mountain ranges===
{{see also|denudation|planation}}
{{Ref improve section|date=June 2009}}
]s take millions of years to erode to the degree they effectively cease to exist. Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode a mountain mass similar to the ] into an almost-flat ] if there are no significant ].<ref name="PitmanGolovchenko1991">{{cite journal|last1=Pitman|first1=W. C.|last2=Golovchenko|first2=X.|title=The effect of sea level changes on the morphology of mountain belts|journal=Journal of Geophysical Research: Solid Earth|volume=96|issue=B4|year=1991|pages=6879–6891|issn=0148-0227|doi=10.1029/91JB00250|bibcode=1991JGR....96.6879P}}</ref> Erosion of mountains massifs can create a pattern of equally high summits called ].<ref>
The first known occurrence of the term "erosion" was in the 1541 translation by ] of ]'s medical text '']''. Copland used erosion to describe how ] developed in the ]. By 1774 'erosion' was used outside medical subjects. ] employed the term in the more contemporary geological context, in his book ''Natural History'', with the quote
{{cite book |last2=Chorley |first2=Richard J.|last1=Beckinsale |first1=Robert P. |title=The History of the Study of Landforms |volume=Three | orig-year = 1991 | date=2003|publisher=Taylor & Francis e-Library |chapter= Chapter Seven: American Polycyclic Geomorphology|pages=235–236}}</ref> It has been argued that ] during ] is a more effective mechanism of lowering the height of orogenic mountains than erosion.<ref name=Deweyetal1993>{{cite journal |last1=Dewey |first1=J.F. |last2=Ryan |first2=P.D.|last3=Andersen |first3=T.B. |s2cid=55985869 |date=1993 |title=Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites |journal=Geological Society, London, Special Publications |volume=76 |issue=1 |pages=325–343 |doi= 10.1144/gsl.sp.1993.076.01.16|bibcode=1993GSLSP..76..325D}}</ref>
:"''Bounds are thus put to the erosion of the earth by water''."

Examples of heavily eroded mountain ranges include the ] of Northern Russia. Erosion of this ] has produced ]s that are now found in the ], including the Cambrian ] near ]. Studies of these sediments indicate that it is likely that the erosion of the orogen began in the Cambrian and then intensified in the ].<ref name=Orlovetal2011>{{cite journal |last1=Orlov |first1=S.Yu. |last2=Kuznetsov |first2=N.B.|last3=Miller |first3=E.D.|last4=Soboleva |first4=A.A.|last5=Udoratina |first5=O.V. |date=2011 |title=Age Constraints for the Pre-Uralide–Timanide Orogenic Event Inferred from the Study of Detrital Zircons |url=https://www.researchgate.net/publication/236585203 |journal=Doklady Earth Sciences |volume=440 |issue=1 |pages=1216–1221 |doi= 10.1134/s1028334x11090078|access-date=22 September 2015|bibcode = 2011DokES.440.1216O |s2cid=128973374 }}</ref>

===Soils===
{{further|soil erosion|pedogenesis}}
If the erosion rate exceeds ], erosion destroys the soil.<ref name=Migon>{{cite encyclopedia|last=Lupia-Palmieri|first=Elvidio|editor-last=Goudie|editor-first=A.S.|editor-link=Andrew Goudie (geographer)|encyclopedia=Encyclopedia of Geomorphology|title=Erosion|year=2004|pages=336}}</ref> Lower rates of erosion can prevent the formation of ] that take time to develop. ]s develop on eroded landscapes that, if stable, would have supported the formation of more developed ].<ref>{{cite book |last=Alexander |first=Earl B. |date=2014 |title=Soils in natural landscapes |publisher=CRC Press |page=108 |isbn=978-1-4665-9436-4}}</ref>

While erosion of soils is a natural process, human activities have increased by 10-40 times the rate at which erosion occurs globally. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in ] and (on ]s) ], both because of loss of the nutrient-rich upper ]. In some cases, the eventual result is ]. Off-site effects include ] and ] of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of ]; combined, they are responsible for about 84% of the global extent of ], making excessive erosion one of the most significant ].<ref name="toy-2002-p1" /><ref name="Springer">{{cite book|author=Blanco, Humberto |author2=Lal, Rattan|chapter=Soil and water conservation|title=Principles of Soil Conservation and Management|publisher=Springer|year=2010|isbn=978-90-481-8529-0|page=2|chapter-url=https://books.google.com/books?id=Wj3690PbDY0C&pg=PA2}}</ref>

