Attrition (weathering)
Encyclopedia
Attrition is a form of coastal or river erosion
, when the bed load
is eroded by itself and the bed. As rocks are transported downstream along a riverbed, the regular impacts between the grains themselves and between the grains and the bed cause them to be broken up into smaller fragments. This process also makes them rounder and smoother. Attrition can also occur in glaciated regions, where it is caused by the movement of ice with embedded boulders over surface sediments.
Pebbles are more affected by attrition further upstream, as the rivers' velocity tends to be higher, and therefore its competence (ability to carry sediment) is increased. This means that the load rubs against itself more and with more force when suspended in the river, thus increasing erosion by attrition, though there is a point after transport over a certain distance that pebbles reach a size that is relatively immune to further attrition. Grain-size distribution of sediments produced by attrition will also be controlled by the lithology
of the rock from which they are derived.
The effects of attrition can be mistaken for the effects of sorting
, in which the grain size of sediments is affected by sediment transport mechanisms e.g. suspension v. bed load. This effects pebble beaches the most as the pebbles smash into each other and causes it to smooth. In most cases attrition is helpful on pebble beaches because it will make sure the pebbles do not cut people and harm others.
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
, when the bed load
Bed load
The term bed load or bedload describes particles in a flowing fluid that are transported along the bed. Bed load is complementary to suspended load and wash load.Bed load moves by rolling, sliding, and/or saltating ....
is eroded by itself and the bed. As rocks are transported downstream along a riverbed, the regular impacts between the grains themselves and between the grains and the bed cause them to be broken up into smaller fragments. This process also makes them rounder and smoother. Attrition can also occur in glaciated regions, where it is caused by the movement of ice with embedded boulders over surface sediments.
Pebbles are more affected by attrition further upstream, as the rivers' velocity tends to be higher, and therefore its competence (ability to carry sediment) is increased. This means that the load rubs against itself more and with more force when suspended in the river, thus increasing erosion by attrition, though there is a point after transport over a certain distance that pebbles reach a size that is relatively immune to further attrition. Grain-size distribution of sediments produced by attrition will also be controlled by the lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
of the rock from which they are derived.
The effects of attrition can be mistaken for the effects of sorting
Sorting (sediment)
Sorting indicates the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Poorly sorted indicates that the sediment sizes are mixed ; whereas well sorted indicates that the sediment sizes are similar .The degree of sorting may also indicate the energy...
, in which the grain size of sediments is affected by sediment transport mechanisms e.g. suspension v. bed load. This effects pebble beaches the most as the pebbles smash into each other and causes it to smooth. In most cases attrition is helpful on pebble beaches because it will make sure the pebbles do not cut people and harm others.
Sources
- Strasbourg, Universite, Institut de Geographie, Laboratoire, Etudes de geomorphologie dynamique sur le Var inferieur, Geologische Rundschau, vol.43, no.2, pp.371-383, 1955
- Bird, Eric C F, Lateral grading of beach sediments; a commentary, Journal of Coastal Research, vol.12, no.3, pp.774-785, 1996