Roundness is the degree of smoothing due to abrasion of sedimentary particles. It is expressed as the ratio of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere.

Measure of roundness

Rounding, roundness or angularity are terms used to describe the shape of the corners on a particle (or

clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...

) of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

. Such a particle may be a grain of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

, a

pebble A pebble is a clastic rocks, clast of rock (geology), rock with a grain size, particle size of based on the Particle size (grain size), Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than Granule (geology), gra ...

, cobble or

boulder 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 ...

. Although roundness can be numerically quantified, for practical reasons

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

s typically use a simple visual chart with up to six categories of roundness: *Very angular: corners sharp and jagged *Angular *Sub-angular *Sub-rounded *Rounded *Well-rounded: corners completely rounded This six-fold category characterisation is used in the Shepard and Young comparison chart and the Powers chart but the Krumbein chart has nine categories. Rounding of sediment particles can indicate the distance and time involved in the

transportation Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...

of the sediment from the source area to where it is deposited. Speed of rounding will depend on composition,

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

and mineral

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

. For example, a soft

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

pebble will obviously round much faster, and over a shorter distance of transport, than a more resistant

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

pebble. The rate of rounding is also affected by the

grain size Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...

and energy conditions. Angularity (A) and roundness (R) are but two parameters of the complexity of a clast's generalised form (F). A defining expression is given by: F=f(Sh, A, R, Sp, T) where f denotes a functional relationship between these terms and where Sh denotes the shape, Sp the

sphericity Sphericity is a measure of how closely the shape of a physical object resembles that of a perfect sphere. For example, the sphericity of the ball (bearing), balls inside a ball bearing determines the quality (business), quality of the bearing, ...

and T the micro-scale surface texture. An example of this practical use has been applied to the roundness of the grains in the Gulf of Mexico in order to observe the distance from the source rocks.

Abrasion

Abrasion occurs in natural environments such as

beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from Rock (geology), rock, such as sand, gravel, shingle beach, shingle, pebbles, etc., or biological s ...

es,

sand dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

or stream beds by the action of current flow, wave impact,

glacial A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

action, wind, gravitational creep and other erosive agents. Recent studies have demonstrated that aeolian processes are more efficient in the rounding of sedimentary grains. Experimental studies have shown that the angularity of sand-sized detrital quartz can remain virtually unchanged after hundreds of kilometers of fluvial transport.

Paleogeographic value of determining the degree of roundness of clastic material

Roundness is an important indicator of the genetic affiliation of a

clastic rock Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...

. The degree of roundness points to the range and mode of transport of clastic material, and can also serve as a search criterion in mineral exploration, especially for

placer deposit In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish language, Spanish word ''placer'', meaning "alluviu ...

s. Alluvial debris in major rivers tend to exhibit a high degree of roundness. Alluvium from small rivers is less rounded. Deposits of ephemeral streams exhibit little rounding with angular clasts.

Clast rounding in non-sedimentary environments

Pebble dike A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...

s are dikelike bodies found in intrusive environments, usually associated with porphyry-type ore deposits, which contain variably rounded fragments in a finely-ground matrix of pulverized rock. The clasts originate in deeper formations in

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

systems, and have been brought up explosively by diatreme or intrusive

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

s as

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

and/or magmatic water flash boils. The clasts have been rounded due to thermal spallation, ''milling'' action, or corrosion by

fluids.Guilbert, John M. and Charles F. Park, Jr., ''The Geology of Ore Deposits,'' Freeman, 1986, pp.83-85 The ore deposits of Tintic mining district and White Pine mining district, and East Traverse Mountain,

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

; Urad, Mt. Emmons, Central City, Leadville, and

Ouray, Colorado Ouray ( ) is a List of municipalities in Colorado#Home rule municipality, home rule municipality that is the county seat of Ouray County, Colorado, United States. The city population was 898 as of the 2020 United States census, 2020 census. The ...

;

Butte In geomorphology, a butte ( ) is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and table (landform), tablelands. The word ''butte'' comes from the French l ...

, Montana; Silver Bell; and

Bisbee, Arizona Bisbee is a city in and the county seat of Cochise County, Arizona, Cochise County in southeastern Arizona, United States. It is southeast of Tucson, Arizona, Tucson and north of the Mexican border. According to the 2020 United States census, ...

; and the

Kiruna (; ; ; ) is the northernmost Stad (Sweden), city in Sweden, situated in the province of Lapland, Sweden, Lapland. It had 17,002 inhabitants in 2016 and is the seat of Kiruna Municipality (population: 23,167 in 2016) in Norrbotten County. The c ...

iron deposit in Sweden, Cuajone and Toquepala in Peru; El Salvador in Chile; Mt. Morgan in Australia; and Agua Rica in Argentina contain these pebble dikes.

References

* Mountain Encyclopedia. - Moscow: Soviet Encyclopedia, 1987. - Vol. 3 - S. 553. * Geological dictionary . - M. : Nedra, 1978. - T. 2. - S. 29. * Kulik, NA, Postnov AV Geology, petrography and mineralogy in archaeological research . - Methods of Earth and Man in archaeological research : Comprehensive Training Manual. - Novosibirsk: Novosibirsk State University, Institute of Archaeology and Ethnography SB RAS, 2010. - S. 39–96. * {{DEFAULTSORT:Roundness (Geology) Geomorphology Paleogeography Sedimentology

Measure of roundness

Abrasion

Paleogeographic value of determining the degree of roundness of clastic material

Clast rounding in non-sedimentary environments

See also

References