Often in the United States, farmers cultivating ] must comply with a conservation plan to be eligible for agricultural assistance.<ref>{{cite web|title=Farm and Commodity Policy: Glossary|url=http://www.ers.usda.gov/briefing/farmpolicy/glossary.htm#conscompliance|publisher=United States Department of Agriculture|access-date=17 July 2011|archive-date=2 September 2011|archive-url=https://web.archive.org/web/20110902194840/http://ers.usda.gov/Briefing/FarmPolicy/Glossary.htm#conscompliance|url-status=dead}}</ref>

==Consequences of human-made soil erosion==
{{Main|Human impact on the environment|Environmental impact of agriculture|Soil retrogression and degradation|Land degradation}}


==See also== ==See also==
{{Div col}}
{{Sister project links|Erosion}}
* {{annotated link|Bridge scour}}
<div style="-moz-column-count:3; column-count:3;">
* {{annotated link|Cellular confinement}}
* ]
* {{annotated link|Colluvium}}
* ]
* {{annotated link|Groundwater sapping}}
* ]
* {{annotated link|Lessivage}}
* ]
* {{annotated link|Space weathering}}
* ]
* {{annotated link|Vetiver System}}
* Certified Professional in Erosion and Sediment Control ]
{{Div col end}}
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
</div>


==Notes== ==References==
{{reflist|2}} {{reflist}}
{{refbegin}}
* World Bank 2001: China: Air, Land, and Water.
*
{{refend}}


==Further reading== ==Further reading==
{{Refbegin}}
* {{cite book|last=Boardman|first=John|coauthors=Poesen, Jean|authorlink=J. Boardman|title=Soil erosion in Europe|edition=|publisher=]|location=Chichester|year=2006|isbn=978-0-470-85910-0|series=}}
* {{cite book|editor-last1=Boardman|editor-first1=John|editor-last2=Poesen|editor-first2=Jean|title=Soil Erosion in Europe|date=2007|publisher=John Wiley & Sons|location=Chichester|isbn=978-0-470-85911-7}}
* Montgomery, David R. (2007) PNAS 104: 13268-13272.
* {{cite book|last=Brown|first=Jason|coauthors=Drake, Simon|authorlink=|title=Classic Erosion|edition=|publisher=]|location=Venture West|year=2009|isbn=|series=}} * {{cite book|author=Montgomery, David|title=Dirt: The Erosion of Civilizations|publisher=University of California Press|year=2008|edition=1st|isbn=978-0-520-25806-8|url=https://books.google.com/books?id=HSu8r15-nnoC}}
* {{cite journal|last1=Montgomery|first1=D.R.|title=Soil erosion and agricultural sustainability|journal=Proceedings of the National Academy of Sciences|date=8 August 2007|volume=104|issue=33|pages=13268–13272|doi=10.1073/pnas.0611508104|pmid=17686990|bibcode = 2007PNAS..10413268M |pmc=1948917|doi-access=free}}
* {{cite book|editor=Vanoni, Vito A.|chapter=The nature of sedimentation problems|title=Sedimentation Engineering|publisher=ASCE Publications|isbn=978-0-7844-0823-0|chapter-url=https://books.google.com/books?id=TxGTDYz_FnwC&pg=PA1|date=1975}}
* {{cite book |url=http://www.csf-desertification.eu/dossier/item/fighting-wind-erosion |last1=Mainguet |first1=Monique |last2=Dumay |first2=Frédéric |date=April 2011 |title=Fighting wind erosion. One aspect of the combat against desertification |series=Les dossiers thématiques du CSFD |issue=3 |publisher=CSFD/Agropolis International |access-date=7 October 2015 |archive-date=30 December 2020 |archive-url=https://web.archive.org/web/20201230021501/http://www.csf-desertification.eu/dossier/item/fighting-wind-erosion |url-status=dead }}
{{Refend}}


==External links== ==External links==
{{Sister project links|auto=1|wikt=y|n=y|b=y}}
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Latest revision as of 00:14, 9 December 2024

Natural processes removing soil and rock For other uses, see Erosion (disambiguation).

