Attrition is the process of
erosion
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 distin ...
that occurs during
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
collision and transportation. The transportation of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
chips and smooths the surfaces of
bedrock
In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet.
Definition
Bedrock is the solid rock that underlies looser surface mater ...
; this can be through water or wind. Rocks undergoing attrition erosion are often found on or near the
bed of a stream. Attrition is also partially responsible for turning
boulders
In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive.
In co ...
into smaller rocks and eventually to sand.
Attrition erosion allows past and present
geologic
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...
changes to be understood as well as
paleogeomorphic environments to be interpreted. Researchers use particle shapes (a result of attrition) to study erosion and environmental changes.
Mechanism
The amount of attrition is dependent on a number of factors: particle properties such as size, shape, surface, porosity, hardness, and cracks, as well as environmental properties such as time,
velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
,
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
, shear, and temperature.
Generally, particles are more affected by attrition farther downstream, 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. Although 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
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...
produced by attrition will also be controlled by the
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. Physical characteristics include colour, texture, grain size, and composition. Lit ...
of the rock from which they are derived. Particle sizes generally decrease continuously as the river flows farther downstream, in a process called downstream fining. Since attrition affects pebble size it also affects the movement of the pebbles, and the size and condition of pebbles can explain past conditions of waterways, such as flow.
Rate of erosion
Rates of erosion by attrition are affected by the rocks shape, lithology, and the energy of impact, therefore, softer rocks are more susceptible to attrition erosion. As attrition and breakage occur on a bedrock it becomes suspended: down-stream of a river or waterway the bedload increases due to attrition. The suspended sediment is greatly affected by lithology, basin slope,
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, and wildfires;
wildfires
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identif ...
in general significantly disturb environments and therefore their geology. Erosion rates, with respect to attrition, are greatest in waterways that are steep with soft rocks, such as
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
, mudstone, or other common sedimentary rocks.
![River Bed Attrition](https://upload.wikimedia.org/wikipedia/commons/6/66/River_Bed_Attrition.png)
The rounding of rocks and
sand grains happens much slower in water environments than from wind. This is because the
water molecules
Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...
around the sand protects it from collisions and because the
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of water lowers the impact energy. Additionally, the more irregular (compared to rounded) a sediments particle is the more it is susceptible to erosion. However, rounded particles are often the result of severe erosional environments.
Differences in lithology affect how quickly erosion (attrition) changes a
shoreline
A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
. It has been observed that
schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
rocks round quickly but then are quickly eroded away. While
quartzite
Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
stays rounded for a longer time. Quartzite has been found to round easily compared to schist and
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
. Schists often erode into a flat surface under high energy attrition, not a rounded particle. Sandstone erodes to a shape between quartzite and schist. Rocks that have undergone chemical alterations, like lithification, tend to strongly resist erosion.
Cosmogenic exposure
Erosion can affect the cosmogenic exposure dating of boulders by altering the
cosmogenic isotope
Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the nucleus of an ''in situ'' Solar System atom, causing nucleons (protons and neutrons) to be expelled from the atom ...
concentration. So, by finding the cosmogenic exposure for two samples of the same rock the exposure time and rate of erosion can be found. The more accurate and
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
measurement is the more accurate the erosion rate or exposure time will be.
Cosmogenic exposure dating is a powerful tool in understating the process rocks undergo and can lead to a greater understanding in geomorphological studies.
Attrition in ocean environments
Attrition in ocean environments is described as “the oceans consuming the land” because the high impact energy of waves and high sedimentation allow the ocean-land contact points to be significantly eroded. Ocean attrition causes shorelines to retreat and ocean depths are increased to the depth of the wave base.
![Attrition Erosion, Langeland Denmark](https://upload.wikimedia.org/wikipedia/commons/3/37/Attrition_Erosion%2C_Langeland_Denmark.png)
The rising of
sea levels
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
has led to an increase in
coastal erosion
Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward ...
. This causes concern to policymakers, coastal researchers, and real estate planners due to erosions effect on flooding.
Rocky coastlines tend to lack vegetation: this leads to little or no
humic acid Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th ...
(organic compounds like soil). No humic acid means there is less chemical erosion, so the erosion on coastlines are almost exclusively caused by particle collisions.
Attrition in volcanic settings
As ash and volcanic
pyroclasts
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
erupt from a
volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
they undergo attrition. Attrition is one of the reasons why
volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
is very fine-grained. The more attrition erosion the more fine-grained ash produced. The consequence of this is the disruption of a volcano's geologic stability, varied
tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
(rock and particles expelled from the volcano during the eruption), and more particles in the atmosphere affecting climate.
The rate of attrition on tephra is affected by the size of the volcano- specifically the depth and height of the volcanic column.
Similar processes
The effects of attrition can be mistaken for the effects of sorting, in which the grain size of sediments is affected by
sediment transport
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural system ...
mechanisms e.g. suspension vs. bed load. This affects pebble beaches the most as the pebbles smash into each other, which causes them to smooth.
Attrition of
particulate
Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ter ...
material is also observed in the chemical industry, where it is undesirable. Products can be lost through the process and
contaminants
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
Wi ...
can be created, requiring additional filtration.
Attrition occurring in an industrial application stems from a wide range of mechanisms: mechanical, thermal, and chemical.
In
abrasive blasting
Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove su ...
, the useful life of the
abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...
(sand or shot) is limited by attrition because as the workpiece surface is worn down, the abrasive breaks down as well.
Attrition contributes to other types of erosion such as deflation and
corrasion
Corrasion is a geomorphological term for the process of mechanical erosion of the earth's surface caused when materials are transported across it by running water, waves, glaciers, wind or gravitational movement downslope. An example is the wear ...
.
[Anim, Mabel, and Benjamin Kofi Nyarko. “Lithological Responses to Sea Erosion along Selected Coastlines between Komenda and Saltpond, Ghana .” Ghana Journal of Geography, vol. 9, no. 1, 2017, pp. 109–126.] Although attrition is often considered a type of corrasion they differ because attrition does not move stationary surfaces and instead erodes them through transited materials.
References
{{Reflist
Sedimentology
Erosion