Abrasion (geology)

Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time. It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles. Abrasion generally occurs in four ways: glaciation slowly grinds rocks picked up by ice against rock surfaces; solid objects transported in river channels make abrasive surface contact with the bed and walls; objects transported in waves breaking on coastlines; and by wind transporting sand or small stones against surface rocks. Abrasion, under its strictest definition, is commonly confused with attrition and sometimes hydraulic action however, the latter less commonly so. Both abrasion and attrition refers to the wearing down of an object. Abrasion occurs as a result of two surfaces rubbing against each other, resulting in the wearing down of one or both of the surfaces. However, attrition refers to the breaking off of particles (erosion) which occurs as a result of objects hitting against each other. Abrasion leads to surface-level destruction over a period of time, whereas attrition results in more change at a faster rate. Today, the geomorphology community uses the term "abrasion" in a looser way, often interchangeably with the term "wear". Abrasion in a stream or river channel occurs when the sediment carried by a river scours the bed and banks, contributing significantly to erosion. In addition to chemical and physical weathering of hydraulic action, freeze-thaw cycles, and more, there is a suite of processes which have long been considered to contribute significantly to bedrock channel erosion include plucking, abrasion (due to both bedload and suspended load), solution, and cavitation. In terms of a glacier, it is a similar principal; the moving of rocks over a surface wears it away with friction, digging a channel that, when the glacier moves away, is called a U-shaped valley.