An actively eroding rill on an intensively-farmed field in eastern Germany. This phenomenon is aggravated by poor agricultural practices because when ploughing, the furrows were traced in the direction of the slope rather than that of the terrain contour lines.

Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

Agents of erosion include rainfall; bedrock wear in rivers; coastal erosion by the sea and waves; glacial plucking, abrasion, and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes in steep landscapes like landslides and debris flows. The rates at which such processes act control how fast a surface is eroded. Typically, physical erosion proceeds the fastest on steeply sloping surfaces, and rates may also be sensitive to some climatically controlled properties including amounts of water supplied (e.g., by rain), storminess, wind speed, wave fetch, or atmospheric temperature (especially for some ice-related processes). Feedbacks are also possible between rates of erosion and the amount of eroded material that is already carried by, for example, a river or glacier. The transport of eroded materials from their original location is followed by deposition, which is arrival and emplacement of material at a new location.

While erosion is a natural process, human activities have increased by 10–40 times the rate at which soil erosion is occurring globally. At agriculture sites in the Appalachian Mountains, intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, this leads to desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide.

Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils.

A natural arch produced by the wind erosion of differentially weathered rock in Jebel Kharaz, Jordan
A wave-like sea cliff produced by coastal erosion, in Jinshitan Coastal National Geopark, Dalian, Liaoning Province, China

Physical processes

Rainfall and surface runoff

Soil and water being splashed by the impact of a single raindrop

Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four).

In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles. The distance these soil particles travel can be as much as 0.6 m (2.0 ft) vertically and 1.5 m (4.9 ft) horizontally on level ground.

If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs. If the runoff has sufficient flow energy, it will transport loosened soil particles (sediment) down the slope. Sheet erosion is the transport of loosened soil particles by overland flow.

A spoil tip covered in rills and gullies due to erosion processes caused by rainfall: Rummu, Estonia

Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers.

Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth. A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations.

Extreme gully erosion can progress to formation of badlands. These form under conditions of high relief on easily eroded bedrock in climates favorable to erosion. Conditions or disturbances that limit the growth of protective vegetation (rhexistasy) are a key element of badland formation.

Rivers and streams

Further information on water's erosive ability: Hydraulic action
Dobbingstone Burn, Scotland, showing two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the stream's banks are glacial till that was left behind as ice age glaciers flowed over the terrain.
Layers of chalk exposed by a river eroding through them
Green land erosion
Green land erosion

Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside, creating head cuts and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V-shaped cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles, and boulders can also act erosively as they traverse a surface, in a process known as traction.

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as scour. Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.

Thermal erosion is the result of melting and weakening permafrost due to moving water. It can occur both along rivers and at the coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials. Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a 100-kilometre (62-mile) segment of the Beaufort Sea shoreline averaged 5.6 metres (18 feet) per year from 1955 to 2002.

Most river erosion happens nearer to the mouth of a river. On a river bend, the longest least sharp side has slower moving water. Here deposits build up. On the narrowest sharpest side of the bend, there is faster moving water so this side tends to erode away mostly.

Rapid erosion by a large river can remove enough sediments to produce a river anticline, as isostatic rebound raises rock beds unburdened by erosion of overlying beds.

Coastal erosion

Main article: Coastal erosion See also: Beach evolution
Wave cut platform caused by erosion of cliffs by the sea, at Southerndown in South Wales
Erosion of the boulder clay (of Pleistocene age) along cliffs of Filey Bay, Yorkshire, England

Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.

Sea-dune erosion at Talacre beach, Wales

Hydraulic action takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping and grinding of organisms, a process termed bioerosion.

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent supply of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of deposition. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a buildup of eroded material occurs forming a long narrow bank (a spit). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.

Erosion of a coastal surface, followed by a fall in sea level, can produce a distinctive landform called a raised beach.

Chemical erosion

See also: Karst topography

Chemical erosion is the loss of matter in a landscape in the form of solutes. Chemical erosion is usually calculated from the solutes found in streams. Anders Rapp pioneered the study of chemical erosion in his work about Kärkevagge published in 1960.

Formation of sinkholes and other features of karst topography is an example of extreme chemical erosion.

Glaciers

The Devil's Nest (Pirunpesä), the deepest ground erosion in Europe, located in Jalasjärvi, Kurikka, Finland
Glacial moraines above Lake Louise, in Alberta, Canada

Glaciers erode predominantly by three different processes: abrasion/scouring, plucking, and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes, such as erosion also control cross-valley variations. In a homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created. Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in glaciated valleys. Scientists also provide a numerical estimate of the time required for the ultimate formation of a steady-shaped U-shaped valley—approximately 100,000 years. In a weak bedrock (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, the amount of over deepening is limited because ice velocities and erosion rates are reduced.

Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. Differences in the height of mountain ranges are not only being the result tectonic forces, such as rock uplift, but also local climate variations. Scientists use global analysis of topography to show that glacial erosion controls the maximum height of mountains, as the relief between mountain peaks and the snow line are generally confined to altitudes less than 1500 m. The erosion caused by glaciers worldwide erodes mountains so effectively that the term glacial buzzsaw has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges. As mountains grow higher, they generally allow for more glacial activity (especially in the accumulation zone above the glacial equilibrium line altitude), which causes increased rates of erosion of the mountain, decreasing mass faster than isostatic rebound can add to the mountain. This provides a good example of a negative feedback loop. Ongoing research is showing that while glaciers tend to decrease mountain size, in some areas, glaciers can actually reduce the rate of erosion, acting as a glacial armor. Ice can not only erode mountains but also protect them from erosion. Depending on glacier regime, even steep alpine lands can be preserved through time with the help of ice. Scientists have proved this theory by sampling eight summits of northwestern Svalbard using Be10 and Al26, showing that northwestern Svalbard transformed from a glacier-erosion state under relatively mild glacial maxima temperature, to a glacier-armor state occupied by cold-based, protective ice during much colder glacial maxima temperatures as the Quaternary ice age progressed.

These processes, combined with erosion and transport by the water network beneath the glacier, leave behind glacial landforms such as moraines, drumlins, ground moraine (till), glaciokarst, kames, kame deltas, moulins, and glacial erratics in their wake, typically at the terminus or during glacier retreat.

The best-developed glacial valley morphology appears to be restricted to landscapes with low rock uplift rates (less than or equal to 2mm per year) and high relief, leading to long-turnover times. Where rock uplift rates exceed 2mm per year, glacial valley morphology has generally been significantly modified in postglacial time. Interplay of glacial erosion and tectonic forcing governs the morphologic impact of glaciations on active orogens, by both influencing their height, and by altering the patterns of erosion during subsequent glacial periods via a link between rock uplift and valley cross-sectional shape.

Floods

The mouth of the River Seaton in Cornwall after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode
The mouth of the River Seaton in Cornwall after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode; leaving behind a tall sand bank in its place

At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called rock-cut basins. Examples can be seen in the flood regions result from glacial Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington.

Wind erosion

Árbol de Piedra, a rock formation in the Altiplano, Bolivia sculpted by wind erosion
Main article: Aeolian processes

Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation, urbanization, and agriculture.

Wind erosion is of two primary varieties: deflation, where the wind picks up and carries away loose particles; and abrasion, where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) surface creep, where larger, heavier particles slide or roll along the ground; (2) saltation, where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) suspension, where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%).

Wind erosion is much more severe in arid areas and during times of drought. For example, in the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.

Mass wasting

A wadi in Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks
Main article: Mass wasting

Mass wasting or mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity.

Mass wasting is an important part of the erosional process and is often the first stage in the breakdown and transport of weathered materials in mountainous areas. It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are always occurring continuously on all slopes; some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope.

Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression, in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence.

Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter by wind along the soil surface.

Submarine sediment gravity flows

Bathymetry of submarine canyons in the continental slope off the coast of New York and New Jersey

On the continental slope, erosion of the ocean floor to create channels and submarine canyons can result from the rapid downslope flow of sediment gravity flows, bodies of sediment-laden water that move rapidly downslope as turbidity currents. Where erosion by turbidity currents creates oversteepened slopes it can also trigger underwater landslides and debris flows. Turbidity currents can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock. Almost all continental slopes and deep ocean basins display such channels and canyons resulting from sediment gravity flows and submarine canyons act as conduits for the transfer of sediment from the continents and shallow marine environments to the deep sea. Turbidites, which are the sedimentary deposits resulting from turbidity currents, comprise some of the thickest and largest sedimentary sequences on Earth, indicating that the associated erosional processes must also have played a prominent role in Earth's history.

Factors affecting erosion rates

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Climate

See also: Climatic geomorphology

The amount and intensity of precipitation is the main climatic factor governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation. This might be during periods when agricultural activities leave the soil bare, or in semi-arid regions where vegetation is naturally sparse. Wind erosion requires strong winds, particularly during times of drought when vegetation is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties. In general, given similar vegetation and ecosystems, areas with more precipitation (especially high-intensity rainfall), more wind, or more storms are expected to have more erosion.

In some areas of the world (e.g. the mid-western US), rainfall intensity is the primary determinant of erosivity (for a definition of erosivity check,) with higher intensity rainfall generally resulting in more soil erosion by water. The size and velocity of rain drops is also an important factor. Larger and higher-velocity rain drops have greater kinetic energy, and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops.

In other regions of the world (e.g. western Europe), runoff and erosion result from relatively low intensities of stratiform rainfall falling onto the previously saturated soil. In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water. According to the climate change projections, erosivity will increase significantly in Europe and soil erosion may increase by 13–22.5% by 2050

In Taiwan, where typhoon frequency increased significantly in the 21st century, a strong link has been drawn between the increase in storm frequency with an increase in sediment load in rivers and reservoirs, highlighting the impacts climate change can have on erosion.

Vegetative cover

See also: Vegetation and slope stability

Vegetation acts as an interface between the atmosphere and the soil. It increases the permeability of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of the plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water and wind erosion. The removal of vegetation increases the rate of surface erosion.

Topography

The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain is also more prone to mudslides, landslides, and other forms of gravitational erosion processes.

Tectonics

Main article: Erosion and tectonics

Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface (e.g., a mountain range) to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to the local slope (see above), this will change the rates of erosion in the uplifted area. Active tectonics also brings fresh, unweathered rock towards the surface, where it is exposed to the action of erosion.

However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause tectonic or isostatic uplift in the region. In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates, for example, beneath the extremely steep terrain of Nanga Parbat in the western Himalayas. Such a place has been called a "tectonic aneurysm".

Development

Human land development, in forms including agricultural and urban development, is considered a significant factor in erosion and sediment transport, which aggravate food insecurity. In Taiwan, increases in sediment load in the northern, central, and southern regions of the island can be tracked with the timeline of development for each region throughout the 20th century. The intentional removal of soil and rock by humans is a form of erosion that has been named lisasion.

Erosion at various scales

Mountain ranges

See also: denudation and planation

Mountain ranges take millions of years to erode to the degree they effectively cease to exist. Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode a mountain mass similar to the Himalaya into an almost-flat peneplain if there are no significant sea-level changes. Erosion of mountains massifs can create a pattern of equally high summits called summit accordance. It has been argued that extension during post-orogenic collapse is a more effective mechanism of lowering the height of orogenic mountains than erosion.

Examples of heavily eroded mountain ranges include the Timanides of Northern Russia. Erosion of this orogen has produced sediments that are now found in the East European Platform, including the Cambrian Sablya Formation near Lake Ladoga. Studies of these sediments indicate that it is likely that the erosion of the orogen began in the Cambrian and then intensified in the Ordovician.

Soils

Further information: soil erosion and pedogenesis

If the erosion rate exceeds soil formation, erosion destroys the soil. Lower rates of erosion can prevent the formation of soil features that take time to develop. Inceptisols develop on eroded landscapes that, if stable, would have supported the formation of more developed Alfisols.

While erosion of soils is a natural process, human activities have increased by 10-40 times the rate at which erosion occurs globally. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual result is desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems.

Often in the United States, farmers cultivating highly erodible land must comply with a conservation plan to be eligible for agricultural assistance.

Consequences of human-made soil erosion

Main articles: Human impact on the environment, Environmental impact of agriculture, Soil retrogression and degradation, and Land degradation

See also

References

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  2. Allaby, Michael (2013). "Erosion". A dictionary of geology and earth sciences (Fourth ed.). Oxford University Press. ISBN 9780199653065.
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